Stratospheric Warming Event Projected To End January

Model guidance continues to indicate that the stratospheric polar vortex will come under further stress as we head toward the end of the month, with a wavenumber-1 event potentially unfolding in the stratosphere and opening the chance for February to be colder than normal.

FU-Berlin

Shown above is the 240-hour forecast of geopotential heights (contours) and temperatures (colored fill) from the ECMWF model, analyzing the 70-millibar slice of the stratosphere. This is around the middle or lower-middle part of the stratosphere, and as a rule of thumb stratospheric warming events tend to be stronger and more able to dislodge the polar vortex when the warming event is seen throughout multiple layers of the stratosphere. While I'm only showing the 70-milliber layer, model guidance has latched on to this warming potential throughout the stratosphere.

In this image, we see the stratospheric polar vortex still pretty much in control, centered almost over the North Pole. The piece to analyze, however, is the swath of warmer air trying to push into the Arctic Circle, with the warmest temperatures located over the Bering Sea. The placement of these warmer anomalies is important, because when stratospheric warming events push into the Arctic Circle from the Bering Sea region, they tend to be more successful at disrupting the polar vortex. I'm sure there's some academic literature on exactly why this happens, but in the absence of reading such literature I take that as a rule of thumb.

We can get a better look at the polar vortex through forecasted potential vorticity charts.

FU-Berlin

In a nutshell, these charts show the orientation of the polar vortex at different levels of the stratosphere. This map shows potential vorticity values at the 550 K level, which is somewhere around the 20-millibar level. Warmer colors depict areas of higher potential vorticity - in this case, the location of the polar vortex. Colder colors indicate where the polar vortex is not, in essence. At hour 240, we see the polar vortex in a rather elongated shape, dislodged from the Arctic Circle and positioned from Greenland across northern Eurasia. This shape of the polar vortex signals that it will be under duress at this point in time, as the polar vortex is 'typically' seen in a more circular shape. This elongated shape suggests pressure being applied from the Bering Sea area in the form of a ridge, which is confirmed by the deeper blue area stretching from the Gulf of Alaska to the Bering Sea. Similar to how higher levels of potential vorticity indicate the polar vortex, lower values (especially in this case) can be indicative of a ridge of high pressure.

So far, we've gathered that model guidance projects the stratospheric polar vortex to come under substantial pressure in the next several days, likely continuing through the end of January. This pressure looks to be applied across much of the stratosphere, boosting the chances that any disruption in the polar vortex could then show up in the troposphere in the form of colder than normal weather. Let's sum it all up with a few graphs.

FU-Berlin

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels, as we discussed with the rule of thumb earlier in this post. However, it can still be valuable to look at, as a reversal in wind direction at the 1-millibar level could then filter down to lower levels. In this forecast image, the 1-millibar wind speed is forecast to continue declining slightly from a recent peak, before strengthening again and then once more weakening. This would take it back to roughly the same level we're at right now, if this forecast verified exactly as shown. Let's see if any signals can be identified in other panels.

The second panel shows the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where both slices of the stratosphere look to see the strength of the polar vortex gradually decline as wind speeds slow down. While the deceleration here appears modest, there are signs that it will continue through the end of January. Both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, which will lead to a slight deceleration across the 1-millibar, 10-millibar and 30-millibar areas. This will then revert back to near-zero, but at the end of the forecast period the ECMWF sees these fluxes strengthening again, leading to further wind speed deceleration in the aforementioned levels of the stratosphere.  Perhaps most significantly, for this second acceleration in the two fluxes, you'll note how the EP-flux on the bottom panel is pointing straight up, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is also forecasted for the end of the forecast period). As such, while the ongoing increase in fluxes may not do much damage to the polar vortex, the re-configuration of the EP-flux in time for the second projected warming event in about ten days could do far more damage, especially if it is a sustained warming event.

As such, I expect a broadly seasonal to warmer-than-normal temperature pattern for the eastern 2/3 of the U.S. through the end of January, but the chances of colder-than-normal weather increasing, particularly after the first few days of February.

To Summarize:

- Model guidance continues to indicate a potentially-disruptive warming event in the stratosphere towards the end of January.
- As this warming event is expected to affect nearly all sections of the stratosphere, effects in the troposphere are likely in the first and second full weeks of February.
- Colder than normal conditions are expected to increase in likelihood after the first few days of February.

Andrew

Long Range Outlook (Made February 19, 2015)

This is the updated long range outlook, made on February 19th, and valid for late February into the first half of March.

Tropical Tidbits
Click to enlarge

We're currently seeing a portion of the semi-permanent Arctic low pressure vortex intruding into the United States, with anomalously low 500mb geopotential height values appearing in the colored shadings and analysis in the graphic above. This anomaly has been forced by a strong ridge pushing into Alaska from the northeast Pacific and west coast of North America. Over the next few days, cold air intrusions should continue as this ridge flexes its muscles and persuades additional Arctic air masses to collapse into the lower latitudes.

ESRL

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...
The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The positive PNA and negative EPO have worked in tandem to indicate this strong ridge blossoming into the Gulf of Alaska and general northeast Pacific. In the upcoming couple of weeks, model guidance is indicating we see the ridge shift further offshore to the west, as the PNA dip from positive to negative shows. Over time, ensemble guidance is telling us that this ridge could keep shifting west, resulting in the EPO actually moving positive for the first time in a while. The positive EPO signal, however, is weak, and will have to be watched for a 'false positive', both literally and metaphorically. If this forecast does verify, we can expect a warm-up in the early days of March.

Tropical Tidbits

Ensembles indicate we will see a ridge of high pressure pushing into Japan on the morning of February 21, as the graphic above shows. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, we can anticipate a warm-up for millions that have undergone a brutal February in the final days of the month, likely into early March. From there, confidence decreases, but a return to an average or cooler than normal pattern may be expected.

You'll notice I haven't used tropical forcing in this post; that's because the ridge in the West is just so overpowering that the Madden Julian Oscillation can't do much of anything.

To summarize:

- A cold pattern is expected to round out February.
- A warm-up is expected for the last days of the month into early March.
- The first half of March should be characterized by a generally average to slightly below-average temperature pattern. There are hints of some stormy weather in early March.

Andrew

Long Range Outlook (Made January 28, 2015)

This is the long range outlook post, made January 28th, 2015, valid for the next 7-31 days.

We'll begin with a look at what's happened in the last week, as well as a verification check on the calls made in our last long range outlook post.

ESRL

Over the last week, we saw rather persistent ridging building over the Western United States into southwest Canada. This forced the development of some colder weather in eastern Canada and the northeast United States. The tropospheric polar vortex continued to be in a disrupted state, with multiple bodies of ridging being forced into the Arctic Circle.

In our last outlook, we predicted a period of cooler than normal weather for much of the Central and East US. That call didn't verify as well as we hoped, with warmth taking hold in the Northern Plains, as well as some rather mild conditions across the Southeast. However, the general idea for cold being maximized in the Northeast did work out well.

JMA

We'll now go over tropical forcing across the globe, and show how our pattern has been affected by it.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

We've begun to see enhanced convection make its way across the waters due south of Eurasia, as our new Madden-Julian Oscillation (MJO) wave forms. This convection is currently located west of India when viewing it from a longitudinal aspect. Such positioning of convection usually favors a colder and stormier pattern in the East US, as was evidenced by the recent blizzard that hit parts of the coastal Northeast. We're still seeing enhanced tropical convection west of the 180 degree longitude line, and this may have led to the unexpected warmth that messed with our call for a cool period this past week. As this convection moves east and dies off, the newly-developing body of convection should take control.

Now that we've discussed what has happened, let's start to go over what will happen.

ESRL

The image above shows 500mb geopotential height anomalies over the Northern Hemisphere, where blues and purples depict negative height anomalies, usually indicative of cold/stormy weather. Similarly, greens and reds indicate the presence of positive height anomalies, a precursor to warmth and generally quiet weather. In this panel of the ESRL ensemble forecast, valid for 9 days from today, we see that the persistent lobe of the tropospheric polar vortex has retracted north into Greenland, a classic positive North Atlantic Oscillation (NAO) signal. However, due to the blossoming ridge in the West, stormy weather still continues in the nation's midsection. This does not last long, however. This forecast matches our projection in the last Long Range Outlook for a brief cold spell around February 6-7.

ESRL

By the 336-hour forecast mark, ensembles are in agreement that the positive NAO will permit that ridge in the West US to bleed east, resulting in a warmer than normal pattern for much of the country. Negative height anomalies are shown along the Gulf Coast, but an unfavorable set-up upstream (to the west) of the United States means that warmth should prevail. This matches our earlier call for warmth after about the first week of February. A consolidated tropospheric polar vortex certainly does not help matters for winter weather fans, either.

Now that we've looked over the next 7-14 days, let's start looking out even further.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of February 2nd. Note the presence of a trough just to the east Japan, with a very powerful ridge just to the west of the country, arguably influencing the nation more than the trough. This trough appears to be a storm system that may impact us here in the United States down the road, just before this ridge may take hold, but that potential will be investigated in a later post. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a general warm period, possibly cooler in the East, around February 8th - 12th.

Tropical Tidbits

Long range models have been hinting at the idea that a strong upper level low or deepening trough will slide into Japan around February 6th or 7th. For now, ensembles are taking a more progressive and generally weaker approach with this potential, as is expected. For now, we'll watch a February 12th - 16th period for cooler weather, throwing a wrench into our outlook from the last post.

ESRL

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA includes a decrease in positive values as of right now, before a spike back well into positive territory in the long range. This works well with the ESRL ensembles we were analyzing earlier. The NAO forecast generally stays positive in the long range, a red flag for that positive PNA ridge to shift east into the Central and East US. A dip to negative territory does occur in the January-February transition, which may create a storm threat (upcoming posts will address this), but nothing too serious. Both the WPO and EPO are negative in the long range, which is good for winter weather fans. However, the consensus has been for this -EPO/WPO regime to weaken heading beyond the 16 day period, which may only enhance the threat for warmer weather in the middle of February.

Finally, let's use the skills learned in our OLR analysis earlier, to forecast the long range.

CPC

As in the JMA graphic, cooler colors define negative outgoing longwave radiation anomalies, which mean the presence of thunderstorm activity. Warm colors depict positive OLR anomalies, otherwise known as suppressed storm activity. In the 6-10 day forecast panel, we begin to see our new MJO wave really strengthen and define itself as it moves east, just south of the Indian subcontinent. Notice in this same panel that our current MJO wave near the 180 degree longitude line is nearly dissipated by this forecast time period. At the 11-15 day forecast mark, the MJO wave has strengthened considerably, and has now switched to pro-warmth phases. These pro-warmth phases can generally be identified when -OLR anomalies move east of the Indian subcontinent, and this should happen even beyond the 11-15 day forecast period. Beyond then, confidence is too low to accurately produce a forecast.

To summarize:

- A period of rather chilly weather should kick off the month of February.
- A brief period of seasonal to cool weather is expected to impact the Central and East US around February 6-7.
- In mid-February, the atmosphere is in favor of a warm pattern. However, disagreement from the Typhoon Rule means this part of the month could really go either way. We will re-examine this in the next long range post.
- An early look at late February continues to show a predominantly warmth-favoring pattern.

Andrew

Long Range Outlook (Made January 21, 2015)

This is the long range outlook post, made January 21st, 2015. This post will address the upcoming pattern over the next 7-31 days.

We'll begin with an analysis of the pattern over the last several days.

ESRL
Click images to enlarge

Over the last week, we saw ridging present across the Western US, as exemplified by the green and yellow colors, with that ridge making its way east to provide warmth for many in the Central US. Ridging was also dominant over Greenland and south, into the North Atlantic and Canadian Maritimes. These two ridges combined to force an upper level low into northern Canada, rather than further south towards the United States, a likely scenario if the former ridge out west had been more dominant.

On a more synoptic scale, we recognize high pressure has forced the tropospheric upper-latitude vortex to weaken and splinter, with the prevailing lobe located in western Europe, and other splinters scattered across lower latitudes. This tells us that the cold air up in Canada is more free to move around, rather than maintain its position, locked in the Arctic Circle.

JMA

Let's now discuss tropical forcing across the globe to recognize how it has affected our pattern the last several days.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

Over the past several days, we've seen an area of enhanced thunderstorms make its way eastward from Oceania, and we now see it beginning to weaken off the western coast of the South and Central Americas. You weather enthusiasts may recognize this as a weakening Madden-Julian Oscillation wave. However, look towards southern Africa. We see a new plume of thunderstorms beginning to develop. It is expected that these storms will propagate eastward and form the new MJO wave that will also push east with time, but that's for discussion later on in this post.

Now that we've gone over the pattern developing in the last several days, let's start looking into the future, beyond the 7 day window.

ESRL

This is the ESRL ensemble projection of 500mb geopotential height anomalies for 7 days out. Here, we see a strong ridge pumping north in the Western US, resulting in a deep trough (and associated cold weather) in the East US. This fits in well with the expected progression of our new MJO wave east, as the placement of tropical convection just southwest of India supports this type of cold regime. Looking across the northern hemisphere, we can also identify an upper level low over Greenland, a signal for the positive phase of the North Atlantic Oscillation (NAO). The +NAO supports a more low-amplitude jet stream, which is technical-speak for weaker ridges and weaker storms, as well as a faster progression of those ridges and storms to the east. However, at least for now, that +NAO is being overruled by the strong ridge in the West, ensuring a cold period to end January (something we've been discussing for over 2 weeks now).

ESRL

Going ahead to 11 days from today, the 500mb geopotential height anomaly shows a different picture than the one we analyzed above. The ridge in the West US has been swallowed up by a more dominant ridge stretching across the Bering Sea and into the North-Central Pacific. With support from an inferior ridge along the western North America coastline, it appears a cross-polar flow situation would be in the works, where cold air would be transported from Eurasia, across the Arctic, directly down into the Northeast and Canadian Maritimes, based on the placement of that trough. Things start to improve in the Central US, for those wishing for spring. That +NAO signal we discussed earlier is now in play, as we see the aforementioned West US ridge 'bleeding east' into the Plains. This fits in with the continued progression of the MJO wave east (weather enthusiasts know this part as Phase 3, the image above this one resembled a Phase 2 event). However, I do think we see the ridge retract itself a bit west as the time between this forecast and present day decreases.

We've examined the outlook for the next 7-14 days, so let's start digging into the outlook in the next 14-31 days.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of January 22nd. Note the presence of a trough in Japan, in the midst of two ridges on either side of the country. This trough appears to be a storm system that may impact us here in the United States down the road. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a storm threat around the January 28-February 1 timeframe. As of now, this storm wouldn't be particularly strong, and should be followed by a warm-up. However, it could perk the interest of some severe weather enthusiasts.

Tropical Tidbits

If we fast-forward a bit, we find ourselves looking at 500mb height anomalies on the evening of January 25th, with a much different prognosis than what we saw in the image above. A strong ridge now encompasses all of Japan, bringing a 'heat wave' to the region. This ridge persists for more than a couple of days, which could very well validate my outlook for a mild middle of February. Using the Typhoon Rule here, we look to find ourselves in a warm period around a January 31st - February 4th period, possibly for longer, based on the looks of ensemble guidance further out. This warmth is then interrupted by a brief cool shot before that warmth may return.

ESRL

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA is positive for the next two weeks, and a sustained strong positive signal at that. This tells me we're looking at that ridge sticking around the West US for a prolonged period of time into February, though it may very well bleed east into the Central US as we already discussed. The NAO forecast is sustained at a moderate level for the entire period, meaning the possibility of that +PNA ridge bleeding east is rather high.
The WPO forecast starts negative, goes positive, and then goes into strong negative territory as that intense ridge on the ESRL ensembles takes over. This should continue for a bit as the MJO wave moves through Phase 3. The EPO follows a nearly-identical path, and both should permit the persistence of cold in the East US. I will refrain from including the Central US in that cold forecast due to the risk of that ridge in the West US bleeding east.

Let's now use tropical forcing to see what we may expect later on in February.

CPC

This four-panel image shows OLR anomalies, using the same color definitions as the JMA chart we discussed earlier in this post. In the top panel, we see current OLR anomalies, and that dying MJO wave is observed moving eastward in the next 1-5 days. By the 6-10 day period, our new MJO wave evolves in Phase 2, favoring the cold weather we have discussed earlier in this post. By by the Days 11-15 panel, our MJO wave has shifted east, to Phase 3, favoring a warmer nation as we move into mid-February. As this wave moves eastward over time, it is expected that the wave will go into phases even more favorable for warm weather, which is why I'm maintaining my call for a warm period in mid-late February. Beyond that 31 day benchmark, confidence is too low to forecast further.

To summarize:

- A period of colder than normal weather across the Central and East US is favored next workweek, likely from Tuesday to Friday to round out February. This cold will be maximized in the Northeast.
- A period of warmth may overtake the Central US in the opening days of February, though the Northeast will remain cold.
- Cooler weather should return to the Central and East US for a brief period around February 6th or 7th, before warm weather takes over.
- A warm pattern may persist into the middle portion of February, possibly into the later part of the month, for much of the nation.

Andrew

Significant Arctic Outbreak Threat Rising for Late January

Model guidance is combining with the long range tools commonly used on this blog (namely the Typhoon Rule) to portray the country's best shot at true wintry weather to end the month of January.

We'll start off with the Typhoon Rule.

Tropical Tidbits
Click on any image to enlarge

The above image shows the GFS forecasted 500mb geopotential height anomaly forecast in the West Pacific region, valid for the morning of January 17th. In this image, we see warm colors, indicative of positive height anomalies / typically warm and quiet weather, but we see a much different picture over Japan. In Japan, a strong upper level low is impacting the country, sliding to the east as it does so. Notice how the center of this upper level low appears to go directly over far northern Japan.

The Typhoon Rule can be used to predict weather phenomenon in the United States. The rule of thumb indicates that weather phenomena impacting Japan is reciprocated in the US about 6-10 days later. So, if we see a strong (and very cold) upper level low crossing northern Japan on January 17th, we can extrapolate this upper level low to "re-appear" in the US on a January 23-27th timeframe.

Does this solution have any support from ensemble guidance? Let's find out.

Weatheronline

The image above shows 500mb geopotential height values (not anomalies) from the prestigious ECMWF ensembles set, valid on January 25th. These types of charts are generally used to identify the presence of large-scale ridges or troughs. In this image, we see a very pronounced depression in the contour lines, bringing the 520gpdm line all the way down just south of the Canada/USA border. This depression of contours shows the presence of a strong upper level low, which is actually centered all the way up in northern Canada.

The strong upper level low is being forced southward due to that strong ridge of high pressure developing along the West Coast. Other guidance eventually does much bigger things with that ridge even further down the road, but we'll get to that a little later on in this post. For now, the takeaway from this forecast is very cold conditions might be on deck for the late January period. I have discussed this end-of-the-month timeframe for some cold weather in earlier posts, but its significance is quickly becoming more realized.

ESRL

Next up, we'll analyze the ensembles off of the ESRL agency, a physics-based modeling branch of the NOAA body. In this image, showing 500mb geopotential anomalies for January 26th, we see a similar layout as the ECMWF ensembles showed. Strong negative height anomalies are developing in the Central and East US as the ridge along the West Coast continues to build and push northward. In these sorts of situations, the ridge may force itself so far north that it becomes a 'blocking' mechanism.

What is 'blocking'? My favorite example is to imagine a highway, with traffic moving along at an even pace. That's a good representation of the atmosphere during 'normal' flow. Now, imagine something happens on the highway that forces the cars to stop - a back-up, perhaps. The cause of this 'back-up' is analogous to the blocking ridge of high pressure; the ridge forces itself towards the North Pole and blocks the atmospheric flow from moving things along. This has been known to produce flooding, long-lasting cold, or intense warmth, depending on the season and who is affected.

ESRL

The upcoming pattern is very similar to that shown by the negative phase of the West Pacific Oscillation (WPO). In the image above, we see typical temperature (bottom panel) and 500mb geopotential height anomaly (top panel) values for a positive WPO phase. Notice how stormy conditions persist just west of the Bering Sea during a +WPO phase, leading to warmth across most of the Central and East US.

To see the typical conditions during a negative WPO phase, just flip the color scale. We then see intense ridging just to the west of the Bering Sea, leading to sustained cold flooding the Central and East US. The eventual alignment of this ridge into the waters near the Bering Sea, as the GFS ensembles are indicating, would lead to this -WPO pattern.

The final (and what I consider the most surprising) part we have to go over is the analog forecast.

CPC

This image shows projected 500mb geopotential height anomalies, valid for about 11 days from today, based on the top ten analogs (dates with the atmospheric pattern similar to the one forecasted to occur 11 days from today) produced at the Climate Prediction Center. In this image, we can see that strong ridge along the West Coast of North America into Alaska, resulting in below-normal anomalies over the Central and East US, eastern Canada, and into Greenland. Just for kicks, let's see what the top analog of 20090118 (January 18, 2009) shows, since it has been deemed the most similar to the forecast down the road.

ESRL

Above, we see temperature anomalies from January 16, 2009. I went back two days as the cold wave had already pushed east by January 18th; we want to get a diagnosis of the cold wave itself. This graphic is in units of Kelvins, but we can easily convert to Fahrenheit.

The core of the cold extends from southeastern Minnesota into far western West Virginia. A spot of values below -17.5 degrees below normal (Kelvin) shows up along the border of Indiana and Ohio. Let's see how -17.5 degrees below normal in Kelvins translates to Fahrenheit. Doing the math, that area saw temperatures 32 degrees (Fahrenheit) below normal for this cold wave, deemed the most similar to what we could see to end January. That's very impressive; for a scale of how impressive it is, consider that the average temperature in Richmond, Indiana (very close, if not inside that -17.5 Kelvin anomaly) to end January is 37 to 38 degrees F. In this cold snap, temperatures likely dipped down to just above zero, with even colder conditions to the north.

By no means does this mean we'll see something as cold as what happened on January 16, 2009, but it gives you an idea of the type of pattern that's coming down the pipe to end January.

To summarize:

- Model guidance and the Typhoon Rule are in agreement concerning a potentially significant cold blast at the end of January.

Andrew

December 31 - January 5 Potential East Coast Winter Storm

I'm watching for the risk of a storm system, possibly associated with wintry weather given the pattern, along the East Coast on December 31 - January 5.

ESRL

Though we can't see out to January 5th, we can diagnose the weather pattern leading up to this timeframe.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA and NAO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The PNA looks to remain positive throughout the entire forecast period, even rising into well above normal territory towards the end of December. This bodes well for a pattern permitting the entrance of cold into the Central and East United States. The positive NAO, an enemy to these East Cost storms, looks to drop to neutral, and possibly below normal with time as we head to the final days of December. We'll need the NAO to get to its negative state for this storm to even have a chance to hit the region.
Also with time, the West Pacific Oscillation and East Pacific Oscillation (WPO, EPO) look to sink from their positive states to negative states. When the EPO and WPO are negative, the risk for cold weather in the eastern two-thirds of the nation rises substantially. As we saw last winter, a sustained negative EPO can lead to some very cold weather.

All in all, the general weather pattern will gradually become more favorable for an East US winter storm as December draws to a close. Let's keep pushing ahead to see what we can find.

CPC

The above image shows the Madden-Julian Oscillation (MJO) forecast from the European model suite (ECMWF), valid from December 6th to December 20th. This chart is complicated for some, so I'll try to break it down. The MJO has eight phases; each phase indicates a different location of enhanced tropical convection/thunderstorms, and those locations for each phase are shown on the perimeter of this diagram. Long story short, when the MJO is in phases 8, 1 and 2 for the winter months, the risk of cold weather rises. Similarly, the MJO in phases 4, 5 and 6 bodes well for warm weather.

The ECMWF suite shows the MJO hitting one phase in particular before moving to the circle in the middle of the image, meaning the MJO is too weak to affect the weather pattern. Can you figure out what phase it is? If you guessed Phase 7, you're correct!

Nicholas Schiraldi

This graphic shows 500mb height anomalies on a time-by-longitude map, averaged out between 40N and 55N latitude. The legend on the left indicates the number of days before (negative numbers) and number of days after (positive numbers) the MJO hits Phase 7. For an example of how to use this map, we might say the region by the 30-W meridian will experience a stormy period about 15 days after the MJO hits Phase 7, due to the deep blue colors in that timeframe and longitude.

Applying this to our situation, if we look up at the MJO phase space diagram above, we see that the ECMWF has us entering MJO Phase 7 in four days, since there are four black dots from the last observation of the MJO to the time when it hits Phase 7. That would put us on December 10th.

Look back for a moment at this composite map immediately above. If you look between the 90W and 60W meridians (about 75W), and move your eyes up to the +25 day level, you'll see a diagonally-moving swath of below normal height anomalies, eventually strengthening quickly at that 75W level. If you're still have trouble understanding, this is saying about 25 days after the MJO strikes Phase 7, a storm system may trek across the United States, and rapidly strengthen when it hits 75W. 75W is located on the East Coast.

Putting all the pieces together, 25 days after December 10th puts us on January 4th, give or take a day. At that time, the pattern may favor a strong storm around the 75 west meridian/East Coast of the US.

So, we now have the MJO supportive of an East Coast storm in early January, with an increasingly-favorable pattern closing out December. But the evidence doesn't stop there...

Tropical Tidbits

The image above shows the forecasted 500mb height values (colored shadings) and mean sea level pressure contours for December 14th in the West Pacific and Bering Sea. Notice a strong storm system crossing into the northern Pacific on this date, with a minimum central pressure of 963 millibars. If you've read this blog in the last month, or follow Joe Renken, you're probably familiar with the Bering Sea Rule. The Bering Sea Rule (BSR) states that weather phenomena occurring in the Bering Sea (typically around Shemya, AK)  correlates to similar weather phenomena here in the US 17-21 days later. Shemya's rough location is in the red circle, and we see this projected storm system just east of that circle.
Work done by Renken suggests Shemya correlates to a location in the Missouri area (I can't remember off the top of my head exactly where), but a storm east of Shemya might suggest the storm appears about 3 weeks later in the Ohio Valley, or even in the East Coast.
As luck (or a little something more?) would have it, extending this forecast graphic's December 14th date out 17-21 days puts us at a potential winter storm in the East US around December 31-January 4th. How convenient!

But that's not the end of the evidence stream just yet...

Those of you on my Facebook and Twitter pages may have taken notice of a new analog system I developed recently, which I then began discussing the other day on the blog. As I had discussed, the system has its good and bad moments, but generally has good success rates from the sample trials I've conducted.

After seeing that there may be the threat for a storm in this timeframe, I decided to see what the analogs were showing for this same timeframe, just for fun.

ESRL

The image above shows mean sea level pressure values averaged out from all analog years used in this weekly forecasting method. These analogs are centered on January 4th of their respective winters, around the tail end of our December 31 through January 5 forecast timeframe. Notice the swath of blues located in where else but the East Coast, indicating the presence of a storm system.

ESRL

Precipitation rate anomalies centered on January 4th from the same analogs shows precipitation falling across much of the Eastern Seaboard, with more still falling in the days prior when the storm moved northward from the Southeast region.
I'm still working on these analogs to bump up accuracy rates, and this is by no means correct. Regardless, it is impressive to see the BSR, Analogs, MJO, and teleconnections combine to suggest a potentially active period for the start of January 2015.

To summarize:

- The atmosphere appears to be gradually becoming more favorable for a colder, more active period to close December and start January 2015.
- Analogs and composites suggest this storm threat is definitely a possibility, primarily for the East US.
- Confidence remains very low due to the long range nature of this event.

Andrew

November 25-29 Potentially Significant Winter Storm

We are beginning to nail down how this potentially significant winter storm may evolve across the country in the days surrounding Thanksgiving.
For my humor and your reading pleasure, I've divided this post into multiple sections, each concerning a different timeframe of this storm.

I. The Instigator 

Let's begin with a refresher on how this potential has come about.

OPC

On the morning of November 8th, we saw the remnants of Typhoon Nuri reach peak strength via Ocean Prediction Center observation. As the chart shows here, the massive storm bottomed out at 924 millibars. This means the storm, located at about 170E and 55N, didn't break the record for strongest extratropical cyclone on record, but it certainly came close.
Why do we care about this storm? The Bering Sea Rule states that a strong storm that hits the Bering Sea can produce a consequential storm in the United States about 17-21 days later. This is the same for high pressure in the Bering Sea. You probably get what I'm getting at here with this observed strong storm in the Bering Sea, so let's keep reading...

NWS

The graphic above, produced by the National Weather Service in Alaska, shows the observed mean sea level pressure of the storm at that 924mb reading, while the NWS office measured it at 930mb on the chart on the left, where strongest extratropical cyclones in the North Pacific are depicted. For multiple reasons, including the possibility that past storms may have been stronger than that 924mb reading, this storm was not declared the strongest on record in the North Pacific. Despite this, it's quite clear this storm was a historically-strong one, relative to storms in the last 60 or so years.

NOAA

Purely for comparison purposes, the team at KOPN Weather identified a strong storm in the Bering Sea on April 7th, 2011, bottoming out at 936mb, that was about 10 degrees W of where this very strong storm was observed yesterday in the Bering Sea. If you recall what happened about 2-3 weeks after the date of April 7th, we saw a certain tornado outbreak strike the South US, devastating thousands across the country, and causing millions on millions of dollars of damage. If we look at where the resultant storm ended up in mid-late April, utilizing the Bering Sea Rule, we find the storm situated in the Ohio Valley.

WPC

Now, keeping in mind that this sort of correlation is a tough one to use at best, not to mention all the caveats associated with long range forecasting, we could theoretically juxtapose the remnants of Typhoon Nuri and this storm in the Bering Sea on April 7th to get an idea of where the consequential storm in the US may end up. Recalling that the storm in 2011 in the Bering Sea was at about 180 degrees longitude, and almost the exact same latitude as the one observed in the last day or two, we find the remnants of Nuri placed about 10 degrees west of that 2011 storm. If we take the location of that storm system in late April (pictured above) and move it west, like the remnants of Nuri were west of that 2011 Bering Sea storm, we end up with a map like this:

WPC

Continuing this correlation, just to see what would happen, we notice that the remnants of Typhoon Nuri are moving eastward (a bit northeast in the process) in the Bering Sea right now, slowly at that. If this storm somehow does end up in that potential location outlined above, and if enough cold air is available (this will be discussed later), a significant winter weather event may strike the Central Plains, Midwest, Great Lakes, and Ohio Valley. Similarly, if the correlation works out, a severe weather event may strike the South US. Confidence remains low, but the potential for this correlation to verify is on the rise.

II. The Japan Connection

Now that we know where this potential is coming from, let's start to use it to our advantage, in the form of the Typhoon Rule (click here for explanation on the rule).

Tropical Tidbits

The image above shows the ECMWF model's forecast of 500mb geopotential height anomalies over the West Pacific. Here, cool colors denote stormy and cold weather, while warm colors depict mild and generally quiet weather. If we take a look at this forecast graphic, valid on November 17th, we find a rather strong trough/storm system pushing eastward into Japan. We can see this trough by the depression of height contours, and associated blue shadings. As this trough pushes through Japan, it looks to close off, a phrase used to describe when those contour lines literally close off and make a circle, indicating a closed low. Until then, we see this negatively-tilted trough hitting Japan around the 17th and 18th. Extrapolating that out using the Typhoon Rule, we find the potential for a storm hitting the US around November 23rd to 28th, oddly enough right around that timeframe that we saw with the intense Bering Sea low.

Tropical Tidbits

Moving on to the next graphic, we find the GFS-Parallel model forecast for November 17th, again forecasting 500mb height anomalies. The GFS-Parallel is not the same as the regular GFS; this Parallel model is the new, enhanced version of the current GFS model, which is set to be 'retired' in the next few months, where the new GFS will take its place. Among new corrections are bias fixes and increased accuracy, etc. The difference from the ECMWF model to this GFS-Parallel forecast is the Parallel model closes off this trough a bit quicker than the ECMWF, as that circular contour line over Japan shows. At this point, it's not so much a question of if this storm will strike Asia, so much as it is what strength will it be and when will it close off. Regardless, the ECMWF and GFS-Parallel both support this storm threat.

Tropical Tidbits

To add in a bit of diversity, let's check out the ECMWF ensemble mean 500mb height anomaly forecast for November 18th. In this image, we see the average of all 52 - yes, as in fifty two separate ensemble members - forecasts favoring a non-closed trough over Japan. This is a bit surprising, as the guidance we went over above has this trough closing off over Japan or just after it leaves the "mainland" of the island nation. Despite this disagreement, which will no doubt be worked through as the time between now and November 17th/18th approaches, the ensembles agree with the other two models on this being a substantial storm crossing Japan, with a consequential storm in the US around November 23-28.

III. The Set-Up

We're now in the timeframe where we can get a view of model projected set-ups for the storm environment (with typical low-confidence, of course). Let's go through the projected set-ups.

Tropical Tidbits

The image above may seem confusing, but it's not that difficult to interpret once you get a feel for it. This image shows the GFS ensemble forecasted 500mb height anomalies, averaged out across the ~20 ensemble members, valid on November 26th. Let's first begin with the positive height anomalies in the West US into British Columbia. We see what are interpreted to be slightly above normal height anomalies, but if this forecast verifies, you'll see these anomalies increase to more extreme values as confidence among all ensemble members increases. For now, confidence is low, so the anomalies aren't as pronounced.
Those positive height anomalies in the West are enabling a positive Pacific-North American (+PNA) pattern to set up. In a positive PNA pattern, ridging in the West forces the jet stream south. If resultant ridging forms in the East, the jet stream then bends north, to enable frigidly cold air to strike the Central/East US, also driving the storm track through that area. This pattern across North America is a classic +PNA pattern, and should be treated as though the Great Lakes/Plains may see the brunt of this storm... initially.
My concern rests with that big upper level low stationed just west of Greenland. If that low becomes too strong and pushes too far south (which is a plausible result), the storm track may be suppressed, and the big winners could end up being the Ohio Valley and interior Northeast. This is something to watch closely in coming days. For now, due to how the Pacific appears to be controlling the pattern, I would favor a Midwest/Great Lakes impact, but let's keep analyzing.

Tropical Tidbits

This graphic is the same type of forecast chart as the one we just discussed, but now comes from the Canadian ensembles, and is valid for November 25th. Despite this slight time difference, note that trough in the Central US, which could be our storm (though the timeframe's a bit fast for my liking). Once again, we see a positive PNA pattern, somewhat suppressed by lackluster ridging in the West US, but compensated by intensified ridging along the East. This would bend the storm track in favor of the Midwest and Ohio Valley for any big snows, and the Gulf Coast could then see some severe weather. Again, this will all sort itself out in due time, and this is merely something to watch for now.

IV. The Storm

It's all been leading up to this, folks. While I don't trust individual model guidance to show what the storm will be like (since it changes from forecast to forecast; no consistency), I would like to show the GFS ensembles' thoughts.

WeatherOnline

What you see above is a 'cluster' forecast from the GFS ensembles, valid on November 26th (technically the evening of the 25th in our time zones), forecasting precipitation and sea level pressure values over North America. I'll show the description of 'cluster' modelling below from the Weather Prediction Center, then I'll try to interpret it for others who may not understand at first.

Sometimes the ensemble members tend to group themselves into two or more solutions.  For example in the image above the ensembles cluster in two solutions off the Pacific NW coast  of the U.S. (a trough south of the Aleutian Islands and a trough off the Pacific Northwest coast of the U.S.).
CLUSTERING is an automated method that identifies and extracts like members and derives output from these like solutions (of which there are different methods to identify clusters).

If you didn't understand that, let's go through an example. Recall that the GFS ensembles have around 20 members, each of which produces its own, different forecast. Let's say that on November 26th, a certain number of ensembles are showing a relatively similar forecast. The 'cluster' method combines these similar forecasts, and does the same with other, like forecasts into 'clusters' of ensemble members. This allows us to narrow down how many ensembles are showing what type of solution for a certain time frame.

Going back to the GFS ensemble image above, approximately 30% of ensemble members' forecasts for this timeframe show a solution like the one above. This solution means a very strong storm would push north and east across the Midwest, like that positive PNA pattern may induce. The result? Heavy snow likely in parts of the Midwest and Great Lakes, while the South may see severe weather.
At first, you may think 30% is not that high. And you'd be right. But for a forecast 324 hours out, about a third of the ensembles showing this sort of solution isn't a bad thing to see if you're hoping for snow in the Central US.

Here's a good representation of what a scenario like the one above may result in.
I'm going on record and saying this is not a forecast! Please don't treat it as such!

Representation of one possible track for this storm.
Again, this is not a forecast, and it shouldn't be treated as such.

To summarize:

- Model guidance continues to support the idea of a strong storm system in the United States around Thanksgiving.
- The set-up for this storm looks to favor a storm track over the Central/East US.
- One model's representation might show heavy snow for the Midwest and Great Lakes.
- Low confidence, high caveats remain present.

Andrew