Thursday, January 29, 2015

January 31 - February 2 Potentially Significant Winter Storm

A potentially significant winter storm is taking aim at a large swath of the country this weekend into the start of next workweek.

Tropical Tidbits
Click images to enlarge
The image above shows the latest GFS forecast for precipitation, mean sea level pressure (MSLP) values, and 1000-500mb thickness values (dashed blue and red lines) on the evening of February 1st. The prognosis calls for energy coming from the Southwest, moving eastward where interaction with additional energy out of Canada may occur. The more interaction these two systems have, the more snow & overall precipitation may fall. The caveats associated with this storm include storm track, strength, snow amounts, interaction between the two pieces of energy, among other items. We'll address some of these here today.

I'll start off right away and say that I think we see this storm either maintain its current position, or even shift south a bit more. Why? I'm glad you asked.

Tropical Tidbits
Here's a look at 500mb vorticity values from January 22nd, over the West Pacific. Notice how we see a positively-tilted piece of energy making its way eastward over Japan, with another piece of energy sliding southward from higher latitudes. Does this set-up look familiar? It should, because the Typhoon Rule says this will be how our storm will evolve. After this point in time, the two pieces of energy approached each other, but did not phase/interact until they were well east of Japan. To me, this means our storm coming up this weekend may not phase as easily as model guidance is saying. It is known that weather models tend to phase storms too readily, and this could be a prime example of that bias showing through. If that is true, this storm would likely stick further south.

CPC
Additionally, and perhaps most importantly, this storm will occur under predominantly northwest flow, with a strong lobe of the tropospheric polar vortex pressing southward from Canada. In this 500mb height contour map, valid for February 1st, we see that lobe of the vortex creating for a very tight gradient in the North US. This could very well act to suppress the storm further south, possibly more than what is currently being advertised. Second, notice how the height contours in Canada seem to be diving southeast into the US. This is what is called 'northwest flow', where the air in the United States is coming from the northwest. Because this is happening, I find it rather plausible we see the storm pushed further south, as strong surface high pressure dropping southward from Canada will also be playing a role here. In case you haven't guessed yet, I'm not finding many factors favoring a north shift.

Instant Weather Maps
One positive thing I am seeing is that model guidance is likely under-forecasting precipitation for this storm. Above, we see the GFS model forecast for low-level relative humidity on February 1st. You can see just how moist this storm will be, with high RH values stretching from the eastern Rockies all the way to the waters offshore the East Coast. This moisture will be driven from the Gulf, hence why I find it plausible, if not likely, more precipitation occurs in this storm than what is being modeled right now.

The question then arises, could this favor prove to stab the wintry portion in the back? Thunderstorms in the warm sector of winter storms have been known to rob the wintry sector of moisture, leading to more rain than forecasted and less snow than forecast. This will be something we'll need to watch when the storm commences.

Tropical Tidbits
Storm track forecasts should really begin to solidify by Sunday(!!), when both pieces of energy relevant to this event move into the United States. Right now, the energy from the Southwest is slowly moving east onshore California, which should aid in model accuracy, gradually, in coming forecasts. An interesting question being posed is, what if latest model guidance inching north in the last day or so has something to do with that energy near California coming onshore? Could it spell a further north track down the road? Anything's possible, sure, but I'll stick with a southern track for now.

Here's my forecast graphic for this event.
The Weather Centre

To summarize:

- Model guidance is in agreement on a potentially significant snowstorm impacting the North US this weekend.
- Amounts in the hardest-hit regions may exceed 12", but this may need revision in later updates.
- High uncertainty still exists.

Andrew

Wednesday, January 28, 2015

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

Sunday, January 25, 2015

January 28 - February 1 Potential Winter Storm

We're watching for a potential winter storm in a January 28th through February 1st timeframe.

Tropical Tidbits
Click Images to enlarge
The above graphic shows mean sea level pressure (MSLP) contours superimposed on 500mb geopotential height values (colored shadings), valid from last Thursday morning. Last Thursday, we saw a positively-tilted trough forcing a rather strong low pressure system to form just east of Japan, dipping below the 1000 millibar mark on this image. When we apply the Typhoon Rule, which states weather phenomenon occurring in Japan is reciprocated in the US 6-10 days later, we come out with a potential winter storm in the January 28 - February 1 timeframe.

Instant Weather Maps
Initially, the ECMWF takes this system down through the Midwest and Ohio Valley, as the above image shows on the evening of January 29th. We see a minimum sea level pressure value of about 1008 millibars, if not a bit below that. This won't be a significant storm, per current forecasts, but could drop some wintry precipitation across the aforementioned regions.

Instant Weather Maps
By the evening of January 30th, the ECMWF model sees this storm transferring offshore the Mid-Atlantic, strengthening at an appreciable pace to a minimum sea level pressure value of ~997 millibars. This would likely produce accumulating snowfall for parts of the Northeast, as the GFS model is also alluding to, but again, nothing incredibly significant.

To summarize:

- A winter storm may affect the US between January 28th and February 1st.
- This storm may affect the Midwest, Ohio Valley, and Northeast the most, if at all, given the relatively-weak strength of this storm.

Andrew

Friday, January 23, 2015

Short Range Outlook (Made January 23, 2015)

This is the latest Short Range Outlook post, where the pattern for the next 0-7 days is addressed. This forecast is valid for January 23rd to January 30th.

ESRL

500mb geopotential height anomalies (right panel) and means (left panel) over the last 7 days show a general ridging pattern across the western portion of North America, extending a bit into the Central US and southern Canada. This pattern resulted in generally mild conditions for much of the nation, particularly in the Plains region. This warmth was not as enthusiastic in the New England region.

NOAA
Satellite analysis of the Northeast Pacific shows abundant moisture making its way onshore in the Pacific Northwest from as far west as Hawaii. This will lead to rainy weather in that region for the foreseeable future, while drier conditions should persist in the Southwest. Some of this moisture is associated with a system that will be moving into the Midwest and Ohio Valley over the next few days, dropping accumulating snow as it does so.
Instant Weather Maps

Model guidance sees the aforementioned piece of energy sliding southeast-ward as a weak snow-producing system. From the most recent NAM model, the heaviest snow would appear to fall in northern Indiana, central Ohio, and into the mid-Atlantic, with scattered snow also showing up in Wisconsin, Minnesota, Iowa, Illinois, and points east, all falling this Sunday through Tuesday. A different storm system looks to drop plowable snow in the Northeast in the same timeframe.

PSU
After this weak system moves eastward, model guidance sees another small storm system dropping into the Midwest, likely laying down some low-accumulation snows. Latest model guidance indicates this would be another 1-3" snow event.

To summarize:

- Heavy snow looks to impact the Northeast in the next four days.
- A weak storm system will drop from Canada and produce accumulating snow in the Midwest and Ohio Valley in the next four days.
- Following that storm, a weak clipper may lay down another 1-3" of snow in similar regions.

Andrew

Wednesday, January 21, 2015

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

Monday, January 19, 2015

Short Range Outlook (Made January 19, 2015)

This is the introductory Short Range Outlook post of our new posting plan, where the pattern for the next 0-7 days is addressed. This forecast is valid for January 19th to January 26th.


ESRL
500mb geopotential height anomalies (right panel) and means (left panel) over the last 7 days show a general ridging pattern across the western portion of North America, extending a bit into the Central US. A separate ridge was also observed in the East US. The former ridge permitted the movement of an upper level low further to the south, which gave the Great Lakes and Northeast a somewhat-cool pattern, if not average.

NOAA
Satellite analysis of the Northeast Pacific shows a healthy storm system moving onshore in southwestern Canada. Abundant moisture from this particular system is being spread across southern Canada, before additional moisture is seen across the Rockies. We can pick out a few more storm systems in this satellite shot, but the main concern will be with that system crossing into Canada.

PSU
Model guidance sees the aforementioned piece of energy sliding southeast-ward as a weak snow-producing system. From the most recent NAM model, the heaviest snow would appear to fall in southwest Michigan, with scattered snow also showing up in Wisconsin, Minnesota, Iowa, Illinois, and points east. This will then introduce a period of cooler than normal temperatures.

PSU
After this weak system moves eastward, model guidance allows the storm to strengthen quickly along the East Coast, which may drop accumulating snow in the Northeast and parts of the Mid-Atlantic. Amounts are still in question, but this high-resolution model indicates amounts in excess of 6" may impact the region.

To summarize:

- The cool pattern we have observed the last seven days is currently being broken by warmth, but should return in the next week.
- A weak system will drop out of Canada to produce snow in the Great Lakes, re-introducing cool weather.
- Scattered storms can be expected across the Gulf Coast.
- Accumulating, possibly significant snow may impact the Northeast.
- Generally wet and warm conditions may be expected for most of the West.

Andrew

Sunday, January 18, 2015

The Weather Centre Updated Posting Plan

We’re announcing a new plan of action for the way we post here at The Weather Centre. Effective immediately, we’re beginning a fixed posting scheme, the continuation of which will be based on feedback and ‘success' of posting in this format. The plan is detailed below. The goal of this plan is to end the current erratic method of posting I employ, where I’ll discuss my thoughts on the long range one day, a storm system for a few days, then go quiet for a few days, etc. I’m aiming for continuity to enhance the way this blog operates.

The plan is as follows:

Winter Storm Posts: As Needed
Severe Weather Posts: As Needed
Tropical Cyclone Posts: As Needed
Drought Status Posts: Pending Decision


---WINTER POSTS (September 15 to April 1)---

Short Range Post (0-7 days): Every Monday & Friday
Long Range Post (7-31 days): Every Saturday & Wednesday


—SUMMER POSTS (April 1 to September 15)---

Severe Weather Short Range Post (1-8 days): Every Monday & Friday
Tropical Weather Long Range Post (7-31 days): Every Saturday

Andrew

Thursday, January 15, 2015

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

January 19th Potential Heavy Snow Event - Northeast

Model guidance is indicating heavy snow may strike the Northeast, particularly in Maine, on January 19th.

Tropical Tidbits
The image above, off the latest GFS model forecast, shows precipitation type, mean sea level pressure (MSLP) and 1000-500mb thickness value projections for Monday afternoon, on the 19th. We see a storm system offshore New England with a minimum central pressure of about 991 millibars, dropping extensive precipitation across the region. This storm should move out quick enough so the event is over by Tuesday morning or early afternoon.

Tropical Tidbits
The snow map from this storm shows about 6-12" of accumulation in New Hampshire, with amounts then escalating to the 36" mark in Maine, which is where the storm is expected to reach peak intensity/impact. Prior to this storm, about 6" falls across the region, lowering the totals shown here. Despite that, this storm still looks to be impressive.

To summarize:

- A heavy snow event is expected to impact the Northeast on January 19th.
- Amounts of 6-12" may be expected in New Hampshire.
- Amounts of 12-24+" may be expected in isolated parts of Maine.

Andrew

Wednesday, January 14, 2015

January 21-25 Potentially Significant Winter Storm

It's looking as if a winter storm will impact the United States between January 21st and January 25th. As of now, this storm has potential to be a strong one.

Tropical Tidbits
The above image shows mean sea level pressure (MSLP) and precipitation values for the morning of January 15th over the West Pacific. In this graphic, we see a strong low pressure system moving up the eastern coast of Japan, delivering heavy precipitation to areas offshore of the island nation. A look at 500mb vorticity values (not shown) depicts this storm phasing with another piece of energy to strengthen and mature the energy into a substantial event.

If we recall that the Typhoon Rule states weather phenomenon occurring in East Asia is replicated here in the United States about 6-10 days later, we should expect a storm system, possibly strong, to hit the US in a January 21-25 timeframe. The orientation of this storm striking the east coast of Japan tells me it may come up from the South US and hug the East Coast here in the US.

Interestingly enough, model guidance is approving of this theory.

Tropical Tidbits
The new GFS (the old GFS model was retired with this morning's 12z / 6AM central time runs, and replaced by what was known as the GFS-Parallel model) is showing a storm system developing in the Southeastern US on January 23rd. Here, we see a large swath of heavy rain, likely containing thunderstorms draped across the Gulf Coast into the Mid-Atlantic, as well as a heavy snow swath spread across the Ohio Valley and into the Northeast. This is something like I would expect to see happen with the way our energy acts when it skirts around Japan in the earlier graphic we analyzed.

Tropical Tidbits
By the evening of January 23rd, we find our storm has moved offshore, dragging an impressive liquid precipitation shield behind it, which then drags behind it a very cold airmass to introduce the anticipated cold blast to close out January. A strong snow swath remains present, but the most intense snow appears to strike Long Island in New York, as well as other coastal spots. Lighter, but still impressive snows then impact more inland regions.

To summarize:

- A storm is expected to impact the United States between January 21 and January 25.
- This storm has the potential to be strong, per current guidance.

Andrew

Monday, January 12, 2015

Long Range Outlook: Dynamic Pattern Unfolding into February

This is the Long Range Outlook, covering the period from mid-January (present day) to around the middle of February.

Tropical Tidbits
The above image shows 500mb geopotential height anomalies across the West Pacific, projected from the GFS ensembles and valid on January 13th. Notice the strong ridge forcing itself north to the west of Japan, likely bringing substantial warmth along with it. If we apply the Typhoon Rule to this situation, which states weather phenomenon occurring in Japan is reciprocated 6-10 days later in the United States, we should expect a period of warm weather commencing around January 15th to 17th, evidenced by a gradual warming trend unfolding in short range forecasts here at home. The Typhoon Rule supports such warmth lasting for about 3-5 days, before a storm system ushers in another wave of cool air. From here, we go to our impending cold blast.

Paul Roundy
A look at the hovmoller diagram above will help us diagnose the atmosphere. We see our current Madden-Julian Oscillation (MJO) wave moving eastward to the 180 degree longitude mark by around the 18th of January. From here, typical MJO phase space diagrams (not shown) show the index moving into territory too weak to identify the phase, but that's just because another MJO wave is forming out by the 50E longitude line, around January 21st. This positioning around the 50E line puts us in a Phase 1 MJO event, as seen below.

BOM
This chart shows typical alignment of outgoing longwave radiation (OLR) values by MJO phase in the west-central Equatorial Pacific. Notice that we see negative OLR anomalies (blue colors) in the hovmoller diagram around January 21st near that 50E longitude line, which matches up with Phase 1 (top-left panel) in this graphic immediately above. This, along with some Typhoon Rule support, should enable a sharp Arctic blast to round out January, possibly to begin February. I do have concern that this wintry return may be weaker than what is currently advertised, as stormy weather over Japan only lasts about 24 hours, if that. This should be a cold event lasting anywhere from 2-5 days for the USA.

As we move ahead into February, things get a little dicey.

JMA
We saw a notable stratospheric warming event strike to kick off the New Year, with temperatures at the 30hPa level only now settling back down to normal levels, as the graphic above displays. Let me please clarify: This was not a Sudden Stratospheric Warming (SSW) event. In order to deem an event a true SSW, wind direction in the stratosphere must either significantly slow or even reverse, in addition to intense warming of temperatures. We saw the latter, to some degree, while the former wasn't as prevalent as we needed. For this reason, it's referred to as just a notable warming event.

The concept right now is that this warming event will lead to more wintry weather about 2-4 weeks down the road from when the warming occurred, which brings us to that late January-early February period. With unfavorable indications from Japan, though, I worry that the cold (while intense) may not stick around for long. In order for that to happen, we may need another warming event, ideally a true SSW. However, even that prospect is quickly fading, as we see a strengthening of the upper polar vortex in coming days and weeks.

Let's dive into February a bit deeper.

Kyle MacRitchie
The above image once again shows forecasted outgoing longwave radiation (OLR) anomalies, but now forecasted well out into the spring. Due to such low confidence, we're only going to look at the long range OLR forecast as an indicator of what may come, not necessarily as something that's "set in stone". Notice how we see our ongoing MJO wave fading near the 21st of January with the dissipation of negative OLR anomalies, as well as our second wave forming well off to the west. As the Paul Roundy hovmoller showed, this second wave slowly shifts east as we move into February, but then note what this forecast predicts in the middle and late stages of February. We see the MJO stalling out around the 160E longitude mark. Going back to our MJO OLR composite image earlier in this post, we see this correlating to Phase 6, also known as the warm weather phase. Here's a graphic displaying a Phase 6 MJO event in February, to give you an idea of what we could see if this forecast verified.

Meteonetwork
We see a strong ridge unfolding across the East US into the Ohio Valley and Great Lakes during this February Phase 6 MJO event, with stormy weather impacting the Pacific Northwest. Ridging is shunted south and east across the Northeast Pacific, into the Southwest US. This should be particularly concerning for winter weather fans in the East, because this set-up does have additional support. Model guidance is expecting the atmosphere as a whole to dive into a La Nina-esque pattern, which is shown like the image below.

NOAA
During a typical La Nina pattern, we tend to see strong high pressure across the Gulf of Alaska, before the jet stream buckles south and delivers cold weather into the North Plains and Northwest. Ridging then is provoked in the East US (the reason why cold weather fans in the East see La Nina's in their nightmares). This, of course, is not a perfect correlation, but you can make key connections between the typical La Nina environment, as well as the projected Phase 6 MJO event in February. This would not bode well for a cold and stormy end to winter for most east of the Rockies, unfortunately.

On a side note, many have asked why I am using the MJO in my forecasts, since we are currently in the pro-warmth Phase 6, but cold weather is prevailing. I continue to use the MJO because it usually provides a good idea as to where we are going in the future. The MJO's effects were being overruled by a very strong ridge along the West Coast; remember that the MJO is not the sole player in this atmosphere, and that's being exhibited well. We are currently seeing warmth return to the US as that ridge breaks down in accordance with the warm MJO phase.

Even when the MJO seemed to be failing, it still managed to reflect itself well in the atmosphere. Shown below is a side-by-side comparison of the last few days of January 500mb geopotential height anomalies (left), as well as typical 500mb height anomalies during a January Phase 6 MJO event, in an ENSO atmosphere reflective of this one. The comparison isn't perfect, by any means, but it's there.

ESRL and Meteonetwork



To summarize:

- Warmer than normal weather is expected around a January 15-23rd period.
- A storm system may impact the country around January 21-25.
- Cooler than normal weather is expected in the final days of January, possibly into February.
- There is increasing potential for a warming trend in the middle of February back to above-normal temperatures.

Andrew