Brief Pattern Change Expected Next Week

A change in the current weather pattern, albeit a brief one, is expected next week.

PSU
Image Valid: January 12

Ensemble forecasts see strong ridging building over Alaska and extreme northwest Canada, extending from northern Siberia all the way across the Arctic Circle to the waters immediately south of Greenland. Consequentially, this will force the tropospheric polar vortex to lower latitudes, and it just so happens that ensemble guidance sees a piece of this vortex will dip south into Canada, and will exert its influence on the United States.
It's important to note that the tropospheric polar vortex will not enter the United States, it will stay well north of the Canada/US border. Cooler air will still result, but extreme cold should not be expected right now.

PSU
Image valid: January 15

By January 15th, the pattern is progressive enough so that the piece of the tropospheric polar vortex shifts east into eastern Canada. We also see strong ridging dominating the Arctic Circle, which will drop the Arctic Oscillation (AO) index well into negative territory, as we'll see a little later on in this post. Note the strong trough modeled over the Gulf of Alaska- this will maintain a positive phase of the East Pacific Oscillation (EPO). While I won't discuss the EPO in this post, that trough in the Gulf of Alaska is what will return our pattern to a more seasonal, and possibly warmer than normal pattern after this brief change to chillier temperatures.

Above 3 images from CPC

Just a brief look at the teleconnection forecasts, we see the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) both expected to drop into negative territory. This is a sign for cooler than normal weather, and as we saw earlier in this post, ensemble guidance believes a brief shot of colder weather will hit the Central and East US. The Pacific-North American (PNA) index is expected to stay positive, possibly heading more towards  neutral territory in the medium-long range, but generally should remain positive. This is likely a result of that trough in the Gulf of Alaska. That trough will keep the pattern progressive, which means the ridge that forms over the West US as a result (the positive PNA indicator) will likely drift east and spread warmer weather east.

To Summarize:

- A brief flip to colder weather is expected next week, mainly between a January 10-15 period.
- Warmer weather is expected to return after January 15th.

Andrew

Long Range Lookout: Cooler Than Normal July Ahead

This post is dedicated to the servicemen and women who are serving our country, who have served our country, and those who have made the ultimate sacrifice for our country.

It appears that a cooler than normal July is on the way for the nation.

We'll begin by looking over shorter-range indicators; in this case, the Typhoon Rule.

Tropical Tidbits

The above image shows forecasted 500 millibar anomalies over the West Pacific. Here, blues indicate negative height anomalies, symbolizing a trough or storm system in the area. Oranges and reds correlate to positive height anomalies, indicative of warmer and quieter weather.
This forecast is valid for July 14th, and we see a system just southeast of Japan, making its way north and east. This system was previously a typhoon, which looks to develop in coming days further to the south. As the typhoon approaches Japan, it will curve and head out to sea. Using the Typhoon Rule, we can expect a cooler bout of weather about 6-10 days after this happens. Recognizing that this forecast is for July 14th, we can extrapolate that to forecast a cool-down around July 20th to 24th, probably spilling over those dates a bit.

However, it's also likely that the entire month will end up cooler than normal for many in the country, due to what is happening in the Central Pacific.

BOM

A quick overview of the columns, going from left to right: The date for the data, the daily barometric pressure reading in Tahiti, the daily barometric pressure reading in Darwin, the daily value of the Southern Oscillation Index (SOI), the 30 day averaged SOI, and the 90 day averaged SOI.
In a nutshell, when the SOI is negative, El Nino conditions are favored in the United States in the following couple of weeks. We see that the SOI has been consistently negative since June 19th, and it's no coincidence that the United States has been predominantly cool since late June.
You can see how El Nino conditions, which are signaled by a negative SOI, make for a cooler than normal summer in the composite chart below:

ESRL
Typical temperature anomalies during an El Nino summer

Since we remain in a negative SOI state this 4th of July, I'm expecting the rest of the month to end up cooler than normal for much of the country. Those in the Southeast and East could see slightly warmer conditions, however.

To summarize:

- A cooler than normal month of July is expected for much of the United States.

Andrew

Second Round of Harsh Arctic Air Flowing Southward

The second round of intense cold air in under a week is on the way for millions in the Northern United States.

Tropical Tidbits
Click images to enlarge

The above image shows the latest GFS model forecast for Monday morning. On this chart, we see temperatures in the negative-teens across Wisconsin, Illinois, Minnesota, Iowa, Michigan, Indiana, and Ohio. The most intense cold is projected to hit northern Wisconsin and southwestern Michigan, but that doesn't take away from the cold in the other aforementioned areas.

Tropical Tidbits

The GEM model, with its notorious cold bias, agrees with the GFS model. Here, we see a swath of bone-chilling cold in southern Minnesota and Iowa, where temperatures are forecasted to approach -30 degrees. This is more than likely overdone, but the general trend of very cold temperatures similar to the GFS stands in Wisconsin, Illinois, Michigan, Indiana, Ohio, and even into the Dakotas.

Other model guidance, mainly the NAM model, is favoring a warmer solution than the two depicted above. It comes out to the GFS/GEM models and their ensembles, against the NAM/SREF models/ensembles. The National Weather Service appears to be favoring the NAM model / warmer forecast, even though morning observations in the Dakotas and Minnesota support a more GFS-like evolution. Regardless of what happens, another very cold morning is in store tomorrow.

To summarize:

- Another bout of harsh Arctic air is on the move south.
- Temperatures could drop into the negative-teens in some spots.
- Uncertainty still exists with how cold it will get.

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

Chicago, Milwaukee Bracing for Severe Arctic Cold

Midwestern cities such as Chicago and Milwaukee are bracing for cold that could drop temperatures well into the double-digit negatives. This sort of cold has been seen recently east of Lake Michigan, but will make its first appearance of the month in more western regions this workweek.

Tropical Tidbits
Click images to enlarge

The above image shows the latest GFS model forecast for air temperatures on Thursday morning at 6AM, fresh off the presses. We see temperatures near Madison, Wisconsin plummeting to -15 degrees Fahrenheit, with Minneapolis seeing similar readings. Chicago looks to be under the gun for -10 to -15 degree temperatures, while Springfield, IL may see relatively warmer values.
There is model discrepancy with respect to how cold this air mass will be. The GFS model is favoring a much colder solution, as we see above, while the short-range NAM model (not shown) is favoring a warmer solution. One model has to cave at some point; it's a waiting game to figure out which one. For now, better safe than sorry.

Tropical Tidbits

Friday morning is the tricky forecast for these Midwestern cities. Skies will be clear for swaths of the Midwest, and with much of the area under a snow pack, temperatures could drop even lower than Thursday morning readings. However, in addition to the NAM model favoring a warmer air mass, warm air advection will be ongoing across the Plains into the Midwest. Discrepancies remain with respect to how fast this warmer air will push into the Midwest, and how intense it will be. For now, I'm taking more of a wait-and-see approach, but preparing for the worst. Again, better safe than sorry.

Purely for eye candy, as in this will not verify, check out the Canadian model air temperature forecast for Friday morning.

Tropical Tidbits

To summarize:

- The Midwest may undergo its most severe wave of cold weather this season, with cities such as Chicago, Milwaukee and Minneapolis going below the -10 degree F benchmark.
- Unusually high uncertainty still exists surrounding the intensity of this cold air on both Thursday and Friday. Any updates to this post will be made on either our Facebook or Twitter pages.

Andrew

Anomalous Upper Level Low to Deliver Intense Cold in East US

An anomalous upper level low connected to the swirling mass of cold air in the Arctic looks to deliver a punch of brutally-cold air, impacting the East US the strongest.

PSU
Click images to enlarge

The above image shows the GFS ensemble mean spread forecast for the 500-millibar field on the top panel, with increased spread/uncertainty among ensemble members shown by shaded colors. That spread can also be seen in the individual member colored lines. The bottom panel shows anomalies for the 500-millibar field, with cool colors indicating negative anomalies, and warm colors depicting positive anomalies.

In this image, we note a strong upper level feature pushing south from Canada, with anomaly values reading 3.27 units below normal, maximized in western New York into western Pennsylvania as the purple colors show. Judging by the numerical denotations on that bottom panel, 500-millibar values could flirt with the 500-dm benchmark along the US/Canada border, indicative of a very strong (and very cold) upper level low.

Tropical Tidbits

Forecasts from computer models suggest sub-zero temperatures will be the theme throughout a significant swath of the North US. Latest guidance suggests temperatures in west New York and Pennsylvania will easily drop below -10 degrees Fahrenheit, with some locations getting very close to -20 degrees F. Locations further to the west, particularly in the Midwest and Great Lakes, may also come close to -15 degrees, with some areas dropping down to that -20 degree F benchmark. All in all, this is looking dangerously cold, potentially life-threatening.

To summarize:

- The first part of a multi-wave cold blast looks to impact the North US this weekend.
- Cold weather will be maximized in the Northeast, where temperatures could reach lows of -20 degrees Fahrenheit.
- Travel is strongly advised against due to the life-threatening nature of this cold air.
- Make sure to seek out assistance now for neighbors, friends, and/or family that may have trouble sustaining heat in their homes, or could be adversely affected by this cold.

Andrew

Upcoming Pattern Very Favorable for East Coast Snowstorms

The upcoming pattern appears to be very favorable for snowstorms along the East Coast.

Tropical Tidbits

The above image shows the 500mb geopotential height anomaly field over North America, forecasted by the GFS Ensembles from February 11th to February 16th. In this graphic, we see a strong upper level low dipping down over the Bering Sea, forcing a strong ridge to blossom along the Western US. A Rex Block then forms in the Southwest, as an upper level low slides under the ridge. Usually, a Rex Block upstream creates zonal flow downstream, but instead of rather calm weather in the East US, we see a deep upper level low pushing its way into the Northeast. This looks to be the pattern for the next 2 weeks or so.

In this sort of pattern, sustained northwest flow (winds from the northwest) often leads to a 'clipper train', where a multitude of Alberta Clippers slides southeast-ward into the Central US. These clippers can then move east off the coast, and can intensify to produce heavy accumulating snow for the East US. I believe that this is a possibility, and could happen more than once in the next couple of weeks.

Unisys

A look at sea surface temperatures shows how potent this environment is. For now, we'll focus on the East Coast. Notice all the oranges and reds immediately along the coastline, extending a bit east. Those are very warm waters, with well-above normal anomalies being detected. Consequentially, any clippers that move off the coast and threaten the East will have unusually warm waters to work with, which could very well enhance snowfall totals.

To summarize:

- The upcoming pattern is favorable for accumulating snowfall in the East US.
- This pattern may support more than one event of plowable snow.
- Extreme cold may affect the Northeast, at times.

Andrew

February 16-20 Potential Arctic Outbreak

There appears to be the risk of an outbreak of cold air between a February 16-20 period.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies over the West Pacific, forecasted by the GFS model and valid on the morning of February 10th. In this image, we see a very strong upper level low displaced just north of Japan, with contours telling me this is a sub-500dm upper level low- a very strong storm. When we use the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, a strong upper level low could approach the United States from the north in a February 16-20 period. It's too early to tell definitively, but this could correlate to a lobe of the tropospheric polar vortex plummeting south towards the mid-latitudes. Again, it's too early to tell right now.

ESRL

Long range ensemble forecasts for the middle of this timeframe are already beginning to see strong upper latitude blocking leading to a new wave of cold weather for the Central and East US, as those blue colors shown over North America. We will need to watch upcoming forecasts to see how they change.

ESRL

For comparison, here's the chart of 500mb geopotential height anomalies over Japan on December 27th, 2013. We saw a very strong upper level low drop south into southern Japan, which then correlated to the infamous cold blast which originally made the phrase 'polar vortex' go viral. Forecasts above don't see the impending upper level low dropping as far south, which is why I'm thinking this upper level low will approach the region, and not necessarily induce a period of incredibly frigid temperatures, like we saw last winter.

To summarize:

- A strong upper level low looks to drop south to scrape northern Japan in the next week.
- As a result, we could see a period of below-normal, possibly well below-normal temperatures between February 16th and February 20th.

Andrew

Christmas Eve Potentially Significant Snowstorm

Model guidance continues to struggle on the concept of a winter storm around Christmas Eve.

Tropical Tidbits
Click images to enlarge

The image above shows the ECMWF model's projection of 500mb geopotential height values (shaded colors) and mean sea level pressure values (contours) for the morning of Christmas Eve. On this image, we see a deepening low pressure system pushing northward into western Ohio, as a strong trough (seen by the blue colors) begins to negatively tilt (seen in this image as those blue colors "pointing" southeastward) and drag the storm northward.

This forecast solution is one of (quite literally) tens of ideas for this storm that have come out of recent model runs, so individual model projections are being weighted unusually low, due to unusually low confidence. Though I don't have the maps to confirm, this projection from the ECMWF would likely deliver a hefty snowstorm, with strong winds, to portions of the Upper Midwest and Great Lakes, particularly Michigan.

Tropical Tidbits

We now move on to the Canadian GEM model's projection, once again showing 500mb geopotential height values and MSLP contours, and once again valid for the morning of Christmas Eve. In this forecast, the storm system in question becomes negatively tilted quicker than the ECMWF model, which then leads to this more westward solution. Here, the GEM has the storm in central / west-central Indiana, instead of the ECMWF's west-Ohio idea.

Tropical Tidbits

As a consequence of this change, the GEM places a swath of accumulating snow in a north-south orientation, from eastern Wisconsin down through Illinois, barely scraping far western Kentucky. Additional snowfall hits Ohio and Indiana, but the amounts in Illinois are the most significant, where totals surpass the 6" benchmark. Recent model guidance has been looking more into this westward shift, but there's no consensus at this time.

Tropical Tidbits

The last model projection we can analyze, owing to an ongoing massive NOAA data outage, is the Parallel GFS model. Once again, the forecast shows 500mb geopotential height and MSLP values, and this projection is once again valid on the morning of Christmas Eve. This forecast is similar to that of the GEM model, in that we see quicker strengthening of the storm system, leading to a more westward solution.

It should be noted that ensemble guidance is a bit further east of these western tracks.

All in all, we have a very difficult forecast still evolving, which might not be resolved for another few days. For now, those across the entire Central and Eastern US should be prepared for potentially significant weather, especially if you are traveling immediately prior to, or after Christmas.

To summarize:

- Model guidance continues to suggest a powerful storm moving through the Central/Eastern US around Christmas Eve.
- Guidance remains very inconsistent, with little to no consensus currently built around a certain track.
- Regardless of where this storm tracks, significant and adverse weather remains possible, if not likely.
- Travel plans may need to be re-evaluated.

Andrew

Christmas Eve Potentially Significant Winter Storm

I'm watching the increasing potential for a significant winter storm in the Central US, created by what could be one of the strongest low pressure systems in the last few years, if not longer.

Instant Weather Maps

The above image shows 500mb geopotential height contours over the United States from the prestigious ECMWF model, valid for December 23rd. In this image, we see two pieces of energy in the Central US. One is a closed low, placed in the Northern Plains, while the other is a deepening trough in the Southern Plains, shown by the depression in contour lines. In a situation similar to that I described in my December 10th post, we see this trough begin to lift northward into the Central US, but not before the "bomb" goes off...

Instant Weather Maps

Just 24 hours later, on the morning of Christmas Eve, we find that the two pieces of energy have combined, and the trough as a whole has now attained a negative tilt, indicating it has reached its mature phase. As a result, the storm undergoes rapid strengthening, very near the criteria of bombogenesis. Bombogenesis is a meteorological term used to describe the phenomenon when extratropical cyclones rapidly strengthen, and their minimum central pressure values decrease by 24 millibars in a 24 hour period. Switching between December 23rd and 24th, we find that pressure values from the ECMWF lower by about 19.3 millibars, only a little ways off from being a true 'bomb'. Even though this storm doesn't fit the criteria, the strengthening is nothing to shake a stick at.

This solution is a tricky one. I don't have access to pay-to-view weather model graphics (yet), so I cannot see the precipitation pattern from this system. However, from what others are discussing, it appears that not only will the nearly-due-northward movement of this system foul up the precipitation shield, but temperature profiles are above freezing in many spots that would otherwise see snow. This could be placed on model error, or it may be a legitimate forecast. For now, it's just too far out to tell one way or another.

Weather Online

What is interesting, however, is the difference between the ECMWF model and the ECMWF ensembles. The above image shows mean sea level pressure values, valid on Christmas Eve (the same time as the second image we discussed). By simple comparison, note how the ECMWF model places the center of this storm somewhere in southern Ohio, while the ECMWF ensembles put northeastern Indiana in the center of this cyclone.

A further east track of this storm system could result in more of a snow impact to the Great Lakes, instead of primarily wind-driven snow in the Midwest and Ohio Valley from current runs of the ECMWF. But as I said earlier, we are still quite a while away from nailing down these details.

To summarize:

- Model guidance is beginning to sniff out a very strong storm system impacting most of the Central and East US on Christmas Eve into Christmas Day.
- This would severely impact travel.
- Snow would be confined primarily to the north-central Great Lakes into Canada.
- Very high uncertainty still exists.

Andrew

December 19-21 Potential Winter Storm

Model guidance is supporting the risk of a winter storm in the December 19-21 timeframe.
I'm trying to decide how to start off this post, since simply going through model guidance won't work. I'll begin by explaining each graphic, doing a compare/contrast as we do so.

Tropical Tidbits

The latest GFS model has a trough pushing into the Midwest on December 20th, as the blue colors and depression of contour lines shows. This trough is beginning to close off after becoming negatively tilted, and is pushing northward as a result. The surface low takes a track through the Ohio Valley on this run, dropping appreciable snows from east Kansas into southern Michigan. North Missouri sees snow over 6" from this system.

In this overview of the pattern, we can diagnose a few items arguing for this more inland track, as future guidance I'll show you will depict an East US track. First of all, we have another strong trough dropping into the West US, trying to work its way southward. This will try and force a ridge to develop in the central and eastern Rocky Mountains, as we can already see above. However, as we see a deep upper level low over Greenland, this ridge won't be able to exert too much influence (that ULL over Greenland defines the positive phase of the North Atlantic Oscillation/NAO, notorious for keeping the jet stream very 'non-wavy')
What does make a ridge form, however, is the troughing in the Bering Sea into the Gulf of Alaska, a textbook positive East Pacific Oscillation (EPO) pattern, which will act to limit cold air reserves in Canada, but also try to direct storm systems northward. The latter influence is seen well in that ridge centered over New York, just east of the storm system in question. That same northward influence could happen along the East Coast, but for now, the GFS favors this solution.

Tropical Tidbits

Here's the ECMWF 500mb geopotential height anomaly forecast, valid at the same time as the GFS graphic. We see more than a couple significant differences here.

First and foremost, the storm system in question seems to be developing a second piece of energy, signified by that second dip in the contour lines along the Gulf Coast. Looking at the forecast following this timeframe. it looks as if that second piece of energy will act to pull the main trough east, and develop it into an East Coast system. This is far different from the GFS, which maintains a single trough.
Additionally, we see the entrance time of the second trough into Western North America has been slowed by about a day. This allows the ridge in the central and eastern Rockies to flourish well into Canada, where the ridge up there is also quite a bit stronger than its GFS counterpart. Consequentially, the storm system is suppressed to the south in the ECMWF forecast. Also, note the lack of a ridge to the east of the storm system in the ECMWF image, compared to the GFS. This, combined with the extra piece of energy along the Gulf Coast, appears to favor an East Coast solution. The solution results in this pattern, valid 24 hours after the image above:

Tropical Tidbits

Purely for comparison, here's the GFS snowfall forecast I mentioned earlier, expressing the solution in opposition to the ECMWF:

Instant Weather Maps

I've been looking back and forth between the ECMWF and GFS images I've shown above, trying to figure out which one I think is the most valid, and I can't decide.

On one hand, the ECMWF projection appears to be obeying the positive EPO signal, as exhibited by troughing along the west coast and a strong ridge in central Canada.

On the other hand, the GFS is doing well with the emergence of a ridge just east of the trough, possibly as a result of the negative PNA orientation out west, as the aforementioned second storm system drops into the West.

It will ultimately depend on the timing of when the trough drops into the West, just how strongly the atmosphere responds to the +EPO signal, and (of course) if that secondary piece of energy forms along the Gulf Coast. It's worth noting none of the model guidance is having any consistency with the ridge in Canada, purely by looking at run-by-run comparisons. Additionally, today's 12z ECMWF run (examined above) is the first one to have that secondary piece of energy develop to the south of the storm, something that does not bode well for any consistency that either the GFS or ECMWF may have built up. I'll pass on giving my opinion right now, because this is a truly grotesque set-up.

To summarize:

- A winter storm is possible for the Central or East US in the December 19-21 timeframe.
- Model guidance is expressing little to no consistency on a defined track for this storm.
- Cold air availability will eventually become a concern.
- Anomalously low confidence exists.

Andrew

Christmas Potentially Significant Winter Storm

I'm now watching the risk of a strong winter storm rise as we move towards Christmas.

Tropical Tidbits

The image above shows 500mb geopotential height values (colored shadings), with mean sea level pressure (MSLP) values superimposed. In this graphic, valid on December 16th, we see a pair of very strong storm systems, one on each side of Japan. The prognosis is that a strong system will push into the Sea of Japan (located west of the country), while a second storm system will develop in the southern part of the country and move northeast-ward, skirting the eastern fringe of Japan as it does so. It is this second storm that we need to keep a close eye on, and is the one we will be discussing here today.

Using the Typhoon Rule, which states that weather phenomena occurring in East Asia is recipricated in the US about 6-10 days later, we can extrapolate this December 16th date out and predict a storm in the United States in a December 22-26th period... right in the Christmas rush.

But we can expand on this quite a bit more. The storm will be shooting north along the eastern coast of Japan. This does have an impact on the expected storm in the US. As you might expect, it raises the chance of this consequential storm also moving northeast-ward rapidly, and from there, we come out with two prevalent/possible storm tracks:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to merely skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

The graphic above only shows the GFS model view... let's head on over to the European model projection.

Tropical Tidbits

Wow! Can you see the change?

This graphic, showing the same parameters as the GFS image, and for the same timeframe, portrays that strong storm in the Sea of Japan, but now the second storm skirting eastern Japan is more inland. It hasn't shifted much, but it has shifted nonetheless.

What does this mean? It means it's time for East Coasters to throw in the towel.

Not really, but a more inland storm does favor an inland track when the storm comes around in the US. The interesting thing is, this more inland track is an idea. Here's why.

Recall that, whether you learned it or just know it through logic, low pressure areas will try to move towards areas with the least resistance, in this case the least high pressure. My theory here is that the storm in the Sea of Japan, the stronger of the two (shown on the GFS as 993mb, 980mb on the ECMWF), is trying to pull the storm skirting east Japan towards itself. Down the road, model guidance shows the second storm absorbing the stronger Sea of Japan storm, rather than vice versa, and that's also a possibility.
My point here is, there is the possibility of a phased storm.

For those who aren't as knowledgeable with weather lingo, a 'phased storm' is a storm system which is made up of previously-two or more pieces of energy. Typically, phased storms end up stronger than either of the first two pieces of energy were. I'm not holding my breath on this Christmas storm phasing, but it probably isn't a bad idea to keep it in the back of your mind.

Regardless of if this storm phases, remember that the storm on the east coast of Japan is projected to be below 1000 millibars, so it's likely to be a nice little storm in itself.

Tropical Tidbits

We've now confirmed that not only are looking at a storm in the Christmas time period, but model guidance has amped up that threat since yesterday. Now, we have to diagnose the weather pattern here at home in that December 22nd - 26th timeframe, to see if we can pull any hints out.

I've posted the image above from the GFS ensembles, showing 500mb geopotential heights on Christmas morning. Warm colors depict ridging/high pressure, usually indicative of warm and quiet weather. Similarly, blues indicate troughing/low pressure, accompanied by colder and stormier weather. We have more than a few things to talk about with the above graphic.

First and foremost, we're looking at the Pacific driving our pattern to round out December. Tropical activity in the Equatorial Pacific will be dying off in the next few days (more knowledgeable weather folks know this as the MJO weakening), which will shift the weather pattern 'responsibilities' to the North Pacific.

We look to have a positive Pacific-North American (PNA) index pattern in place for this event. We can observe this positive PNA as a ridge forming in the West US, which allows the jet stream to buckle in the Central US. Such a pattern is climatologically favorable for a Central US storm track. In addition, a positive East Pacific Oscillation (EPO) signal will begin showing up, as ridging overtakes Canada. This will be in part due to that positive PNA, but the further east you go, the more the EPO influence takes over. A positive EPO doesn't affect the storm track so much as it does temperatures (above normal in the North US). When we factor into account slight ridging along the Eastern Seaboard, we start to see that signal for a storm system in the Central US, favoring development in the Central US.
I'm a bit skeptical, however, Many Northeast weather buffs may know that winter storms are favored in the East when the North Atlantic Oscillation (NAO) changes phases from positive to negative, or vice versa. Guess what the forecast for the NAO is around this storm's timeframe:

ESRL

We've got a dilemma on our hands, with some variables favoring a Central US storm, and others favoring an East US storm. So is the world of forecasting...

To summarize:

- A winter storm appears to be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Rather high confidence in the threat of a storm in this timeframe, but low confidence in who will be most affected.

Andrew

December 22-26/Christmas Potential Winter Storm

I'm watching the potential for a winter storm in the December 22-26 timeframe.

Tropical Tidbits

The image above shows the forecasted 500mb vorticity values on the morning of December 16th, valid over the West Pacific. In this graphic, we see a negatively-tilted upper level low pushing east into Japan, as the elevated vorticity values indicate. We can observe the negative tilt by the height contours seeming to "dig" in a southeast-ward direction. When a trough/storm system is at its mature stage, usually its strongest stage in a storm's life, the storm is said to be negatively tilted.

Using the Typhoon Rule, which states weather phenomena occurring over Japan is reciprocated in the United States 6-10 days later, we might expect this storm to impact the United States in a December 22-26 timeframe... possibly right around Christmas.

But there's more to this story.

Tropical Tidbits

Take a look at this image, forecasting 500mb geopotential height values in colored shadings, as well as mean sea level pressure (MSLP) values in the contoured lines. In this image, valid just six hours prior to the time period in the first image we analyzed, we see a strong storm system in the Sea of Japan, correlated to the strong upper level low.

But wait... there's something else there.

Check out that sagging of contour lines just south of Japan, right where the arrow is pointing. It almost looks like another low pressure system! Other model guidance confirms this idea of a second body of low pressure forming south of Japan and skirting the nation to the east before phasing with the very strong storm in the Sea of Japan. The implications here could be huge.

If that really is another low pressure system, then we have a very interesting scenario on our hands for Christmas. We can't tell for sure just yet, but the storm shooting northward to the east of Japan could mean a few possibilities.
Let's first hypothesize that the item outlined above is indeed a storm system. The track to the east of Japan could either mean we would be looking at:

- A Panhandle Hook storm, where the system shoots north from the Southern Plains. These storms are climatologically favored to bring heavy snow to cities in the east-central Plains and Lower Great Lakes. This scenario is a possibility, as that strong storm in the Sea of Japan would likely correlate to a strong North Plains cyclone. This would keep that body of low pressure east of Japan in an area close-by, as the storms would eventually phase (not to mention low pressure areas are attracted to other low pressure areas).

- An East Coast storm. Because this body of low pressure is forecasted to skirt the eastern side of Japan, this could be a plausible scenario. We won't know if either of these are correct until we have more model runs to access.

ESRL

Long range ensemble model guidance for the evening of December 23rd shows a relatively favorable pattern for a storm. We see strong ridging high pressure in the West Coast of North America, but another swath of high pressure also looks to exist in southern Canada. This could boost temperatures, or worse, suppress and kill the storm system altogether. Even though this is quite far out and should not be taken at face value, those in the Midwest looking for a winter storm should feel rather good from what model guidance is projecting:

Tropical Tidbits

This image from the GFS ensembles shows what I'm talking about. In this image, also valid for the evening of December 23rd, we see a few factors at work here. A strong positive PNA is currently agreed upon by these ensembles, which permits a strong ridge to form in the Western US. This ridge then allows a colder pattern to develop in the Central US, as the jet stream drops southward to accommodate the +PNA ridge. Sweetening the pot is evidence of a very slight ridge in the Southeast, something that could allow any potential storms to move more northward. That final piece to the puzzle will be a bit more difficult to come by, as that strong upper level low over Greenland will try to keep the flow over North America very zonal (west-to-east, jet stream not wavy). There's still, of course, plenty of time for this to change.

To summarize:

- A winter storm may be in the cards for December 22-26th, likely impacting Christmas travel plans.
- A second storm system may need to be watched for the Northern Plains.
- The primary threat here may become a storm favorable for heavy snow, either in the Central/East US (ideally the Ohio Valley/Midwest) or along the Eastern Seaboard.
- Relatively low confidence still exists due to the long-range nature of this threat.

Andrew

Updated December Long Range Forecast

In an effort to cut down on confusion that has been stirred up in recent days, I'm publishing this post to give an idea of my thoughts for the rest of the month.

To stay as accurate as possible, we'll start off with the weather overseas.

Tropical Tidbits

The image above shows forecasted 500mb height anomalies over the West Pacific. In this image, valid on December 6th, we see a deep trough over Japan. Those of you who have followed us for a while know that stormy weather over Japan correlates to stormy weather here about 6-10 days later. This graphic tells us that the weather will likely be colder than normal on a December 12-16th timeframe, possibly with an accompanying storm system (but I wouldn't hold my breath for it).

Tropical Tidbits

By December 10th, the trough has moved out and a ridge has filled in. This ridge won't be of a spectacular magnitude, but it looks strong enough to indicate quiet and warm weather here in the United States in a December 16th-20th time period.

Tropical Tidbits

The last image I'll show you with relations to Japan is above, valid on December 13th. Looking at East Asia, notice an elongated trough to the north of the island nation, inducing below-normal height anomalies in the country. Using the Typhoon Rule, we can approximate a cold/stormy period following the aforementioned ridge, namely in a December 19-23 period, if not for longer. It should be noted that this trough doesn't "dig" south into Japan, raising concerns that a zonal flow (average to slightly warm) pattern may evolve instead of this cold weather. I will issue additional updates as needed.

ESRL

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.

Glancing over this four-panel forecast, we find that the PNA is expected to stay positive throughout the entire forecast period. This will allow for high pressure to reside in the West, which would usually give way to a cold period in the Central and East US. However, on the next panel, the positive NAO projection tells us this ridge will be allowed to bleed east into those aforementioned areas, keeping most of the country warmer than normal.
The positive WPO and EPO, both Pacific teleconnections, are good for cold weather prospects in the Central and East US, but until that positive NAO ends, we're likely stuck with a warm period.

Wait... so you're predicting some good cold shots, but also a warm period for the same timeframe?

The idea I'm getting right now is that the next 10-15 days of December are looking predominantly warm. However, in between those warm spells, some cold air outbreaks (not intense, but still noticeable) may be expected. This is a pretty turbulent forecast period, since I'm experiencing a good deal of uncertainty.

* For those of you who have been following on Facebook and were confused by my analog postings last night, please disregard those postings and go with this article as your guide to my expectations.

To summarize:

- A cold spell may be expected in a December 12-16 period.
- Warmth will then follow in December 16-20th, roughly.
- Around December 19-23 and beyond, the risk of below-normal temperatures may return, but uncertainty is high.
- The overall pattern remains generally unfavorable for sustained cold weather.

Andrew

New, Successful Analog Guidance Predicts Warm December, Cold January

A new system of analog guidance I composed in recent days, showing success in the limited number of trial runs I have conducted, is forecasting a predominantly warm month of December, followed by a cold January.

ESRL

The image above shows analog guidance's temperature forecast for the month of December. As the image shows, millions across the country would average out over a degree (Celsius) above normal, primarily in the Plains and Midwest. Only the Southwest would see a colder than normal start to winter.

Now, how accurate is this analog guidance? I just got this system up on its feet last night, and these are the first results of the forecast. Some development is still needed, but that will come with time. I've noticed the analogs seem to understand the general pattern (i.e. negative Arctic Oscillation, positive PNA, etc), but still do struggle with less-synoptic features. For now, I'm presenting the data here, allowing it to verify and examine how it does later in January.

ESRL

In January, my analogs see the mass of cold air being stored in Canada on the last image bulldozing southward, bringing below-normal temperature anomalies to much of the nation, especially the northern Plains. The South then looks to see slightly above normal temperatures. Oddly enough, this pattern is surprisingly La Nina-like, even though I made sure to factor in El Nino analogs.

ESRL

The 500mb height forecast anomalies for January show sustained ridging over the Arctic, forcing what appears to be the brunt of the Polar Vortex south into western North America. This then bleeds east, allowing for those chilly temperature anomalies in the Central US.

I'll end this post with a look at the analog forecast for the stratospheric portion of the polar vortex, since this image only shows the tropospheric version.

ESRL

If you look closely, you'll see the polar vortex has been nearly obliterated...

To summarize:

- A new set of successful analog years is predicting a warm month in December, followed by a cold month in January.

Andrew