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

December 31 - January 5 Potential East Coast Winter Storm

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

ESRL

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

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

A quick refresher on the PNA and NAO...

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

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

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

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

CPC

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

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

Nicholas Schiraldi

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

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

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

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

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

Tropical Tidbits

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

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

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

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

ESRL

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

ESRL

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

To summarize:

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

Andrew

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