Long Range Lookout: Stormier Pattern On The Way

The pattern for the next while is looking much stormier in the near future.

Above is a 4 panel forecast of two teleconnections, the North Atlantic Oscillation (NAO) on the bottom half and the Pacific North American (PNA) index on the top half. The left half forecasts are from the ESRL/PSD, a physics-based branch of the government's weather services, and the right half forecasts are from the NCEP, which gives us the GFS model. If you didn't catch that, here's a breakdown:

Top left: ESRL/PSD PNA Forecast
Top right: NCEP PNA Forecast
Bottom left: ESRL/PSD NAO Forecast
Bottom right: NCEP NAO Forecast

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. These forecasts call for a deep negative PNA, which typically is not that favorable for cold in the East. We'll discuss that prospect later, but keep in mind that negative PNA.
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 ESRL and NCEP are varying on their forecasts, but I have a feeling that the NAO will stay negative, so HP systems caused by the -PNA may be deterred slightly by the -NAO.

This is the Arctic Oscillation (AO) forecast. In its negative phase, you will see high pressure forming over the North Pole, displacing cold air and letting it flow south into North America. The positive phase of the AO brings about a very strong polar vortex (low pressure system that sits on the North Pole), which locks up the cold air in north Canada. Multiple models and ensembles are predicting that the AO will go positive for a while before dropping negative during the second to last week of December, finally trying to push positive in the days leading up to New Year's. There seems to be good agreement among the models, so cold shots are likely to be shooed away in the positive phase and enhanced in the negative phase.

All in all, a negative PNA, weak negative NAO and rollercoaster AO support a much stormier pattern but not a huge potential for a cooldown. Unless we see a sturdy -NAO and -AO, don't expect to see any sustained cold.

Andrew

December 17-20 Potential Significant Winter Storm (Updated 12/12)

There remains potential for a significant winter weather event to occur over some part of the central and eastern US. Let's see what the models are saying.

The National Weather Service's GFS model starts out with our system in the early morning hours of December 17th. These images will show sea level pressure (SLP) anomalies, with cool colors showing areas of low pressure, and warmer colors depicting higher pressure areas, hence the terms high and low pressure systems. We can see a nice swath of dark green across the Gulf Coast showing the presence of a storm system. I'll just come out and say it- at this point in time, with a lack of a strong high pressure system immediately offshore the Southeast, it's obvious this system wants to go up the East Coast.

In the afternoon of December 17th, the main storm system has now strengthened, with light green anomalies blossoming across the Southeast. The low pressure system is now apparently in the southern Ohio Valley, or the western Mid Atlantic region. Again, the absence of a high pressure system and a negative NAO (more on that later) make me think that this system will try and slide offshore.

In the early morning hours of December 18th, the Northeast is greeted by a rapidly strengthening system. In a 12 hour period, our storm system has dropped its central pressure from 1001 millibars to 993 millibars- nearly a 10 millibar drop. This is no doubt due to the Atlantic Ocean's influence, something that will work to strengthen the system further as time goes on.

The afternoon of December 18th brings about an even stronger storm system, with a central minimum pressure of 984 millibars. The system is located just onshore. While many of you may jump for joy at the potential of a huge snowstorm, the GFS predicts a small corridor of intense snow well inland, with lighter amounts towards the coast. Immediate inland areas would receive rain.

While I think the GFS is too strong, it has been consistent with a forecast of a storm going up the coast.

We now take a look at the European's ECMWF model, known to slightly outperform the GFS on a regular basis. The ECMWF, however, has a very similar solution to the GFS, with a storm system originating near the Gulf Coast at a minimum pressure of 1002 millibars. Again, the absence of a big ridge in the Southeast tells me a move offshore and up the coast is possible.

On the morning of December 19th (keep an eye on these timing issues, they could be crucial), we see that the ECMWF has indeed taken this storm towards the coast, and we now see a strong system hitting NC/VA. The tight alignment of isobars near these two states tells me that winds will be strong, regardless of precipitation. I should mention that this looks to be a rain event at this point in time.

Going ahead further to the morning of December 20th, our storm system has fully bombed out and is immediately offshore the New England area. Very strong winds are predicted across the region. Now, I know what you Northeast folk are thinking: snowstorm. And to that I say: hold on. This system came from the South, which means it is carrying a warm and humid influence with it. That doesn't help snow prospects in the least. But, to see if this is the case, let's see the forecast for 850mb temperatures.

In order to get snow, we want to see 850mb temperatures below 0, or in colors that are not light orange. Personally, I would want to see dark green/light blue to be confident of a snow potential. All we see in New England is dark green and spots of light blue. That's not encouraging to me. I'm not buying this scenario just yet of an all out snow storm in the Northeast. Now, if I were to combine the GFS/ECMWF and make a forecast out of that, yes, there is potential for accumulating snow in the Northeast. However, the next segment explains why I am hesitant to believe such an idea.

This is the ECMWF Ensemble Prediction Center, also known as the ECMWF EPS. Imagine the ECMWF model, but there are 51 different forecasts of the ECMWF, each with its parameters slightly changed to give diversity to the forecast. This image and the next will show the average of these 51 ensemble members. This first forecast, valid the morning of December 18th, sees our system in the Ohio Valley rather than the Gulf Coast. That difference in placement, while it may seem small, is pretty darn significant. This particular scenario would give snow to the Midwest and Ohio Valley rather than just into the Northeast.

24 hours later, the ECMWF EPS sends this storm out into the Atlantic, just offshore of the Mid-Atlantic region. Cold air looks to be sufficient to give snow potential to the Northeast as a whole. If you haven't already guessed, the GFS/ECMWF are at war with the ECMWF EPS to see whose forecast will verify.

Usually, I would side with the ECMWF EPS, but the consistency of the GFS and the new trend in the ECMWF brings me confidence too low to make a call.

Preferred Forecast: Indifferent To All
Confidence: N/A

Andrew

December 18-21 Potential Significant Winter Storm (Updated 12/11)

The potential is rising for a major winter storm to affect the Central and East US. Let's see how this could all unfold.

This is the Hour 168 forecast from today's 12z (6:00 AM) run of the National Weather Service's GFS model. This is the sea level pressure (white lines) and 500 millibar level (colors) forecast for the morning of December 18th. In this forecast, we see a large blue circle in the southern Plains, as well as a low pressure system near the Gulf Coast. The aligning of white lines (isobars) in a circular formation around the 'L' near the Gulf Coast, as well as the blue area in the South Plains suggests the presence of a storm system. This does look to be the storm system we are watching for this time period.

Going ahead a bit to the morning of December 19th, we now see that our storm system is predicted to move due east and become a strong coastal storm in an event that would be all too familiar to Superstorm Sandy, which I will discuss further later on. In a few words, I don't believe this forecast. The atmospheric pattern is much more conductive for a storm track either through the Plains or into the Ohio Valley/Midwest, the latter of which I think is more likely.

We now move on to the European weather service's ECMWF model, known as the most reliable forecasting model man has made to date, although recently this model has been slipping. The same rules apply for colors and white lines. This is the forecast from the ECMWF, valid on the morning of the 18th. Here, we see our low pressure system in the South Plains just like the GFS. Pay attention to what happens in this next forecast frame for the morning of the 24th.

The ECMWF takes this storm into the Southeast and off the coast! The reason I am so shocked is because this is a huge change from what the ECMWF was previously showing. Before, we had multiple forecasts of this system going into the Ohio Valley and Midwest, bringing rain and possibly good snows to those areas. When I come to situations like these, where the models suddenly change their forecast, I say 'Look at the trends'. So, I don't think I'll take this forecast too seriously unless tonight's new ECMWF forecast shows a similar idea. I will describe more why I don't think this forecast is correct further down.

Now here's where things get serious. This is the forecast from the 12z ECMWF ensemble prediction system (also known as the ECMWF EPS). To think of this the right way, imagine that forecast of the ECMWF above, but now there are 51 of those forecasts, combined into one, and that's the ECMWF EPS. Given, each different forecast has at least some change in their parameters so there is a difference from the actual ECMWF model, but when it comes to the ECMWF EPS, there's no model that can match that skill, so take that into account when seeing this forecast.
The ECMWF EPS starts out the morning of December 18th with our storm system in the southern Midwest, rather than the South Plains. Additionally, the system is not as strong as the ECMWF/GFS are showing. Why is this significant? Because if the ECMWF EPS goes up against the ECMWF/GFS, who do you think will win: 2 models with very high skill, or 51 models with the same high skill?

We now move ahead to the morning of December 19th. We now see the ECMWF EPS has taken this system into the waters just offshore the New England/Mid Atlantic area. It is stronger than the past forecast image. To me, this seems like the more obvious solution, because this is essentially what the ECMWF model itself was showing for quite a while before today's sudden change.

Now, let's discuss just why I'm so confident in the ECMWF EPS, and am disregarding the ECMWF and GFS.

(Originally posted Dec. 9, NEW INFORMATION ADDED on 12/11) The Lezak Recurring Cycle, or LRC is involved in this situation. Using the 53 day LRC timetable, I traced back December 18th to October 28th, and here is the 500mb pattern observed that day.

Look at that! We see a strong system in the Ohio Valley on this day, as well as a weak high pressure system in the Southwest indicating a possible positive PNA. There is a negative NAO in place, though we can't tell whether it is west or east based. This is our storm system that we could see hit on Dec. 18-21, and is the same system that the ECMWF was doing so well with before it got lost.

There is a method to the ECMWF and GFS' madness, and it is shown below.

Our storm system has now moved to the east, wobbling a bit as it did so, and combined with Sandy. Notice we still had our positive PNA showing itself with high pressure in the West US. If we were to follow this system and discount Sandy, I still believe an eastward movement would have prevailed, likely not as fast if Sandy were not present. The lack of Sandy in this potential storm system, combined with a positive PNA and east-based negative NAO leads me to believe that this forecast system should go a little more to the north and east than it did in October. 
The ECMWF and GFS are pulling the idea of a Sandy repeat, not in strength but in track. Personally, we all know Sandy isn't going to happen every 53 days, so we won't see this system ride the South US and shoot up the coast. If it DOES end up happening, it won't be a HUGE surprise, but it will be something I was not initially predicting.

Preferred Forecast: ECMWF EPS
Confidence: 45%

Andrew

Bering Sea Signaling at New Year's Day Big Storm Potential

I got a lot of criticism for releasing my original post on the potential for a New Year's Day storm, but such criticism doesn't deter me. I do these posts in the interests of YOU, so you won't be stranded on a big day like New Year's after a surprise storm comes through.

Above, we see the 24 hour forecast from the Ocean Prediction Center (OPC), the oceanic branch of the National Oceanic and Atmospheric Administration (NOAA). This chart depicts 500 millibar forecasts, with depressions and 'L' signs in the image characterizing low pressure areas, and arcs and/or the letter 'H' signifying high pressure areas. In this forecast, we see not one, not two but THREE separate storm systems in the western Bering Sea. (If you want to get technical about it, there is only one, but eventually the other two will find their way in there.)

Those of you who have followed me for a while know that I look to the Bering Sea for estimates on long range storms. This is no exception. The folks over at the AccuWeather Forums have found a correlation between a strong storm in the Bering Sea, and a strong storm in the US 2.5 to 3 weeks later (18-21 days later).

This forecast map is valid December 11. If we take that date out 18-21 days, we end up with a timeframe of December 29-January 1: Right in the middle of holiday traffic, potentially right on New Year's Day. If this rule (called the Bering Sea Rule) is put into effect here, and these three storms follow said rule, we could see quite a stormy period in the days leading up to New Year's Day.

Before you ask questions, no, I do not know the track, and no, I do not know what regions or cities could even possibly be affected by this. What I typed above is all I know.

Andrew

December 18-21 Potential Major Winter Storm

There is growing potential for a winter storm during the timeframe of December 18-21. Let's see what the models are saying.

This is the European's ECMWF model, and here we see a storm system coming out of the South Plains with a central minimum pressure of 996 millibars. Looking at the atmospheric set-up surrounding this system, we see a strong high pressure system in south central Canada, so that makes my enthusiasm for an Ohio Valley hit go down slightly. However, watch that extended dark orange color, as we could see a high pressure try to form in that region, which could provoke this storm slightly to the north.

Advancing 24 hours to the morning of December 19, we see that this system has moved north and actually into the Ohio Valley, thanks in part to a developing high pressure system in the Gulf Coast and what appears to be another high pressure system to the east of this system. The storm system in question is now predicted by the ECMWF to be 991 millibars, a pretty strong storm. Notice how it is more elongated than compressed, with those 991mb values trying to push east into the Mid Atlantic. This could be a sign that the system wants to split, but that's too far out for me to be confident in deciphering.

I can't say I'm sure what the GFS forecast for the afternoon of December 18th is showing. It does have a low pressure system in the picture, but the GFS has the system just offshore Florida in a bizarre set-up that could turn into a nasty Nor'easter. I have no idea what the GFS is doing, but to entertain those in a snow drought, let's move on.

Wow. The GFS bombs out a storm just off the Mid-Atlantic coast on the afternoon of December 19th. Again, this is pretty bizarre. I'm just not even sure what to say about this forecast other than... wow.


(Originally posted Dec. 9, edited slightly) The Lezak Recurring Cycle, or LRC is involved in this situation. Using the 53 day LRC timetable, I traced back December 18th to October 28th, and here is the 500mb pattern observed that day.

Look at that! We see a strong system in the Ohio Valley on this day, as well as a weak high pressure system in the Southwest indicating a possible positive PNA. There is a negative NAO in place, though we can't tell whether it is west or east based. Does this look familiar to the first image shown? You bet. It looks like our 53 day cycle is working.


The models are having some trouble, so let's go a day ahead to October 29th and see what happened.

Our storm system has now moved to the east, wobbling a bit as it did so, and combined with Sandy. Notice we still had our positive PNA showing itself with high pressure in the West US. If we were to follow this system and discount Sandy, I still believe an eastward movement would have prevailed, likely not as fast if Sandy were not present. The lack of Sandy in this potential storm system, combined with a positive PNA and east-based negative NAO leads me to believe that this forecast system should go a little more to the north and east than it did in October.

Looking at this 53 day timetable, I think I know why the GFS is acting so weird. It is trying to imitate Sandy in its forecast, with a strong storm system sliding up the coast. However, I highly doubt such an event will happen- the GFS is likely playing its games.

Preferred Forecast: ECMWF
Confidence: 35%

Andrew