December 20-23 Potential Significant Winter Storm

**This post is dedicated to those who lost their lives in the tragic massacre in Connecticut. All information below is typed and published in honor of the children and teachers killed in this horrible event.**

The new models have come in for a December 20-23 potential significant winter storm. Let's get into it.

We start with the European weather service's ECMWF model. This is a forecast for December 19th, and we see two things happening here: a pair of storm systems in New England, as well as a low pressure system beginning to emerge from the Rocky Mountains. Our system is that one emerging from the Rockies, with a central pressure of 993 millibars, meaning it is a fairly strong storm. Let's hone in on the atmospheric set up at this point. We see that dual storm system departing, as well as a weak high pressure system in the Southeast. A high pressure system is also shown in the West US. Not really anything significant to note at this time other than a progressive negative PNA pattern showing up, something likely to change as this system moves east.

24 hours later, we see our system has now moved into the Midwest, pushed in that direction by the Southeast ridge being shown to the east of Florida. We now see a high pressure system in the West US and the Rockies. Note the compact-ness of the low pressure system in the Midwest- that means our system is small but strong, as the central minimum pressure is at 996 millibars. Now, I don't trust the ECMWF path at the moment, and I'll tell you why. The pattern that was ongoing when this system emerged from the Rockies was a negative PNA, meaning that the system goes into the Plains or into the Deep South. Now, the system emerged from the Rockies, and we see the ECMWF project it moving into the Midwest as seeing the image above. As this system moves into this region, it's likely we will see a positive PNA set up, something I will discuss later on.

Now, on the morning of December 22nd, we see our system has shifted into the Northeast, with a minimum pressure nearing 980 millibars. Check out that incredible pressure gradient in the Midwest. The closer those lines (isobars) are together, the stronger the surface winds will be. If that's any indication of what we could see, December 22nd could be a very windy day if the ECMWF pans out.

Here's what I think will be crucial in determining if the ECMWF will work. As I said above, the system will leave the Rockies in the midst of a brief PNA phase change, from negative to positive. Above is the ECMWF's 500mb height anomaly forecast for the morning of December 21st (Oddly enough, the day the world is predicted to end). We can see the storm system in the Ohio Valley, but take a closer look at the ridge in the Rockies. That is a positive PNA, people! Such a positive PNA, combined with the presence of a Southeast ridge presents us with the general idea that a Midwest/Ohio Valley hit is "likely", but only if the pattern works out as the ECMWF is predicting it to.

Before we move on to the GFS model, the ECMWF did show some snowfall in the WI/MI regions from this system, on the order of several inches at most.

This is the American weather service's GFS model, a forecasting model that is always in a tight race with the ECMWF to see which is better. Historically, the ECMWF has a better forecasting record, but the GFS has been catching up recently. Above, we see the December 19th afternoon forecast of sea level pressure (SLP) anomalies. The cool colors show lowered pressures (indicative of a storm system), and warmer colors show raised pressures, which typically show a quieter pattern. This forecast from the GFS shows our storm system in the Midwest at this time. Our Southeast ridge is present, meaning the East Coast isn't so likely to see this storm. Note the timing differences between the GFS and ECMWF for when the storm system moves into the Plains- this GFS forecast is for the afternoon of Dec. 19, and the ECMWF is for the morning of December 20th. As small as 12 hours' difference may seem, I can tell you firsthand that such a difference can bring about severe track changes.

Moving into the early morning of December 21st, we see the storm system has now entered the East Great Lakes with a minimum pressure 993 millibars. Also note the storm system in the Southeast that is likely to swing up the coast. As this storm system moves into the Great Lakes, if the GFS verifies, we could see a nasty cold outbreak across the Plains, Midwest, Great Lakes and Ohio Valley if cold air is indeed available. That's not a guarantee, but something could come out of this.

Here's the thing that really shocks me, but not in the way you think. On the morning of December 22nd, we see our storm system has now bombed out to 971 millibars- a very strong storm system for an on-land system. This is what shocks me: the nature that this storm system takes. We see it move into the Northeast, but suddenly stop moving and retrograde (move west in the North Hemisphere) slightly, essentially staying put. If such an event happened, we could see significant cold and snow across the northeastern quadrant (not specifically limited to the Northeast- imagine cutting the nation into 4 pieces) of the United States.

Total accumulated snow from the GFS through the morning of Dec. 22 shows who could get slammed:

Forecast Preference: ECMWF
Confidence: 30%

Andrew

Major New Year's Storm Looking More Likely

(Originally posted December 10th) 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.

(The following is new information)

The above is a 500mb map from November 8th, 2012. On November 8th, a strong storm system was in the immediate vicinity of New England, and high pressure was stationed over the Central US, with another low pressure (LP) system on the West Coast, similar to a negative PNA pattern.

The reason why I bring this up is simple: This is the best forecasting tool we have at the moment. This is the Lezak Recurring Cycle (LRC) in action. Using a 53 day cycle time developed by the folks at AccuWeather Forums, I traced December 31st to November 8th. Considering the Bering Sea is in agreement at this stage, and to see a strong LP system in the Northeast in the 53 day timeframe is greatly enhancing my optimism for a big New Year's Day storm.

Now, the atmospheric pattern is not precisely the same every cycle of the LRC- the storm track will change, meaning it may not end up in the Northeast. It could hit the Plains as a very weak storm. I doubt the idea of a weak storm as the Bering Sea is onboard here, but yes, any region of the Lower 48 is still in the target range; the Northeast just has a slight advantage.

Andrew

Stormy Pattern Could Solidify, Watch For Cooling Trend Towards January

I think now is the time to seriously sit down and address what's likely to happen with the atmosphere in coming weeks and into the end of Dec.

I'd like to start off with the potential for slightly cooler temperatures emerging over the nation. Above is a chart of observed 70mb stratospheric temperatures from 2011 and 2012. The dashed green line is the average stratospheric temperature for that time of the year. Let's take a look on the right side of this chart, where the newest observations are in. See that spike in temperatures? That was a recent stratospheric warming event, known as a significant stratospheric warming event (SSW). An SSW shows that warm air has penetrated into this layer of the atmosphere, and is rapidly increasing the temperature, key word being rapidly, hence the 'sudden' in sudden stratospheric warming. I have learned that there is roughly a 10 day gap between an SSW and observed cold weather at the surface. Considering this cold air must push through several hundred millibars (several thousand feet) in the atmosphere, such a gap is not surprising. If we estimate that this SSW happened a few days ago (we'll say Dec. 8-10), one would expect a trend towards colder weather to occur near the 18th-20th. It is possible that this cold air arrives shortly after the departure of a potentially significant winter storm during this time period, but more forecasts are needed to confirm this.

Adding to the potential for a cooling trend towards January is the raggedness of the polar vortex. The top image shows pressure and temperature forecasts from the ECMWF model at 1 millibar- the highest integer you can get in the stratosphere. The bottom image is the same pressure and temperature forecast in the 100mb layer, considered the end of the stratosphere and the beginning of the troposphere. If you look closely at these images, you can see big capital 'L' symbols. That marks the position of the lowest pressure readings in that area, a.k.a. the Polar Vortex (PV). If you try to match up the two 'L's in these two images, you'll find that they are not near each other. This is equivalent to an uneven Jenga tower- the bottom blocks are more populous on one side, and the top blocks are plentiful on another side. If you play Jenga, you know that such a recipe spells a very unstable tower. In a similar sense, such a mismatched polar vortex means that the cold air that the PV holds onto is more able to be released south. If such a mismatched PV goes on for some more time, we could see a complete collapse of the vortex altogether, leading to frigid air across the board, but that's for another time.
Continuing to analyze the 100mb layer of the stratosphere, check out the light orange colors over North America. This means that the stratosphere is warm in that area. As we know, a warm stratosphere forces cold air south. If cold air is able to propagate south from the lower stratosphere, I see it more than plausible for a cold air mass to settle over Canada. The North Plains and Upper Midwest could also be involved. If a good storm system were to come along and hold onto some of that cold air, I see it possible for a sharp temperature crash in the North half of the US as a result of the warming of the stratosphere. If you are wondering how this could all perfectly happen, just ask Russia and Europe: They're experiencing a warm stratopshere, and currently suffering from extreme cold.

This is the Madden-Julian Oscillation (MJO) forecast from the ECMWF Ensembles. We can see that the forecast calls for the MJO moving into a weak Phase 1. Typically, such a phase is supportive of deep cold over the nation, but the negative PNA says otherwise (we will discuss this next). Anyhow, the lack of strength in this Phase 1 is not reassuring, I believe that the MJO will have little effect on the weather for some time, especially in part thanks to the negative PNA, which we will now discuss.

A mixed blessing. A double-edged sword. Only a few of the terms used to describe the negative PNA. On one hand, the negative phase of the Pacific North American index leads to storms in the West, which can then transfer energy across the Rockies and into the East. On the other hand, a depressed height anomalies in the West also translate into raised height anomalies in the East in a typical equal-and-opposite-reaction example. Today's 12z suite of models and ensembles reaffirms my thoughts of the negative PNA sitting in place for a while, but a positive PNA could emerge later on. But this time, many may not want to see this factor switch back to positive. The negative PNA looks to be composed of multiple strong pieces of energy that are pushing into the West Coast. As this happens, they swing into the Southwest and through the South Plains, and, depending on other atmospheric factors, can go into the Plains, Midwest and East Coast. The negative PNA tends to divert storm tracks into the Plains and down through the Gulf Coast, hence the recent snowstorms in the Upper Midwest/North Plains. But the latter storm track will be falling into place soon. As we see some pieces of energy start to follow that Gulf Coast track, a negative NAO will pick it up and try (most likely succeed) in picking up the storms, shooting them up the coast, and bombing out into the (in)famous Nor'easter.

12z models and ensembles forecasting the NAO show a good consensus with a weak negative NAO eventually pushing into a moderate negative NAO before some signs of a backtrack into a weak negative NAO occur. Note the GFS ENS/GFS/GGEM ENS showing the NAO trending into neutral territory. Before you get upset, New Englanders, this will also be the time when the PNA may go positive, so that Gulf Coast track wouldn't be present anyhow.

The North Atlantic Oscillation is a little more complicated than just positive and negative. When the high pressure system that causes the negative NAO is west of Greenland, we call that a west-based negative NAO (for obvious reasons). Likewise, the ridge being east of Greenland results in an east-based negative NAO.

This is the same 12z model suite forecast, now for the west-based negative NAO. Notice how all models and ensembles are pushing heavily on a deep negative west-based (WB) negative NAO, before trending into neutral territory in the long range. The effects of a west-based negative NAO include that storm track being shoved into a position that favors more coastal storms, something many in the East Coast are looking forward to. Cold air is also realigned in a pattern that favors chilly days in the East US.

Let's take a step back and regroup all of this information. We have the PNA going negative, which will incite pieces of energy moving into the Gulf Coast. We have a negative NAO that will then take these pieces of energy and move them up the coast. The tendency for a west-based negative NAO will then amplify these storms. Adding to the mix, the idea of a storm taking a quick detour north from the Southeast results in a trend towards a more meridional flow for the region, something that enhances the storm system and thus the precipitation. An example of this meridional flow is shown below:

8-10 day ECMWF 500mb height forecast. Possible
atmospheric pattern outlined in black. Note the
sharp detours and turns that happen as
the flow continues through the nation. This is
meridional flow.

Something else intriguing me is the sudden crash of the Southern Oscillation Index (SOI). You avid weather followers may know it as a predictor of the El Nino or La Nina, but it serves another purpose- showing Kelvin Waves.

The most recently taken observation of both observed underwater values and anomaly underwater values are shown above, with depth of the water on the left and location on the bottom. If you look in the upper left corner of the anomaly chart, you can see a blob of orange and yellow. The placement of this swath at 100m-150m underwater, as well as its tendency to be longer than taller, tells me that this is a Kelvin Wave. A Kelvin Wave is a warm body of water that moves along the equatorial Pacific between 50m-200m, and shows up as a blob of red. The presence of a Kelvin Wave forces the waters to warm, and warm waters means rising air (a.k.a. convection). This enhanced rising air as a result of the warm Kelvin Wave waters blows the bottom out of the SOI, hence why we have seen the index crash in daily observed values below.

Columns (from left to right): Date, two columns of observed pressures,
daily SOI values, 30 day averaged SOI values, 90 day averaged SOI values.

As I have recently learned, when this SOI crashes, the pattern then will try to regulate itself by enhancing cold weather over the North Hemisphere. And why, you ask? Because, as the warm air shoots into the atmosphere from the Kelvin Wave, the polar vortex is affected. Much like the SSW, the polar vortex is injected with warmer air, and the vortex weakens, releasing cold air south, possibly into the Lower 48. Now, I wouldn't nearly place all my money on this, considering the Kelvin Wave/SOI is one in dozens and dozens of atmospheric factors, but such an event could have interesting implications as we move towards January.

Well, there you have it folks. The pattern is looking stormier and cooler as we move through December and into January. As soon as the October correlation in Siberia cooperates, Jan/Feb are looking even chillier than all of this could do. Exciting times ahead.

I would like to thank the Severe Weather Centre, a fantastic weather blog not unlike mine, and the AccuWeather Forums for detailing some information used here.

Andrew

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