Wednesday, November 28, 2012

December 10-12 Long Range Potential Winter Storm

I am officially opening up the discussion on a potential December 10-12 storm system that has been in many of the last couple days of the GFS model forecast.

Top left: 500mb heights. Reds are low pressures, purples are high pressures.
Top right: Pressure values. High/Low pressure systems are outlined.
Bottom left: 700mb relative humidity. Higher humidity values are in darker greens.
Bottom right: Precipitation. Any values above the red line are snow, any values below the line indicate rain.
For quite some time now, the GFS model, as pictured above, has been projecting a strong storm system to develop in the Plains and move northeast and strengthen, in a path all too familiar to those that went through the 2011 Groundhog Day Blizzard. This particular image above is of the 12z GFS for December 11th, depicting a strong storm system in the top right image. Precipitation in the bottom left image is widespread, with much of the Midwest getting snow while the Southeast is hammered with rain and storms. But first, let's determine the likelihood of such an event even happening.

It should be noted that 9 of the last 11 GFS model forecasts have had this storm system- a good sign as far as reliability goes.

This is something called the Pacific-North American Index, or PNA. In positive phases, storms are directed at the Midwest, like the above example shows, and negative phases generally induce warmer spells over the East US and storms diverted to the Plains or Deep South. These two images are separate forecasts of the PNA, with the left image being from the ESRL/PSD agency and the right image from the NCEP, essentially the GFS Ensembles. Because the NCEP is just composed of the GFS Ensembles, I favor the ESRL/PSD ensemble forecasts. That agency has the PNA just as negative as the NCEP, but has the index spike around December 8-12 to only weak negative values, which would help out this storm system into taking the track the forecast at the top of this post is showing.

However, when I take a look at the individual 12z GFS Ensembles for this same timeframe, it occurs to me that sometimes, you can't trust forecasts like these. The ensembles have a strong ridge over the northeast Pacific and West Coast, typical of a positive PNA. While it is not necessarily a positive PNA, it is apparent to me that such a set-up could indeed support a storm of this magnitude and track.

Also enhancing my investigation into this particular storm is the West Pacific Oscillation (WPO) and East Pacific Oscillation (EPO). These two indices affect the Bering Sea, which is currently experiencing an Omega Block. An Omega block involves a high pressure forming in any given area. In response to the sudden increase in height anomalies (a.k.a. the formation of a high pressure system), a stormy pattern develops in the immediate vicinity of the high pressure system, forcing height anomalies down on either side of the high pressure system. This creates the Greek symbol Omega (if you were to look at the 500mb height chart), hence the name 'Omega Block'. The WPO/EPO, both of which affect this block which just happened to form in the Bering Sea, are looking to be on the move near this storm's potential occurrence.

The EPO looks to be more or less positive during the storm's potential arrival. Such a solution would mean that a disturbance would try to form over the Bering Sea, thus hurting the high pressure system stationed over that body of water. If we want cold weather to enter the nation, it is an absolute must that we get that Omega block out of the Bering Sea, and this positive EPO could be helped by the WPO. The WPO index is looking to be trending to weak negative territory by the middle of the second week of December, which is when we could expect this strong storm system to hit. This more positive trend to the WPO also enhances the atmosphere to attempt and create a stormy pattern in the place where this Bering Sea Omega Block is.

The 12z GFS Ensembles have the Omega block retrograding north into the Arctic Circle, which would weaken the polar vortex and help cold air flow south. If such a solution were to happen, I see little reason to why we should give up the second half of December.

All in all, keep your eyes peeled. I'm seeing changes in the teleconnections, and when you get changes in the teleconnections, it's not all that difficult to see a brief stormy pattern as the pattern shifts from one phase to another. If the WPO/EPO/PNA forecasts verify on their forecasts as shown above, there could indeed be potential for a storm like this to blow through. Encouraging signs are also emerging from ensemble and model forecasts, as shown above by the GFS/GFS Ensembles.



Anonymous said...

I understand that it's pretty far out, but is their a chance that this could be a east coast storm or take a track up the coast? You've explained in many posts about the NAO in a week or two sinking so hopefully this could be for the northeast! Thanks keeping the fingers crossed!

Anonymous said...

What about the EURO model andthe others? Are they in agreement with this/ This is really long range.

Anonymous said...

Lets all take a look at this. It wont happen . In fact winter probably will never happen either. It will be like last year. I have a feeling. Just look at it.

Anonymous said...

This winter will not be all winter. It will be a mix of winter a few days followed by spring a few days and the pattern will stay that way until late spring. Just hoping when its cold it snows and when its warm there are no storms. I hate rain during winter months.

Anonymous said...

5 days later and GFS and some EURO models are still showing this storm developing with some variation on the date, but now only about a week away. Smells like a change is in the air. Finally winter will arrive in the Midwest?