Wednesday, February 13, 2013

February 21-23 Potential Significant Winter Storm

The potential for a storm system between the 21st and 23rd continues to unravel as the time draws closer.

Pictured above is the ECMWF model (henceforth called the European model), forecasted for Day 9 (9 days away). The image shows mean sea level pressure values for this system, and the system is defined in the Midwest with a central minimum pressure of just above 1001 millibars. This is not a particularly strong system, but is still worth watching closely, as this model is projecting widespread amounts over 6 inches for many big cities in the Great Lakes and Midwest.

Now pictured above is the GFS Model (henceforth called the American model), also valid for Day 9. This time, however, the American model is projecting mean sea level pressures to be as low as 993 millibars, indicating a very strong storm system. Storm placement is relatively similar, with both models indicating the storm will be somewhere in the central half of Illinois. There is definitely that big strength difference, and this strength difference will eventually determine just where the storm goes and how much snow is produced. The American model prints out nearly a foot of snow in the Upper Midwest in response to this storm system, higher than the European model.

Now we see the American model's ensembles for the same timeframe. Now shown are 850 millibar temperature values and mean sea level pressure contour lines. The ensembles take the system to a very similar placement as the European model; not surprising given that most forecast models tend to follow the European model. Notice how the system is weak and spread out in this forecast. This denotes uncertainty among the models- typically, uncertainty in ensembles is denoted by a very broad low pressure system, and that is what is being observed here. Personally, the addition of this ensemble set to the pack of forecasting systems supporting such a snowstorm enhances confidence that this event will actually happen.

Now shown above are the European model's own ensemble set, forecasting the Day 9 forecast of 850 millibar temperatures and mean sea level pressure contours. The ensembles are slightly further south when matched up with the American model, American ensembles and European model. This is to be expected; all models and ensembles, no matter their forecasting accuracy will have problems in the long range. Nonetheless, its presence in the relative same area as the other models is indeed supportive of this event coming to fruition.

I could go on and on about other models and ensembles supporting such a solution, but we need to take a step back and analyze the atmospheric pattern that will be in place when this storm potentially occurs.

This is the American ensemble forecast, yet again for 9 days away from today. The colors show 500 millibar pressure anomalies, with blues signifying below normal heights (low pressure) and reds depicting above normal heights (high pressure). First and foremost, this system will be originating from the Pacific. If we look at the Pacific ocean, we find a very tight jet stream, shown as the 500mb pressure contours grouped very close together. This tightened jet stream adds increased energy to disturbances that flow through this jet stream. The pieces of energy may start off in the Bering Sea, where persistent low pressure is holding strong in this forecast. As they swing out from the Bering Sea, strong high pressure in the Gulf of Alaska tightens the Pacific Jet stream further. In response to the extreme above normal height anomalies in the Gulf of Alaska, low pressure would be tempted to develop in the Southwest. If we recall Newton's Third Law of Motion, every action produces and equal and opposite reaction. The presence of extreme high pressure in the Pacific will not only enhance low pressure in the Bering Sea, it will also do so in the Southwest. A concern from those two anomalies of pressure is that a Rex Block may set up. A Rex Block occurs when high pressure is stacked north of low pressure. We can see this trying to happen in the West US, with that Pacific high pressure apparently trying to progress into the Pacific Northwest region. Persistent low pressure in the Southwest would finish out the Rex Block. A Rex Block induces a more zonally-oriented (west to east) wind flow that is more favorable for warm temperatures and not big coastal storms. Fortunately, these concerns are alleviated when looking at other ensemble forecasts near this timeframe.

What is apparent is that a negative Pacific North American index (PNA) will be present during this timeframe. A negative PNA acts to produce persistent low pressure in the West US, and this then instigates high pressure formation in the Southeast US. This high pressure formation is a barricade to Nor'easters and any winter weather for the East US. We do see a pretty solid negative PNA in the forecast by February 21st as shown in the image above. If this does happen, we can expect high pressure formation in the Southeast US in response.

However, all hope is not lost for those in the Midwest. Rather, it is enhanced. This is the forecast from several global models and ensembles for the North Atlantic Oscillation (NAO). In the negative NAO, high pressure forms over Greenland and stays there. In response, the jet stream will dip south into portions of the US and produce an environment favorable for coastal storms (aka Nor'easters). We see a fair consensus of a negative NAO during the timeframe of this potential storm. The strength of this negative NAO is TBD, but the general idea of a negative NAO is in place.
The North Atlantic Oscillation then gets more specific with the West-based and East-based phases. The West-based negative NAO means that high pressure over Greenland is centered to the west of that land mass, hence west-based negative NAO. It is this specific phase that is best for Nor'easters. The East-based negative NAO means that high pressure is strongest to the east of Greenland, and this means storms and cold are preferred in the Midwest over the Northeast. Looking over more model forecasts, the negative NAO will be nearly evenly distributed among both regions, but the East-based is definitely a favored phase by the time this storm's timeframe rolls around.

The negative PNA will set off the storm originally going through the Southern US, but high pressure in the Southeast will try to force it north into the Northern Plains, a common storm track this winter. However, the east-based negative NAO will suppress that high pressure to the point that the storm system is able to stay on a more eastward track that then goes through the Midwest and Lower Great Lakes, delivering snow to those areas. For those wondering if the Southeast ridge could disappear and the storm goes up the coast, chances are nil. The negative PNA, having a stronger influence on the US weather as it is upstream from the nation, will make the Southeast ridge prevail to an extent that matches my description in the first few sentences of this paragraph.

Here's my current prognosis: