Monday, November 19, 2012

November 27-29 Potential Winter Storm (Updated 11/19)

There is potential for a winter storm between November 27-29 for much of the northern and eastern parts of the nation. Let's take a look.

This is the GFS forecast for the evening of November 29th. On this chart is 6 hour accumulated precipitation, as well as the rain/snow line (blue), and low and high pressure systems superimposed. Basically, this is the whole deal. On the evening of the 27th, the GFS model is projecting our big storm system to be in a pretty messy place. I'm not sure if I can pinpoint just where this system actually is on this picture. Precipitation looks to be ongoing across wide swaths of the nation, including the North Plains, the Rockies and the Midwest into the western Great Lakes. The blue rain/snow line is to the south of a heavy area of precipitation across the Dakotas, Minnesota and other states to the west, meaning that residents in those areas could see snow if this forecast verified. That would be a good 2-5 inches of snow in just 6 hours.

Fast forward 12 hours and we are on the morning of November 28th. At this point, our storm system has moved far off to the northeast and is now in southeastern Canada at a minimum pressure of 998 millibars. Precipitation is ending across Wisconsin and Michigan as snow, with Nebraska and Kansas also getting some snowflakes as the cold air fills in behind the system. The blue rain/snow line remains north of many locations in the Ohio Valley, meaning this would be a rain event for them. At this point, Canada is now getting the snow and the New England area is getting in on some heavy rains.

Personally, the fact that the cold air does not wrap around behind the storm system into the Midwest and Ohio Valley is a red flag for me. If it's happened in the Rockies and Plains, why can't it happen in the Midwest and Ohio Valley as well after the system has passed through?

This is the ECMWF's forecast for the morning of November 27th. The European model is portraying a large area of low pressure across the Plains and southwestern Midwest, indicating that our storm system is beginning a move to the northeast. If I were to estimate where the precipitation would be falling, I would say Iowa, Nebraska and Kansas would be getting in on some precipitation (likely snow in the latter two states).

The image above is 850mb temperatures for the same time. See how the cold air is wrapping behind the system as it moves to the northeast? That is how storm systems operate. I find the GFS solution of not pulling in cold air pretty sketchy. It's possible, but in these circumstances, I find it unlikely. As stated above, if precipitation is ongoing in Nebraska and Kansas, it would likely be snow. The ECMWF confirms this suspicion, showing as low as negative 15 degrees Celsius only 5000 feet off the ground.

On the morning of November 28th, 24 hours later, we see a very strong low pressure system placed in the western Great Lakes. The bottom right corner of this chart tells me that it is as low as 983 millibars- very strong for a system on-land. The tendency for isobars to be so close in the above image tells me that any precipitation of any kind in Wisconsin, Illinois, Michigan, Indiana, Ohio or Indiana would be wind-driven. Let's look below to see what type of precipitation would fall where.

850mb temperatures for the morning of November 28th are showing a deep wrapping of the cold air into the heart of the system. Like I was saying, it's only natural for storm systems to pull in cold air behind themselves. The GFS does not have the correct forecast. If I were to say there is precipitation ongoing in this timeframe, I would say it would be in Wisconsin, Minnesota, Michigan and Ohio into Indiana. The first 2 states would likely be snow, while the last 3 states would be iffy, especially Michigan.

(Originally posted November 18) I know what you're thinking: "Why should I get excited over a forecast over 10 days away? It'll never happen- the model is too far out!" But there is substantial evidence that this isn't just another 'fantasy forecast' (a forecast that looks great but doesn't happen).

This was the observed surface map for November 13th, just 5 days ago. This surface map is for the northern Pacific and Bering Strait. As you can see, there is a very strong storm system swirling around in the waters just off the end of the Aleutian Islands on this date.

Why is this significant? There has been a connection found between strong Gulf of Alaska (GOA) storm systems, and a strong storm system hitting the Lower 48 around 17-21 days later. If we take November 13 and apply that 2.5-3 week gap, we find that this strong storm system would hit...

...between November 30 and December 4. It just so happens that the storm system forecasted above falls near this timeframe.

Is this a coincidence? I doubt it. While the GFS may be over-exaggerating some aspects of the storm (i.e. strength, precipitation, air temperature), I have a feeling that the GFS is catching on to this Bering Sea Rule, as it's called.

I can't say I am willing to pinpoint the exact track and effects of this storm system because the models are all over the place. One cannot figure all those factors out just by seeing a strong GOA system, either- all you can determine is that a strong storm system will be around the Lower 48 roughly 17-21 days later.


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