Saturday, December 29, 2012

Sudden Stratospheric Warming Begins

As expected, the sudden stratospheric warming has begun over Canada as of the day after Christmas. This sudden warming is signified by massive warm temperature anomalies flooding into the Arctic.

In these sudden stratospheric warmings (SSW), warm air penetrates the stratosphere and is forced up high into the stratosphere. When the warm air is forced up, cold air originally from the stratosphere is forced down to the surface. There is a couple week lag between sudden stratospheric warmings and cold air reaching the surface. I'm not confident of the exact number of days it takes for this cold to hit the surface, but I think we should see this SSW's effects by mid to late January, something that could bring downright frigid air to much of the nation IF the atmospheric pattern cooperates.

It should be mentioned that SSW's cause trough formation later on. Like we saw with the Dec. 1-6 SSW in the first several frames of the animation above, we are now seeing forecasts of low pressure building in the Bering Sea, right where that first SSW occurred. If the same rule applies, the end of January could see anomalous low pressure building over Canada, possibly extending into Greenland- something that could ignite the infamous, anti-cold-and-snow positive NAO. But that's a different post for a different time.


January 11-14 Potential Winter Storm

The slim potential there is for a storm in this timeframe appears to be rising. Let's take a look.

This is the 0z GFS forecast for this timeframe, the evening of January 12th. We see a strong storm system lifting northeast, producing a good swath of snow across the Midwest and central/eastern Great Lakes. Heavy rain is ongoing in the Ohio Valley and Mid-Atlantic regions with this system, and a severe threat may be evolving on the Gulf Coast if this is a linear storm formation. A quick look at snowfall forecasts reveals a nice 6''+ swath of snow from Iowa to Illinois to Michigan. Behind this system, cold air stretches as far south as Corpus Christi, Texas, as shown by the solid blue freezing line (32 degrees F).

I would not post this if it were just a single GFS run. It has now been at least 4 model runs of the GFS that a storm has been shown to go through the Ohio Valley and bring accumulating snow to either the Midwest, Ohio Valley or both. Each image is a different model forecast.

6z GFS forecast

12z GFS forecast

18z GFS forecast
I'm the sort of person that needs to know why things happen; I can't just see a forecast and accept it, I need to know what would make that forecast work. I'll do the same here.

This is the GFS Ensemble 500mb height anomaly forecast for Hour 384, the furthest the GFS model and ensembles can go. We see lower heights over much of the United States, as well as high pressure in the Gulf of Alaska and low heights in the Bering Sea.

The lower heights in the Bering Sea creates a positive West Pacific Oscillation (WPO). In response to these lower heights, high pressure then forms in the Gulf of Alaska, and in response to that, low pressure forms in the West US. This West US low pressure causes a negative Pacific-North American index (PNA) to form, which typically diverts storms to the Gulf Coast and North Plains. But the big thing is how low pressure is not confined to the West US, but is present across the entire nation. This then results in higher heights (high pressure) to form further out in the Atlantic. In a typical -PNA, the ridge sets up in the East US. This high pressure in the Atlantic then tries to propagate down into the waters off the Southeast US, meaning the infamous Southeast Ridge could set up. If this happens, the storm track would greatly favor the Midwest and Ohio Valley for snow, and that's what I think will happen going into the end of the second week of January. It should also be noted that many individual GFS Ensemble members project this Southeast ridge to form at Hour 384:

This massive low pressure will emerge in the United States by the splitting of the polar vortex. Not collapse, but a split. This will send pieces of the vortex into lower latitudes, and in this case possibly the US.