Sunday, November 3, 2013

Flash Weather: 2013-14 Winter Weather Forecast (Part 2)



Followup Notes: With only a few weeks of meteorological fall left, I continue to wonder about the -EPO and how it may be a saving grace for the Tennessee Valley this winter. With the QBO in a positive and falling state, I expect the polar vortex to start out strong but weaken as we approach winter, which is part of the reason I expect the second half of winter to be stronger and more intense than the first half. Since the AO/NAO is partly tied to the QBO, and since a +QBO would likely lead to a +AO/+NAO, I can definitely see warmer than normal temperatures to abound in December for the third consecutive year, with each subsequent month lessening in terms of positive temperature anomaly; however, with a strong -EPO, the pacific pattern allows troughing to dig into the southeast quadrant. My guess at this point is we could see a winter, temperature wise, similar to 2007-08. During this winter, December was a torchfest, but January evened out and February and early March featured some memorable snow events. I will note that we near the launch of Yuletide, December is looking more and more like a coin toss.

Furthermore, there’s been an interesting trend emerging in the arctic regions in recent weeks. Notice how the two dark red regions (areas of rising heights or higher pressures in the upper atmosphere) explode near the Aleutian region (Gulf of Alaska) and off the coast of Greenland in the North Atlantic. The key effect of these expanding height zones lies in their ability to buckle the jet stream, forcing colder air south into areas on either side of the block. This is essentially the definition of a –NAO, as defined in my first winter weather video. These blocks tend to form and reform, as they oscillate around the globe.


From this animation, you can clearly see how the two blocks develop, ‘wobble’ and then trap the cold air in between. When this happens, the cold air is essentially channeled southward towards the Lower 48. Early indications reveal the trending -NAO pattern appears east based, though this doesn’t mean the –NAO will remain that way.



When a –NAO is ‘east-based’, this means the strongest part of the high pressure system is to the EAST of Greenland. This allows the jet stream to focus the cold air into the Plains and Midwest, while the southeast corner is brushed with cooler than normal temperatures

When a –NAO is ‘west-based’, this means the strongest part of the high pressure system is now to the west of Greenland, and allows the cold and snow to expand into the eastern US. This type of –NAO is more favorable for snowstorms in the southeast, although wintry precipitation is almost always enhanced when the NAO is negative in either phase.

Note: EPO graphic from DT Wxrisk. Check out his Thanksgiving video here:


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