Monday, September 30, 2013

Flash Weather: 2013-14 Winter Weather Forecast (Part 1)


This is the first installment of an ongoing series this fall concerning the winter weather season ahead. Although the meteorological elements are still coming together, there is a convergence of signals indicating this winter will be one heck of a ride...

Note: With respect to the video, I forgot to mention the difference between El Nino and La Nina when discussing the ENSO forecast for the upcoming winter. If you note the image to the left, you'll see a general map depicting the variances we see during both occurrences (ex: the northern Pacific low during an El Nino versus its high pressure counterpart in a La Nina).

Thus, the greatest opportunities for a colder winter in the southeast take place during a weak El Nino or weak La Nina, as the jet stream is allowed to buckle down further south to promote colder temperatures. If the ENSO is notably displaced from neutral territory, the jet streams tend to not engage each other, thus, keeping the moist air from merging with the colder air.

For this winter, I expect the two jet streams to collide with each other at times, with the more consistent jet stream being the pacific jet, which should supply an abundance of moisture to the Tennessee and Ohio River valleys. The wild card is how the AO/NAO cooperates, as this has a large influence on the movement and position of both jet streams.

                   Image from NEWXAction
The most ideal ENSO setup for middle Tennessee has historically been a weak El Nino, as this allows the subtropical jet stream to collide with the dip in the polar jet over the Ohio and Tennessee River valleys. As lows ride the active southern branch of the jet stream, it interacts with the cold shots coming down from Canada. The combination of polar, continental air from the north with the moist flow supplying above-average precipitation to the southeast, provides a promising setup for wintry conditions in the mid-south. If the jet stream position is compromised as a result of the Bermuda high and sneaks a ridge into the southeast, lows (like the Colorado Low) coming out of the four-corner region can take a I-44 track, leaving Tennessee in the warm sector, preventing any snow or ice to make an impact.

You can see how the storm track is dependent upon jet stream placement. If an omega block pattern can develop and establish itself off the coast of Alaska, this can likely lead to the polar jet bending further south, allowing the eastern two-thirds of the United States to receive more arctic blasts. The area of high heights in the northern Pacific is partially influenced by the region of warmer than normal sea-surface temperatures. When the jet responds to western ridging caused for the sea-surface temperature anomalies, this is known as a +PNA (Pacific-North American Index), which coincides with a colder pattern for the Great Lakes, Ohio Valley and Northeast.

It's interesting to note how the American Ensembles are picking up on an imminent cool down for our part of the county later this month, which could have lasting implications as we approach winter. If this pattern continues into late fall, we may see weak-El Nino conditions last into the first month of winter, which part of the reason I expect a colder December than the past two years for the Ohio and Tennessee River valleys. Although I do not expect moderate El Nino conditions, I still believe we can have a couple weeks of merged jet stream action, which could lead to some interested winter weather for the eastern two-thirds of the nation.

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