MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
Severe Weather Season Is Here (Intermountain Region And Great Plains Will See Wintry Weather With Blizzards); The "Double Whammy" Effect!
Plymouth State University Weather Server (5)
This is very much a "loaded gun" scenario for much of the nation, with sharp temperature conflicts and at least two important storms. The first major center of low pressure in the series will bring widespread severe weather and heavy rain from Texas and Oklahoma into the Ohio Valley and through much of the Old South. Since the ECMWF and GFS panels seem to placing more energy in the primary cyclone and tracking that feature into the Great Lakes, precipitation totals in middle Appalachia, the Mid-Atlantic and New England regions may be rather light. Coastal secondary cyclogenesis is NOT assured, so it appears to me that most serious risks for heavy snow will be across parts of the Intermountain Region and central/northern Great Plains.
"Storm Number 2" seems a good bet for the February 23 - 28 time frame. Once again taking the "Colorado/Trinidad A" option, the quick return of warm, moist and unstable air will mean that an initial display of strong to severe thunderstorms with this feature in California and the Southwest is likely to reform in the south central U.S. and much of Dixie. The path of moderate/heavy snowfall across the various western U.S. mountain ranges should continue through the middle Missouri Valley, then east-northeast into Minnesota, Wisconsin, and Michigan. While redevelopment seems a good bet over the Delmarva Peninsula by February 27, snow and sleet may not form south of upstate New York and New England.
Cold West Vs. Milder Gulf Coast, Eastern Seaboard Alignment Taking Shape
There may be some difficulty in getting a good fix on the threats of strong storms to the lower 48 states during the following 10 days, largely because there are so many impressive disturbances in the sequence. But if we use the ensemble packages of the various computer models seem to be showing a defined pattern for trajectory of these impulses, carving out a trough across the western half of the nation. A weak blocking signature remains over Hudson Bay and Quebec, while a subtropical high locks in across the Greater Antilles. With this arrangement, temperatures should trend mild or occasionally warm over the Deep South and Eastern Seaboard (when cold air damming does not set up). The West, Great Plains and to some degree the Great Lakes would be colder than normal, implying some potential for important snowfall.
EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
Active Pacific Ocean Storm Sequence Traverses U.S. Between Weak -NAO Signal (Baffin Island) And Growing Subtropical High (Greater Antilles)
University Of Nebraska HPRCC
Pennsylvania State University E-Wall
As we continue to progress through late winter into spring during a neutral ENSO episode, it is important to understand that volatility is usually the biggest "player" concerning the temperature spread and chances for precipitation. There is no important synoptic scale feature on the map in the Northern Hemisphere which would act as a "game changer". The Madden-Julian Oscillation is strengthening, but once again its greatest concentration of energy is over Indonesia and Malaysia (with only a weak linkage to the polar westerlies). If you had to look toward one particular item which will be the biggest influence in weather across North America, it would have to be the rather impressive sequence of storms stretching from Siberia through the northern Pacific Ocean.
I suspect that the 500MB longwave pattern developing now over the U.S. and Canada will likely be the controlling feature in apparent weather through the first two weeks of March. The idea of a persistent, though weak, Rex blocking feature over Hudson Bay and Quebec appears to be legitimate (although the numerical models have been calling for this type of signature since November, with disastrous results). The scenario involves a series of deep storms digging from the Gulf of Alaska into southern Colorado, then recurving into the lower Great Lakes. In some cases these disturbances may undergo a redevelopment process over or near the Delmarva Peninsula. If that is the case, then snow or sleet may have "equal time" with liquid types along the populous Interstate 95 corridor north of Wilmington DE. Mostly this pattern will be of a cold West/mild East type though, since I expect a strong input from a subtropical high situated over the Greater Antilles. You will notice that for all of the comments recently about cold and snow threats, the coldest values have again largely lost out to warmth in the middle of the nation as well as the Old South. Even with a massive snowstorm thrown in, the Northeast has averaged near seasonal normals.
There is one big "danger factor involved in this forecast which needs to be addressed. A close review of the 12z model runs shows the potential for a broad-based, intense winter-type storm moving from the lower Great Plains into the Mid-Atlantic region. While I think the results of the operational GFS scheme at 360 hours + borders on either ridicule or fantasy, the American and European ensemble packages shows the threat for a blockbuster disturbance to be real. More than likely, such an event would involve transport of deep tropical moisture and warmth into the Ohio Valley and Mid-Atlantic regions, with another brief intrusion of very cold values into the Great Plains and Mississippi Valley. It is important to remember that there have been several extremely intense cyclones at various points in the Pacific, North America, and Atlantic theaters since mid-October, and the threat could again be realized in the U.S. somewhere besides the Eastern Seaboard (where Sandy and the February 8-9 storm struck).
Prepared by Meteorologist LARRY COSGROVE on
Saturday, February 16, 2013 at 7:30 P.M. CT
The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.
Copyright 2012 by Larry Cosgrove
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