MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
-NAO Styled Rex Block Introduces Cold Weather To Right Of The Rocky Mountains; (Lesser) Threat Of Major Winter Storm Remains In Play For Eastern Seaboard
Plymouth State University Weather Server (3)
Like it or not, there are still many questions about the evolution of weather across the eastern half of the nation in late week. Yes, it is likely to be cold, as all of the numerical models and ensemble packages show a Rex block in eastern Canada and just south of Greenland forcing the formation of a closed 500MB low and cold pool centered over the Mid-Atlantic and New England states. The presence of a modest +PNA ridge complex over the West would also seem to favor dominance of cold air from the Great Plains to the Eastern Seaboard.
But what happens to that troublesome upper shortwave seen on the computer schemes, entering the middle/lower High Plains by February 27? This feature still seems well-organized, and may be able to maintain its strength as it enters the mean 500MB trough axis across the Southeast on Thursday afternoon. Yes, all of the operational models have dropped this feature and show no concerns for heavy snow along the Interstate 95 corridor above Raleigh NC. However, I can tell you that a relatively tranquil and cold solution has relatively low confidence. With increased ridging across Quebec, teleconnections would favor a deeper, possibly closed low near the 40 N 70 W "benchmark" on Friday. Time will tell, but do not let this possibility out of your sight if you live from Virginia into Maine!
Flattening Of +PNA Ridge Allows Mostly Mild, Dry Conditions Over The West Coast, Intermountain Region To Migrate Into Texas And Old South
Since projections of long-lasting cold air have been hideously overdone by the computer schemes during the entirety of the winter season, I am skeptical of recent calls (the usual +PNA, -AO, -NAO chatter in the weather community) for continued widespread placement of Arctic values in the eastern two-thirds of the U.S. While I think a cold spell of 3 - 5 day duration is probable to the right of the High Plains, the signs shown by the operational ECMWF version and the ensemble packages of the major models show a flatter upper flow taking shape by March 5. A westerly trajectory aloft should allow transport of warmer air in the Southwest and Intermountain Region to infiltrate Texas, then perhaps cover the rest of the Old South by some point in the extended period. That arrangement may allow for some storms, too, but the lack of a high-latitude ridge over the western half of North America favors return of spring-like temperatures to the Midwest and East Coast after the first week of March.
EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
After A Cold Start, Teleconnections Paint A Milder And Somewhat Semizonal Pattern For North America
Pennsylvania State University E-Wall
As indicated in the previous paragraphs, I am leery of accepting the colder March solution offered by the NAEFS and CFS (monthly) projections. If you review the ensemble means of the GGEM, ECMWF, and GFS equations, you find complete agreement on the scenario of a semizonal flow at 500MB crossing the lower 48 states. Weaker ridging at high latitudes and frequent arrival of shortwaves form the northern Pacific Ocean are milder teleconnections, and the CFS weeklies for March 9 - 22 seem to support at least a neutral if not increasingly warm look to the temperature map over much of the continent. Since the Madden-Julian Oscillation will be slipping into a position over the Indian Ocean and Indonesia (all the while showing little injection into the polar westerlies), I will argue for a milder outcome over much of the U.S. in the 11 - 15 day period, and for the entire month of March on average.
I suspect, however, that tropical forcing from between Hawaii and Baja California will support more strong mid-latitude cyclone formation, taking either a Panhandle Hooker "B" or Colorado/Trinidad "A" track. That would essentially split the nation between colder values (with snow) from the Intermountain Region into the Upper Midwest and Great Lakes with warmer air (and with it, chances for severe weather and heavy rain) from the lower/middle Great Plains into the Old South, Ohio Valley and Eastern Seaboard.
Don't give up on the arrival of Spring just yet....
Prepared by Meteorologist LARRY COSGROVE on
Saturday, February 23, 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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