WEATHERAmerica Newsletter, Sunday, April 1, 2007 at 2:45 A.M. ET

[Larry_Cosgrove]

Highly Amplified 500MB Longwave Pattern Yields A Severe Weather Outbreak In The Corn Belt And Dixie, Followed By A Long Spell Of Cold Temperatures Over Eastern Half Of North America

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WEATHER SUMMARY THROUGH NEXT 72 HOURS

Welcome to April, known for erroneous computer forecasts, herky-jerky temperature trends, increasing chances for precipitation and some whopper tornado outbreaks.

True to form, there is a little bit of everything in the predictions for both the near term and the medium range. The short range outlooks is actually quite troubling, with the potential for a noteworthy severe weather event (see map below). Two disturbances, one over the eastern Pacific Ocean and the other in BC/AB, are undergoing a phasing process. At the same time, the regrouping subtropical jet stream is streaming northeastward from the equatorial regions and will arc across Mexico into E TX and LA by tomorrow morning. With atmospheric height falls in place across the Intermountain Region and a surge of hot, dry values from Chihuahua into the lower High Plains, the stage will be set for a convective barrage easily the equal of, and probably surpassing, the thunderstorm episode in the Great Plains and Midwest of recent days.

As the 500MB component undergoes deepening and digging in the Upper Midwest, surface cyclogenesis should get underway on Monday morning over SE CO. The eventual lowest pressure of the cyclone is an important parameter when forecasting a "historical" outbreak of severe weather as opposed to a 'notable" type of event. If you review past major explosions of tornadic supercells, the core depth of the low center is at or below 992MB. Most of the NWP depictions show a central barometric reading of no less than 998MB with the current feature, but other factors could play a major role in promoting the development of intense convection. Notable is the subtropical jet stream, now reforming after playing a huge role in the torrential rainfall and tornado development in TX, OK, KS, and CO during the past 72 hours. As the higher low latitude velocity maximum reaches the western Gulf Coast Monday night, the nose of the windfield will promote the formation of thunderstorms. One or two complexes of convective cells may produce hail, tornadoes and/or microbursts from E TX and LA into MS, then expand into AL, GA and the FL Panhandle on Tuesday.

Further north, the complex interaction of surface convergence (cP, cT, and mT) with the digging polar westerlies (32 unit vorticity maximum at 500MB) virtually ensures an outcropping of supercell thunderstorms. Instability measures (using LI and CAPE) are extremely unstable on Tuesday into Wednesday from the eastern Great Plains through most of the Ohio Valley and lower Great Lakes. Since the surface low will be undergoing deepening in the 48-72 hour time frame, initiation of severe cells will probably be greatest along, and just below, the storm track. The warm sector of the low will be directly in between the subtropical and polar jet streams, indicating a favorably divergent pattern aloft. Meanwhile, the cold sector over E MT, ND, and N MN, a raging blizzard will get underway with up to 2 feet of snow buffeted by wind gusts as high as 55 mph.

The system currently bringing showers and thunderstorms to Appalachia and the Eastern Seaboard should exit the Atlantic coastline tonight. While an mP wedge will likely keep severe weather threats to a minimum north of the Potomac River, the atmosphere could be unstable enough to allow for some strong thunderstorms from OH and the Virginias into the Gulf Coast this afternoon and evening. Further north, much of E ON, QC and then New England will be mired in dreary, wet conditions over the course of the next day or so.


SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
E OH....WV....VA....C, E NC....SC....GA....FL Panhandle....S AL....S MS....S LA

STRONG WINDS (Pressure Gradient Derived)
BC....AB....SK....MB....N ON....C QC


HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)

Isolated Locations In
E OH....WV....VA....C, E NC....SC....GA....FL Panhandle....S AL....S MS....S LA
(QPF 1-2")


WINTER WEATHER OUTLOOK
(potential for 2-4 inches or more of snow, or significant ice/glazing)

Scattered Locations In
MN Boundary Waters....W ON
(Wet Snow; 4-12")

Isolated Locations In
WA Olympic Peninsula....BC....AB
(Snow; Above 4000 Feet; 4-8")


COLD TEMPERATURES OUTLOOK
(potential of daily maxima staying below 32 deg F/0 deg C; blue shading indicates dangerous wind chills)

WA Higher Elevations....BC....AB....SK....MB....N, W ON....N, C QC....NL

MEDIUM RANGE OUTLOOK
(weather trends through the next 6 to 10 days)

The look of the 500MB longwave pattern is, for the most part, what snow enthusiasts were wanting to see this past winter. A dual blocking signature, composed of complex Rex ridging over western Canada and NL/LBR into southern Greenland, is taking shape. The two positive height anomalies work to suppress the latitude of the jet stream, and help to convert the developing storm now over the Pacific Northwest into a cA vortex. So after a day of heavy rain (north of the Mason-Dixon line) and strong to severe thunderstorms (MD....DC....VA into N FL), colder air will sweep across most of the U.S., reaching as far south as the Gulf Coast.

Since this is April and the sun angle is growing ever stronger, the cold intrusion will not have the same impacts as would be the case in January or February. That said, the lower temperatures will have a wintry impact with nocturnal+elevation snows in the interior Northeast on Day 4, as well as periodic snow and graupel showers in the lee shores of the lower Great Lakes. The 40 deg F maximum isotherm may drift as far south as N KY into VA, and nocturnal record low readings are possible into much of the Mid-South.

This colder pattern should last through Day 10 before finally breaking down. There are signs from some of the NWP schemes of a shortwave rotating around the upper low as it lodges over QC and northern New England in the 192-216 hour time frame; the 0z Apr 1 GGEM actually shows a monster winter type storm near the MA Islands. But most of the computer outlooks are relatively tranquil during the period of lower temperatures, with the only truly inclement weather likely to occur around Atlantic Canada and the Gulf of St. Lawrence. Across the West, downslope flow to the left of an unusually strong surface high pressure cell (1045MB) should create warming with near-record heat in the Desert Regions.

The presence of a strong sequence of storms from the western Pacific Ocean and Aleutian Islands is likely to mean a shift to a -PNA configuration beyond April 15. Moderation of the temperature patterns will commence as disturbed weather spreads eastward from CA on Day 10, possibly culminating in a renewed threat of severe weather in parts of the lower Great Plains in the 240-264 hour time frame.


Prepared by Meteorologist LARRY COSGROVE on
Sunday, April 1, 2007 at 2:45 A.M. ET

Disclaimer:

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 2007 by Larry Cosgrove

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