WEATHERAmerica Newsletter, Sunday, April 8, 2007 at 4:00 A.M. ET

[Larry_Cosgrove]

"Winter Mode" Switches To "Just Plain Stormy" Phase As Three Powerful Spring Cyclones Set To Affect Most Of The Lower 48 States Between Now And April 20

(Disclaimer And Copyright Statements At End Of The Discussion)

WEATHER SUMMARY THROUGH NEXT 72 HOURS


With the dual blocking configuration showing signs of collapsing, sensible weather around the U.S. and Canada will trend milder, if only slightly so, over the near term. That is small comfort to those of you in the Maritime Provinces, buffeted by wind, heavy snow and record cold in the past 12 hours. But the storm in NS is consolidating the area of cold air aloft, so moderation is a given for the Midwest and Northeast, if only for a little while.

But while temperatures are likely to be a bit warmer, especially across parts of the Great Plains and Dixie, the overall 500MB longwave pattern will still display chilly elements and include increasing potential for precipitation. A prominent storm over the Gulf of Alaska is linking with the subtropical jet stream (origin: the equatorial Pacific Basin) to bring rain and higher elevation snows to the Intermountain Region today and tomorrow. As the low tries to get around the vestiges of the blocking signature in Canada (which itself turns into a moderately strong Rex signature in NU and MB), rain will become focused over BC, WA, and OR and spread inland. The mPk regime will support nocturnal and mountain snows, good for any lingering spring action at ski resorts along and to the right of the Continental Divide. By tomorrow evening the low will show signs of redeveloping over SE CO.

On Tuesday the disturbance should be intensifying over W OK, and linking with the thermal boundary now causing heavy rain (bordered by sleet and snow!) over TX and LA. Interaction of three air masses should bring about severe thunderstorms from the Ouachita Plateau through a good portion of the Deep South. Some snow may fall across the Upper Missouri Valley, but most of the precipitation will be in the form of a cold, stratiform rain across the Corn and Tobacco Belts. Yet another storm will approach the Pacific Northwest on Day 3, replete with heavy rain and snow along the coastal ranges of BC, WA, and OR.

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

STRONG WINDS (Pressure Gradient Derived)
NL....E QC....NB....NS

STRONG WINDS (Pressure Gradient Derived)
E AB....SK....C, E MT....E WY....W SD....NE Panhandle....C, E CO....E NM


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

Isolated Locations In
Coastal, BC, WA, OR
(QPF 1-2")


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

Scattered Locations In
NL/LBR....Extreme E QC....E NB....PEI....NS
(Snow; 4-16"; Blizzard)

Isolated Locations In
Lower MI....N IN....N OH....NW PA....W NY....ON Peninsula
(Snow; In Squalls; 4-12" Prone Leeshore Locations)


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

NL/LBR....QC....ME....NB....PEI....NS
MB....W, C ON


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

Attention all wild weather enthusiasts. As we are about to embark on our adventure, known as the "Rock and Roll Longer Range Forecast", you are advised to buckle your safety belts. Because the next twelve days, my friends, promise to be one heck of a ride.

The longer term models remain steadfast in predicting the passage of three strong storms through the lower 48 states between now and April 20. Since the ECMWF version has been the most consistent with its forecast of the disturbances in terms of track, geographic scope, and intensity, I choose its depiction of events as the background for this outlook. Essentially, the three storms over the northern Pacific Ocean will undercut a Rex structure over northern Canada (which will shift to an -NAO type of block by Day 10). Each low will bring widespread heavy precipitation. Frozen types are possible within the respective cold sectors over the Intermountain Region and Great Lakes; in the overrunning pattern along the storm track, heavy stratiform rains should occur. Severe weather is a huge risk factor within the warmer mT regime south of the impulses. And each system should undergo redevelopment as convective circulations initiate secondary cyclogenesis over NC and VA. This may result in a dreadful soaking rainfall for the Northeast and Maritime Provinces, spaced apart every 2 or 3 days.

The first system is already in view approaching the Pacific Northwest. Digging southeastward to dodge the Rex block in NU and MB, the system will spread mountain snows into MT and WY in the near term before redeveloping over SE CO. Initially dry upon hitting the Great Plains, the low will link with the subtropical jet stream and spread moisture well to the north, and east, mostly as a steady moderate rain. Convection firing along the warm front may expand southward into the mT fetch, and severe thunderstorms are entirely probable in much of the Deep South in the 96-108 hour time frame. The new low should organize in NC before taking a track close to the shoreline, spreading its depressing array of rain and wind along the Interstate 95 corridor and then the Maritime Provinces by April 14.

Then Act 2 gets underway, probably on April 10 along the West Coast. I suspect that this will be the strongest of the series, and may well provide a memorable severe weather outbreak affecting the lower Great Plains and much of the Old South. The latest renditions of the GGEM and ECMWF show the strong subtropical high feeding in moist, unstable air before collapsing as the giant trough and low progress eastward. Once again a redevelopment scenario will get underway in the VA Capes, providing gales, torrential rain and even some higher elevation snows from WV and VA into ME and NB. The entire disturbance should be pulling away into the Atlantic Ocean on April 16.

The third low in the sequence should be hitting CA and OR around Day 9. Responding to a building Rex block in western Canada (which will exert as much influence as the corresponding ridge south of Greenland), the 500MB low will close off earlier in AZ and NM before responding to a kicker shortwave from BC around 240-264 hours. Early indications are that the storm may weaken some, but still provide another severe thunderstorm event in the south central states, Mississippi Valley and Corn Belt by April 20.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, April 8, 2007 at 4:00 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

All rights reserved.

This publication may not be reproduced or redistributed in whole or in part without the expressed written consent of the author.