WEATHERAmerica Newsletter, Saturday, July 26, 2008, 6:45 P.M. CT

by LarryCosgrove | July 26, 2008 at 03:53 pm

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WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
ISOLATED Severe Thunderstorms
(Microbursts, Isolated Tornadoes)
Coastal LA, MS, AL....FL....S GA....E SC....NC....VA....DC....MD....DE....PA....NJ
NY....CT....RI....MA....VT....NH....ME....NB

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
AB....SK....E MT....ND....SD....SW MN....W, C IA....MO....AR....W TN....N MS

Some Thunderstorms May APPROACH Severe Limits
W TX....NM....AZ....E NV.....UT....CO....W, C NE....W KS

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

Isolated Locations In
Coastal LA, MS, AL....FL....S GA....E SC....NC....VA....DC....MD....DE....PA....NJ
NY....CT....RI....MA....VT....NH....ME....NB....QB
(QPF 1 - 3")

Isolated Locations In
W TX....NM....AZ....E NV.....UT....CO....W, C NE....W KS
(QPF 1 - 3")

Isolated Locations In
AB....SK....E MT....ND....SD....SW MN....W, C IA....MO....AR....W TN....N MS
(QPF 1 - 2")

EXCESSIVE HEAT OUTLOOK
(potential for maximum temperature to exceed 95 deg F within the next 24 hours)

Numerous Locations In
W LA....W AR....TX....OK....C, E KS....NE....SD....ND....MT....WY....S ID....SE OR
CA....W AZ....NV

Isolated Locations In
MS....AL....FL....GA....SC....NC....TN....S KY....W IL....MO....IA

TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Eastern Equatorial Pacific Ocean)

Eastern Equatorial Pacific Ocean

Formative tropical depression nearing International Dateline. Approaching area of strong vertical wind shear associated with TUTT signature makes further development unlikely.

Weakening Tropical Storm Genevieve is 800 miles southwest of Cabo San Lucas Baja CA, moving westward into cool, stable regime. Dissipation likely within 24 hours.

Formative tropical depression below Panama. This feature should organize into a tropical cyclone with threats for excessive rainfall in much of Central America and the Mexican Riviera. Also, there is a danger that this feature could recurve northward into southern Mexico within 72 hours.

Atlantic Basin

Remnants of Hurricane Dolly moving northward along Rio Grande River. See more details in the Short Range discussion below.

Convective cluster over the Bahamas must be watched for further development.

Impulses spun off from ITCZ are drifting westward over the eastern equatorial Atlantic Ocean. The presence of wind shear and Saharan dry air makes major strengthening unlikely, but the most southern disturbance could approach the Leeward Islands within four or five days.
African ITCZ

Very active ITCZ with widespread convection between the Sahel and the Equator, signifying a northward shift and expansion in the Saharan heat ridge. Impulse over Ghana and Ivory Coast has the best chance of survival and tropical development over the Atlantic Ocean.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Dolly Remnants, Monsoonal Moisture Fetch Combine To Pummel Rio Grande Valley With Flooding Rains

Like many hurricanes that fail to make major impacts through wind and surge, Dolly is creating huge problems through output of heavy rainfall. The circulation of the storm has survived by tracking more or less along the Rio Grande River, and has linked with the "last legs" of the monsoonal fetch from the equatorial Pacific Ocean, through central Mexico. Torrential rainfall, getting an orographic assist, will likely continue in extreme western Texas, New Mexico, eastern Arizona, before ending over Utah and Colorado on Tuesday. Some communities may see their entirely yearly rainfall budget received within the next 48 hours.

Excessive Heat Plagues Parts Of The Intermountain Region, Great Plains, And Deep South

As might be expected, the last week of July, heat is the major issue confronting forecasters around North America. The concentration of the hottest values, however, is remarkable in that so much of the continent is in above normal temperatures. Were it not for the last gasp of the monsoon over the West, the Intermountain Region would be dealing with oppressive conditions from the Pacific shoreline to the Great Plains, the mean center of the 500MB subtropical high. The hottest readings are making inroads through most of the Deep South as well, since penetration of cold fronts has slackened with the resurgence of a "Great Smokies" ridge.

Midwest, Northeast Dodge The Heat With Cold Fronts And Thunderstorms

While the Midwest and (especially) the Northeast have gotten blasts of hot air from either the Sonoran or Bermudan heat ridges, cold frontal passages and resultant convection have largely made a dent in temperatures for those two regions. This is indeed the case for the near term, what with the intrusions of modified polar air seen today and yet another entering the Upper Midwest on Monday night. The drawback to cooling, of course, is the frequent blasts of heavier showers and thunderstorms, which may raise dangers for severe weather and flooding from the Missouri Valley to the Atlantic Coastal Plain during the following four days.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

An All-Important 500MB Weakness....

The gap between the Sonoran heat ridge and Bermuda High has generally been situated over the Great Lakes and Mississippi Valley during the month of July. So where high heat has often been found over the western half of the nation or along the Atlantic Coastal Plain, the Midwest and Appalachia has become a dumping ground, of sorts, for intrusions of cool air from Canada. The 500MB weakness shows up repeatedly in the forecasts for the medium range, but this time a bit more eastward (from Quebec to the Gulf of Mexico). So if model forecasts verify, at least one cold front, with thunderstorms, will arrive in the Great Lakes and Eastern Seaboard late next week.

....Against Two Persistent Heat Ridges

Another trend seen this summer season has been the presence of two very strong heat ridges, which alternate between a merged state and separate formations over the High Plains and below the New England coastline. Some linkage between the subtropical highs may occur briefly around August 5 and 6. But for the most part the anticyclones will remain distinct, and could provide potential for impacts from tropical disturbances on the Gulf Coast or the Eastern Seaboard during the next two weeks.

Watching The Increasing Polar Westerlies (And Storms) Over The Pacific Ocean

A possible contributor to apparent weather in North America is the increasingly active polar westerlies over the Pacific Ocean. There are two very impressive disturbances riding along the jet stream, and the numerical models suggest that both systems may reach Canada and the U.S. at lower latitudes than has previously been the case. As a result, the West Coast may see a frontal passage as far south as the Bay Area of CA in about seven days. The low over the Gulf of Alaska could be an important thunderstorm producer in the Midwest and Northeast around or slightly after the middle of the coming week.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Who Will Be The Hottest In August?

Judging by the alternation between the Sonoran and Bermudan heat ridges, there should be plenty of hot air to go around in two clusters: the High Plains and Old South, and the Atlantic Coastal Plain. After a period of locally fierce heat across parts of the Intermountain Region and California, lower 500MB heights should favor more normal conditions along the length of the West Coast. Any heating of the Eastern Seaboard will be tempered by a cold frontal passage around August 5 or 6.

Deep Mean Easterlies May Carry Tropical Disturbances Into The Deep South

One aspect of August is the tendency for mainly easterly flow aloft and at surface from western Africa into the Dixie States. This signature shows up nicely on ensemble forecasts, with waves on the underside of the subtropical ridge complex acting as a kind of warning of impending warm-core cyclone possibilities. Given the activity along the African ITCZ and the shift of the Saharan heat ridge into the Mediterranean countries (thereby reducing infiltration of hot, dry air and dust), potential for a tropical cyclone impact will be rising along the Gulf Coast and Eastern Seaboard during the 11-15 day time frame.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, July 26, 2008 at 6:20 P.M. CT

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 2008 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.

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