WEATHERAmerica Newsletter, Saturday, August 30, 2008 at 7:40 P.M. CT

by LarryCosgrove | August 30, 2008 at 06:06 pm

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WEATHERAmerica Newsletter, Saturday, August 30, 2008 at 7:40 P.M. CT

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TODAY'S FUN LINKS:
Special Report For The Oil Drum
Gustav Storm Surge To Present Biggest Threat To Central Louisiana
Forecast Model Performance for GUSTAV AL072008

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, Large Hail, Isolated Tornadoes
E ID....W MT....SE BC....S AB....S SK....S MB....W ON

Some Thunderstorms May APPROACH Severe Limits
W, C TX....NM....AZ....UT....WY....W, C CO

Some Thunderstorms May APPROACH Severe Limits
N LA....E AR....N MS....N, C AL....FL....GA....SC....NC....S VA....S WV

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

Scattered Locations In
E ID....W MT....SE BC....S AB....S SK....S MB....W ON
(QPF 1 - 2")

Isolated Locations In
W, C TX....NM....AZ....UT....WY....W, C CO
(QPF 1 - 2")

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

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

Numerous Locations In
C CA....NV....UT....WY....E MT....ND....extreme S SK....S MB....W ON
W Upper MI....WI....MN....SD....NE....KS....OK....TX....LA....AR....MO....IA....IL
IN....KY

Isolated Locations In
FL....GA....SC....NC....TN....AL....MS....LA

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

Eastern Equatorial Pacific Ocean

No disturbances; highly sheared wind profile across easternmost Pacific Ocean due to monsoonal fetch from equator into western U.S.

Atlantic Basin

Dangerous Hurricane Gustav entering Gulf of Mexico northeast of Pinar Del Rio, Cuba.

Tropical Storm Hanna over the southeastern Sargasso Sea.

Invest 97L near the Cape Verde Islands.
African ITCZ

Five active impulses including the 97L circulation. Strongest feature are over southern Mali and southern Chad into the Cameroons.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Gustav Targets The Central Gulf Coast

To put it bluntly, the central Gulf Coast of the U.S. will be in dire straits on Monday into Wednesday. With an upper level trough over the western half of the Gulf of Mexico weakening and drifting southward, Gustav, already a dangerous storm with Category 4 winds, will contort to the northwest again and make a landfall in about 60 hours just to the right of Grand Isle LA. The hurricane may briefly become a Category 5 as the center passes over very warm waters, with speeds of 140 mph or more likely when Gustav reaches the coastline of the Bayou State. Due to the huge size of the system and its approach from the southeast (steered that way by a strong high pressure area in the Ohio Valley which will slow the forward motion while setting up a pronounced gradient), Gustav could produce a disastrous surge into New Orleans, with high winds, torrential rainfall, and tornadoes making a bad situation worse. Baton Rouge LA will also be hard hit, with winds of 100 mph probable for a few hours as Gustav winds slowly into the center and northwest corner of Louisiana. Eventually the Gustav remnants will link with a strong cold front over the Great Plains, and may (in about seven days) interact with what may a strong hurricane along the Eastern Seaboard (Hanna).

If the 18z GFDL and HWRF packages are correct with track and intensity scenarios, what unfolded in the Crescent City 3 years ago may be rivaled. The only way out of this "doomsday" perspective is the dwindling hope that the upper trough in the western Gulf of Mexico holds in place. That might shear the circulation and induce a path correction further east. But given the consistency of the computer outlooks, Gustav may be one for the ages.

And that statement is not a historical compliment by any means.

Heat Ridge Builds Eastward....

The strong heat ridge that has evolved over the Great Plains and Midwest will begin to shift eastward, with parts of the Corn Belt and the Mid-Atlantic region seeing a return to 90 deg F (or more) readings over the next several days. Keep in mind that the coming shift in the 500MB longwave pattern will soon sweep away chances for extreme heat (while bringing some other weather challenges), so enjoy this small dose of summer while you can.

....While Strong Cold Front Charges Into The Intermountain Region

The cold front that is poised to sweep into the Pacific Northwest will bring an sharp decline in temperature to much of the Intermountain Region, breaking down the Sonoran heat ridge and triggering strong to severe thunderstorms from the Rocky Mountains into the High Plains over the following three days. It is this trough and frontal structure that will interact with Gustav, pulling the circulation and moisture of the tropical cyclone remnants into the central Great Plains next week and perhaps setting up a severe weather episode from the Missouri Valley into the lower Great Lakes.

Monsoonal Fetch A Sign Of Impending Weak El Nino Episode

The strong warming trend in SSTs over the eastern equatorial Pacific Ocean is often associated with the genesis of low latitude southwest flow aloft. You can see this effect on GOES WEST imagery, with a fetch of deep moisture from the Mexican shoreline into the western U.S. A mimic of the so-called "summer monsoon" over the Intermountain Region, this wind and dewpoint field is also symptomatic of a developing El Nino episode. Should this moist axis remain into the coming winter, a scenario involving strong storms and unseasonably cold temperatures could evolve over the southern and eastern rim of the U.S.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Hanna May Pay A Visit To The Eastern Seaboard, As A Hurricane; Impressive Shift To Cooler Air As Gustav Remnants Link With Cold Front

The full details on possible track scenarios and potential weather effects are yet to be settled, but Tropical Storm Hanna may end up turning into a significant event for the Eastern Seaboard. The numerical models have backed away from a Gulf of Mexico entrance, but now seem to be clustering on a fairly dangerous trend of taking a major hurricane into the Carolinas and then up along the Atlantic shoreline. I believe that the most viable option is a conversion to extratropical status after a damaging wind and rain event along much of the Interstate 95 corridor. The operational GFS version from 18z suggest that the vestiges of Gustav, together with an oncoming cold front, could act to bring much cooler air across all of the Old South and East Coast in 6 to 7 days.

Another Strong Cool Intrusion?

The 500MB longwave pattern will exhibit ridging along the West Coast, some blocking in the NAO position, and strong shortwaves moving a long the Canadian border. Yet another shot of polar air is forecast to enter the Upper Midwest in the late medium range, with the briefest of warming trends over parts of Dixie, Appalachia and the Atlantic Coastal Plain in 10 to 11 days.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Will Threat From Invest 97L Be A Factor?

The sprawling circulation over the Cape Verde Islands may yet become a hurricane, but I am unsure if the 12z ECMWF version is correct in its handling of this feature. Whereas the American scheme has consistently seen 97L as a threat to become a monster storm over the middle of the Atlantic Ocean, its European counterpart has tended to show the gyre as a potential risk to the Eastern Seaboard. The latest scenario, involving a Hanna-like approach to Florida, seems unlikely if only because the subtropical high over the Atlantic Ocean is out of position in ten days, well east of Bermuda. I suspect that a "middle ground" approach may work best, with the disturbance becoming well-organized but making a turn toward New England and/or the Maritime Provinces during the extended period.

Your Basic +PNA, -NAO Jet Stream Configuration

The appearance of a +PNA/-NAO alignment in the 500MB features was noted during some parts of August, and if GFS and ECMWF ensembles are correct could once again occur in the coming weeks. A positive height anomaly over Greenland and a somewhat flatter ridge complex across the western U.S. supports frequent and strong cool intrusions into the nation. At the same time, vigorous shortwaves may occasionally interact with either tropical moisture or disturbances from the lower latitudes, opening up the possibility for major precipitation events in the eastern half of the country between September 10 and 14.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, August 30, 2008 at 7:40 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

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