WEATHERAmerica Newsletter, Saturday, August 23, 2008 at 5:15 P.M. CT

by LarryCosgrove | August 23, 2008 at 04:20 pm

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WEATHERAmerica Newsletter, Saturday, August 23, 2008 at 5:15 P.M. CT

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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)
Some Thunderstorms May APPROACH Severe Limits
TX....LA....MS....AL....FL....GA....SC....Coastal NC

Some Thunderstorms May APPROACH Severe Limits
S CA....AZ....E NV....UT....WY....NE....KS....CO....NM

Some Thunderstorms May APPROACH Severe Limits
OK....N AR....S MO....S IL....C, S IN....OH....N PA

ISOLATED Severe Thunderstorms
(Microbursts, Large Or Plentiful Hail)
ON Peninsula....W, C NY....VT....NH....N, C ME....QB Eastern Townships

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
BC....W WA....NW OR

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

Scattered Locations In
BC....W WA....NW OR
(QPF 1 - 3")

Scattered Locations In
TX....LA....MS....AL....FL....GA....SC....Coastal NC
(QPF 1 - 4')

Isolated Locations In
S CA....AZ....E NV....UT....WY....NE....KS....CO....NM
(QPF 1 - 2")

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

Scattered Locations In
C, E BC....AB....MT....ID....E WA....C, E OR....CA....NV....AZ....UT....W CO

Isolated Locations In
NM....TX....OK....W, C KS....W, C NE....W, C SD....W ND

Isolated Locations In
N GA....TN....KY....S MO....AR

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

Eastern Equatorial Pacific Ocean

A thunderstorm cluster just south of Tropical Depression 11E has some potential for further strengthening as it tracks northward. A TUTT signature in Chihuahua and Coahuila states, TD 11, and the new impulse may all contribute to major flooding in the California Peninsula, Sinaloa State of Mexico and parts of the Intermountain Region during the next six to seven days.

Tropical Depression 11-E

Tropical Depression 11-E is entrained in a pronounced south-southwest flow aloft, and may be declared a tropical storm shortly. the system poses a danger to the California Peninsula and communities bordering the Sea of Cortez mainly due to sea swell and flooding triggered by heavy rainfall. Eventually the moisture associated with TD 11E may target the American Southwest and Rocky Mountains.

Atlantic Basin

The clutter on the GOES EAST satellite image is an example of the fairly extreme activity of the tropics in the Atlantic Basin. Complicating potential for warm-core cyclogenesis are TUTT signatures in the southern Caribbean Sea and adjacent to the Gulf Stream, along with a considerable amount of dust and dry air from the Sahara Desert to the right of 40 W Longitude. A very impressive convective cluster above Suriname (to the right of Invest 94L) also may organize further.

Tropical Storm Fay

Tropical Storm Fay continues to progress westward along the Interstate 10 corridor. Fay is sheared on its southwest flank, but otherwise continues to draw in plentiful moisture from the Gulf of Mexico. As a result, dangers from flooding rainfall will continue to lurch to the west, eventually including Louisiana and Mississippi on Sunday and Monday. A trough and cold front should finally capture the circulation and open up Fay into a frontal wave with significant rains for the Tennessee and Ohio Valleys.

Invest 94L

Since none of the major baroclinic or tropical models properly initialized 94L or is sister convective cluster to the southeast, eventual track and intensity scenarios are a matter of pure conjecture. As of now, the trajectory shown on ensembles of the major numerical models seems viable: a path through the northern Caribbean Sea followed by a turn into the Gulf of Mexico with possible landfall in TX or LA. Of course, until the low center is actually captured, possibilities ranging from a Belize or Yucatan Peninsula (GGEM) or Atlantic Ocean fish storm (Operational GFS) cannot be discounted.

Invest 95L

While showing a fairly well-defined circulation center, Invest 95L is having terrible issues with dry air and Saharan dust. The TUTT signature to its northwest is also impacting the left side of 95L, which is predicted by the 12z ECMWF version to become a major hurricane with some potential to target the Mid-Atlantic or New England states in the 11 - 15 day time frame.

African ITCZ

The Inter-Tropical Convergence Zone remains in a hyper-active state, with five impulses stretching from the eastern equatorial Atlantic Ocean into Ethiopia. Note that the disturbances are all experiencing entrainment of dry air. At the same time, the tropical waves are geographically large in scope, with the most vigorous circulation in western Mali showing up in the operational GFS run at 12z as a likely candidate for hurricane status (while never making landfall).

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Lingering Fay Circulation Brings Needed Heavy Rainfall

Caught in weak steering currents, Tropical Storm Fay continues to lumber westward through the Florida Panhandle. Since a trough across the Midwest will miss picking up Fay, it is likely that the circulation center and moisture fetch will remain an issue for the Deep South through Tuesday. Torrential rainfall is actually not a bad thing for portions of MS, AL, and GA, helping to ease a long-standing deficit. By the middle of next week the Fay remnants should begin to lift out into the Ohio and Tennessee Valleys, linking with the strong cold front now approaching the Pacific Northwest.

Severe Weather, Then Strong Cooling In Pacific Northwest

The mP frontal structures seem to be getting stronger and stronger, in accordance with a deep trough complex over the Gulf of Alaska. While locally strong warming will occur to the right of the cold front (see map in the next paragraph below), there is no question that the postfrontal regime will be very chilly for late August. The arrival of the much lower readings will be accompanied by strong to severe thunderstorms in the Pacific Northwest, with heavy rain and strong winds likely in the Vancouver BC, Seattle-Tacoma, and Portland OR metro areas.

A Weak Cool Intrusion For the Midwest And Northeast

The most recent shots of cooler (cP) values have made some in the Midwest and Northeast think that summer was over. Another intrusion of lower temperatures is slated to arrive Sunday and Monday. But this air mass is not as chilly as its predecessors, and may have only a brief life-span. Note that while many maximum values across eastern Canada and New England are in the 50 - 70 deg (F) range, the Mid-Atlantic states remain warm. And major heat will be building ahead of the next cold front to affect the Pacific Northwest. So before that cool domain reaches the Corn Belt and Eastern Seaboard on Wednesday into Thursday, the word "hot" may yet creep back into the vocabulary in Columbus OH and Springfield PA. If only for a short time.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Complications From Tropical Cyclone Threats Loom Large

What makes the medium range and extended outlooks such a risky proposition is the very obvious threat of tropical cyclone development, and landfall potential, from disturbances over the Atlantic Basin. I would warn everyone with a vested interest in inclement weather that "the danger is there", and should not be taken lightly. Cliches aside, both the investigated areas of convection, plus the system immediately southeast of 94L, could (in a worst case scenario) impact the Gulf Coast and Eastern Seaboard. The baroclinic models experience great difficulty with coverage of the Caribbean Sea, an area which will go from being a "dead zone" to "favored area" during the next week or so. With that in mind, and a 500MB weakness forecast to linger over Texas and Louisiana, close attention is needed to see just where the lower latitude impulses travel to.

Is There A Heat Ridge In YOUR Future?

In the wake of Tropical Storm Fay and the cold front now approaching the West Coast, the numerical models are pointing toward a semizonal flow setting up along the border between the U.S. and Canada. A fairly large subtropical high is predicted to take shape from the lower Front Range into the Great Lakes and Mid-Atlantic region, undercut by a weakness over parts of Texas and the Deep South. In a nutshell, an upper air configuration like that seen by the 12z GFS and ECMWF versions is at once strongly supportive of hot temperatures over the eastern two-thirds of the nation while also allowing for tropical cyclone strikes along the Gulf Coast.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

After Heat And (Possible) Tropical Cyclone Landfalls, Cooling Returns

I compiled a composite anomaly for Septembers in years of neutral ENSO status which later turned to a weak El Nino. Note that the temperatures across the western half of the nation were below normal, while only minor warmth was noted in the central Gulf Coast and lower Appalachia. The thinking is here that the combination of a tropical disturbance (of whatever intensity) and a breakdown of the heat ridge cause by intrusions of cooler air will result in a lower temperature profile around the U.S. I suspect that the lower values will be centered more from the Great Plains to the East Coast this time around, owing to the tendency in late summer for a +PNA configuration to develop which may keep the West Coast and Intermountain Region in the warm to hot spectrum.

Threat For Major Hurricanes Should Decline Sharply After September 9

Warming has been very impressive in the 1,2 ENSO sector, with the neutral 3,4 region of the Pacific Ocean slowly edging toward a weak El Nino status. A look at MTSAT and GOES WEST imagery (and for that matter, GOES EAST) shows a preponderance of lower latitude westerlies and TUTT signatures. Thus, vertical shear is increasing and spreading into the Atlantic Basin. So while the tropics are "open for business", it will not be long before upper level wind patterns will limit the potential for warm-core cyclogenesis. Shearing of convection, recurvature of active storms and increasing amount of cool intrusions from Canada (all symptoms of the equatorial SST signature shown above) will all act to sharply curtail hurricane potential after the seasonal climatological maximum of September 9.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, August 23, 2008 at 6:15 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.