WEATHERAmerica Newsletter, Saturday, August 9, 2008 at 7:15 P.M. CT

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WEATHERAmerica Newsletter, Saturday, August 9, 2008 at 7:15 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)
SCATTERED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
W TX....OK....KS....C, S MO....AR....NW MS

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
SE WV....N OH....SE ON....QB....NB....ME....NH....VT....MA....RI....CT....NY....PA
NJ....DE....MD....DC....VA....NC....SC

Some Thunderstorms May APPROACH Severe Limits
SK....MT....W, C ND....SD....NE....WY....CO....NM....E AZ

Some Thunderstorms May APPROACH Severe Limits
S FL


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

Scattered Locations In
BC....N AB
(QPF 1 - 5")

Scattered Locations In
W TX....OK....KS....C, S MO....AR....NW MS
(QPF 1 - 5")

Isolated Locations In
SE WV....N OH....SE ON....QB....NB....ME....NH....VT....MA....RI....CT....NY....PA
NJ....DE....MD....DC....VA....NC....SC
(QPF 1 - 2")

Isolated Locations In
SK....MT....W, C ND....SD....NE....WY....CO....NM....E AZ
(QPF 1 - 2")

Isolated Locations In
S FL
(QPF 1 - 2")


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

Numerous Locations In
CA....NV....W AZ....UT....S ID....C, E MT....W ND....W, C SD....WY....NE

Scattered Locations In
CO....NM....TX....OK....S AR....LA....S MS....S AL....N FL....S GA

TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Eastern Equatorial Pacific Ocean; centered positions and forecasts available from NRL Tropical Cyclone Page)

Central And Eastern Equatorial Pacific Ocean

Tropical Storm Kika is moving west-northwest and is not expected to affect any land area before dissipation in about 48 hours.

Formative tropical depression seen 800 miles southeast of Hawaiian Islands; stable marine layer to north of system should prevent any growth, or recurvature toward the Hawaiian Islands

Major hurricane Hernan will move over colder waters and into vertical shear axis shortly, with dissipation likely within 48 hours.

Two small impulses 500 miles south of Mexican shoreline are not likely to organize.

Strong tropical wave over and below Honduras and Nicaragua is likely to be reach named storm status in about 24 hours, then turn northwestward along coast of Mexican Riviera. High damage potential in central America and Mexico due to flooding rainfall.

ITCZ-born impulse over Panama and northwestern Colombia may become a tropical depression with flooding threats to Pacific resorts in Central America.

Atlantic Basin

Mesoscale convective complex over Ozark Plateau is forecast by some models to become a tropical depression after reaching the warm waters of the Gulf of Mexico. Minimal threat seen at this time.

Disturbed area over the Gulf Stream is associated with aging frontal structure. Any development in this zone should move quickly out into open waters of the Atlantic Ocean.

Two impulses ejected from the African ITCZ are now located over the equatorial Atlantic waters. High amount of vertical wind shear and massive ingestion of Saharan air mass should destroy the most westward of the disturbances. Low below the Cape Verde Islands may organize further is southwest flow aloft and the heat ridge complex to north weaken some.

African ITCZ

There are SIX strong convective circulations in the vicinity of equatorial Africa. While the most westward of these impulses are unlikely to have any future effects on the Americas, the three middle areas of disturbed weather (southern Mali, Togo and Benin, Nigeria and Cameroons) are forecast by some of the baroclinic models to develop into named storms with possible impacts on the major islands rimming the Caribbean Sea as well as the U.S.

See EXTENDED PERIOD FORECAST For important details about tropical cyclone threats to the U.S.


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Touch Of Autumn For Midwest, Eastern Seaboard....

A reminder that the calendar is a mere 43 days from the start of calendar fall: the low temperature in Bloomington IN will reach 50 deg F! Actually, impressive cooling will be the rule in the near term across much of the U.S., created by a polar frontal passage in the Pacific Northwest, a late redux for the "monsoon" across the Intermountain Region, and a prominent trough over the eastern third of the U.S. Recovery of temperatures is unlikely until next weekend, when the 500MB longwave pattern undergoes another tumultuous shift favoring heat ridge formation in and near North America.

....While Parts Of Dixie And The Southwest Fry Either Side Of The Monsoon

But a rebuff to the cooling is present, and in the days ahead will ultimately bring many of us back to the stark reality that August (and this summer season, which on the whole has been a hot one) is still upon us. Heat will survive over parts of the Southwest, Texas and Deep South. And as can be seen on the above graphic, readings across the Prairie Provinces and Great Plains will be returning to the much above normal category, a harbinger of the coming return of a "two heat ridge" 500MB longwave pattern.

Thunderstorm Potential Greatest In South Central, Northeastern U.S.

With so much cold air aloft against the still-strong sun angle, convection is bound to occur with severe or very rainy results. Evidence the effects of a frontal structure which will settle into an area from northern Texas into Florida (possibly spinning off a tropical disturbance). The same relationship exists under the trough complex in eastern Canada and the Northeast, which will be visited by hail and downpour threats well into the coming week. The resurgent "monsoon" over the western states could also act as a moisture source for the next front arriving in the Prairie Provinces, with thunderstorms prevalent as far north as the Great Slave Lake.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Western U.S. Heat Ridge, Eastern U.S. Trough

For much of the time between August 13 and 19, a persistent +PNA configuration will be in place over North America. Essentially this favors a continuation of current precipitation and temperature anomalies. The hottest weather will be over the western and southern states, with cooler values dominating an area from the northern High Plains into the Eastern Seaboard. Thunderstorms will be a real problem along the lines of temperature conflict, stretching in an arc from the Black Hills into the Ohio and Tennessee Valleys. with other heavy rain risks present in the Mid-Atlantic and New England regions.

Subtle, Yet Major, Change In 500MB Longwave Pattern Around August 19
There is excellent agreement among the various numerical models and the respective ensemble members of a critical change in the 500MB longwave pattern as we exit the medium range. A lateral heat ridge, a sort of elongated Bermuda High, becomes established from the Gulf of Mexico into the central Atlantic Ocean, while a broad weakness lines up from the Gulf Coast into eastern Ontario. The operational GFS has been the most bullish of the versions with a small negative height anomaly over the Great Lakes, while the other schemes show a stronger, bulkier subtropical high across much of the Deep South. With a tropical cyclone threat looming in the 11 - 15 day time frame, the position of the anticyclone and the discontinuity or trough will prove critical to the track, and impact, any landfalling warm-core disturbance.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Teleconnections Over Indian Ocean, The Orient Favor Heated, Stormy Scenario In U.S.

August is getting off to a cool start, what with the 50 deg F minimum isotherm likely to reach into the Great Lakes and Northeast in the next few days. That lower temperature pattern looks not to last, as atmospheric conditions upstream would seem to indicate a major reversal in the jet stream configuration over and near North America in the last 12 days of August. In a nutshell, a tremendous transfer of energy and moisture is building up from the Indian Ocean and across the western Pacific theater. This huge fetch, associated with the monsoon trough complex in eastern Asia is linking with the polar westerlies, driving up a strong ridge complex that should become situated below the Aleutian Islands. A trough should then form along or just to the left of the West Coast, with ridging dominant through much of the lower 48 states. Allowing for a weakness between the Sonoran and Bermudan ridges (which will prove critical to the track of any tropical cyclone approaching the U.S.), heat should return between the Continental Divide and Mississippi River, with another area of hot temperatures from the Carolinas into southern Quebec and New Brunswick. The window for ridging may collapse following the departure of any warm-core disturbance, with August 27 inaugurating another round of cooling east of the Rocky Mountains under a resurgent +PNA, -NAO alignment in the 500MB longwave pattern.

Prominent Threat For Hurricane Strike In The Southeast

It would be unwise of me to make any kind of offhand guess about where, if at all, a tropical cyclone may strike the U.S. after August 20. Model guidance, as might be suspected this far out, offers many possible scenarios for both path and intensity. What we do know is that, taken literally, a solid majority of the extended period NWP schemes favor a landfall by a disturbance originating in the vicinity of Cape Verde during the next 96 hours. The consensus operates on the idea that as the subtropical high extends westward, deep mean easterlies will transport a strong low pressure area to a position about 400 miles from the southeastern shorelines on August 19. The operational GFS scheme has vacillated wildly between Florida and an off-the-East-Coast trajectory, while the European outlook has long hinted at an entry through the Caribbean Sea. The Canadian ensemble members seem similar to the forecast presented by the standard American model, while the GFS ensemble package shows the Sunshine State, then Appalachia, as areas of primary risk for effects from this potential storm.

What needs to be said is simply this: with so many equations screaming "named storm approaching", anyone living over or east of the Mississippi Delta is in at least some risk for major weather problems out of the tropics in the extended period.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, August 9, 2008 at 7:15 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

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