WEATHERAmerica Newsletter, Sunday, September 9, 2007 at 7:00 A.M. ET

WEATHER HAZARDS (During The Next 24 Hours)

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

STRONG WINDS
(From Tropical Storm Gabrielle)
NC Outer Banks

Some Thunderstorms May APPROACH Severe Limits
C, S FL

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
W TX....NM....CO....WY....NE

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
OH....PA....NY....VT....NH....ME

Some Thunderstorms May APPROACH Severe Limits
C, E TX....W LA....AR....N MS....N AL....TN....KY....W WV


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

Isolated Locations In
C, E TX....W LA....AR....N MS....N AL....TN....KY....W WV
(QPF 1-2")

Scattered Locations In
OH....PA....NY....VT....NH....ME
(QPF 1-3")

Scattered Locations In
C, S FL
(QPF 1-4")

Isolated Locations In
W TX....NM....CO....WY....NE
(QPF 1-2")

Isolated Locations In
E NC
(QPF 1-3")


EXCESSIVE HEAT ADVISORY
(potential for daily maxima above 95 F)

Scattered Locations In
SW OR....CA....NV....AZ....UT....CO....NM....TX....OK ....AR....LA....MS....AL....FL
GA....SC....NC....VA....TN

FROST Or FREEZE POTENTIAL
(possibility of killing frost or hard freeze)

N, C ON....N, C QC


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Intense Heat For California (And Still Pretty Hot In The Deep South)

California has had to deal with some extremely intense heat of late, and the "savior marine layer" has sometimes been pushed back, allowing the Los Angeles and San Diego metro areas to fry in the late summer oven. Unfortunately for the beleaguered SoCal power grid, the cTw regime will be in complete control through the near term. The 90 deg F isotherm should make it to the beaches of the Golden State, and readings should surpass the century mark (100 deg F) anywhere along the Interstate 5 corridor.

The heat, along with the requisite high relative humidity, will also be in evidence through the Desert Regions, TX and the Deep South as well. Thunderstorms may cool off some in the cT+mT domain in the southern tier of the U.S. But the fact is the next 72 hours will be miserable and stinking hot anywhere from the lower Colorado Valley into the Carolinas.

First Meaningful Shot Of Polar Air From Canada Of The Season

Yes, this is a serious shot of cooler air, created by a strong -EPO styled ridge that should last well into the medium range. Digging shortwaves have already brought the first intrusion of cP values into the northern High Plains, and the first frost or freeze of the season is likely in parts of Ontario and Quebec tonight. The upper air pattern lacks a positive height anomaly in the NAO position, so the polar westerlies will probably not be able to deliver the truly chilly air into the Northeast before the medium range. Still, most of the Midwest and Great Lakes can count on a breathtaking autumn air mass taking hold in the next few days.

Hot Vs. Cold = Severe Thunderstorms In the Great Plains And Midwest

The press of cPk values into a wide field of hot temperatures and high dewpoints can only mean excessive production of thunderstorms. Convection will be especially intense from the central Great Plains into the Corn Belt off and on through the next six or seven days. This is partially good news, since many of the communities in the Ohio and Tennessee Valleys need the rain. However, the vigorous shortwave will carry with it plentiful vorticity and upper dynamics, increasing the danger for severe weather along the leading edge of the attendant frontal structure.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Eastern U.S. Trough Will Suppress The High Heat, But For How Long?

The strong -EPO styled ridge complex will likely drift into the PNA position (BC and YT) before breaking down after Day 9. In the process, the shortwave forecast to drop out of NT and N AB will dig, temporarily, a fairly deep trough complex over the eastern half of the continent. This trough, in turn, will obliterate the heat ridge which has undergone a renaissance and sent temperatures into the 90s (F) over much of Dixie. Some of the hotter readings may linger close to the Gulf Coast and certainly in the Desert Regions. But below normal values will control apparent weather in the Midwest and Northeast for the better part of the medium range.
Sonoran Heat Ridge Hangs Tough In The Southwest

Western Atlantic Basin Ridge Complex Favors Tropical Cyclone Development

An elongated lateral ridge complex is not a good thing to see during the hurricane season. But unfortunately, the longer term will be dominated by a subtropical high stretching from the Azores into the FL Peninsula. So while a good deal of eastern North America enjoys a spate of cool, dry weather, disturbances of the Cape Verde, Caribbean, and Gulf of Mexico families are likely to organize in a shear-free environment over warm water. The anticyclone will steer these disturbances westward, with the chief landfall threats over the Greater Antilles, FL and the Gulf Coast around or after Day 10.

TROPICAL WEATHER OUTLOOK
(Tropical Features In The Atlantic And Pacific Basins)

Tropical Storm Gabrielle

Gabrielle remains a relatively small, weak tropical storm, with a cluster of strong thunderstorms located just north and west of the center. The convective shield and 40 mph winds associated with Gabrielle will be an irritation for the eastern third of the Tar Heel State, while the 1-3 inches of rain that falls will of course be welcome to the parched farmland to the east of Interstate 95. Seaward recurvature processes are already underway, making it doubtful that any real effects from the tropical storm are felt in VA and MD, both of which could use the moisture.

Thunderstorm Complex Over Cuba, Western Caribbean Sea

Incredibly, this disorganized but expansive area of convection persists after being spun off from Hurricane Felix nearly a week ago. A TUTT signature over the Bahamas seems to be repressing convective outflow over Cuba, while a circulation occasionally pops up over the western Caribbean Sea. Tropical development is unlikely, but this feature could become better organized while moving toward TX and northeastern Mexico over the following 72 hours.

Two Tropical Waves Of Note In Equatorial Atlantic Basin

While the small impulse approaching the Leeward Islands is dwarfed by its larger counterpart over the eastern Equatorial Atlantic Ocean, it is this wave which appears to have some symmetry and a midlevel circulation. Since strong ridging (reducing shear and enabling westward transport) will be developing over the northern and western sections of the Atlantic Basin, both disturbances pose a threat to the Greater Antilles and southern U.S. during the medium range.

African ITCZ Remains Very Active

There are still five strong tropical waves in place along the Inter Tropical Convergence Zone in Africa, stretching from near the Cape Verde Islands to Ethiopia. It is the abundance, and strength, of these impulses which tells us that at least one disturbance in the series will survive the cool SST field over the easternmost Atlantic Ocean. If I had to hazard a guess, I would say that the convective circulation over Nigeria probably offers the best chance at becoming a mature tropical cyclone after September 19.

Huge Closed Extra-tropical Low Southeast Of The Grand Banks

This system IS a legitimate threat to undergo either transformation to subtropical status or undergo warm-core cyclogenesis. Cut off from the polar westerlies and with a closed circulation, this cut-off low is like many past storms which transition from a cold center to a highly convective squall banding. If this feature does become a hybrid or purely tropical disturbance, the greatest threat would be to shipping and marine interests and perhaps the Azores Chain in a few days.

Tropical Storm Danas

Danas is approaching Japan in the wake of the deluge left behind by Typhoon Fitow. However, computer forecasts insist that the tropical storm will not reach Honshu, but instead recurve toward the northeast and link with the polar jet stream. This action will further bolster a storm complex over the Aleutian Islands, thereby pumping up a strong (for September) EPO styled ridge in the Gulf of Alaska. For this reason, we will see a trough (with much cooler air) forming downstream over the central sections of North America.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, September 9, 2007 at 7:00 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

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