WEATHERAmerica Newsletter, Saturday, September 20, 2008

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

Some Thunderstorms May APPROACH Severe Limits
Coastal LA, MS, AL....FL....SE GA....Coastal SC, NC

Some Thunderstorms May APPROACH Severe Limits
AZ....NM....CO....UT....ID....WY....MT....W ND....W SD....NE Panhandle

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

Isolated Locations In
W AB....BC....WA....N ID
(QPF 1 - 2")

Isolated Locations In
AZ....NM....CO....UT....ID....WY....MT....W ND....W SD....NE Panhandle
(QPF 1 - 2")

Isolated Locations In
Coastal LA, MS, AL....FL....SE GA....Coastal SC, NC
(QPF 1 - 3")

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

Isolated Locations In
S CA....S NV....S UT....CO....W KS....OK Panhandle....W TX....NM....AZ

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

Indian Ocean And The Orient

Tropical Storm Hagupit approaching the Philippines; may strengthen to minimal typhoon status with ultimate landfall in Taiwan and eastern People's Republic Of China.

Two ITCZ waves west of the International Dateline, progressing westward with some potential for tropical development within 48 hours.

Eastern Equatorial Pacific Theater

Invest 90E has a well-defined circulation and should be upgraded to depression or storm status within the next 24 hours. Positioned south of the Gulf of Tehuantepec, this disturbance will bring torrential rains and flooding to much of southern Mexico and nearby Central America. There is a small chance that this feature could survive its trip over land and emerge as a potential tropical cyclone in the Bay of Campeche next week.

Atlantic Basin

Convective circulation noted over the Bay of Campeche is unlikely to organize due to vertical wind shear.

Another thunderstorm grouping over the southern Caribbean Sea has potential to become a tropical depression as it moves generally northward.

Invest 93L above Guyana is becoming better organized as it tracks north-northwestward. 93L may at some point merge with the disturbance north of Panama, and may become implicated in a threat to the Eastern Seaboard late next week (see more below in the MEDIUM RANGE OUTLOOK).

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

The "Rest And Relaxation" Period !

With no major cold fronts, debilitating severe weather outbreaks or true temperature extremes, the near term over most of North America should not be taxing on most of us. So get some rest and get ready for challenges down the road. Some of which are less than a week away!

Warming Over The Great Plains And Dixie....

Southwest flow at most levels will bring continental air from Mexico or the Desert Regions, so a warmer forecast is in store from the Rocky Mountains to the Mississippi Valley. This of course is a dry and down-sloped regime, producing mild overnight readings while allowing for (in some cases) near-record maxima from N TX into VA.

....Cooling In The East And The Intermountain Region

While perhaps lacking any sort of drama, strong high pressure cells are a part of the current synoptic pattern. Mainly because of the inherent dryness of the atmosphere (with a mean westerly flow aloft across the U.S.), but aided by intermixture with the cold pool building over Canada, potential exits for chilly nocturnal values behind the passage of two cold fronts. Whereas the temperature drop in the West is enhanced by elevation, readings across much of the eastern third of the U.S. on Tuesday morning will be low enough for some locations to turn on the heat.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Nor'easter Threat (With Some Help From The Tropics)

There have been strong hints from the numerical models for four days now of a major-impact weather event along the Eastern Seaboard. Unlike a true "Nor'easter" where a purely extratropical cyclone organizes off of the coastline of the Carolinas, the predicted system appears to have three components in formation: a strong, slow to lift out polar high pressure cell over Nova Scotia, a weak 500MB low in lower Appalachia, and inclusion of tropical moisture and energy by the arrival of a disturbance from the Caribbean Sea (probably Invest 93L). A blend of all numerical versions suggests this storm, which could be a prolific rain and wind producer through overrunning and orographic processes, will move just to the left of the Atlantic shoreline. But there are differences among the various schemes, with the GFS model suite keeping the precipitation shield offshore and the ECMWF variants predicting a northeastward turn of a decidedly warm-core feature. The Canadian outlook, meanwhile, promotes a passage into the Appalachian Mountains. So a compromise path is offered which, if it verifies, may mean a flooding and wind damage episode from NC into parts of NS and NB next weekend.

500MB Mean Trough Over Eastern North America?

With high-latitude blocking becoming an issue in the NAO and AO positions during the 6 to 10 day time frame (more on this below in the EXTENDED PERIOD FORECAST), and the prospect of a strong storm heading into Newfoundland by October 1, it seems likely that a mean 500MB trough will set up from Hudson Bay into the southeastern states by late in the medium range. This trough complex would have the effect of channeling much cooler values into the Midwest and Eastern Seaboard, eliminating the benign character of sensible weather now in place east of the Rocky Mountains. In its place would be readings that would trend below normal as far south as the Gulf of Mexico, with the prospect for a significant rain event in parts of the Northeast at the end of the month.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Large -AO And/Or -NAO Signature Means....

While the GFS model suite at 18z continued the recently established trend of showing a positive height anomaly over Greenland, the ECMWF and GGEM ensemble means at 12z on Saturday projected a much larger ridge complex across much of northern and eastern Canada linking with what appears to be a Rex signature from Baffin Island into Iceland. If correct, this ridging would force the westerlies further south into the lower 48 states during the 11 - 15 day time frame. And with a prominent storm in the Gulf of Alaska, chances for significant precipitation and temperature extremes will rise during the first week of October.

.....A Prominent, COLD Trough East Of The Rocky Mountains!

Eventually, the aforementioned deep low over the northeastern Pacific Ocean will pass through the Great Lakes and New England. In its wake, teleconnections on the large -NAO ridge complex would favor an unusually prominent full-latitude trough across eastern North America after October 4. If so, drainage of chilled air from Nunavut A.R. and the Northwest Territories would leave an area from Ontario and Quebec into the Deep South saddled with unseasonably cold air!

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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