WEATHERAmerica Newsletter, Saturday, October 6, 2007 at 4:50 A.M. ET

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WEATHERAmerica Newsletter, Saturday, October 6, 2007 at 4:50 A.M. ET

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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
SE KS....E OK....E TX....S LA....S MS....S AL....FL....S GA....E SC....E NC

Some Thunderstorms May APPROACH Severe Limits
N WI....N MI....S ON....N PA....S NY


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

Isolated Locations In
SE SK....S MB
(QPF 1-2")

Isolated Locations In
SE KS....E OK....E TX....S LA....S MS....S AL....FL....S GA....E SC....E NC
(QPF 1-2")

Isolated Locations In
N WI....N MI....S ON....N PA....S NY
(QPF 1-2")

Isolated Locations In
Coastal BC, WA
(QPF 1-2")


EXCESSIVE HEAT ADVISORY
(potential for daily maxima above 90 F)

Scattered Locations In
FL....S GA....S AL....MS....W TN....W KY....W IN....IL....C, S WI....MN....E ND....E SD
IA....MO....AR....TX....E NM....E CO....NE....KS....OK


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Is That Great Smokies Heat Ridge Supposed To Be There?

Instead of stories of falling leaves, campfires, and breaking out the jackets, most residents of the U.S. east of the Rocky Mountains are still swatting bugs, sweating profusely, and ruminating about the air conditioning bill. And this situation should remain through the next 72 hours.

Although many might find the presence of a strong heat unusual for October, in truth this configuration can appear as late as November. The current anticyclone is exceptional, however, in that its "reach" extends from the Great Plains into the Northeast. This is essentially the same subtropical high that has been observed since late July, and has been responsible for record heat concentrated in the Ohio Valley.

Pressure from the exiting trough complex over the western states will likely result in weakening and eastward translation of the ridge next week. But a caveat applies: with a slow moving low or trough setting up from the Corn Belt into the Deep South, it is not out of the question for a tropical feature to be pulled northward from the Greater Antilles. Such a disturbance might well become enmeshed with the upper low, causing a very stormy, rainy end to the heat wave across the eastern third, or perhaps half, of the nation in the medium range.

And What About That High-Elevation Snowfall In Parts Of The Intermountain Region?

Just as the heat ridge is keeping summer in play over the eastern half of the nation, so is the deep storm over the Bonneville Basin producing chills and snowflakes (the latter mainly at higher elevations). Moderation is already underway, however, as the trough complex moves slowly eastward and is cut off from its sources of cold air in the Gulf of Alaska and western Canada. By the time the trough axis crosses the Front Range on Monday, all potential for meaningful snowfall in the ranges of UT, CO, and WY will be coming to an end.

Severe Weather Threat For Sunday And Monday

Were the trough moving out of the Intermountain Region exhibiting a negative or neutral tilt, the risk for a vicious outbreak of severe thunderstorms and tornadoes would be quite high. But although the disturbance is expected to close off upon entering the Great Plains tomorrow, the orientation of the energy will still be positively tilted, with thunderstorm formation being mainly of a linear nature. There WILL be areas of intense convection from the Red River Valley into the upper Mississippi Valley in the 48 hour time frame, with some of the stronger cells redeveloping in the Midwest on Monday. However, formation of true supercells is expected to be fairly limited with the main emphasis in forecasts being the potential for heavy rainfall.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Slow-Moving Low Complicates Rainfall Forecast F or The Midwest And East Coast

There is some variety among the numerical models concerning the eventual position, and structure, of the upper low forecast to develop over the Great Lakes and Ohio Valley beyond 96 hours. That said, the presence of a blocking signature north of the low, and a wide-open field of tropical moisture, may spell a shock ending to the ongoing drought conditions in parts of the Corn Belt and Mid-Atlantic regions. And should an actual tropical cyclone develop and be thrown into the mix (still a possibility), the chances for an excessive rainfall event would increase dramatically. Keep in mind that in all record precipitation occurrences along the Eastern Seaboard, a trough is present along and to the left of the Appalachian Mountains, with a strong heat ridge in the western Atlantic Ocean.

Later in the week, the presence of the slowly retreating upper level low and ridging in central Canada could produce some very cool temperatures across much of the Midwest, Tennessee Valley, Appalachia, and the Atlantic Coastal Plain.

Rex Block In Canada: Unusual For A La Nina Autumn

A typical fall during a moderate La Nina episode features a unified, progressive flow with some tendency for trough development over the Pacific Northwest and northern Great Plains. That type of configuration is clearly not present in North America now, but may tend to take shape after October 14. Before that time, the merger of a retrogressive blocking signature from Greenland and a weak +PNA signature in the Prairie Provinces will create a Rex block stretching across central Canada. This alignment should result in advection of very cool (cPk) values into the U.S. with relatively high surface pressures. So after a long spate of warm or hot readings, the word "chilly" should return to the lexicon of residents of the Midwest and Northeast about a week from today.

Intense Storm Sequence Over The Northern Pacific Ocean Takes Aim At West Coast

Inevitably, events upstream across the northern Pacific Ocean will have an impact on North America. A strong, semizonal, yet active polar jet stream stretches from Mongolia into the Gulf of Alaska, Bolstered by subtropical disturbances merging in with the westerlies, the fast flow signature will send a series of powerful shortwaves into the West Coast during the middle of the month. The result? Think about cool, cloudy and windy conditions with heavy rains from N CA into BC late in the medium range.

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

Threat Remains Over Cuba And Florida

Although a convective circulation in the Louisiana coastal waters amounted to nothing, there are still concerns regarding convective blow-ups in the eastern Gulf of Mexico, near Cuba and over the FL Peninsula. The most probable outcome of this feature will be nothing more than severe thunderstorms and heavy rains. However, if the complex takes a course to the left of Key West FL, further organization is possible with landfall threats to the eastern Gulf Coast.

Tropical Low Across The Greater Antilles

I suspect that the better potential for tropical development lies within the field of convection straddling the Caribbean Sea, Greater Antilles, and Turks/Caicos chain. Occasionally a circulation will form, and wind profiles in the vicinity of the thunderstorm area are no worse than marginal in terms of shearing impact.

In the event that a depression or named storm does form in this vicinity (odds are about 1 in 10), there is a high danger of landfall and incorporation into a trough complex expected to move into the eastern third of the U.S. next week.

Typhoon Krosa Near Taipei, Taiwan

The island of Taiwan is taking a beating from Typhoon Krosa as I write this. The storm is weakening somewhat due to interaction with land, but should still be a formidable system as it approaches the eastern shoreline of the People's Republic of China below Shanghai tonight and Sunday morning. The r emnants of Krosa will then break up over Korea early next week, with little impact on the course or energy of the polar westerlies.


Prepared by Meteorologist LARRY COSGROVE on
Saturday, October 6, 2007 at 4:50 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

All rights reserved.

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