WEATHERAmerica Newsletter, Saturday, August 16, 2008 at 6:25 P.M. CT

by LarryCosgrove | August 16, 2008 at 04:35 pm

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WEATHERAmerica Newsletter, Saturday, August 16, 2008 at 6:25 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
AZ....NM....TX....W OK....W KS....C, E CO

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
ON....MI....WI

Some Thunderstorms May APPROACH Severe Limits
FL....AL....GA....SC....NC....C, S VA

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

Isolated Locations In
AZ....NM....TX....W OK....W KS....C, E CO
(QPF 1 - 3")

Isolated Locations In
ON....MI....WI
(QPF 1 - 2")

Isolated Locations In
AZ....NM....TX....W OK....W KS....C, E CO
(QPF 1 - 3")

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

Scattered Locations In
C, S BC....S, C AB....S SK....SW MB....ND....SD....N NE....WY....MT....ID....NV
CA....AZ....UT....W CO....NM....W TX

Isolated Locations In
FL Peninsula

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

Eastern Equatorial Pacific Ocean

Dwindling remnants of Iselle are destined to spin down southwest of Cabo San Lucas Baja CA within the next 24 hours.

Other disturbances well south of Mexico and west of Colombia represent possible risks for tropical development, but with little or no threats to populated areas.

Atlantic Basin

TUTT signature is interacting with vestiges of tropical wave over the eastern Sargasso Sea. Tropical cyclone formation is unlikely.

Vigorous tropical wave southwest of the Cape Verde Islands has ingested a great deal of low-dewpoint, dust-laden air from the Saharan heat ridge. With developing and strengthening layer of deep mean easterlies, however, this feature stands a decent chance to organize into a tropical depression in about two days as it progresses toward the Leeward Islands.

Tropical Storm Fay

Tropical systems are often difficult to forecast with regard to track and intensity, and Fay appears to be no exception. Fay should gradually reorganize after its damaging encounter with the mountains of Hispaniola, and if the center of circulation passes between Jamaica and Cuba as appears likely, core wind speeds should exceed hurricane force by Sunday morning. I base this primarily on the very warm water temperatures present in the northern Caribbean rim and the somewhat favorable upper atmospheric signature which lies to the right of a dry air intrusion and cyclonic flow axis.

A view of the water vapor imagery is included here to establish two points. One is the trajectory of Fay. Note the frontal structure and fairly prominent westerly flow along the Gulf Coast, and the TUTT signatures to the northeast and west of the tropical storm. The upper low center, although small, is important because its upper right quadrant will be difluent, stimulating outflow and deepening. As ridge building occurs to the northwest of the other disturbance in the Sargasso Sea, the environment will support rapid intensification and a northward turn (between the conjoined weakness and the briefly expanding Bermuda High). Thus a path through Cuba below the Florida Keys seems likely.

A consensus of the 18z tropical model forecasts, weighted by the rightward outlier (GFDL) seems the best way to go for predicting where Fay will move. My prediction for entry into Florida is over Monroe County just below Marco Island, with a 50 mile window either side of the outlined track. Keep in mind that even a minor leftward shift will imperil Fort Myers and/or Naples. And a "Charley (2004) " explosion scenario cannot be ruled out, although the shearing potential may remain higher if the frontal structure and west wind profile aloft are not entirely eradicated by the expansion of the heat ridge.

I suspect that Fay will pass close to the Melbourne FL vicinity before re-entry into the Atlantic Ocean and a final landfall just east of Beaufort SC on early Wednesday morning. Read below in the Short Range Outlook how Fay may become a flood producer in Appalachia and the Piedmont next week.

African ITCZ

The ITCZ remains active with five well-defined impulses stretching from southwest of the Cape Verde Islands into Ethiopia. Ingestion of dust and Saharan air is limiting the most westward of the disturbances, but this feature shows up prominently in many of the longer-term baroclinic models with an approach to the major islands and/or the Eastern Seaboard of the U.S. Using September 9 as a "shut-down" or "slowing down" date for this current tropical cyclone season (because of an ENSO profile that favors low latitude shearing flow in the upper atmosphere as we progress into autumn), we still have more than three weeks for African tropical waves to have a significant impact on locations in and to the right of North America.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Upper Air Disturbance Makes For Rain, Lower Temperatures In Texas, Oklahoma

The presence of steep lapse rates and prolific thunderstorm activity in the lower Great Plains, rare for August, has meant reductions in temperature and easing of the ongoing drought conditions in parts of Texas. Associated with an upper level low and weakness stretching from Colorado to the Gulf Coast, the potential for cloud cover and heavy rainfall is expected to continue through the next 96 hours south of the Kansas/Nebraska border to the Rio Grande Valley.

A Tropical Deluge For The Southeast?

Largely dependent on the track and strength of Tropical Storm Fay, there appears to be potential for a prolonged period of heavy rainfall from Florida into lower Appalachia and the Piedmont beginning Monday. The oncoming high pressure cell will likely set up an oceanic fetch into Georgia and the Carolinas, and the sluggish trajectory of Fay could create a set up for torrential rainfall in an area which has long been hit by drought. Caveat: a west-of-Florida-Peninsula path of the tropical cyclone would probably confine the deepest moisture into the Peach State and eastern Alabama below Interstate 20.

What The Great Lakes Cold Front Means To You

A fairly impressive cold front is showing up in computer model charts, progressing from the Prairie Provinces into the Mid-Atlantic and New England States by Tuesday night. Besides bringing some spectacular late summer weather to the Northeast, the moderately strong high pressure cell accompanying this intrusion may be a steering mechanism for the incoming Fay circulation, while also creating an orographic banking east wind over the foothills of the Appalachian Mountains which could enhance rainfall totals from GA into W VA.

After The Heat, A Cooling Trend For The West

The impressive burst of heat throughout the western states looks to be short-lived, what with an mP cold front lurking offshore. The marine-based air mass will encroach of the hotter readings early next week, dislodging the hotter regime entirely in the Pacific Northwest and northern Intermountain Region by Wednesday.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

How Meaningful A Cool Intrusion For The Midwest?

The cold front and 500MB shortwave that will break the back of the Pacific Northwest hot spell will be a significant player in the Midwest and Northeast during the medium range. Rushing into the 500MB weakness stretching from Ontario to the Gulf Coast, the front may pick up the remnants of Fay as it slows approaching the Atlantic Coastal Plain. This scenario, seen by the 12z ECMWF version, would suggest that the best cooling will occur between the Rocky Mountains (mostly under a Sonoran heat ridge) and Appalachia (where the temperature boundary should reside for much of the time between August 24 and 27).

A Wet Pattern For The Eastern States?

Note the 500MB weakness showing up from the Bahamas into the trough over the Great Lakes. There are loops in this feature (seen on this ECMWF chart covering the period from August 23 through 26). Tropical waves or upper disturbances may rotate around the Bermuda High, which will go into a weakening stance for much of the medium range. Deep tropical moisture will flow towards a lingering frontal structure, potentially setting up yet another barrage of showers and thunderstorms from North Carolina into New Brunswick.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Watch The Tropics, Please!

There have been subtle hints from many of the numerical models covering the longer term that the 11 - 15 day time frame (and just beyond) could be particularly threatening for the Eastern Seaboard and/or the Gulf Coast. Not only is the ITCZ in overdrive with the production of strong convective circulations in Africa (some of which may survive and become tropical cyclones in the Atlantic Basin). But the consensus of computer forecasts suggests the formation of a strong anticyclone in eastern Canada with a permeable subtropical high covering the southern two-thirds of the U.S. and the adjacent waters of the Gulf Stream. This type of 500MB longwave pattern is often seen in major hurricane landfalls along the southern and eastern rim of the nation, and can allow for problems with both wind and rainfall output due to orographic effects and linkage with frontal structures. A small vestigial anticyclonic circulation shows up over NC on the variants of the GFS and ECMWF schemes by September 1, which could allow for a "split weakness" pattern at 500MB allowing entrance by warm-core disturbances in either the Deep South or the Mid-Atlantic and New England states.

Lingering 500MB Trough, Weakness In The Midwest

It would appear that with a Sonoran heat ridge and Bermuda High in view for much of the extended period, the discontinuity between the two anticyclones would enable cool intrusions to drop into the Great Plains and Midwest. A mean (although weak) trough looks to settle into the upper Mississippi Valley and Great Lakes, with a fast jet stream flow running from Washington to the Dakotas and then into Quebec. This is a very warm, but not really hot, configuration for much of the U.S., and would enable deep tropical moisture to be present in most locations from the Great Plains into the East Coast. Thunderstorms would be a continual risk for the Corn Belt and the interior of the Northeast.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, August 16, 2008 at 6:25 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.