WEATHERAmerica Newsletter, Saturday, October 4, 2008 at 7:20 P.M. CT

TODAY'S FUN LINK: SOLARCYCLE 24.com

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
FL Peninsula

Some Thunderstorms May APPROACH Severe Limits
NM....W TX....W OK....W KS....C NE....C SD....ND....C, S MN....S WI

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

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

Isolated Locations In
NM....W TX....W OK....W KS....C NE....C SD....ND....C, S MN....S WI
(QPF 1 - 2")


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

Isolated Locations In
S FL

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

Indian Ocean And The Orient

Tropical Depression Higos is moving ashore in southern China, and will trigger heavy rainfall along a frontal structure that stretches northeastward into Japan.

Invest 93L should be declared a tropical depression within the next 24 hours. Probable targets for heavy rainfall are Luzon Island and Taiwan before recurvature into Kyushu by October 11.

The Madden-Julian Oscillation is showing signs of life with a convective belt building over the equatorial Indian Ocean. Should this area of energy expand and translate through Indonesia into the western Pacific Ocean, linkage to the polar westerlies may occur with downstream amplification of the 500MB longwave pattern over North America (in other words, a strong storm followed by important cold advection from Canada into the U.S. in about two weeks).

Eastern Equatorial Pacific Ocean

Tropical Storm Marie is rapidly weakening about 800 miles southwest of Cabo San Lucas Baja California.

Tropical Depression 15E should intensify and be declared Tropical Storm Norbert. There is a fair chance that this feature may reach hurricane status, and could graze Socorro and clarion Islands of Mexico before breaking up in five to six days over cold waters to the west of Baja California.

Invest 92E is a growing convective circulation below Central America. Strengthening is likely, with torrential rainfall and surge impacts into southern Mexico over the following 72 hours.

Atlantic Basin

Invest 96L near Belize is heading west-northwestward toward the Yucatan Peninsula. This feature will likely track into the Bay of Campeche within 48 hours, then turn northward as it is captured by a trough moving out of the Rocky Mountains. Some potential exists for organization into a tropical depression or named storm before the circulation encounters shear and cooled waters over the central and northern Gulf of Mexico in about five days.

Two TUTT signatures are seen over the Caribbean Sea and eastern Sargasso Sea, respectively. Only the disturbance below Puerto Rico has any (slight) potential for surface translation; this system will target Hispaniola and later Jamaica and Cuba with strong thunderstorms and torrential rainfall.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Western U.S. And Great Lakes, Northeast Quite Chilly....

The cold advection pattern across the western states has a compliment further east. The presence of a Rex block across Nunavut AR is helping to establish a digging mechanism for a shortwave from Ontario and Quebec, driving a wedge of very cold air into the Northeast. As is the case with the mPk surge across the Intermountain Region, the cooling above the Mason-Dixon line will be impressive but very short-lived, with milder readings appearing in the Mid-Atlantic and New England states by October 8.

....While Warmth Returns To The Southwest And Old South

As impressive as the cold advection pattern over the U.S. appears now, the mPk domain will weaken considerably and lift out into the Great Plains early in the coming week. Ridging should appear over the Desert Southwest behind the trough complex, while a subtropical moisture stream and anticyclone will act in combination to warm much of the Deep South and a part of the Ohio and Tennessee Valleys.

Early Western Chills Spark Thunderstorms From Texas, Louisiana Into The Midwest

The surge of very cold air across the Intermountain Region will weaken (although likely replaced by an even stronger placement of polar values within five days), reaching the Great Plains on Tuesday. But the injection of cyclonic vorticity, marginally cool, stable values and a Gulf of Mexico moisture plume will raise the potential for heavy showers and thunderstorms from the Gulf Coast into the Upper Midwest. Should the disturbance currently just east of Belize become entrained into the frontal structure, there is a very real possibility for a flooding episode in the lower Mississippi and Tennessee Valleys by the end of the coming week.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Great Smokies Heat Ridge Reappears....

While the medium range starts off with a strong cold intrusion across the Intermountain Region, a return of the Great Smokies heat ridge seems likely to warm most of the Old South, Midwest and the Eastern Seaboard. This signature is actually common in October, accounting for autumn season haze and the re-formation of the 90 deg F isotherm. That latter parameter may advance as far north as the Ohio River and Mason-Dixon line on October 10 - 11 if the ECMWF model forecast verifies.

....But Progressive Trough, Baffin Island Rex Signature Bring The Cold Eastward

The GFS version has remained consistent in acknowledging both the positive height anomalies over northeastern Canada as well as the fast eastward translation of a strong trough and cold front during the medium range. So after an initial warming over the eastern states, the next surge of mPk+cPk air into the western U.S. will translate to the Atlantic Coastal Plain in about nine days. Word of warning: with so much in the way of strong dynamics (note the negative tilt of the 500MB vorticity axis) and inflow of deep moisture from the Gulf of Mexico, a severe thunderstorm and heavy rain event could occur during an October 12 - 13 time frame over parts of the Corn Belt, Tennessee Valley, and Mid-Atlantic regions.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Progressive 500MB Longwave Pattern Has A Cold Look To It

After October 14, and despite the lingering presence of a -NAO signature, the 500MB longwave pattern takes on a clearly progressive bent with at least two strong storms during the 11 - 15 day time frame. In a set-up which mimics, to some degree, what transpired across the U.S. last winter and spring, the midlatitude cyclones will be accompanied by two strong (albeit transient) intrusions of polar air. The arrangement as shown by the GFS ensemble mean (shown here) support a very chilly temperature regime for the Pacific Northwest, the Great Lakes, and the Northeast.

Alignment Of Moisture Fetch Plays An Important Role In Forecasts For Later Autumn And Winter

I have found that an important predictor of apparent weather in the later autumn and winter months is the strength and position of equatorial moisture fetches. These ribbons of high dewpoints and energy act as a boost to disturbances in the polar westerlies, and also serve as a focus for storm tracks (with midlatitude cyclones moving along the left edge of the cloud shield). Last year there were two prominent transport fields from the tropics to the higher latitudes: from Indonesia into systems entering the Gulf of Alaska, and another axis of mT flow from the Mexican shoreline of the Pacific Ocean into the lower Great Lakes. Both windfields helped to produce robust storms and cold intrusions that produced, despite a mainly progressive 500MB longwave pattern, a fairly cold and extremely snowy (not to mention prolonged) winter in the Great Lakes and St. Lawrence Valley.

While it is still "early in the game", we can clearly see two dominant moisture axes at work on GOES and METEOSAT imagery. One banding of clouds runs from the equatorial eastern Pacific Ocean through Florida, then over the Gulf Stream to the Grand Banks. Another area of enhanced flow starts off the coast of Brazil, then works into central Europe. If we apply seasonal retrogression to these "quasi-subtropical jet streams), you can make an argument for a fairly cold, stormy trend this winter over the Old South and Eastern Seaboard, while most of the European subcontinent experiences relatively balmy conditions with frequent rains from the Iberian Peninsula into Scandinavia.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, October 4, 008 at 7:20 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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