WEATHERAmerica Newsletter, Saturday, August 4, 2007 at 5:15 P.M. ET

[Larry_Cosgrove]

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes Or Waterspouts)
N ON....NU Islands Of Hudson And James Bays....Far W QC

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes Or Waterspouts)
Extreme E WI....MI....ON Peninsula....W NY....W PA....DE....MD....DC....N, C VA
WV....OH....N, C IN....N, C IL....N MO

Some Thunderstorms May APPROACH Severe Limits
S CA Deserts....S NV....AZ....E UT....CO....NM....extreme W TX

Some Thunderstorms May APPROACH Severe Limits
E BC....AB....W SK....MT....WY....W SD....ND....NW MN

Some Thunderstorms May APPROACH Severe Limits
E TX....LA....MS....AL....FL....GA....SC....NC....S VA....S KY....TN

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

Isolated Locations In
N MB....N ON....NU Islands Of Hudson And James Bays....Far W QC
(QPF 1-2")

Isolated Locations In
Extreme E WI....MI....ON Peninsula....W NY....W PA....DE....MD....DC....N, C VA
WV....OH....N, C IN....N, C IL....N MO
(QPF 1-2")

Isolated Locations In
S CA Deserts....S NV....AZ....E UT....CO....NM....extreme W TX
(QPF 1-2")

Isolated Locations In
E BC....AB....W SK....MT....WY....W SD....ND....NW MN
(QPF 1-2")

Isolated Locations In
E TX....LA....MS....AL....FL....GA....SC....NC....S VA....S KY....TN
(QPF 1-2")


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

Numerous Locations In
Interior CA....SE OR....C, S ID....C, S MT....ND....ON Boundary Waters....MN
WI....Lower MI....OH....KY....SW VA....NC....SC....N GA....TN....N AL....N MS....N LA
N TX....AR....OK....NM....AZ....NV....UT....CO....WY ....SD....NE....KS....MO....IL....IN


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Yes, It Is Hot, But....

Although temperatures will be above normal in many sections of the lower 48 states through the next three days, the hot spell developing around the U.S. is not particularly impressive in terms of output of 100 deg F maximum readings. Higher dewpoints and thunderstorms may cut down afternoon highs in the Old South, with the worst heat stretching from the High Plains into the Ohio and Tennessee Valleys. The marine layer should be strong enough to withstand any advances from inland cT values, and a frontal structure along the Canadian border should (through thunderstorm production) eliminate excessive heat in the upper reaches of the Great Lakes and the Northeast.

Potential For Strong Thunderstorms, North, South....

Convection during the near term will largely follow a "ring of fire" configuration, that is around the perimeter of the 500MB heat ridge. Best chances for severe weather will be close to the Canadian border, where jet stream dynamics, surface convergence and upward vertical motion will be most favorable. Other thunderstorms, mainly of a diurnal nature, will develop through the southern tier of the nation, accented by mesoscale cold pools and seabreeze interactions. Since the character of the anticyclone involved is "dirty" (punctuated by areas of moisture and smaller disturbances). some late day potential for thunder exists even close to the core of the subtropical high, over the Mid-South.

....And West, But The Summer Monsoon Is Not Really A Strong One

To be sure, there IS a "summer monsoon" ongoing across the Intermountain Region, as you can tell by looking at the GOES image above. But compared to past years, this season's display of diurnal+orographic convection tends to be irregular and mainly confined to the states adjacent to the Mexican border. Some expansion of moisture is possible over the following 72 hours. But convective activity will be most likely to the right of the Continental Divide and more sporadic than in past years.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

How Strong Of A Heat Ridge?

I could say, "well, I've seen worse", and be justified (think the 1976 mega-ridge over the Great Plains, the 1980 "Dallas Inferno" and last year's "Coast to Coast Bakery"). But what the subtropical high forecast over the next 240 hours lacks in depth or even coverage, it may make up for in duration. Looking at the 12z Saturday August 4 runs of the operational models and the respective ensembles, little variation in position or strength of the heat ridge is noted through Day 15! In short, much of the interior of the U.S. is in for oppressive heat into the third week of the month, and the hot temperatures will be accompanied by high dewpoints on all sides of the positive height anomaly.

Get set....to sweat!
For The Pacific Northwest And The Northeast, Position Is The Thing

While much of the interior of North America will be baking in hot temperatures between now and August 15, it is important to note that two regions will largely escape the extreme heat: the Pacific Northwest and the Northeast. This is not to say that either part of the U.S. will totally escape the torrid conditions; indeed, one or two day displays of readings above 95 deg F are probable. But the placement of the core of the heat ridge on all of the numerical models runs mainly dead center of the nation; therefore trough axes in western Canada and QC into New England may occasionally direct cooler values of either mP or cP sources southward, thereby nullifying the scorched earth pattern likely to be experienced in places like Ottumwa IA, Hillsboro OH, and Union City TN.

Severe Weather Threats Galore In Southern Canada, Northern Tier Of U.S.

Because of the "locked-in" nature of the 500MB heat ridge across the lower 48 states during the medium range, the polar westerlies should remain close to their climatological position for August, near the U.S.-Canada border. Shortwaves will traverse the jet stream, ridging roughly along 55 N Latitude, and in the process set up "three regime" (that is cP, cT, and mT air masses) convergence zones that will favor development of intense convection. Output of severe weather should occur roughly every 48 hours along a path from BC and WA to QC and New England; development could be either in supercellular fashion or include groupings such as derechoes, MCCs, or MCSs. Two concerns to note: this is the time of year when we see "northwest flow" severe thunderstorm potential in the Great lakes and Northeast. Also, the threat is present for right-turning, nocturnal groupings of cumulonimbi which can trigger wind and lightning damage in communities from the Upper Midwest into the Hudson and Delaware Valleys.

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

The Ever-Active ITCZ

A review of METEOSAT imagery shows five vigorous tropical waves percolating along the Inter Tropical Convergence Zone. Since we are now in "Cape Verde Season" of the tropical cyclone formation calendar, these impulses must be monitored as possible candidates for development into depressions and named storms. There is a layer of cooler, relatively stable mP values just above the equatorial Atlantic Basin which may dampen chances for further organization of the disturbances during the next 72 hours. When the waves reach warmer SST values closer to the Lesser Antilles, however (which ordinarily takes five to six days after leaving the shoreline of western Africa), renewed threat for development may occur.

Western Atlantic Basin Is Quiet....For Now

Contrary to rumor, the tropical wave over the westernmost Caribbean Sea is not dead and still has a marginal chance of becoming a tropical depression. The impulse is skirmishing the coastline of Central America, and by Sunday morning should be approaching Belize. With relatively warm waters over the western Gulf of Mexico and Bay of Campeche, strengthening of this feature is possible in a low-shear environment.

Other convective clusters are present over the western Atlantic Ocean, and these disturbances may track into the FL Peninsula and Gulf of Mexico over the next 72 hours. But no computer model shows any signs of warm-core cyclogenesis near the U.S. in the near term or medium range.

La Nina May Be Squashing The Tropical Cyclone Output Of The Central, Eastern Equatorial Pacific Theater


Even though the ongoing La Nina episode over the equatorial Pacific Basin is relatively weak (note the warm striations interspersed with the cooler anomalies in the ENSO sectors), there are still notable shifts in sensible weather which correspond well with a colder signature in the 3,4 and 1,2 regions. most notable is the relative lack of tropical cyclone development between the Mexico/Central America shoreline and seas below the Hawaiian Islands. Lower SSTs and a prevalence of lower latitude westerly winds are not conducive to warm-core cyclogenesis, hence the trend for minimal tropical cyclone activity to the left of the Americas and east of the International Dateline.

Orient And Oceania; From Convective Chaos, More Typhoons?

While Typhoon Usagi is "out of the equation", converting to extratropical status below the Kamchatka Peninsula. there are viable tropical waves across the western Pacific Ocean which pose a threat for development. One huge convective shield over the Marianas Trench seems to be organizing, and may become a depression within another day or so. Other impulses near and beyond the International Dateline must all be monitored, as a weak Kelvin Wave (associated with the Madden Julian Oscillation) is now exiting Indonesia and will favor thunderstorm growth along and about 15 deg north of the equator.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, August 4, 2007 at 5:15 P.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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