WEATHERAmerica Newsletter, Saturday Morning, June 23, 2007 at 3:35 A.M. ET

[Larry_Cosgrove]

Breaking Down The Forecast: Intense Thunderstorms, Searing Heat, Intense Thunderstorms, An Autumn Chill On Tap For The Midwest And Northeast; Intermountain Region And Dixie States Look To Be "Deep Fried" Through July 4th

(Disclaimer And Copyright Statements At End Of The Discussion)


WEATHER SUMMARY THROUGH NEXT 72 HOURS

This being early summer, two points are likely to stand out in the forecast: heat and thunderstorms. The hottest readings will likely not be at greatest expanse until the medium range. Convection, however, will be a major player for the next three days, with ample chances at severe weather as well as flooding in several sections of North America.

The risk for severe thunderstorms is greatest during the next 48 hours over southernmost Canada. A sequence of two 500MB shortwaves is moving eastward just above the international border, very close to a zone of maximum surface convergence (mP+cP/cTw/mTw). The general pattern will be favoring supercell formation in S AB....S SK....S MB....W, C ON during the later afternoon. Then, after nightfall, the convection should congeal into either an MCC or full-blown MCS moving along the warm front into the lower Great Lakes and the Northeast.

Further south is a persistent (more than two weeks!) 500MB weakness across the Great Plains. There are mesoscale disturbances embedded within the trans-ridge gap; one of these impulses will work northward today and increase thunderstorm intensity in parts of S WI and IL; another piece of energy along the TX Gulf Coast could regroup north and westward with hail and microburst threats from the TX Big Country into the Front Range of NM and CO. This alignment of thunder should change little through next week, keeping the south central states out of the crippling summer heat wave while presenting the ongoing danger of floods and ruined agriculture to the lower rung of the Interstate 35 corridor.

And yes, that heat! The smaller area of hot weather across the interior of Dixie will expand early next week, merging with the building/ejecting Sonoran heat ridge (which is effectively frying the West). While it will probably take until late Monday before the cT+mT regime to be felt in the major cities of the Northeast (re: thunderstorms and frontal structure nearby), the sulfurous area of heat and very high humidity will be on the move during the 48-72 hour time frame. The +100 deg F readings noted in the western third of the nation could make an appearance in places like Birmingham AL, Bardstown KY, and Defiance OH as early as June 25.

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


NUMEROUS Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
S AB....S SK....S MB....C ON....Upper MI

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
C CO....C, E NM

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
MO....SE IA....S WI....IL....W IN


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


Isolated Locations In
C CO....C, E NM....TX....LA....MS....N, C AL....W, C GA....W, C TN....AR....OK....E KS
MO....SE IA....S WI....IL....W IN....W KY
(QPF 1-2")

Scattered Locations In
S AB....S SK....S MB....C ON....Upper MI
(QPF 1-3")


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

Numerous Locations In
CA....NV....AZ....UT....S ID....MT....WY....CO....NM....W TX....W OK....W, C KS....W, C NE
W, C SD....W, C ND....S SK

Isolated Locations In
LA....MS....AL....N FL....GA

MEDIUM RANGE OUTLOOK
(weather trends through the next 6 to 10 days)

The apex of the heat ridge over the lower 48 states will be on Tuesday, when most of the nation is under the subtropical high (588dcm or higher at 500MB) and core values of the anticyclone will be rivaling historic levels seen in historic hot spells of August 1976, July 1995, and July 2006. Those living in the Midwest and Northeast may be tempted to think "not such a big deal", since coverage of those regions by the ridging will probably last no more than three days. But if you factor in the extremely high surface humidity (it will be a dirty ridge, with the Great Plains weakness acting as an enhancer for moisture from the Gulf of Mexico), and the notion that the abnormally torrid conditions may persist in the Dixie and Intermountain States through AT LEAST July 3rd (and probably much longer), then you come to see just how special this event is.

Much of the Midwest and Old South has been in the "hot and dry" category for a while now, with only occasional interruptions from rain or cP regimes drifting out of the north. In fact, even before Day 3, when the true Sonoran "hot blast" starts to circumvent the persistent weakness across the national heartland, readings may exceed 95 deg F in a good portion of Dixie. The action of a particularly strong but geographically small 500MB shortwave in the Prairie Provinces early next week should pump up 500MB heights through the Corn and Tobacco Belts into the Mid-Atlantic states. Even parts of ON, QC and New England should see at least one day (Tuesday) where maximum temperature records could be threatened. And with dewpoints climbing close to 70 deg F, power usage will be at high levels as people try to cool off.

Sonoran heat ridges typically undergo a slow retrogression into the western U.S. after a time, and upstream signals favor this trend occurring late next week. One of the impulses in the polar jet stream over the northern Pacific Ocean is forecast to slow and deepen in the Gulf of Alaska as a lateral Rex block in Greenland and Baffin Island attach to a small +PNA signature in AB and YT. This will allow very cool air to pool across E MB, ON, and QC, and the cP regime will follow a procession of three shortwaves helping to form a trough complex in eastern Canada and the lower Great Lakes. Intense thunderstorms should progress from the ON....MN Boundary Waters into IL....IN....OH....ON Peninsula....NW PA....W NY at midweek, with the risk of intense convection slowing and then settling into the Ohio and Tennessee Valleys, middle Appalachia, and the Interstate 95 corridor during Thursday and Friday. Not until the last shortwave in the series moves through NY and PA on Day 9 will the chill Canadian air reach its farthest south destination (close to Interstate 40, or Memphis TN to Wilmington NC).

The ridge complex will then lock in across the western half of the U.S., with a core at or above 595dcm at 500MB near Denver CO between June 30 and July 3. The ridge axis will connect with the positive height anomaly in northern and western Canada, enabling drainage of the cP domain into much of the Midwest and Eastern Seaboard (interior NY and New England could see record low temperatures during the nights of Day 8 and 9). The regime will be highly subsident and stagnant west of the Missouri River, meaning critical pollution and heat issues for the populous Salt Lake City UT and Denver CO metro areas into the Independence Day holiday. Points west of the Continental Divide may get into a cooler mP regime, but the parched Desert Regions will have no escape from the ongoing drought and excessive heat (yes, it is hotter than normal in the desert these days....). If NAEFS, GGEM, and GFS longer range data verifies, the heat ridge will make another incursion over and east of the Great Plains in about two weeks from now.

TROPICAL WEATHER OUTLOOK
(important features over the tropical sections of the Atlantic and Pacific Basins)

There are three main reasons why tropical development will be hard to come by in either the Pacific or Atlantic theaters during the next week or so. One is the dominance of west or southwest flow aloft, evident on images of the GOES W, GOES E and METEOSAT satellites. Between 40 W Longitude and the International Dateline, shearing flow can be found anywhere between the Arctic Circle and the Equator. Since most warm-core cyclogenesis occurs and translates east to west, this opposing wind pattern destroys incipient circulations and thunderstorm clustering needed for a tropical cyclone to organize.

This is not to say that the 2007 Hurricane Season will be a dull one like 2006 was. In fact, the parade of ITCZ waves moving through sub-Saharan Africa has been quite impressive, with both mesoscale and synoptic-scale impulses with clearly defined circulations in evidence for the past three weeks. And while the strength of the cTw regime and anticyclone in the Sahel is greatly limiting chances for intensification and growth of the disturbances, the ridge is expected to continue a northward expansion which will allow for less dry, dusty air to advect into the tropical waves. So during July, we should not assume that emergence of convective clusters off of the coast of Sierra Leone should be interpreted as a death warrant for those waves.

I am concerned about the SST configuration over the Atlantic Basin, however, since we know that any formative tropical cyclone needs values above 79 F for the convective engine to accelerate. Readings over the Gulf Stream are abnormally cool, and even in the best case scenario it will be about three weeks before a tropical depression or storm could be sustained by traveling over waters to the right of the U.S. But I reiterate that we have already had two designated tropical storms, and as ridging takes over across the lower 48 states next week the deep mean easterlies could be carrying some sort of tropical surprise into FL and the Deep South.

As the late Karen Carpenter once put it, "We've only just begun...."

Prepared by Meteorologist LARRY COSGROVE on
Saturday Morning, June 23, 2007 at 3:35 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

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