WEATHERAmerica Newsletter, Sunday, May 20, 2007 at 6:45 A.M. ET

[Larry_Cosgrove]

Severe Weather, Flooding Rains Likely In Parts Of Great Plains, Midwest this Week; Tropical Disturbances May Play A Part In The Forecast For The Dixie States

(Disclaimer And Copyright Statements At End Of The Discussion)

WEATHER SUMMARY THROUGH NEXT 72 HOURS


The near term forecast can best be described as a storm chaser's dream, but the average homeowner's nightmare. I say this because this is the height of the chase season, and nature will be providing a spectacular show of convection this week, starting with the High Plains today and culminating in a prolonged siege across much of the Midwest by Days 3 and 4. The prospect for damage, either from thunderstorm effects or from flooding, looms large in this outlook however, which cannot be good news to communities hard hit by excessive rains and storms about two weeks ago.

Part of the concern about severe thunderstorm potential arises from the fact that a very strong disturbance is entering BC and WA from the Gulf of Alaska. Formerly a stationary mPk vortex, this feature has abundant cold air and vorticity aloft and will be running into a wide area of three regime surface convergence (cP, cT, mT) starting this afternoon. The core of the trough complex will likely not be in place over the upper and middle Missouri Valley until Day 3, so we should see a gradual "ramping up" of intense convection during the course of the next 72 hours.

The surge of colder air should gradually push across the Intermountain Region, with hail and microburst threats mainly in the Pacific Northwest and northern/central Rocky Mountains through tomorrow. Some intense cells could form along a dryline in the High Plains today and Monday. In the arid regime west of the dewpoint discontinuity, covering parts of the Bonneville Basin and Desert Regions, temperatures may exceed 100 deg F through the next three days. A truly dangerous threat emerges on Tuesday when the trough hits the aforementioned "Danger Zone" of colliding air masses. A corridor from N OK into SD will be at highest risk, with latest NWP data suggesting that the best chance for tornado strikes would be along and perhaps 200 miles either side of a line from Concordia KS to Valentine NE on the afternoon and evening of May 22.

While the Northeast has seen some unseasonably chilly, raw conditions this weekend (daily maxima in the 40s F, lots of rain and showers), the pinwheeling low pressure center east of LI NY should begin to pull northeastward this evening. The cold, unstable nature of the regime, with a spinning environment, could actually help to trigger some thunderstorms with hail in parts of NJ, NY, CT, RI, MA and VT during your Sunday afternoon.

But a change is coming as the trough complex in the western states helps to "pump up" 500MB heights to the right of the Mississippi River. Drawing in a weakness through the Old South, the incoming trough will consolidate moisture while allowing for a more stable, dry domain to take hold from Appalachia into the Eastern Seaboard on Day 3. And while the ensuing anticyclone should not be considered a true "heat ridge" (588dcm core at 500MB), temperatures will warm considerable across the Corn Belt and Mid-Atlantic states, with the 80 deg F isotherm actually poking up into ON and NY on May 22.

__________________________________________________ _________________________

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

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
W TX....NM....OK Panhandle....W KS....CO....W NE....W SD....ND....S MB....S SK....MT
ID....WY

Some Thunderstorms May APPROACH Severe Limits
PA....NJ....NY....CT....RI....MA....VT

ISOLATED Severe Thunderstorms
(Microbursts, Waterspouts)
S FL


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

Isolated Locations In
W TX....NM....OK Panhandle....W KS....CO....W NE....W SD....ND....S MB....S SK....MT
ID....WY
(QPF 1-2")

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

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

The forecast through the end of the month has some positively scary overtones. In the sense that not only is there the "usual" spring season concerns of severe weather and flooding, but also due to possible interplay of tropical features which normally would not be an issue for at least another month. Complicating the outlook is a distinct +PNA ridge configuration which will help to solidify cold air drainage from the northern reaches of Canada into a zone stretching from the Upper Midwest into the St. Lawrence Valley. So as storms undercut the ridge complex (starting in BC and WA and ending up in the Great Lakes and New England, the baroclinic zone figures to be convectively active during the following 240 hours.

As for the surge of heavy to severe thunderstorms expected across the Intermountain Region and Great Plains during the short range, the trough complex associated with the convection will make only gradual progress eastward. A deep oceanic low will form well east of the Mid-Atlantic shoreline on Day 3, tracking slowly northward into the Gulf of St. Lawrence with rain and wind increasing for parts of NL and the Maritime Provinces. With the elimination of this low center, the ridge along the Eastern Seaboard should start to break down. But enough of the anticyclone will remain in the western Atlantic Ocean to start a new process of ridge building, culminating in a gigantic subtropical high to the north of Bermuda by May 30 (595dcm core at 500MB). This is where the trouble starts, as the trough advances into the Ohio Valley and gets locked into a death struggle with the incipient heat ridge.

We could be looking at about three or four days of continued convective heavy rainfall for much of the Midwest before the trough settles into the lower Great Lakes and Ohio Valley around the 192 hour time frame. Then, with the ocean ridge building, a flow of deep tropical moisture will develop from the lowest reaches of the Caribbean Sea (near Trinidad and Tobago) into the Appalachian Mountains and Atlantic Coastal Plain. The numerical models have consistently painted a picture of repeated heavy rainfall from N VA into NY and southern New England between May 28 and 30. An x-factor here is the possibility, featured by the last few runs of many of the numerical versions, for a tropical disturbance to be drawn into the mix. Since there is absolutely no agreement among the various schemes on the point of cyclogenesis and strength of the system, it would be pointless to spotlight track and intensity scenarios. But if such an impulse does take shape, the possibility for enhancement of heavy rainfall in parts of Dixie and the Interstate 81 and 95 corridors will have to be looked at.

The satellite views of the Pacific Basin show the upcoming threats for North America. A sequence of three storms will probably track through southern Canada from now through the first week of June. Notice the possible interaction with Typhoon Yutu and a new tropical ave in the vicinity of the International Dateline; both systems could energize the polar westerlies, speeding up the already fast flow and bringing a vigorous storm into the Pacific Northwest and upper Missouri Valley late in the medium range. Of particular interest is a new feature over Szechuan province in China which, if it merges with the new warm-core low could form an unseasonably cold, strong mPk vortex below the Aleutian Islands before the end of the month. Such a gyre, if it verifies (and ensemble means have been calling for its formation for about six days now...) would pump up the +PNA signature and lead to two major consequences for apparent weather in the lower 48 states: record cold temperatures in the Upper Midwest and a calamitous severe weather and flood episode from the Ohio Valley into the Mid-Atlantic region.

Stay tuned!

Prepared by Meteorologist LARRY COSGROVE on
Sunday, May 20, 2007 at at 6:45 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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