WEATHERAmerica Newsletter, Sunday, May 4, 2008 at 7:10 P.M. CT

TODAY'S FUN LINKS: Anniversary Of Greensburg KS Tornado

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)
C, E NM....TX....W, C OK....W, C KS

ISOLATED Severe Thunderstorms
(Microbursts, Waterspouts)
Coastal SC, NC

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
CA....S NV....W AZ

(Tuesday)
SCATTERED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
N, C TX....OK....KS....E, C CO....NE....SW IA....W MO....W AR

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
SE IA....C IL....C IN....OH....W PA

(Wednesday)
NUMEROUS Severe Thunderstorms
(Microbursts, Large Hail, Tornadoes)
N, C TX....C, E OK....C, E KS....C, E NE....IA....W, C MO....W AR....NW LA

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
C, S WI....C, S Lower MI....ON Peninsula....SW NY....W PA....N OH....N IN....N IL

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

Isolated Locations In
C, E NM....TX....W, C OK....W, C KS
(QPF 1 - 4")

Isolated Locations In
Coastal SC, NC
(QPF 1 - 2")

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

Isolated Locations In
S CO....NM....W, S TX

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

More Heartbreak For Mid-America: Severe Weather Outbreak May 5 - 7, 2008

This has been a tough year, with far more tornadoes than average for the U.S., and an upswing in fatalities related to the twisters (77 so far for 2008). And the news is not good, particularly for the areas hardest hit by the storms during the winter and spring. While there is some disagreement between the GEM, NAM, and GFS model suites on just exactly what locations will be hardest hit, one thing is glaringly clear from even the most casual look at the computer outlooks. That is, a widespread outbreak of severe thunderstorms and tornadoes will hammer the Great Plains, Midwest, and parts of the Old South during the next 72 hours.

Moist advection is already getting underway form the Gulf of Mexico into the south central U.S., the only disruption on an otherwise quiet weather map. They key feature of note is the closed low now swirling over the Southwest. With a formative subtropical jet stream and energy incoming from a strong disturbance over the Gulf of Alaska (which is undercutting a Rex signature over the Prairie provinces), the low will slowly move eastward and set up a highly difluent, energy-laden flow axis from northern Mexico into the mid-Missouri Valley. The result of this motion will set up a vorticity advection pattern into a convergence zone (mP/cT/mT) that should result in developing convection from NM and TX north into KS on Monday. While temperatures aloft are expected to be too warm for meaningful hail formation, microbursts and heavy downpours in the lower Great Plains will set the stage for a far more dangerous event unfolding Tuesday and Wednesday.

The main body of the upper low should pass from S AZ into the TX/OK Panhandle Region on Tuesday. Very unstable air from the Bay of Campeche will intersect the incoming cold core and dry tongue, with increasing supercellular thunderstorm activity from N TX into NE. The warm frontal structure may activate, sending strong convection into the Corn Belt. Heavy rainfall is also a consideration with this system, of course deluging flooded communities that do not need any additional moisture.

Wednesday may well bring about an awful day filled with tragic consequences of the upper level low emerging from the Rocky Mountains and slamming head on into a super-unstable cauldron of clashing air masses. The most recent GFS scheme points toward KS and N OK as the strike zone for multiple, intense and long-lived tornadoes; the NAM version seems to point toward C, S OK into N TX, while the GEM family of equations is a bit more progressive, targeting an are from the middle Missouri Valley into W AR and E OK. If you take an average of the three schemes, the activity of May 7 will probably have the greatest impact in a triangle from Salina KS....Fort Smith AR....Stephenville TX. Since the low aloft will be compressed somewhat by sliding under the weak Canadian block, the corridor of convective development may be fiercest over a concentrated area and not the more spread-out display seen by the Canadian model output.

The Coming Battle: Cold In The Pacific Northwest; Heat Over The Southwest

A trend has been noticed in the past 30 days where heat dominates the Southwest, but the Pacific Northwest and northern Rocky Mountains stay much colder than normal. This pattern looks to continue through the next sixteen days, producing a stark thermal contrast over the western states once the upper low vacates the Great Basin and lower Colorado River Valley. The temperature gradient may also provide a "railroad track" effect which will allow the storm sequence over the Pacific Ocean to move along into the High Plains.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Late Season Nor'Easter?

Don't laugh. Nor'easters can occur at any time of the year, and the atmospheric set-up as visualized by the most recent GFS, ECMWF and GGEM model data is quite favorable for a large, cold storm to affect the Mid-Atlantic and New England states late in the medium range. With a blocking signature taking shape over the Davis Strait, teleconnections indicate a deep trough and closed low centered near Long Island NY by Day 10. The operational European model output may have the correct idea with a deep low center crawling up the shoreline from the Delmarva Peninsula into RI and MA. If this scenario verifies (about a 50/50 chance at this point), colder than normal temperatures and heavy stratiform rain, with wind, will dominate the Interstate 95 corridor above Richmond VA around May 12 -14.

Mexican Heat Ridge Is A Very Important Feature....

A recurrent them in the operational forecast models and the respective ensemble families is a positive height anomaly, core values of around 590dcm at 500MB, that alternates between Chuhuahua State of Mexico and the Gulf Coast. As this ridge may build northward with summer, heat and dryness should follow. Typically, subtropical highs in this position migrate into the southwestern U.S. as summer approaches. But with a weaker ridge signal evident over the southwestern Sargasso Sea, the possibility looms for a dual ridging complex that brings searing heat to both the West and portions of the Old South later this month and in June.

....As Is The Emerging Davis Strait Block

While heat is on the minds of many with the approach of summer, the formation of a blocking ridge over the Davis Strait after Day 8 is a signal for unusually cool weather in the Great Lakes and Northeast. Not only is such a ridge complex linked to storm activity (see above) from VA northward, but the positive height anomaly creates an unusual "inverse" temperature pattern that can produce record warmth in Quebec vs. unseasonable chill from MI into the Mid-Atlantic states and New England.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Hyper-Active Storm Track STILL In Play Over The Pacific Ocean

One of the perplexing trends of the spring of 2008 has been the unrelenting advance of major storms from central Asia all the way into the West Coast of North America. These disturbances have deep connections to the equatorial regions, a trend seen since November of 2007. The presence of subtropical jet streams helps to accentuate a thermal imbalance about the impulses, with very warm and moist air followed by chilly mPk regimes. As each of these systems advances into the U.S., more severe thunderstorms and brief cold snaps can be expected through the next sixteen days.

Heat, And Drought, Loom For Western, Southern Tiers Of U.S.

A troubling signal for extensive heat and drought is the tendency for low soil moisture to continue over much of the western and southern portions of the U.S. In many cases, the dryness of the topsoil is an indicator of the coming of ridge formation. Computer forecasts through May 20 indicate little deviation of the trend for little rainfall in the Southwest, High Plains, and sections of the Old South.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, May 4, 2008 at 7:10 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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