WEATHERAmerica Newsletter, Sunday, March 9, 2008 at 9:10 P.M. CT

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)
TX....LA

STRONG WINDS
(Pressure Gradient Derived)
W BC....W WA....W OR


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

Isolated Locations In
TX....LA
(QPF 1 - 2")

Isolated Locations In
Coastal BC, WA
(QPF 1 -2")


FROZEN PRECIPITATION OUTLOOK
(potential for accumulation of ice rime, freezing rain, sleet or more than 3" total snowfall within the next 24 hours)

Isolated Locations In
E KY....E OH....WV....W PA
(Snow, Sleet, Freezing Drizzle; 2 - 4" with Glazing In higher elevations)

Isolated Locations In
N SK....N MB....N ON....NW QC
(Snow; 3 - 6")

Isolated Locations In
BC....WA
(Above 6000 Feet; 4 - 12")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Diffuse Upper Air Disturbance Targets Texas And The Deep South

The atmosphere over the western and southern portions of the U.S. is trying to warm up, but there are some impediments to a rapid return to Spring. The upper level low over NM and TX is an example of how cold air aloft and returning moisture can combine to stop heating. This impulse is weak, but has a moisture feed from the Gulf of Mexico that creates cloud cover, thus putting the brakes on moderation. This system will continue to trigger showers and (a few intense) thunderstorms from the Lone Star State through the Gulf Coast during the following 48 hours before being absorbed in a shortwave sequence dropping through the Midwest on Day 3.

Warming Across The Western States (For Now)

The start of a REAL warming trend will be in evidence over the southwestern states during the near term. Air from the Mexican Cordillera will be lifted northward behind high pressure and ahead of an oncoming storm complex entering the Pacific Northwest. This cT regime will then spread into the Great Plains behind the aforementioned upper disturbance, with the 80 deg F isotherm entering KS on Day 3.

Midwest And Northeast Go From Cold, Snowbound To Cool And Partly Cloudy

The proverbial good news: the huge snow event that struck the Midwest and interior Northeast this past weekend is long gone, and temperatures are on an upward swing through the next few days. That said, the moderation will be of a limited nature, bringing readings to seasonal normals. And potential for nuisance snow, sleet and rain will continue during the near term, as several impressive but moisture-starved 500MB shortwaves dig southeastward from the cAk vortex over southern Nunavut AR.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
Impressive Storms From Pacific Ocean Sequence Will Yield Weather Extremes



The storm that is forecast to hit the lower 48 states during the period adjacent to St. Patrick's Day (March 14 - 18) looks to be a doozy. Visible now on GOES WEST imagery below the Aleutian Islands, the disturbance will expand and dig into the West Coast before finally redeveloping near Trinidad Colorado on March 16 (secondary lee cyclogenesis). The path of this system will almost certainly take the southern variant of the track type, as there will be blocking taking shape downstream in the -NAO position. If the numerical model depictions of the storm and its structure verify (see above), the chief danger will be a widespread severe thunderstorm outbreak targeting parts of TX....OK....S KS....S MO....AR....LA....MS....AL....GA.....TN....S KY around the 156 - 204 hour time frame.

There are other strong impulses waiting in the wings, and the MTSAT Full Disc image serves the purpose of showing the still-vigorous equatorial moisture and energy connection to the polar and Arctic jet stream coming off of Asia. As long as this linkage remains viable, the potential exists for repeated and intense midlatitude cyclones to strike the U.S. with volatile swings in temperature.

Lots Of Cold Air, But Heat Will Make An Appearance Along The Southern Tier

The above temperature forecast map is an important reminder that summer-like heat will be making more and more inroads into the southern tier states as we move into and through the spring season. The GFS and ECMWF schemes have consistently advertised the possibility of +90 deg F (and maybe even 100 deg F) maxima over parts of W TX precedent to St. Patrick's Day. The huge low coming into the Great Plains will pull up a hot and dry cTw regime from northern Mexico, opposed by incoming mPk and mTw values and displacing the retreating cPk domain. So in addition to the "roaring furnace" winds in the lower High Plains and sweltering heat along the western Gulf Coast, the "fuel" for a severe thunderstorm "fire" will be present from the Interstate 35 corridor into the Deep South and Tennessee Valley in seven or eight days.

Blocking Signatures Return To The 500MB Longwave Pattern

Yes...It's back! It has been a long time since we have seen a strong blocking signature in the NAO position. But there is solid agreement among the various numerical models on the evolution of a positive height anomaly between the Grand Banks and Greenland. Teleconnections on this Omega block favor colder than normal temperatures across the Great Lakes and Northeast. Further south, with the re-emergence of the Cuban heat ridge, the Deep South will be prone to huge swings in air masses and severe weather as storms are forced to take a more eastward path, collapsing the subtropical high.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

How Long Will The -NAO Blocking Signature Last?

No blocking configuration lasts forever, and those that have occurred in recent months have tended to be short-lived. But it is interesting to note that the -NAO signature showing up late in the medium range is still visible in GFS and ECMWF ensemble packages through Day 15. With higher than average 500MB heights in the vicinity of Labrador, Greenland, and over the Grand Banks, any storm entering the Northeast would likely a) deepen and take a track below Long Island NY. So a colder solution favoring mixed or changed precipitation rather than "all rain" outcomes may be a better forecast bet for communities above the Potomac River between March 20 and 25. And with the active series of storms getting an excellent pull of mTw values form the Gulf of Mexico behind the Cuban heat ridge, outbreaks of severe thunderstorms in the lower Great Plains and Old South are probable in the extended period.

What About Severe Weather Season?

Because of the continued size and intensity of the storm sequences emanating from central Asia, the potential for large-scale outbreaks of severe thunderstorms and tornadoes will probably be ongoing through the first half of the spring season. Since we know from climatology that the disturbances will take more northward variations on current track scenarios, then it is logical to expect that the position of intense convection will gain in latitude over the next 90 days. Surface convergence, plentiful vorticity, difluence and steep lapse rates have been the trademark of the severe weather outbreaks during the winter. And with the increasing sun angle, warmer and more unstable air within the lower right quadrant of the cyclones can only increase an already dangerous potential for hail, high winds and tornadoes.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, March 9, 2008 at 9:10 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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