WEATHERAmerica Newsletter, Saturday, March 1, 2008 at 6:20 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)

SUNDAY


MONDAY



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

Isolated Locations In
TX....OK....KS....NW MO....S IA
(QPF 1 - 3")

Scattered Locations In
Coastal BC
(QPF 1 - 3")


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

Scattered Locations In
W, C BC
(Snow, 4 - 24")

Scattered Locations In
E SK....MB....W ON....NW MN
(Snow, 4 - 8")

Scattered Locations In
ID....W MT....W, C WY....W, C CO....E, C UT....NE AZ....N NM
(Snow; 4 - 8")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Effects Of Major Winter Storm, Now Through Wednesday



The huge storm now taking shape over the "Four Corners" Vicinity offers many forecast challenges during the course of the next 96 hours. Accompanied by a major shift in temperature from warm to cold, this low will have a multi-faceted precipitation shield. Some locations starting out with a warm, convective setting may shift suddenly to cold and snow, particularly along the immediate track of the disturbance (see map above). Other communities are in for anything from flooding rains to near-blizzard conditions.

An area of snow organizing over the Intermountain Region will follow the north side of the main path of 500MB vorticity into the central Great Plains on Sunday. The cold and unstable air mass accompanying the digging, closing trough axis will allow for locally strong thunderstorms, the chief issue being downbursts which could be accompanied by small hail along with snow or sleet. Using the trajectory predicted by the most recent ECMWF, GFS Ensemble suite, and NAM versions (the GFS Operational model seemed off kilter with its surface to 500MB alignment), the axis of heavy snow may shrink to about a 200 mile corridor through the heart of the Corn Belt and lower Great Lakes. It would not surprise me if many locations in the designated area started as rain, given the scenario of a cold frontal passage before arrival of the main disturbance.

Severe thunderstorms should begin to fire along the convergence zone between cPk, cT, and mTw air masses in early afternoon on Sunday. There may actually be a fairly widespread episode of strong convection, as the frontal structure will be accompanied by mesoscale vorticity maxima and somewhat unstable air from S WI into TX. But where the strongest advection of vorticity meets the most buoyant regime and parceled strong vertical motion, over parts of TX, OK, and KS, tornadic supercells may form. It is interesting to note that the numerical models keep up the convective threat overnight and early Monday through TX, implying a sizable microburst, large hail, and flash flood threat in the Lone Star State until the low and cold front pull to the north and east before noon on March 3.

I suspect that the lagging cold dome at upper levels will produce a changeover to snow immediately to the left of the storm track. So some locations in the Mid-South and Appalachia may get see a surprise quick hit of wet flakes after a period of rain and thunder. Severe thunderstorms will continue to be an issue from the lower Mississippi Valley on Monday into much of the Southeast (intense upper dynamics running into a warm and humid regime) along with heavy rainfall.

Warmth For The Southeast

One interesting side the deep storm reaching lower latitudes in the west and central states will be to establish a connection of warmer and more humid air. The position of the surface and 500MB ridge is also favorable for a southerly component to infiltrate as far north as the Mid-Atlantic region. So communities to the right of the storm track can probably expect about 48 hours of above normal (if not much above normal) readings precedent to the low and cold frontal passage.

Not One, But TWO Intrusions Of Cold Air


Despite its apparent strength, the disturbance now organizing over the West will not establish a long-lasting shot of colder temperatures to the Old South. Instead, after a 24 to 36 hour fall in temperatures, milder values will begin to work northeastward from Mexico into Texas and the rest of Dixie. A cold front will form, however, from the High Plains to the Virginias, however, and this thermal boundary should be on the move southward by the start of the medium range, bringing with it a longer-lasting chill more reminiscent of January than of March.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Is The "Pacific Storm Express" Ever Going To Quit?

If you have been following satellite imagery this winter, you will have known that the past three months are NOT typical of La Nina episodes. Reason being that a strong subtropical jet has accompanied a series of very powerful storms from central Asia into the lower 48 states. The "usual" -ENSO state is fairly mild and dry along and below 40 N Latitude. And colder air would stay entrenched only in the very northern tier with few "breakouts" into California and the Old South. Instead, the interaction of the lower latitude windstream, much like that in an El Nino year, has allowed disturbance to maintain or even to add on to intensity and deliver some impressive, albeit short-lived, cold intrusions which in a few cases have reached far into Mexico.

Current information from INDOEX, MTSAT, and GOES WEST satellites would suggest that the trend of the winter is not letting up just yet. Five impressive cyclonic signatures are visible, and each of these impulses shows up in numerical model data through Day 16. Note the emergence of jetlets just above the equator (the STJ connection again), and the linkage of the tropical convective complex over Indonesia and Malaysia to the polar westerlies continues as well. It is this phasing with energy associated with the Madden-Julian Oscillation that has created the volatile environment in North America during the past 90 days. And while the next system in the storm sequence around Day 9 and 10 (now exiting Japan) may take a more northern route into the Ohio Valley, I fully expect the routine of extreme weather followed by brief cA intrusions to continue through at least the third week of March.

Souther Tier Of U.S. Eyes Return Of Normal, Or Warmer, Temperatures

While the general alignment of the jet stream configuration through 240 hours can be classified as a +PNA type, the core of the polar westerlies may migrate far enough north for a respite for cold intrusions in the Old South after Day 7. The Midwest and Northeast, however, near to the ever-present Hudson Bay Vortex, are likely to continue in a figurative "icebox" through the next ten days.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Keeping in mind the above comments about the storm sequence emanating from central Asia into the U.S., the middle of March will probably be quite active in terms of both severe weather (lower Great Plains and Old South), heavy rainfall (lower Missouri and Ohio Valleys into the Mid-Atlantic region), and frozen precipitation (Intermountain Region, northern Great Plains, Midwest and interior Northeast.

Several areas of concern at this time of year are flooding and ice jams (intermediate warm-ups melting the enormous snowpack over the northern third of the U.S.) and continued cold intrusions (note the deep negative anomalies attendant with each storm and the mean trough axis that straddles the Great Lakes, The GFS and ECMWF ensemble suites show two prominent storms, both of which may follow a track from Colorado to Kentucky, then off the New England coastline.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, March 1, 2008 at 6:20 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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