WEATHERAmerica Newsletter, Sunday, January 20, 2008

[Larry_Cosgrove]

TODAY'S FUN LINKS: CoCoRaHS - Community Collaborative Rain, Hail & Snow Network

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
Some Thunderstorms May APPROACH Severe Limits
S TX

Some Thunderstorms May APPROACH Severe Limits
S FL


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

Isolated Locations In
CA
(QPF 1 - 2")

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

Isolated Locations In
S FL
(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
N, C CA....N NV
(Snow; Above 2000 Feet; 4 - 12")

Scattered Locations In
UT....N CO....N WY....NE....S SD....IA....S MN....C, S WI....N IL....Lower MI....S ON
(Snow; 4 - 8")

Isolated Locations In
KS....MO
(Snow to Sleet, Freezing Rain; 1/8" Glazing Possible)

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Misery For The West Coast

The pool of unstable cold air and vorticity over N CA will either stay put or sag to the south during the course of the near term. This disturbed area means a prolonged period of chilly, damp weather for most of California. Mountain snowpack will build as much as 10 feet by the time the low exits on Day 6, and thunderstorms with heavy rain and hail are almost a certainty for the more southern sections of the Golden State. Since no kicker is evident on satellite east of the International Dateline, sunshine and warmth will be a sparse commodity from San Diego to Ukiah this week.

Another Shot Of Bitter Cold Coming Up!

The -EPO signature is still strong enough to channel broad northerly flow into Canada and the U.S. Shortwaves in the Yukon and Northwest Territories will drop into the Great Plains and points eastward during the next 72 hours. As a result, the current cold dome will be replaced by another Arctic intrusion on Day 3. Numbing cold, snow showers and gusty winds will be the rule for the Midwest, Northeast, and Mid-South at midweek, with only the southernmost reaches of Texas and Florida escaping the force of the cAk domain.

Subtropical Jet Stream Keeps The Southern Tier Of U.S. Miserable

Funny, this was not supposed to be an "El Nino" winter! While we have a pronounced La Nina signature in SST anomaly charts over the equatorial Pacific basin (albeit one which is oriented over the western and central sectors), the very active subtropical jet stream is again gathering moisture and energy and taking aim at northern Mexico and the southern U.S. besides feeding disturbances at higher latitudes, this banding of stronger winds will bring another round of heavy precipitation to the now-clear Deep South within the next 36 hours and beyond.


MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Active Storm Track From Southern Great Plains To Northeast

The combination of energy exiting California and a digging impulse from the Gulf of Alaska will spell trouble for much of the nation during the course of the medium range. After inundating (and chilling) the western third of the U.S. in the next five days, low pressure will be kicked eastward as a new and stronger trough with origins over the western Pacific Ocean. From this distance, it would appear that the correct solution for track and apparent weather lies somewhere between the scenario of the 12z GFS (S TX to southern New England) and that of the concurrent ECMWF run (E CO to the Great Lakes, with redevelopment over the Gulf Stream).

My concern is that in a highly volatile 500MB longwave pattern, conditions could change to favor a colder, icier development where two or three impulses eject instead of just one big cyclone. The extreme warmth over the Dixie states tells us that this feature will have a convectively active warm sector, as well as the potential for overrunning-born heavy rain and ice in some sections of the Great Plains and Midwest. The time frame is somewhat sketchy, but January 26 - 30 seems as good a period as any that would be vulnerable to the broad disturbance.

Warmer Temperatures In Store For The Deep South

We have not had many "constants' in this crazed winter of 2007-2008. But there is almost universal agreement among the numerical models covering the medium range that some outrageously warm temperatures will set up from TX and NM into the Ohio Valley and Old South between January 26 and 28. At a time when the trough is collecting over the western U.S., the strong southwesterly flow stretching from the lower Great Plains into the Mid-Atlantic states may send temperatures to astonishingly high levels along and below the Interstate 40 corridor. It is not out of reach for daily maxima to surpass 80 deg F in parts of the Lone Star and Bayou states on Days 6 and 7.

Cold Air Locks In From Intermountain Region Through The Upper Midwest

Just as a massive warm-up is likely in the southern and eastern reaches of the U.S. at the start of the medium range, a deepening of colder values seems probable in the western and northern parts of the country after January 28. The passage of an immense winter storm should carve out a new mean 500MB trough in the Great Plains and Intermountain Region, with deepening of the circumpolar vortex over northern Canada acting to push colder air into the Upper Midwest and Great Lakes. While the milder readings will be suppressed somewhat, the Arctic domain is not likely to expand into the Deep South, owing to that region being close to the persistent subtropical high over Cuba and the Bahamas.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Expansion Of Baffin Island Vortex Is The Key....
One signal which warns of very turbulent conditions is the southeastward shift of the circumpolar vortex from near the North Pole at the end of the medium range to a broad gyre covering Baffin Island and much of Nunavut AR. The ensemble forecasts of the GFS and ECMWF indicate that the motherlode will attach to a mean trough complex stretching from NM and TX into ON by Day 15. If this scenario verifies, a baroclinic zone is likely to set up from the western Gulf of Mexico into New England. This strong boundary would separate very warm and humid air across the Southeast and coastal Mid-Atlantic states from a moderately cold regime encompassing the western two-thirds of the U.S. and most of Canada (excepting Newfoundland and the Maritime Provinces). The transition area would then be a favored track line for storms of the Galveston Bay Spin-Up type, favoring heavy rain and ice in Appalachia and the Eastern Seaboard as well as important snowfall in much of the Midwest and Great Lakes between January 31 and February 5.

....Along With Pacific Ocean Storm Sequence

The satellite image attached shows the rather intense storm sequence stretching from just to the right of the International Dateline back into Mongolia. Note also the convective array from the Philippines into Oceania, an indication that the Madden Julian Oscillation is still a factor in (possibly) amplifying the polar westerlies. If the impulses over the western and central Pacific Ocean continue to tap energy from the disturbed weather in the equatorial regions, then a rather inclement, if not colder, forecast is in store for the lower 48 states as we enter February.

Growing Snowpack, Weakening La Nina Translate To....


These two parameters would seem to favor a colder and stormier end-of-winter pattern. Consider that the snow cover across North America is at its farthest south point in the past few years (I even saw some sleet at my home near Houston TX the other night...), and that expanse of ice and snow serves as a reflective blanket that aids in cold formation. And with the very frayed appearance of La Nina in its eastern sectors near the Galapagos Islands, the forecast gradual demise of the -ENSO signature in late winter should enable a greater-amplitude, colder/snowier weather trend during the last three or four weeks of calendar winter.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, January 20, 2008 at 6:15 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 2007 by Larry Cosgrove

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