Weatheramerica Newsletter, Saturday, December 13, 2008
LarryCosgrove | December 13, 2008 at 05:20 pmby
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SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
(Pressure Gradient Derived)
HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)
Isolated Locations In
C, E ON
(QPF 1 - 2"; Some Freezing Rain Possible)
WINTER WEATHER OUTLOOK
(potential for minimum temperature to drop below 10 deg F, or for excessive/dangerous frozen precipitation events within the next 24 hours)
Isolated Locations In
(Light Snow, Sleet, Freezing Mist)
Scattered Locations In
W QB....C ON....N, W MN....SD....ND....MT....WY....NE Panhandle....CO....NW NM
N AZ....UT....N NV....ID....WA....OR....N CA
(Snow, 2 - 24"; Near-Blizzard Conditions; Some Ice E SD into N MN)
TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Equatorial Pacific Ocean)
Western Pacific Ocean
Tropical Storm Dolphin is moving northward, passing the island of Luzon well to the east. Dolphin will likely become a typhoon but recurve northeastward, thus avoiding interception with Taiwan and Japan. This feature may energize a frontal structure now entering the western Pacific basin from the People's Republic Of China, thus having implications for amplification of the jet stream over North America (and with it another huge cold intrusion) during the medium range.
SHORT RANGE OUTLOOK
(Through The Next 72 Hours)
Massive Blizzard, Cold Wave Envelop The Northern Intermountain Region
The channeling of bitter cold Arctic air from Canada into the Pacific Northwest and northern High Plains is underway, and will culminate with the freeze line reaching as far south as the suburbs of the Los Angeles CA and Las Vegas NV metro areas by Sunday night. With no less than three strong cyclonic circulations present over the West at 500MB, the potential for heavy snowfall, ice, and strong winds will be present. The worst of the winter storm conditions (near blizzard) will migrate from Montana and northern Wyoming across the Dakotas into northern Minnesota through Sunday. Much of central Ontario and western Quebec will also see significant snow along with the cold temperatures. Two branching-off areas of wintry precipitation will develop within the following 24 hours: from Washington into northern California (mostly orographic snows) and a smaller post-frontal patch from Colorado through much of Kansas and northwestern Missouri that may cause numerous traffic hazards Sunday afternoon and evening.
Rapid Warming Trend Gets Underway From The Lower Great Plains To East Coast
At the same time the Arctic regime is surging into the Great Plains, a much warmer (and more moist) air mass will emerge over Texas and begin to spread rapidly northeastward. The 50 deg isotherm may advance into Ohio and Maryland before retreating Monday, with showers and thunderstorms occurring along the approaching cold front.
New Surge Of Arctic Air Targets Midwest By Monday
The orientation of the cold air dome will begin to turn more eastward early in the new week. Snow threats will diminish rapidly Monday across much of the West, while a dry cold settles in over the Midwest and Great Lakes. By Tuesday the lower temperatures will begin to overpower the moderation in the New England and Mid-Atlantic states, leaving only the Deep South in (relatively) mild temperatures.
MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)
Colorado/Trinidad Storm Track Dominates With Snow, Ice Threats For The Midwest....
The 500MB low which closes off over California during the next few days will be kicked out by a shortwave digging out of British Columbia. Starting with a small but heavy precipitation event over the Golden State, late next week, the low pressure center will redevelop over southern Colorado around December 19. Taking a track fairly close to the Colorado/Trinidad "A" route, the disturbance will tap moisture from the Gulf of Mexico, creating heavy snow, ice and rain over much of the Midwest before reaching the interior Northeast. There is a fair chance that cities such as Des Moines IA, Minneapolis MN, Chicago IL, Milwaukee WI and Detroit Mi could be impacted by significant snow and ice accumulation. But the path of the cyclone and southwest flow aloft would seem to argue against important frozen precipitation in much of the Interstate 95 corridor (although the New York NY and Boston MA metros could see a period of ice and snow early in the process next weekend).
....While Bitter Cold Dominates From New England through The Intermountain Region
The presence of so much Arctic air virtually guarantees a colder-than-normal medium range over much of the U.S. A note of caution for those noticing the heat ridge over Florida and the Gulf of Mexico! Many of the numerical models and ensemble members weaken that feature in the 6 - 10 day range, with a huge block of cold settling in behind the Colorado/Trinidad cyclone mentioned in the previous paragraph. The worst of the cold will generally be felt along and north of 45 N Latitude. That said, examination of the mean frontal position shows a tendency for a TX....N GA....SE VA axis to be favored. So many locations in the Mid-South, Ohio Valley and even the Mid-Atlantic regions may have lower than normal temperatures for much of the time between December 15 and 23.
Gulf Coast, Southeast May Miss Out On the Arctic Air
Although the European model seems to favor a more eastern-weighted cold pattern in the medium range, there is one feature which argues strongly for keeping the Deep South and even a good chunk of the Eastern Seaboard out of the Arctic air: the heat ridge seen by all of the numerical models over the Gulf of Mexico and Florida Peninsula. The subtropical high will act as a buffer to incoming cold fronts (while at the same time encouraging overrunning and freezing precipitation events in the Corn Belt and interior Northeast). There is some consensus, however, among the equations that the ridge complex will collapse entirely by December 21. That would lead me to believe that calls by the ECMWF ensemble members for a warmer extended period will not verify.
EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)
"Super Vortex' May Dominate U.S. With Cold In Last Week Of December
For some time now, the computer forecasts have been showing a classic "super vortex" or huge cAk gyre centered over southern Baffin Island. A normal cold dome in that position would favor cold only over the northern tier of the U.S. and most of the eastern two-thirds of Canada. But if you review the anomaly charts above, you can see the concern that the expansive motherlode will actually be tapping Siberian air and periodically expanding as far south as the Great Lakes and northern New England. The positive-tilt nature of the trough associated with this upper low will allow for shortwaves to travel along a route from TX into MD and DE, with periodic bouts with light/moderate freezing precipitation may be an issue in the Midwest and the Northeast in the "holiday week" between Christmas and New Year's Day.
Huge Flare-Up Of Madden-Julian Oscillation Over Indian Ocean, Indonesia
There is a break in the interaction between the huge flaring of the Madden-Julian Oscillation (seen from Indonesia into southern India). Therefore, an all-out amplification of the 500MB longwave pattern over North America is unlikely now through the next ten days. But if the Kelvin wave complex follows the pattern seen so far since October, linkage with the polar westerlies should be re-established by December 27. That turn of events, with the possibility of another Kona Low and sub-Aleutian mAk vortex, could lead to strong ridging in the EPO or PNA position. Ridges along the West Coast, even in progressive patterns, will favor intrusions of bitter cold air into the eastern two-thirds of the continent.
A No-Blocking Scenario For Prolonged Cold Temperatures In Eastern 2/3 Of North America
Although many have failed to notice, there has been relatively little blocking in the 500MB longwave pattern outside of the first week of October. What has occurred, despite a 500MB longwave pattern that can best be described as progressive, is the continued formation of Arctic vortices in and near Hudson Bay. The numerical models are hinting that ridging will take hold over the Southwest and Greater Antilles in the 11 - 15 day time frame, with positive height anomaly (NOT a block) near the North Pole. Teleconnections on this triangulate of ridges (along with the predicted cAk motherlode in eastern and central Canada) would support a trough over the eastern half of the U.S. as we approach the New Year. That set-up almost surely will allow for very cold values to permeate far to the south once again, with some of the well-populated areas of the Midwest, Northeast and Dixie dropping well below seasonal normals in the extended period.
Prepared by Meteorologist LARRY COSGROVE on
Saturday, December 13, 2008 at 6:30 P.M. CT
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
All rights reserved.
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