WEATHERAmerica Newsletter, Saturday, December 20, 2008 at 5:10 PM

by LarryCosgrove | December 20, 2008 at 04:50 pm

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WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)

STRONG WINDS
(Pressure Gradient Derived)
WI....MI....ON....NY....CT....RI....MA....VT....NH ....ME....S NB....S QB

STRONG WINDS
(Pressure Gradient Derived)
Pacific Shoreline Of BC, WA

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

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

Isolated Locations In
DE....C, S MJ....NYC, LI NY....Coastal CT, RI....SE MA
(QPF 1 - 2")

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)

NUMEROUS Locations In
E ON....S QB....S NB....ME....NH....VT....MA....N RI....N, C CT....NY....N NJ....PA
(Snow; 4 - 24"; Near-Blizzard Conditions)

Isolated Locations In
C MD....SE PA....C NJ....NYC Metro NY....Coastal CT, RI....Boston MA Metro
(Snow, Ice; 2 - 6"; Change To Rain Then End As Snow)

Scattered Locations In
N MN....WI....MI....C, E ON....N OH
(Snow; In Squalls; 2 - 4", Locally 12"; Near-Blizzard With Chill Hazard)

Scattered Locations In
SW BC....WA....OR....N CA....N NV....ID
(Snow, Ice; 4 - 12", Mainly At Higher Elevations)

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Progressive Arctic Vortex Brings Winter Storm To Great Lakes, Northeast....

I had suspected that the various numerical models were not handling the unusual storm complex heading out across the Great Plains very well in runs prior to that of 12z. Recent developments may be proving my suspicion to be correct. Note the rare linkage of an Arctic vortex with a subtropical jet stream over North America. A full-phase event involving a cAk gyre is rare, and can lead to extreme weather on the synoptic scale. Linkage with deep moisture alone is one reason why the system may have critical impacts on over the populous Great Lakes and Northeast.

Snow and wind output in the cold sectors of this feature appear to be a given, and that blizzard-like condition will encompass most of the Great Lakes region through Sunday evening. High snow output ratios may enable cities such as Milwaukee WI, Grand Rapids MI, and Detroit MI to see as much as 8 inches of dry powder by the time the cold core low exits the scene on Monday morning. Outrageously cold temperatures will reach most sections of the U.S. east of the Rocky Mountains (possible exception: the Florida Peninsula). But the biggest surprise with this feature may be its potential for havoc over the northeastern states.

The various equations have trended colder with the secondary cyclone that is expected to develop near Wallops Island VA on Sunday morning, with a critical (but small) rightward shift in the surface low pressure center off of the shoreline of Northeast. It would appear that the "stay as snow" line may never get more than 50 miles from the coast, which in theory would allow most of the north and west suburbs of the Interstate 95 corridor (Washington DC to Boston MA) to get a healthy accumulation of snowfall and quite possibly some ice coating. With wind a factor late day behind the storm, maintaining power and transportation could be quite a problem on Sunday night and Monday morning, a heavy travel time before the Christmas holiday.

....While Ushering In Period Of Intense Cold Across eastern Two-Thirds Of U.S.

The incoming colder regime may not last more than 48 hours in much of the U.S. to the right of the Rocky Mountains. That said, the increase in heating usage in an economically difficult time is nothing to be ignored. The nocturnal freeze line will very probably reach the Interstate 10 corridor on Sunday night, and wind chills may be as low as -20 in much of the Midwest and Appalachia before southwest flow aloft sets in with warming on Tuesday.

Yet Another Storm For The Pacific Northwest, Intermountain Region

This is a fast and furious weather pattern, and no sooner will the Arctic episode and Nor'easter exit than another Colorado-Trinidad storm take shape. The new feature will actually be an amalgam of two large disturbances: one (the vestiges of Typhoon Dolphin) dropping through the Gulf of Alaska and another west of Baja California. As the Kona Low and sub-Aleutian vortex pump up 500MB ridging over Alaska and the Yukon Territory, the two vortices will consolidate into one powerful storm within 72 hours over or near the Oklahoma Panhandle. Aided by moisture moving around the rear of a strong subtropical high across the Bahamas, the low will tap into Arctic values to return the threat of heavy snow and ice from the Pacific Northwest to the Rocky Mountains and northern High Plains during the period Sunday through Tuesday.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

How Warm Will It Get East Of The Rocky Mountains? And For How Long?

Using the 12z ECMWF panels as a guide for the intensity for storms (the GFS depictions are very diffuse with the low ejecting out of the High Plains on Tuesday), it would appear that most of the medium rang will be a warm one for the Dixie states and the Eastern Seaboard. The GFS ensemble package shows an unceasing southwest flow aloft across those regions. So, despite the breakdown of the Bahamas heat ridge, locations below a line drawn from Albuquerque NM to Columbus OH to Providence RI can probably count on temperatures that run, in some cases, well above seasonal normals in the Christmas/New Year's "hammock week".

"Christmas Storm" Mired In Uncertainty For Midwest, Northeast

Following up on the storm exiting the OK panhandle on Tuesday, the much stronger solution shown by the ECMWF scheme is preferred. The low center keeps its definition on a track that follows the "Panhandle Hook B" scenario. Ice and snow may present problems over the Intermountain Region, western Corn Belt and Great Lakes, while showers and locally strong thunderstorms occur across the lower Mississippi, Tennessee, and Ohio Valleys. I do NOT expect a sweeping surge of colder air behind this disturbance, largely because the character of the upper low will turn from longwave to shortwave, and the wind fields over the eastern half of the continent will stay mainly from the south and west.

New Storm Will Target The Western Two-Thirds Of U.S.

Not to be outdone, yet another storm is likely to affect the U.S. during the medium range. The next storm in the series will start out as a "trough type" system, following the "Panhandle Hook A" pattern. Snow and ice concerns should be limited to the Intermountain Region and High Plains. As the low center moves toward the Great Lakes (WI through C ON), warmer air and wind will overspread the Midwest. Computer forecasts of this feature are supportive of a convective aspect within the warm sector, with a highly difluent pattern aloft and strong (cA/cT/mT) aspect. For that reason, severe weather may be a big problem from E TX....C, E OK....NE TX into the Ohio and Tennessee Valleys during December 26 - 27.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

December Pattern So Far....

December has differed greatly from the period September 15 - November 30 on a couple of counts. One is the 500MB longwave pattern, where a repeated tendency for ridging in northern Canada has been replaced by a series of Arctic vortices (most often falling into the Hudson Bay classification). Blocking has been relatively rare; the current Rex signature over AK, YT, and BC is the exception, with the only true block formation being over northern Russia. The drought situation over the Southeast is rapidly disappearing, with the suppression of ridging and the mean elongate trough from ON into the Deep South. Despite recent bouts of warmth over the Old South, this is a cold-dominant configuration. The x-factor here: the very visible Kona Low near Hawaii, which has occurred frequently and helped to pump up a ridge into Alaska and the Arctic Sea (-AO).

....And What Is Ahead Through The First Week Of January

Despite the -PNA configuration forecast to exist off and on through the next ten days, the outlook for the extended (11 - 15 day) period seems to return to a warm West, cold East scenario as we head into the first week of the 2009. I have been noticing a tendency in the various computer schemes to retrogress the positive height anomaly over the British Isles back to just below Iceland and Greenland. There are also subtle hints in the ECMWF and GFS ensemble members of a formation of Grand Banks vortex after December 31. So the storm complex ejecting out of the Rocky Mountains in about ten days may play a role in returning colder air to the Great Plains, Midwest and Northeast, with some potential for a winter storm of the Colorado-Trinidad B type. Note that the Kelvin wave over Indonesia and the southwestern Pacific Basin is hooking up with the polar westerlies once more, which may translate to an amplified jet stream configuration and colder values heading southward from Canada into much of the U.S. in about two weeks.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, December 20, 2008 at 5:10 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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