WEATHERAmerica Newsletter, Saturday, January 10, 2009

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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
(Orographically Derived)
S WY....W CO....NW NM....N AZ....SW UT


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

Isolated Locations In
N FL
(QPF 1 - 2")

Isolated Locations In
W WA
(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)

Scattered Locations In
N, C PA....N NJ....NY (excluding SI, BK, QN NY and LI)....CT....RI....MA....NH
ME....S NB....PEI....NS
(Snow; 4 - 12")

Isolated Locations In
S PA....C NJ....NYC and LI NY
(Snow, Sleet, Freezing Rain; 1 - 3")

Scattered Locations In
C, E AB....C, S SK....C, S MB
(Snow; 3 - 6")

Isolated Locations In
BC....E WA....ID....W MT....W WY....NW CO
(Snow; 4 - 8")

Isolated Locations In
C ON....MI....WI....N IL....N IN
(Snow, In Squalls; 4 - 8")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Slowly But Surely, Bitter Cold Is Taking Over North America

For days now, Alaska and much of Canada have seen a build-up of brutally cold temperatures. While it may be argued that it is supposed to be cold in the northern third of the continent in January, the plain fact remains that even for the tundra and boreal forest this has been an extremely cold winter so far.

The colder values have been nudging southward, and as of Saturday afternoon it can be argued that fully two-thirds of the U.S. are under at least a cA designation. With the milder air over Dixie being scoured out with a cold frontal passage, our attention must now turn to the descent of the harshest Arctic air into the lower 48 states. The freeze line will advance well into the Deep South over the next two nights, with subzero readings common across the Upper Midwest, Great Lakes and New England.

Next up for the tundra regime: the Corn Belt, Tennessee Valley, and the Eastern Seaboard!

Small Storms Add To The Snow Cover

While some have decried what they feel to be another "mild, no snow" winter in the U.S., the facts point out something different. When you compare the snowpack from this time in 2008 and what is seen now, it is easy to see that current snow is far more extensive in both depth and coverage. The biggest reflective increases are in the Midwest and Northeast.

The disturbance tracking just below the Mason - Dixon line will exit the Atlantic coastline near Cape May NJ on Sunday morning, then pass just below Nova Scotia and Newfoundland. This is what could be termed a moderate-intensity storm, dumping as much as a foot of snow over the interior Northeast while giving somewhat lesser amounts (with mixture of sleet, freezing rain and rain) to portions of the Interstate 95 corridor above Chester PA.

Another low, an Alberta Clipper (which the current feature is not, but that is another lament...), will usher in bitter cold values while spreading another dose of light snowfall across the Midwest on January 12 - 13. The snow may seem insignificant to winter weather enthusiasts, but it is very important! More snowpack means greater nocturnal refrigeration under clear sky, while at the same time acting as a buffer to warm advection events.

Which is why this January may ultimately go on the books as the coldest first month in the past ten years.

Warm, Dry Air Replaces Snow And Cold Over The Western States

In a +PNA configuration, ridging with relatively warm and dry weather dominate the West, while the eastern two-thirds of North America trend colder than normal with better potential for snowfall. So while the Pacific Northwest is emerging from a long siege of bitter cold, heavy ice and snow, the states further east see a return to "real" January. The ridge axis will be draped from Alaska into the Colorado River Basin, enabling strongly positive thermal anomalies in locations west of the Continental Divide. One pleasant effect for people living in British Columbia and Washington state, beleaguered by recent flooding and torrential rainfall: abundant, drying sunshine.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

The Cold Wave: How Bitter, And Who Will Be Hit Hardest?

The driver in the forecast of bitterly cold air east of the Rocky Mountains is the strong storm between Hawaii and the Aleutian Islands. This feature is creating an impressive display of warm advection into Alaska and British Columbia, in the process forming the +PNA ridge complex. This is NOT a true blocking configuration, as the anticyclonic curvature may deflate by January 18, and the 500MB structure of the ridging seems unstable for much of the forecast time frames.

That said, the cold will be brutal and perhaps create a window for two or three record-setting nights in locations along and west of the Mississippi River. Snow squalls will be prevalent across the still-open Great Lakes and the adjacent shorelines, with west-facing slopes of the Appalachian Mountains from N GA to New England seeing appreciable snows. When winds calm under the expected passage of a 1044MB high pressure cell on January 14 - 17, the 0 deg F isotherm may well reach as far south as 35 N Latitude in a few spots (with cities such as St. Louis MO, Louisville KY, Cincinnati OH, Baltimore MD and Atlantic City NJ the most likely demarcation for subzero nocturnal values next week). There is even a possibility that one of the major metro areas near the Great Lakes stays below zero all day long in the heart of the cold snap!

Moderation From The West To The Rescue After January 18

Despite an overwhelmingly cold jet stream configuration across North America, it seems possible that a brief moderation trend could impact the Midwest and Dixie states around January 19 - 21. The operational ECMWF scheme and the GGEM ensemble package are the most bullish on a period of brief warming, with the entire GFS model suite and the European variants at odds with that scenario. A high-latitude shortwave may erode the +PNA ridge complex, while the southern branch of the jet stream strengthens close to the Gulf Coast. That turn of events, if it transpires, would enable an ImP air mass to move through the Great Plains toward the Eastern Seaboard. The opportunity for milder values looks to end when a low takes shape over GA around January 20 - 21, returning a mantle of cold advection through the eastern half of the nation.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

+PNA Configuration Returns....

A look at MTSAT imagery shows a rather vigorous pulsation in the Madden-Julian Oscillation, with a broad feed of energy into a deep winter storm east of Japan. This disturbance looks to become another mAk vortex below the Aleutian Islands in the next ten days, establishing yet another mechanism for ridge building along the West Coast. With the establishment of a +PNA ridge configuration, north-northwest flow from the Arctic tundra will return. This can only mean a very cold period over the eastern two-thirds of North America in the 11 - 15 day range.

....Via A Possible Major Winter Storm

I looked with interest at computer forecasts last week which seemed to hint at a chance for an intense winter storm to take shape over the Gulf Coast around January 20. The numerical models indicated that this system would be that rare animal not often seen in the U.S. in recent years: a Miller A cyclone that is capable of spreading important precipitation, much of it frozen, from the Dixie states up along the Eastern Seaboard.

The GOES WEST satellite image shows an active subtropical jet stream in place from below Hawaii into the Southeast. If this moisture and energy axis remains in place during a time when the 500MB flow undergoes a "split maneuver" (which some outlooks see around January 19 - 21), the two wind axes could phase. This is on view with the 12z GGEM and GFS ensemble runs, with the European and American variants suggesting that the resurgent +PNA configuration drops the aforementioned cA shortwave into the mix. If so, then chances at a respectable, if not strong, winter weather event could occur along the Interstate 95 corridor January 21 - 22. This disturbance would be followed by another intrusion of Arctic values that may linger until perhaps January 26 across the U.S. from the Great Plains to the Atlantic shoreline.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, January 10, 2009 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 2009 by Larry Cosgrove

All rights reserved.

This publication may not be reproduced or redistributed in whole or in part without the expressed written consent of the author.