WEATHERAmerica Newsletter, Sunday, December 16, 2007 at 5:15 PM ET

[Larry_Cosgrove]

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)
NL/LBR....E QC....ME....NB

STRONG WINDS (Pressure Gradient Derived)
W, C CO....S UT


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

Isolated Locations In
NL/NWF
(QPF 1 -2")

Isolated Locations In
Coastal BC, WA, OR, N CA
(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)


Scattered Locations In
NL/LBR....E QC....ME....NB
(Snow; Blizzard; 4 - 16")


Isolated Locations In
Coastal Ranges Of BC, WA, OR, N CA
(Snow; 4 - 8")


Isolated Locations In
S ON....N OH....NW PA....W NY
(Snow; In Squalls; 4 - 12")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Overview

The most important aspect of the major storm now moving into New England is its deposit of heavy snow cover from the Front Range of WY and CO into the Northeast. This vast, deep snowpack will serve to chill temperatures and maintain the mean storm track from the southern Intermountain Region into the Mid-Atlantic shoreline (see below). Very little change is seen in the presentation of the numerical models for the following two weeks.


The Cold Doesn't Last Too Long

One aspect of the aforementioned deepening low is its trail of colder air, which is limited due to two reasons: the fact that the system is compact and not part of a trough complex, and also the lack of any real blocking signatures at higher latitudes. So when a new disturbance moves out of the Great Plains later this week, the freeze line will jump northward, with mostly rain indicated south of the Interstate 90 corridor from Chicago IL to Boston MA. An area from the lower Great Plains through the Old South should have temperatures well above normal by midweek.

Pray For The Pacific Northwest

La Nina episodes often present a worst-case scenario for the Pacific Northwest, and the attached satellite image shows that a very troublesome period is in store not only for the Puget Sound and Willamette Valley vicinities but for most of the Intermountain Region as well. Strong impulses will reach the western shoreline of North America nearly every other day, dropping southeastward with two prominent effects: adding to snow and rain totals while serving to carve out a very cold trough across the western states. In time, these disturbances will eject into the Great plains with repercussions for the eastern half of the U.S.


MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)


A "More Of The Same" Storm Track Across The U.S....

After a relatively weak low pressure center moves from the Great Plains to the Mid-Atlantic states later this week (note: the ECMWF version shows a stronger system with colder temperatures and more precipitation in the Northeast), the power storm complex over the Bering Sea and Aleutian Islands will eject at least two more (possibly three if the European model output from 12z is correct) strong storms into the lower 48 states over the 6-10 and 11-15 day time frames. The look of these cyclones is VERY similar to the current disturbance, therefore increasing snow cover while generating cold air through the western states into the Northeast. One of the two lows may have a major snow/ice impact on the Interstate 95 corridor, and either could briefly introduce a cold air mass to the Deep South (but the subtropical high over Cuba and the Bahamas should be strong enough to reduce any frost or freeze impacts). See also: attachments describing the GFS ensemble predictions for the next 15 days.


....With Ongoing Temperature Trends Likely To Continue

If there is one trend to follow, it is the idea that extension of the snow cover and building of high pressure across Canada may be enough to force temperatures downward across the western and central U.S. Overall, however, the NWP schemes and ensemble members are very consistent since Friday with storm tracks and maintenance of warmth in the lower latitudes of the U.S.

Note: the 500MB longwave pattern is very favorable for development of Santa Ana events in CA, with warm, dry conditions and frequent bursts of high winds.


EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Keep Watch On The Indian And Pacific Oceans!

The ensemble members are picking up on a very gradual, though profound, change in the jet stream configuration towards New Year's Eve. The core of very cold air over the Kamchatka Peninsula into the Bering Sea (creating the immense storms over the Aleutian Islands which target the U.S.) is forecast to rotate southeastward into BC and WA by January 1. This shift turns the upper level flow to nearly zonal across most of the nation, resulting in much milder air once the second huge disturbance moves out around the turn of the New Year. But this mild, inactive pattern may last less than a week due to two important changes: the cold dome relocates to ON while a convective pulsation over the Indian Ocean migrates to the International Dateline. The latter feature is now associated with tropical cyclone development near Diego Garcia and some immense thunderstorms across Malaysia and Indonesia. As the disturbed area slides eastward, a surge of warm air will be pushed northeastward, setting up a strong clocking ridge into AK and western Canada. The result: around January 5 - 6, a shock of Arctic cold air east of the Continental Divide and the possibility of a major winter storm across the Mid-South and Eastern Seaboard.


Prepared by Meteorologist LARRY COSGROVE on
Sunday, December 16, 2007 at 5:15 PM ET

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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