WEATHERAmerica Newsletter, Saturday, November 22, 2008 at 5:35 P.M. CT

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)
E QB

STRONG WINDS (Pressure Gradient Derived)
N, C Coastal BC

Some Thunderstorms May APPROACH Severe Limits
E TX....SE OK....SW AR....NW LA


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

Scattered Locations In
N, C Coastal BC
(QPF 1 - 3")

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
E QB
(Snow; Blizzard; 4 - 12")

Isolated Locations In
N, C SK....N, C MB....W, C ON....extreme SW QB....Tug Hill Plateau NY
(Snow; 2 - 6")

TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Equatorial Pacific Ocean)

Indian Ocean And The Orient

New flaring of the Madden Julian Oscillation from southern India, Sri Lanka into the Philippines. Invest 99W will likely approach Luzon within the next 24 hours as a strong tropical wave or depression.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Storm Will Bring Volatile Weather Scenario Across Eastern Half Of U.S.

While not fitting into a classic storm track category, a low currently over Saskatchewan will play a big role in the course of weather across the eastern two-thirds of the U.S. (as well as adjacent provinces of Canada) during the next 72 hours. The 500MB signature on the numerical models is very impressive, with a large, closed cold dome that passes through the Great Lakes and comes to rest over the QB Eastern Townships by November 25. A shortwave within the gyre amplifies the southern extent of the upper trough, and proceeds to develop secondary cyclogenesis over MO and IL on Monday morning.

The ultimate track of the surface low will probably be from the Corn belt into the Hudson Valley of NY. That type of path rules out any important snow or ice for the Interstate 95 corridor; heavy rain may be an issue from NC into CT, RI, and MA. But the cold sector of the cyclone will be another "lake effect snow machine", with heavy synoptic-process snows likely in portions of western and northern New York State, northern New England and the lower/middle St. Lawrence Valley on Monday night and Tuesday.

Unseasonable Cold Over Midwest, Northeast....

The tendency for stratospheric warming over Canada has continued in the past week, signaling a long period of colder than normal temperatures in the U.S., Ontario, Quebec and the Maritime Provinces. There seems ample potential for two cores of colder values during the foreseeable future: below the Aleutian Islands and across the Great Lakes region. In all honesty, the general set-up of the jet stream configuration is most similar to November and December of 1976. The western states mostly escape the cold, save for the occasional undercutting of the +PNA ridge complex by upper lows that stall over the Four Corners region or Sonoran Desert. If this trend continues through the upcoming winter, the warm West, cold East alignment will be in place until the middle of February.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Transition In 500MB Longwave Pattern Favors Widespread Intense Cold....

Let us lay out the key 500MB features to see if we can find a common thread:

1) Sub-Aleutian Low
2) Extensive and amplified ridge signatures in the EPO, PNA, and NAO positions
3) Negative height anomalies over California and Ontario merging
4) flat subtropical high over Hispaniola and Puerto Rico

You can see by the extensive area of negative 500MB height anomalies present over the U.S. that the above represents the start of a challenging winter season forecast. The ensemble members of the GGEM, GFS, and ECMWF scheme have been VERY stable in presenting the above alignment of systems, with the result being a very cold presentation for apparent thermal displays in most of the lower 48 states to the right of the Continental Divide. So after a relatively benign period through next Thursday, the proverbial roof falls in with the possibility of an Arctic vortex over the Great Lakes region.

....With Some Signals Of A Major Winter Storm For Dixie, Eastern Seaboard

All of the operational models and quite a few of the respective variants show energy from a disturbance in the Southwest phasing with a shortwave digging southeastward from the Prairie Provinces during the 6 - 10 day time frame. Explosive cyclogenesis does not occur until the resultant storm passes just off of the VA Capes around December 1; previous to that time only scattered precipitation is probable in most sectors of the area of low pressure. However, during the evening of 12/10 and the succeeding day, heavy rain/sleet to snow could occur in much of the Northeast. And the formation of a closed 500MB low in MI and On could mean a banner lake-related snow squall event for the prone locations to the lee of the Great Lakes.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Why Is The Sub-Aleutian Low Such A Big Deal?

It is a common mis-understanding that deep 500MB lows located over or even north of the Aleutian Islands favor a strong +PNA configuration with extensive cold air evacuation across the eastern two-thirds of the U.S. and Canada. That is not really true, as the storm complex must be located SOUTH of that island chain for downstream ridge building to occur. The computer models hold on developing this gyre until December 2, at which point the familiar ridge, warm West vs. trough, cold East configuration takes shape. All of the ensemble forecast show a huge negative height anomaly at a latitude close to Vancouver BC during the extended period, which is compatible with the possibility for a display of very cold air to the right of the Rocky Mountains between December 3 and 8.

Ensemble Forecasts Agree: Classic Scenario For Widespread Intense Cold In December

Considering that ensemble predictions are a summary of many different members (the GFS has 21 variants on its main equation), it is indeed rare when an outstanding signature shows up in the arithmetic mean. That said, the various outlooks and members show what could be termed a very special period in apparent weather for the eastern two-thirds of the continent. Not only is there a cAk vortex over Ontario and Quebec (with the anomaly signature suggesting a position even farther south, but a cross-polar flow sets up for much of the 11 - 15 day period! Add to that very cold look the high probability of a major winter storm forming at the start of the sequence (which may bring the snow line as far south as Raleigh NC by December 3), and you have the kind of forecast that has not been evident in both the Midwest and Eastern Seaboard since 2003! One more item to note: the presence of a defined subtropical high over the Greater Antilles without a declared positive deviation in heights is a sign that the aforementioned storm could be a monster, drawing up deep tropical moisture while enforcing steep 500MB height falls as far south as Florida in the first week of December.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, November 22, 2008 at 5:35 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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