WEATHERAmerica Newsletter, Saturday, November 29, 2008 at 1:40 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)

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
S LA....S MS....C, S AL....C, S GA....C, S SC....S NC

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

Scattered Locations In
S LA....S MS....C, S AL....C, S GA....C, S SC....S NC
(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)

Isolated Locations In
BC....W MT....W WY
(Snow; Above 5000 Feet; 2 - 6")

Isolated Locations In
W, C SK....ND....NE SD....W MN
(Snow; 3 - 6")

Isolated Locations In
C ON....N NY Tug Hill Plateau
(Snow; In Squalls; 3 - 6")

Isolated Locations In
Laurentian Shield QB
(Snow; 3 - 6")


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Full Latitude Trough, Developing Storm Affect Eastern U.S.

The elongated banding of heavy rain and thunderstorms across the Deep South represents the first stage of cyclogenesis. A storm should take shape over Appalachia (probably as a dual-center structure) within 48 hours, spreading mainly moderate rainfall along the Eastern Seaboard. Snow will develop within the cold sector, in both synoptic and lake-related fashion, during Sunday and Monday. While the possibility exists for localized amounts in excess of 8 inches in the Great Lakes and interior Northeast, it is unlikely that a major winter storm will organize. The biggest effect of the trough complex will be to drive the nocturnal freeze line to the Gulf Coast on the mornings of December 1 and 2.

Intrusion Of Cold Air Balanced By Western U.S. Warmth Under +PNA Ridge Complex

While an IcA regime will hit the Deep South, with chills across the Midwest and Eastern Seaboard, the presence of ridging along the West Coast should provide potential for very warm conditions in areas along and to the left of the Continental Divide. A Santa Ana episode is possible in California on Sunday and Monday before the moderately strong surface anticyclone begins to move toward the High Plains, thereby relaxing the pressure gradient across the Golden State.

Another Surge Of Arctic Air Likely To Enter North Central U.S. Early Next Week

Remembering that the 500MB longwave pattern is still wholly progressive (despite the positive height anomalies showing up near Greenland and in northern Canada), it should not be surprising that the strong trough across eastern North America is transient. By Tuesday, all of the numerical models show a prominent shortwave dropping out of the Prairie Provinces. This impulse will be accompanied by gusty winds but relatively little precipitation, owing to the mean northwest flow preventing moisture advection from the Gulf of Mexico into the Great Plains.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Warm West, Cold East Configuration Continues....

Despite an apparent break in the 500MB longwave pattern at the end of the medium range, the overall synoptic setting for the autumn of a +PNA configuration looks to continue. The actual driver for the warmth in much of the West and colder temperatures east of the Rocky Mountains will be a -EPO styled ridge complex that takes shape over the northern rim of the Gulf of Alaska within five days. The trough downstream will amplify, with a strong shot of Arctic air being felt across much of the eastern half of the U.S. through December 8.

Storm Threat For Southern, Eastern Tiers Of U.S. Around December 9 - 10

Since there has not been a major eastern U.S. cyclogenesis event since October 21, our curiosity is focused on recent computer model predictions of powerful winter storm in the December 8 - 12 time frame. As is to be expected, the various equations have different points of origin and track for this system. But a cursory look at the operational schemes would seem to favor a Colorado/Trinidad "A" system. If so, the best potential for important snow would be across the Corn Belt, Great Lakes, and interior Northeast. It is NOT out of the question, however, that this feature could take the southern variant "B" path favored by the 0z ECMWF package, which would leave open the possibility of secondary redevelopment. And, with it, some meaningful chances for frozen precipitation along the Interstate 95 corridor above Fredericksburg VA by the start of the extended period.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Snow Cover Comparison: November 28 2008 Vs. November 28 2007

At first glance, there does not appear to be much difference between the current snow cover map and that seen at the same point in time last year. But the subtle presence of more snow across the Great Lakes and parts of Appalachia is indicative of a +PNA configuration. In time, as sea ice fills in over Hudson Bay, colder air masses will penetrate farther to the south, perhaps dragging the edge of the snow field into parts of the Atlantic Coastal Plain and Ohio Valley by mid-December.

ENSO Signal Remains Neutral

Despite assertions from some forecasters of an impending return to a La Nina episode, the SST signature across the equatorial Pacific Ocean remains neutral with a slightly cool appearance over the western ENSO sectors and generally warm waters closer to the coast of Ecuador. So the greatest influences on the mean circulation across North America will have to come from the Madden-Julian Oscillation (see more on this factor below); the Pacific Decadal Oscillation (PDO) which has been negative, but showing moderation in the Gulf of Alaska water temperature profile; and the hydrothermal configuration over the western Atlantic Basin, which is split between abnormally cool readings south of Newfoundland and positive anomalies from the Grand Banks to Iceland. Put together, the various oceanic signals favor tendencies for -NAO blocking patterns and formation of strong, full-latitude trough complexes across eastern North America. If so, as we move toward the climatological peak of winter (mid/late January), there should be a better possibility for impressive cold, snow and ice threats across the Mid-Atlantic and New England states. That turn of events may take a while to unfold, however. So if you are a frustrated snow enthusiast living in the urban Northeast, patience is in order.

MJO Teleconnection Favors Colder Extended Period Across Eastern Two-Thirds Of U.S....

As the strong Kelvin wave over Indonesia continues its slow eastward march, the potential for interaction with the polar westerlies will only increase. Note that the MTSAT image shows a very clear linkage of the tropical convective belt with a storm growing between Japan and the Aleutian Islands. The typical lag time of connection between an MJO-associated impulse over the western Pacific Ocean and amplification of the jet stream across North America is about ten days, which could greatly impact the intensity of a winter storm (and following cold intrusion) across the eastern half of the U.S. around December 8 - 12.

....But Ensemble Members Offer A (Possibly) Conflicting Opinion

There has been almost no consistency among operational and ensemble forecasts of the various equations in regard to the 11 - 15 day time frame. This "vacillation and variation" has meant shifts from a +PNA or -EPO styled configuration to a cold West, warm East over the past few days. The 0z Nov 29 runs of the numerical models seem to be centering on a volatile weather pattern through the U.S., with a brief cold snap across the Intermountain Region during the medium range giving way to a moderately strong (though non-blocking) ridge and resultant warmth for most of the extended period. Conversely, the eastern states may start out milder between December 8 and 9, only to chill down in the December 10 - 15 range.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, November 29, 2008 at 1:40 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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