WEATHERAmerica Newsletter, Saturday, December 27, 2008, 4 PM CT

by LarryCosgrove | December 27, 2008 at 02:40 pm

257 views | 12 Recommendations | 2 comments

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)
S ID....W WY....W CO....NW NM....N AZ

STRONG WINDS
(Pressure Gradient Derived)
QB

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 - 3")

Isolated Locations In
E Lower MI....ON Peninsula
(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
C ON....N, C QB
(Snow; Some Sleet, Freezing Rain; Near-Blizzard; 4 - 18")

Isolated Locations In
BC....S AB....C, W MT....ID....WA....OR....N CA....extreme N NV....N UT....W WY
(Snow; Mostly Above 3500 Feet)

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Unseasonable Warmth Ending Over Eastern Half Of Nation....

While I detest use of the word "blowtorch" when describing a warm advection episode, the incredible rise in temperatures across the eastern half of the U.S. into Ontario and Quebec may merit use of that term. Consider that rain and thunderstorms could reach into the eastern Great Lakes on Saturday evening, and the rain/snow line may climb as far north as Timmins ON! The linkage of the storm passing through the upper Mississippi Valley into the Laurentian Shield will enable transport of air from the Bay of Campeche. The milder values will last along the Eastern Seaboard through Sunday before, as they say, reality rears its ugly head.

....Is Dislodged By Major Winter Storm, Severe Weather In The Midsection....

The compact but powerful winter storm moving through the Midwest has displayed many forms of weather, from tornadic thunderstorms to severe icing. The huge shift in temperatures, however, is the principal attraction of this disturbance, with 30 deg (F) deviations either side of the frontal structure. Colder values will slowly dislodge the positive thermal anomalies across the Old South and Eastern Seaboard, and the heaviest ice and snowfall should move into ON and QB by Sunday morning.

....Followed By Increasingly Cold Temperatures Advancing Southward From Canada

And then comes the inevitable cooling-off period. Once the disturbance over the Upper Midwest passes into Labrador, the Arctic values controlling the western U.S. and much of Canada will begin to slide toward the south and east. The cold advection process looks to be gradual, and probably not likely to create extreme negative deviations in the near term. Still, the wild stretch of +70 deg F temperatures through the Mississippi, Ohio, and Tennessee Valleys will come to a stop by Monday.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Trend Toward A Colder Pattern Across U.S. Continues....

The GFS scheme and its variants have been steadfast since the December 20 runs concerning a noteworthy 500MB pattern change. The key features forecast to enable this shift are a strong -NAO signature near Greenland and Baffin island; a southeastward-moving cAk vortex originating in Alaska; and a formative +PNA signature. While the GGEM model suite shows almost the exact opposite of the American equation scenario, there is some concurrence with the ECMWF version and ensembles. A series of strong shortwaves are forecast to dig into the U.S. from the Gulf of Alaska, eventually yielding a very cold regime over the eastern half of the nation by New Year's Eve.

....With Threat Of Major Winter Storm Over Eastern Third Of Nation

A disturbance now over the Gulf of Alaska poses a huge forecast challenge as we exit 2008. There is little cohesion or consistency among the numerical models and recent runs, with the track and structure of this system changing radically with every new depiction. That said, with a low-slung semizonal flow and developing blocking near Greenland, this feature could create a lot of havoc in the Midwest and Northeast if a southward deviation in the path of the cyclone were to occur. I am currently leaning toward the ECMWF model idea of a low center that redevelops off of the shoreline of the Delmarva Peninsula, thus exposing New York and New England to the risk of a significant snow or ice threat on or about December 31.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

December Has Been Cold Across Much Of North America....

While the December temperature forecasts for the immediate Eastern Seaboard have not verified well (warmer than average and not far colder as expected), Arctic air has largely prevailed throughout the lower 48 states during the past 3.5 weeks. A reversal in the trends at 500MB occurred around December 10, with a positive height anomaly in Nunavut AR being replaced by a full-latitude trough complex with a core cAk motherlode near Hudson Bay. Ridging over the western Atlantic Ocean acted as a barrier to prolonged cold periods from FL and GA into ME and the Maritime Provinces, while an elongated -EPO and -AO alignment of ridging helped to keep the western two-thirds of the continent in a frigid air mass. Some easing of this jet stream configuration is indicated during the remainder of the month, but little overall change in the outcome of the warm vs. cold is expected.

....And January May Be Even Colder

In reviewing the consistent GFS model forecasts and its ensemble members, one theme emerges for the first twelve days of January. That is of a very cold period along and east of the Rocky Mountains, with only occasional moderation along the immediate east Coast. A caveat is present here: there are strong signals for Colorado/Trinidad storms, possibly of the "B" type. So a temporary rise in temperature (that is, lasting only 24 hours or less) might favor an ice to snow scenario for the Interstate 95 corridor from Richmond VA to Portland ME.

Note that a tendency is seen on the ensemble mean for linkage between the +PNA component and the retrogressive, fading -NAO signature. See also that, despite the distance from initiation of the model, a cAk vortex takes shape over Quebec with a negative 500MB height anomaly over the Mid-Atlantic states between January 10 - 12. This set-up would seem to favor a deep coastal low with widespread Arctic air reaching far to the South (Cuba).

Conflicting Signals Abound Over The Pacific Basin, Indian Ocean

You may have heard that La Nina has made a comeback of sorts, what with a colder signature clearly evident through the equatorial Pacific Basin. The presence of a low-grade -ENSO signature is generally considered to be favorable for widespread cold temperatures over the lower 48 states, particularly from the High Plains to the Eastern Seaboard. If the low-latitude waters to the right of the International Dateline were to chill to a moderate designation, then a tighter north-to-south, semizonal pattern would emerge with those along and north of 40 N Latitude getting slammed with repeated heavy snows and brutal cold.

The presence of an active Madden-Julian Oscillation sequence would seem to be a deterrent to a strengthening La Nina. But despite pulsations of cloudiness and thunderstorms that seem to traverse the area from Sri Lanka to the southwestern Pacific Basin once every 6 to 10 days, the SSTs have dropped. My argument is that the eventual motion of the Kelvin waves towards Oceania should limit any dramatic drop in hydrothermal readings. Also, the frequent linkage of these impulses with the polar westerlies (favoring amplification of the upper flow downstream) tend to produce intrusions of Arctic air into North America. Hence a colder weather scenario seems likely for a good deal of Canada and the U.S.

One more factor which many may be ignoring is the warming of waters in the Gulf of Alaska. Reversing a longtime negative display of the Pacific Decadal Oscillation (PDO), moderation of SSTs west of British Columbia is generally associated with the development of blocking 500MB ridges in the EPO and PNA positions. If this rise in oceanic readings continues, the prospects for a long and possibly harsh winter in most of North America will rise.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, December 27, 2008 at 4:00 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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