WEATHERAmerica Newsletter, Sunday, November 25, 2007 at 6:30 P.M. 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)
ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
C, S AL....FL Panhandle....C, S GA....C, S SC


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

Scattered Locations In
AL....FL Peninsula....GA....NW SC....W NC....C, E TN....C, E KY....IN....OH....W PA
(QPF 1 - 3")

Isolated Locations In
Coastal BC, WA
(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)

Isolated Locations In
BC
(Snow; 4 - 8")

Isolated Locations In
S SK....MB....W ON....NW MN....N ND
(Snow; 2 - 4")

Isolated Locations In
C ON....C QC
(Snow; 4-8")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Heavy Rain, Thunder For Much (But Not All) Of The Deep South

The low pressure center moving out of E TX (type: Galveston Bay Spin-Up) will begin to accelerate and perhaps deflect a little to the right during the following 48 hours. With a shortwave advancing across the Great Lakes picking up the southern disturbance, some deepening is probable. Dynamics could increase sufficiently to generate a swath of heavy stratiform rainfall in parts of the Ohio Valley and Northeast. Instability is somewhat limited within the warm sector, but thunderstorms could be locally heavy with isolated severe cases from the Gulf Coast into the Mid-Atlantic region.

One aspect of this storm which is important to note: despite a tendency for strengthening, the low will NOT incorporate the colder air mass over central Canada. So readings should be in the normal or above normal range in the wake of the system.

The Cold Stays Locked Up In Canada (For Now)

There IS a great deal of bitter cold air building in Canada, and this cAk dome WILL have a fairly wide impact over the lower 48 states during the medium range. But for the near term, the prevalence of mainly westerly flow aloft and the lack of a high-latitude blocking signature insures that it will be another 72 hours before the Arctic values start to encroach on the U.S. A shortwave ejecting out of the western U.S. trough complex will begin to draw on the frigid domain on Tuesday, with the nose of the colder temperatures arriving in the Upper Midwest behind the impulse. Until that time, mild or even warm readings will be in play along and below 45 N Latitude.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)


Coming Attractions: A cAk Vortex....Over Ontario!

With a strong thumb projection ridge turning to an Omega block over Alaska and the Yukon Territory during the medium range, the southward dislocation of the Hudson Bay vortex should commence by Day 6. The deepest part of the cAk motherlode should reach as far south as Ontario, with suppression of the Arctic, Polar, and Subtropical jet streams apparent in the 192 to 240 hour time frame. The resulting cold snap will last through the eastern third of North America will into the extended period. It should be stated, however, that the harshest part of the cold air will stay out of the Deep South, courtesy of the westerly flow aloft and the lingering effects of the flat subtropical high which is now straddling Florida and the Bahamas.

Subtropical High Remains Strong Over Florida And The Bahamas

The persistent heat ridge affecting the Southeast will still be an important player in apparent weather through the next ten days, with some suppression and weakening indicated at the start of the extended period. The subtropical high is now playing a critical role in keeping Arctic values out of the lower 48 states, partly by its presence but also since the ridging helps to maintain a generally westerly flow between it and the Hudson Bay vortex. Until the cAk regime encroaches on the Gulf Coast around December 3 and 4, above normal temperatures will continue in play below the Ohio River and Mason-Dixon line.

Pacific Northwest Forecast: Brutal!

All that is bad in the forecast for the U.S. and Canada will be found in the outlook fro British Columbia, Washington, and Oregon during the longer term. The first portion of the medium range will be dominated by low-level inflow of Arctic air behind a digging shortwave out of BC and AB. Then the Pacific Northwest must face the wrath of the volatile storm sequence stretching from the Tien Shan range into the Gulf of Alaska. The low west of the Puget Sound will have an impact in the near term with rain and wind. But the second and third impulses in the series could bring major impacts from wind and precipitation around December 3 -5. A critical factor in the forecast will be the addition of energy from the equatorial regions in Oceania. Typhoon Mitag and a new tropical cyclone may both interact with the polar westerlies, supplying additional energy and moisture to squalls affecting the West Coast late in the medium range.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Cold Intrusion May Lift Out During Second Week Of December....
At its farthest south point, the cAk vortex may pass through part of the Great Lakes around 216 - 240 hours. It seems likely that the cold air associated with the motherlode may influence conditions in the eastern half of the nation through December 8. The ensemble forecasts, however, detail a shift in the polar westerlies during the extended period. The Alaska/Yukon block weakens and retrogresses toward Siberia, and energy from the Kamchatka Peninsula begins to reach the lower 48 states by Day 11. As a result, after the coming cold period in the medium range, a spell of mild and relatively dry weather seems probable through much of the U.S. by the second week of December.

....But Recharging Aleutian Low May Help Rebuild -EPO or +PNA Ridge Complex
I suspect that any warming over the northern tier of the U.S. in the extended period will prove temporary. The trend for deeper and stronger cyclones ejecting out of Asia continues unabated, and the addition of energy from the equatorial Pacific Ocean shows no signs of stopping. If the active convection belt were to drift westward into the Indian Ocean, then a somewhat flatter and higher-latitude west-to-east orientation of the westerlies would commence. That action may happen for a brief time between December 9 and 12. But I suspect that a return to an mAk vortex below the Aleutian Islands will occur after that time, with a resultant colder and stormier pattern for much of the nation and southeastern Canada.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, November 25, 2007 at 6:30 P.M. 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

All rights reserved.

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

[dyndrite]

Larry,

Trend forecasting is so easy.

Dyndrite

[Larry_Cosgrove]

Originally Posted by dyndrite

Larry,

Trend forecasting is so easy.

Dyndrite

OK...so what do you mean?

LC

[dyndrite]

I was just pulling my hair out at the dynamics with that last little system. I think sometime we think we understand more than we do.

I was reading your forecast and it reminded me of some of the new terminology and back to the old days. I think I am going to bring back an old term, “unsettled.”

Good luck with the next system this weekend. I will update this afternoon.

Dyndrite