WEATHERAmerica Newsletter, Sunday, November 11, 2007 at 5:50 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)
STRONG WINDS (Pressure Gradient Derived)
E OR....N ID....BC....AB....MT....SK

Some Thunderstorms May APPROACH Severe Limits
E TX....W LA....AR....N MS....W TN....W KY....C, S IL....MO....E KS....E OK

Some Thunderstorms May APPROACH Severe Limits
S AZ....S CA Deserts

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

Numerous Locations In
BC....WA....OR....N CA
(QPF 1 - 4")

Isolated Locations In
E TX....W LA....AR....N MS....W TN....W KY....C, S IL....MO....E KS....E OK
(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
Interior And Higher Elevations Of BC
(Snow; Blizzard; 4 - 24")

Isolated Locations In
N ON....N QC
(Snow; 3-6")

Isolated Locations In
Coastal NL/LBR
(Snow; 4-8")

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)


Strong Storm, Brief Cold Intrusion From The Midwest To The Eastern Seaboard

The powerful storm now moving into the pacific Northwest will have a sweeping impact on precipitation potential and temperatures across much of the nation. Since a positive height anomaly is taking shape over the Davis Strait and Greenland, the disturbance will be forced to dig into the Midwest and Northeast, adding higher velocity wind and vorticity profiles during the next 96 hours. A moisture fetch exist over the lower Great Plains, and as the intense dynamics acquire the proverbial fuel, thunderstorms will begin to build over the Mississippi and Ohio Valleys on Monday and Tuesday. Computer outlooks show a negative-tilt trough with an impressive PVA spike reaching the Atlantic Coastal Plain from Virginia into Maine by Wednesday morning, implying wave cyclogenesis with heavy rainfall and briefly strong convection through the middle and northern portions of the Interstate 95 corridor. For many, the biggest issue with the passage of this system will be its introduction of chilly cPk values from the Great Lakes through the Eastern Seaboard later this week.

If You Want Warm And Dry, Check Out The Southwestern States And Great Plains

On the other side of the temperature coin, the rapid east-southeast motion of disturbances from BC and WA into the Great Lakes is almost always accompanied by strong warming across the Desert Southwest and the middle/lower Great Plains. Readings may soar into the 80s (F) as far north as CO and KS on Monday and Tuesday before the cold frontal passage brings values close to normal at midweek.

The Pacific Northwest: Why "La Nina" Can Be Termed An Obscenity

The weather around the Pacific Northwest has sometimes been unfairly stigmatized as being routinely awful, with incessant rains and raw, chilly temperatures. Truth be told, Portland OR and Seattle WA are comparable to New York NY in terms of total precipitation, with both cities escaping the thermal extremes of the major cities along the Eastern Seaboard. However, in weather patterns heavily affected by the presence of a La Nina episode (as is the case currently), the vision of umbrellas being ripped apart by strong winds amidst downpours is likely to be true. One disturbance moving into the Puget sound during the following 36-48 hours could bring almost extreme amounts of rainfall with winds reaching hurricane force through the northern Intermountain Region. And succeeding areas of low pressure are sure to impact locations north of the Bay Area (CA) through the next 10 days (see more on this topic below in the MEDIUM RANGE OUTLOOK).

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

ECMWF Version Says; Colorado Trinidad "B" Yields Major Nor'easter

While all of the medium range forecast models have had difficulty recently, one aspect that the versions, especially the ECMWF have been correct with has been storm track patterns. Both the Noel Nor'easter and the "Sea Monster" of the past 48 hours verified well with respect to pressure and precipitation aspects, so the appearance of a new powerful cyclone on the European outlook during the past two runs is cause for concern. Whereas the 12z Nov 11 GFS shows a well-developed but more northern-path system, its overseas counterpart takes a Colorado/Trinidad type storm under an emerging Rex signature in QC and deepens the feature into a monster, with effects from VA into NB seen between 192 and 240 hours. When placed against the height anomaly profiles shown by the ensemble members, the more aggressive scenario is feasible. So, while using the customary caution needed for dealing with a distant time frame, the possibility of an excessive rain (higher elevation snow in MD, PA, NY, MA), high wind and heavy rain event looms large for the November 19 -21 time frame.

Positive Height Anomaly In NAO Position: What Does It Mean?

With most of the numerical versions continuing to show a prominent -NAO configuration through the next 240 hours, questions may arise as to possible impacts of the positive height anomaly stretching from N QC into Iceland. The most obvious effect is deviation of storm tracks. So with energy coming into the lower 48 states late this week from the Gulf of Alaska, lee cyclogenesis over CO will be forced to take a more eastward path (as opposed to recurving through the Great Lakes). Teleconnections on ridging just below Greenland favor formation of a deep closed low off of the shoreline of New England, accompanied by somewhat colder than normal temperatures in southeastern Canada and the Northeast during the latter half of the medium range.

New Storm For Western States By November 20, 21


The storm which is now heading east from Japan is impressive in its own right, but the inclusion of energy from Tropical Depression 22W will likely turn this feature into a prominent mPk or mA vortex. Within the 7-10 day time frame, the combinant disturbance will likely follow a semizonal path into the Pacific Northwest, with another round of heavy precipitation, strong winds, and cold temperatures for BC, WA, OR and N CA.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

After Another Colorado Low, A Dry And Milder Stretch In Lower 48 States

The broad disturbance associated with Tropical Depression 22W should have its greatest impacts during the medium range. After the low assumes a Colorado/Trinidad type path (this time going through the Great Lakes as the -NAO signature breaks down), the 500MB longwave pattern across the U.S. should fall into a broad trough arrangement, albeit with semizonal overtones. That turn of events should allow for the U.S. between Days 12 and 15 to undergo a transition to mP and ImP values, thus bringing a period of relative calm to the nation during much of the Thanksgiving holiday weekend.

Broad Trough, Kamchatka Peninsula Vortex May Portend A Coming +PNA Ridge Complex

Just how long apparent weather will be in quiet mode is questionable, however, given the predictions upstream of a rather prominent mAk vortex over the Kamchatka Peninsula by 360 hours. With prominent 500MB height falls over and below the Aleutian Islands, the question then becomes: will a -EPO, and then +PNA, ridge complex develop? Typically, a moderate La Nina episode show minimal tendency toward positive height anomalies over western North America. But there can be exceptions, and if the gyre becomes situated between Unalaska and Hawaii, it is entirely possible that a thumb-projection or even an Omega signature could take shape over AK, YT, and BC, thus affording the chance for some genuinely wintry weather over the eastern two-thirds of the continent.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, November 11, 2007 at 5:50 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

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