WEATHERAmerica Newsletter, Saturday, October 25, 2008 at 6:20 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)
NS....NB....PEI....extreme E QB....NL/LBR

STRONG WINDS
(Pressure Gradient Derived)
MB....W ON....MI....IN....IL....MO....NW AR....OK....KS....NE...IA....MN....SD....ND

STRONG WINDS
(Pressure Gradient Derived)
CO....W WY....W MT

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

Scattered Locations In
NS....NB....PEI....extreme E QB....NL/LBR
(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)

Cold Temperatures
MB....W ON....N MN....ND....SK
(Minima Less Than 10 deg F)

Isolated Locations In
MB....MN....WI....Upper MI....W, C ON
Snow (In Squalls; 2 - 4" But Locally Higher; Reduced Visibility In Strong Winds)

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

Indian Ocean And The Orient

Madden-Julian Oscillation remains quite active with large Kelvin wave straddling the Andaman Islands, Malaysia and Indonesia. Three direct linkages with the polar westerlies are evident, implying a trend toward amplification (cold intrusions, storms) of the jet stream over North America at some point in the second week of November. Elongation of this impulse into Sri Lanka portends a long-lived MJO percolation.

Central And Eastern Equatorial Pacific Ocean

ITCZ continues to percolate, but no individual convective circulations are seen at this time. Sub-Aleutian Low has deep connection to the tropics from the International Dateline into the Gulf of Alaska.

Atlantic Basin

No tropical disturbances are evident over the Atlantic Basin. Equatorial moisture connections are established from southern Mexico into the Maritime Provinces as well as from Venezuela to the British Isles.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Transient +PNA, -NAO Configuration Leads To Major Cold Episode Over Central, Eastern U.S.

While not a cohesive, true blocking configuration, the presence of +PNA ridging across western North America with concurrent 500MB height rises over the NAO sectors will produce an impressive full-latitude trough from Cuba into Nunavut AR during the near term. This trough complex will be the driver for rapid, intense cold advection during the near term. It is entirely possible that record minima could be established as far south as Florida by Day 4, with a killing frost or freeze to the Gulf Coast.

Cold Translates To Wind, Snow For The Great Lakes

The vast, accordion-like pressure gradient between the cPk ridge over the Great Plains and the surface low with frontal structure moving through the eastern reaches of the continent will help to produce two significant weather events. One will be the aforementioned cold intrusion over the eastern half of the U.S. The other, no less notable, is the likelihood for a significant lake-effect or related snowfall from Upper Michigan into northern Ohio, northwest Pennsylvania and western New York. keep in mind that the still warm water temperature may enable mixture with rain in lower elevations. At the same time, with 850MB temperatures at or below - 10 C and cyclonic eddies aloft supplying lift and organization, the warm waters will help to create convective squalls that could bring blizzard-like conditions to some of the notorious snow belts around the Great Lakes. The peak time for the event looks to be Tuesday night and Wednesday morning as the cold dome passes from W ON into the Northeast.

Western States Are Still Warm, But Not Hot

Since the surface anticyclone is expected to pass east of the Rocky Mountains, the resultant pressure gradient over California and the Southwest will not be particularly strong. So while temperatures in the Golden State and the lower Colorado Valley will be routinely above normal through the next five days, no Santa Ana conditions or extreme heat is probable. But until the 500MB flow begins to flatten out (Thursday of the coming week), much of the SoCal metro area will still face daily maxima near or in some cases exceeding 90 deg F.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

First The Cold, Then Warming
The end to unusually cold temperatures will be in full swing by Halloween Day, with warmth advancing through the Great Plains into the Midwest and Tennessee Valley. The settling of the surface high pressure area to a position off the coast of Georgia and Florida may enable some negative thermal anomalies to continue in the Southeast into the first two days of November. But to the left of the anticyclone, the 70 deg F maximum isotherm may reach Chicago IL and Cleveland OH next weekend.

Gulf Of Alaska Low Is The Real Deal....

The tendency for strong oceanic storms and frontal structures to pass south of the Aleutian Islands continues, with possible implications for the upcoming winter forecast. A cyclone which is now entering the Gulf of Alaska will probably slow as it approaches the West Coast in about seven days. Energy which is moving into this low center from the tropics are creating a "boost" effect, which in turns pumps up ridging in the PNA position while increasing snow cover over Alaska and northwestern Canada. The presence of this longer lasting storm complex may help, through teleconnections, to limit the amount of time that warm air controls much of the lower 48 states.

....But 12z ECMWF Forecast Of Cold Intrusion, Storm May Be In Error

While the powerful Sub-Aleutian Low may indeed help to build ridging over western North America, the scenario put forth by the 12z ECMWF scheme is purely an outlier. The latest run of the European model creates another cold intrusion into the Midwest and Southeast, closing off a 500MB low over SE VA on Day 10. As I see it, the spell of higher temperatures may be relatively short-lived. But I do NOT see the potential for important cooling and cyclogenesis over the Eastern Seaboard until after November 8.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Warm Start, Cold Finish Scenario Shaping Up In First Week Of November
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There is no question that a strong period of warming will originate over the Great Plains during the medium range. But teleconnections of upstream and forecast features suggest that this round of milder temperatures will last less than a week. Note the ongoing positive height anomaly over the Arctic Sea north of Alaska and the Chukchi Peninsula, and the tendency for a +PNA ridge signature to form across western Canada and the U.S. See also the depressed atmospheric heights below the Aleutian Islands. When combined with the impressive, slow-moving Kelvin wave now entering the southwestern Pacific Basin (see MTSAT image above and storyline below), these factors present a good case for a turn toward colder air to the right of the Front Range by November 8. The possible clincher in this prediction: the GFS ensemble suite has (with agreement from the latest ECMWF and GGEM variants), for four days now, predicted the formation of a southern branch jet stream from Mexico into the western Atlantic Ocean. This configuration supports a southern storm track with implications for cold advection reaching into the Deep South.

Activity Of The MJO May Be Big Player In ENSO Status, Winter Season Forecast

The long-lived, wide-ranging, and slow-moving Kelvin wave which formed over the equatorial Indian Ocean about 10 days from now is edging into the southwestern Pacific Ocean. ITCZ percolation continues east of this impulse, now reaching well to the right of the International Dateline. The flaring of the MJO is creating a linkage to three portions of the polar westerlies, energizing storms and creating windows for amplification of ridging to the right of mid-latitude cyclones. If this disturbed state along the ITCZ continues through the winter, there will be two complications to upcoming forecasts: increased potential for high-latitude ridging and blocking, as well as potential alteration of the ENSO episode (currently neutral) into a positive character (perhaps nearing an El Nino designation, although none of the computer models indicate such a turn of events). Remember that a weak +ENSO measure would favor a +PNA configuration, significant in the occurrence of cold advection for central and eastern North America.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, October 25, 2008 at 6:20 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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