WEATHERAmerica Newsletter, Saturday, October 11, 2008 at 4:55 P.M. CT

by LarryCosgrove | October 11, 2008 at 02:27 pm

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WEATHERAmerica Newsletter, Saturday, October 11, 2008 at 4:55 P.M. CT

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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)
W TX....E NM....E CO....OK Panhandle....W KS....W, C NE....SD....W MN....W ON
S MB....ND

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

Isolated Locations In
FL....GA....C, S SC
(QPF 1 - 3")

Scattered Locations In
W TX....E NM....E CO....OK Panhandle....W KS....W, C NE....SD....W MN....W ON
S MB....ND
(QPF 1 - 4")

Isolated Locations In
C, N Coastal BC
(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)

Scattered Locations In
N UT....ID....WY....MT....S SK
(Snow; 4 - 12")

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

Indian Ocean And The Orient

Strong flaring of the Madden-Julian Oscillation over equatorial Indian Ocean and Indonesia (see comments in EXTENDED PERIOD FORECAST below).

Invest 94W is a strong tropical wave which will trigger heavy rains and severe thunderstorms over much of southern China during the next 24 hours.

Convective circulation has formed over the Marianas Trench. This feature may become a tropical depression or named storm during the next 48 hours as it moves westward.

Central And Eastern Equatorial Pacific Ocean

Hurricane Norbert is weakening over Baja California Sur; a second landfall in western Mexico should occur on Sunday (as a tropical storm or depression).

Tropical Storm Odile is moving westward and away from the Mexican coastline. Heavy rainfall associated with Odile should cease within 24 hours, even though it is probable that Odile may reach Category 1 status.

An ITCZ-related impulse below Panama may become a tropical depression along the Pacific shoreline of Central America on Sunday or Monday. Major concerns are with flooding rainfall from Costa Rica into southern Mexico.

Atlantic Basin

Closed 500MB low just off the coast of GA and SC appears to be acquiring a subtropical character.

Convective impulse off of the coast of Belize is not likely to organize further due to proximity to land.

Tropical wave encountering a TUTT signature (designated Invest 98L) near the Windward Islands will not intensify due to fast southwest flow aloft, inducing shear.

Two disturbances over the equatorial Atlantic Ocean may combine into one storm. Likely trajectory is northward with eventual destination being the Azores Chain.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Impressive "Great Smokies" Heat Ridge....
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Undercut by the developing subtropical low off of the Georgia and South Carolina coast, the heat ridge that currently dominates the eastern half of the U.S. will be centered over the Ohio Valley for much of the next three days. The 90 deg F isotherm may reach into parts of MO, IL, and KY. The signature of the Great Smokies signature is its widespread display of haze, making for high ozone levels as well as abundant pollen counts from the lower Great Plains into the Great Lakes and Mid-Atlantic region.

....Faces Suppression From Cold Western U.S. Trough, Frontal Structure

I have noticed that the trend in model forecasts of the Great Smokies heat ridge is for a somewhat weaker appearance, and always to the left of the Appalachian Mountains. From that tendency we can speculate that warming over the Eastern Seaboard will not be as impressive as is the case in the Ohio and Mississippi Valley. Also that as the trough complex over the West moves out, a steady collapse of the higher 500MB heights will get underway early in the work week. The cold, moist mPk regime across the Intermountain Region will produce generous snowfall amounts from Montana into the Salt Lake Valley this weekend, and in time could set the stage for widespread frost and freeze warnings across the Corn Belt later in the coming week.

Santa Ana Episode For California?

With high pressure building rapidly south and eastward behind the departing storm and trough, the flow around surface high pressure will bank southeastward from Nevada and Arizona into California. The resultant downslope winds will heat and dry upon reaching the western reaches of the Golden State. This "Santa Ana" or "Canyon Wind" configuration may be an incendiary issue for anyone living along to the left of the Interstate 5 corridor on Sunday and Monday.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Cold Intrusion Into Midwest, Northeast At Midweek....

Ultimately, the trough and cold front now straddling the Rocky Mountains will crease the heat ridge, suppressing the higher 500MB heights and warmer surface readings into the Deep South. An intrusion of polar values is forecast to proceed through the Midwest and then Appalachia and the Atlantic Coastal Plain by October 17, bringing chilly nights and brisk days. While precipitation is widespread with the colder air now in the West, the presence of a ridge axis over the Gulf Coast and a semizonal flow aloft argues strongly for a dry frontal passage east of the Great Plains.

....While Heat Ridge Warms The Great Plains, Deep South

Without a true blocking signature at the higher latitudes, the jet stream configuration remains progressive and subtropical highs can re-emerge in between the trough axes. This appears to be the case next weekend, where what is now a Great Smokies heat ridge shifts into Texas and Louisiana. So whereas the Midwest and much of the Eastern Seaboard experience moderate to strong cooling, temperatures could warm considerably from the Gulf Coast into the Missouri Valley for much of the medium range.

Second, Stronger Shortwave Raises Severe Weather Threat By October 21

The jet stream configuration looks to remain in an excited, though somewhat semizonal, state through the extended period. The outlier ECMWF model forecast, which shows a coastal storm forming off of North Carolina and Virginia in the medium range, is a far outlier and is discarded for the purpose of this forecast. A rather impressive shortwave is shown on many of the models and respective ensembles, digging into the Great Lakes in ten days, then shifting toward Newfoundland. If the subtropical high breaks down, or moves into Florida, the opening of a higher dewpoint field from the Gulf of Mexico into this highly dynamic system may produce a severe weather event across portions of the Ohio an Tennessee Valleys into Virginia and the Mid-Atlantic states on October 21 - 23. Note also the possibility of another "Santa Ana" wind/heat episode in California around that time frame. EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Effects Of +AO, +NAO, +PNA Jet Stream Configuration

The look of the 500MB longwave pattern through the next 16 days is purely progressive, showing a strongly positive measure for the Arctic and North Atlantic Oscillations while supporting a moderately positive Pacific North America teleconnection. So while the occasional cold front penetrates ridging in western Canada and the U.S., colder than normal values will ultimately dominate the area east of the Rocky Mountains. The strong warming shown on Days 10 and 11 by the operational GFS gets pushed out by a cold intrusion by October 24; another impressive shot of mPk values is shown building over the Gulf of Alaska during the last week of October. The colder GFS and GGEM variants are favored, largely because the ECMWF scheme and its members have not verified well since modifications were made to the parent equation about two weeks ago.

Signals In The Indian And Oriental Theaters Support Neutral ENSO....

If you look at the IODC satellite view in the TROPICAL WEATHER SUMMARY above, you cannot help but notice the huge flaring of convection that covers the equatorial Indian Ocean and now extends into Malaysia and Indonesia. The upward atmospheric motion in this vicinity signifies the formation of a Kelvin wave, which in time should progress into the central Pacific Ocean and link with the polar westerlies. This type of disturbed area will, after passage, enhance warming of waters. While some have postulated that the ENSO sectors will shift toward a weak or moderate La Nina, the SST profile in most of Oceania is largely neutral. So if the wave remains strong, any subsurface cooling may be offset by the dynamic processes associated with the eastward shift of the equatorial energy signature. Bottom line: a neutral positive measure (0 to 0.5 ONI) is still in play, similar to forecasts put out by NOAA and BOM. And if an active MJO is the case this fall and winter, perhaps a weak El Nino could appear.

....And Colder Eastern U.S. Forecast

On the idea that various Kelvin waves develop over the equatorial Indian Ocean and remain intact upon reaching the International Dateline (as was the case during much of the 2007-2008 winter season), then a case exists for several strong cold intrusions into the eastern two-thirds of North America. The process supposes that the polar westerlies interact with the energy released by the impulse over the western, then central Pacific Ocean. Amplification of downstream ridging in the EPO and/or PNA positions would allow for digging of 500MB shortwaves into the U.S., much like what is shown by the operational GFS at 240 - 264 hours on the 12z run. Northwest flow off of the building snow and ice pack around and above the Arctic Circle may, in theory, help to introduce cold advection into much of Canada and the U.S. in a progressive, undulating flow configuration.

Prepared by Meteorologist LARRY COSGROVE on
Saturday, October 11, 2008 at 4:55 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

All rights reserved.

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

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