WEATHERAmerica Newsletter, Sunday, September 30, 2007 at 5:40 A.M. ET

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)

SCATTERED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
E KS....W, C MO....IA....W WI....MN....W, C ON....W QC

Some Thunderstorms May APPROACH Severe Limits
W AB....BC....WA....OR....NV....ID....W MT

Some Thunderstorms May APPROACH Severe Limits
FL Peninsula


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

Scattered Locations In
W AB....BC....WA....OR....NV....ID....W MT
(QPF 1-3")

Scattered Locations In
E KS....W, C MO....IA....W WI....MN....W, C ON....W QC
(QPF 1-3")

Isolated Locations In
FL Peninsula
(QPF 1-2")


EXCESSIVE HEAT ADVISORY
(potential for daily maxima above 90 F)

Scattered Locations In
S CA....S NV....AZ....NM....TX....LA....S AR....S OK


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Still Talking Heat, And Severe Thunderstorms !

With well-developed 500MB shortwaves smashing into areas of strong surface convergence (cP/cT/mT), intense convection is almost a given during a typical "La Nina" fall season. This autumn is no exception. A rather vigorous impulse in the westerlies exhibits a closed circulation across the northern and central Great Plains, and this feature is running straight toward a regime of unseasonably high temperatures and dewpoints. The spoke of very high vorticity (50 units) progressing from NE through IA would seem to favor a tornado threat in the upper/middle Mississippi Valley today. Since the system is moving into a mean ridge axis across the Ohio Valley, some weakening is likely and the convective threat in the Corn Belt will be greatly diminished on Monday.

Strong, Progressive 500MB Shortwaves Will Not Bring Meaningful Intrusions Of Cool Air

It is important to remember that while individual shortwaves and associated surface cold fronts may appear to be quite strong (and are...), the chances of getting a sharp, lasting reduction in temperature to very warm areas across the Great Plains, Dixie, and Eastern Seaboard is virtually nil. With a tendency for heat ridge formation in the Great Smokies and/or Bermuda position (it's an La Nina thing), each powerful blast of mP values modifies along a cross-country trek. Outside of a 24-48 hour period of lowered readings and dewpoints, the ongoing 500MB longwave pattern favors much above normal temperatures from the Great Plains to the Atlantic shoreline. This conditions will likely last through the next 10 days.

Heavy Rain Threat In Northern Mexico Has Designs On Southwestern U.S.

Combine the huge moisture shield of Tropical Storm Juliette with an axis of deep moisture running along the Mexican Cordillera, and you can see why a substantial threat for flooding rain and thunderstorms is increasing for parts of the Desert Southwest and "Four Corners" vicinity by tomorrow. The area of high dewpoints is moving north-northeast, and by Day 3 should merge with a cold front and trough that is now entering the Pacific Northwest. Excessive rainfall is therefore likely from the Sonoran Desert into the lower Great Plains.

The Pacific Northwest Sees Heavy Rain And Chilly Temperatures

The cold front which is now passing through BC, WA, OR and N CA is loaded with moisture and has plenty of chilly (mP) air in its western quadrants. The extraordinary amount of lifting and forcing, combined with instability, will make for a rather raw, stormy forecast for the northwestern states today. Another "loaded" impulse in the polar westerlies over the Aleutian Islands will barrel into the Pacific Northwest on Tuesday with more inclement weather.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

The Relevance Of Late Season Heat Ridges To U.S. Tropical Cyclone Strikes

Yet another heat ridge is forecast to develop during the first part of the medium range, stretching from the Mid-South into the western Atlantic Ocean. Besides maintaining the run of very warm to hot weather over Appalachia and the Eastern Seaboard, the anticyclone may serve to incubate a tropical disturbance now taking shape over the Turks/Caicos chain. Providing an outflow mechanism, reducing wind shear over a warm water body is a sure recipe for tropical development. It should also be pointed out that as the ridge complex begins to fall apart (note the approach of an impulse in the polar westerlies around Day 7 and 8), the developing feature may turn northward and link with oncoming frontal structure. This sequence of events may result in an important wind and rain event for the Mid-Atlantic and New England states within 192 hours.

-PNA Configuration Is Typical Of A La Nina Episode....

In most autumns represented by a -ENSO anomaly, rainfall in the Pacific Northwest will be above normal and temperatures colder than average. That would seem to be the case in this "La Nina" fall across western Canada into WA, OR, and ID. All of the global models and the respective ensembles show a series of strong mP frontal passages from the Gulf of Alaska into the northern Rocky Mountains, lasting through October 15. Some of the higher elevations may also see snow, and any vestige of summer in the Puget Sound and Columbia Basin looks to be long gone.

....But Strengthening -NAO Signature Is Not !

One item which is not the normal in a cold anomaly over the equatorial Pacific Basin is a prominent NAO-styled blocking signature. Usually, a La Nina episode is characterized by a strong, semizonal flow aloft with little evidence of high latitude blocks. But the numerical models and variants have been consistent for days now in showing a prominent Rex block taking shape over Greenland and Iceland by the end of the medium range. For such a positive height anomaly to form, three things must happen. The Scandinavian ridge must retrogress toward North America, a shortwave in the polar westerlies must amplify, and a tropical feature must conjoin with that northern stream impulse (to produce enough warm advection to pump up 500MB height profiles). Bottom line: if the NAO forecasts are correct, we may see a tremendous storm over the Great Lakes and/or Northeast in the 192 - 240 hour time frame.


TROPICAL WEATHER OUTLOOK
(Tropical Features In The Atlantic And Pacific Basins)

Karen And Melissa: Alive Or Dead ?

The answer is "dead", at least in the "official" sense as Karen has been declared dissipated and Melissa is obviously struggling. In both cases, string TUTT signatures are creating a very sheared vertical profile with thunderstorms being pushed away from the center of circulation. There is a fair chance that as a heat ridge replaces the upper level low over the Sargasso Sea, Karen may regroup and move westward (along the lines of the 0z Sep 30 ECMWF forecast). As for Melissa, the tropical storm is caught up in a trough attached to a very deep storm that is approaching the British Isles. So recurvature and minimal growth of the TS is likely during the following 72 hours.

Disturbance Over Turks, Caicos Chain May Be An Important "Player"

While the satellite imagery shows a highly sheared system, thunderstorms have remained a constant in the southwestern quadrant of the Sargasso Sea. I do not trust the baroclinic model forecasts for this feature, which range from the improbable (NAM outlook seeing a Gulf of Mexico hurricane) to the puzzling (little motion through 240 hours as seen by the latest ECMWF depictions). With rising 500MB heights over the Eastern Seaboard, further organization of the disturbed area is a possibility, as is the idea of a path toward FL and the Eastern Seaboard during the medium range.

Wherefore Art Thou, Juliette?

The tropical storm that is well southwest of Baja California, Juliette, may become a hurricane as it recurves northward. The very fragmented appearance of TS belies its threat to northern Mexico and the American Desert Southwest. The fetch of deep tropical moisture associated with Juliette will help to create a heavy rain and flood threat along and to the right of its path into the U.S. In addition, the higher dewpoints may enhance rainfall amounts ahead of a cold front slated to enter the Great Plains within 72 hours.

Lekima, And Tropical Disturbance Set In Oceania, May Give A Boost To The Polar Jet Stream

Tropical Storm Lekima is expected to move northward toward eastern China and/or Taiwan, then merge with the polar westerlies in about four or five days. Other tropical disturbances in Oceania are expected to follow suit, tracking northward and acting to strengthen the jet stream flow. With energy interacting with strong midlatitude cyclones over Japan and the northern Pacific Ocean, warm advection ahead of the enhanced storm may pump up ridging over western North America, and in turn increase digging and cold advection of cP-based shortwaves into the lower 48 states at some point during the late medium range.


Prepared by Meteorologist LARRY COSGROVE on
Sunday, September 30, 2007 at 5:40 A.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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