WEATHERAmerica Newsletter, Thursday Evening, September 18, 2008 at 8:45 P.M. CT

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 ON....C QB

Some Thunderstorms May APPROACH Severe Limits
LA....AR....W TN....MS....S AL

Some Thunderstorms May APPROACH Severe Limits
Interior CA....NV....AZ....NM....CO....UT

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

Isolated Locations In
NE FL....Coastal GA, SC, NC
(QPF 1 - 2")

Isolated Locations In
W ON....C QB
(QPF 1 - 2")

Isolated Locations In
LA....AR....W TN....MS....S AL
(QPF 1 - 3")

EXCESSIVE HEAT OUTLOOK
(potential for maximum temperature to exceed 95 deg F within the next 24 hours)

Isolated Locations In
S Interior CA....NV....AZ....NM....W CO....UT....ID....W WY

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

Eastern Equatorial Pacific Ocean

Huge convective circulation straddles an area from the Gulf of Tehuantepec through Central America into Colombia. This feature has the potential to cross over into the Caribbean Sea, and may represent a threat for warm-core cyclogenesis in either the Pacific or Atlantic theaters. Abundant torrential rainfall may trigger widespread and dangerous flooding from Oaxaca State of Mexico into Panama.

Atlantic Basin

There are three ITCZ-related impulses (90E, a convective cluster near Panama and a disturbed area (93L) east of the Windward Islands) of interest which may become a threat for tropical cyclone development in the southwestern Atlantic Basin. Any one of those features could move northward under a very well-defined equatorial moisture and energy connection now straddling a belt from Brazil into Newfoundland. This is of critical importance to forecasts for the Eastern Seaboard in the medium range, owing to the threat that a warm-core system and the deep tropical fetch could mesh with an extratropical low near the Carolinas. In concert with a strong high pressure cell predicted to pass off the New England coastline, a widespread rain and wind event (with coastal flooding and beach erosion) may occur around September 25 - 27. More on this threat in the MEDIUM RANGE OUTLOOK below.

African ITCZ

Three important ITCZ waves noted (Nigeria, Central African Republic, Somalia). Notice, however, that westerly flow aloft is present over the western Sahara Desert and adjacent Atlantic Ocean, implying these disturbances will not be a threat for tropical development upon emergence into equatorial waters (due to shearing wind profiles).

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

In The Aftermath Of Ike

As a witness to the power of Hurricane Ike last weekend (two chase trips along the perimeter and the western eyewall), the storm which devastated much of East Texas left a lasting imprint on the 500MB longwave pattern over the eastern two-thirds of the U.S. Ike merged with a trough complex and cold front, and helped to drag down very cool values as far south as the Strait of Yucatan while hammering the Midwest with floods, severe weather, and strong winds derived from its compact pressure gradient.

While the immediate effects of the storm are slowly (and, I might add, painfully) receding into memory, the shift in the jet stream configuration to a +PNA alignment implies that most of the interior of North America will see strong intrusions of cooler air but mostly dry conditions. There are exceptions of note (and you will see one such difference in the MEDIUM RANGE OUTLOOK below), such as the TUTT signature over the Deep South and the prominent thunderstorm episode for parts of central Canada. But overall, locations across the U.S. are starting to have that autumn "feel" of lower temperatures and reduced relative humidity.

More (Pleasantly) Cool Air Along And East Of The Mississippi Valley

The progression of strong, yet mainly dry, cold fronts across the lower 48 states will pay some dividends in beautiful fall weather. Warmings will be brief; lowered temperatures and dewpoints will leave a mark everywhere with the possible exception of Florida. Consider that on Saturday morning, much of Appalachia and the Eastern Seaboard will have readings anywhere between 5 and 15 deg (F) below seasonal averages.

Warmth Builds Into The Great Plains Ahead Of New Intrusion Of Pacific Air

This being (for the most part) a progressive 500MB longwave pattern, the procession of shortwaves and fronts across the U.S. are bound to allow some building and advance of warm, dry air. The very summer-like air mass now over the Intermountain Region will be escorted quickly into the Great Plains, replaced by a period of cool, raw conditions dominating the Pacific Northwest. With a downslope, mostly southwesterly flow from the Front Range to the Mississippi River this weekend, the 90 deg F isotherm may reach as far north as NE and IA.

MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Very Calm Across Much Of The U.S., But....

While the lack of precipitation leading into the 6 - 10 day time frame may cast a tranquil appearance, there is potential for volatility in apparent weather across the U.S. One potential source of impact is the deep trough and cold front sweeping through the Intermountain Region early next week. Another conflict shaping up is between the strong IcP anticyclone passing through the Midwest and Northeast versus the looming threat from the deep tropical moisture/energy fetch out of the Lesser Antilles. Embedded in that flow is Invest 93L, which in time could link with a vestigial frontal structure over Florida and the Gulf Stream (see below).

....Is That A Nor'easter Brewing?

Many of the numerical models and the respective ensemble members have been showing what appears to be a significant wind and rain event along the East Coast from SC into New England and Nova Scotia during the course of the Day 6 - 10 time frame. At first glance, this development is a result of merging a tropical entity (93L, perhaps) with a weak 500MB cutoff low drifting out of lower Appalachia. What makes this situation interesting is the scenario of a long, drawn-out interaction with high pressure slowly giving ground over a period of four days. If true, and the components are in place to create such an episode, something approximating a subtropical or hybrid Nor'easter may affect the Interstate 81 and 95 corridors with high winds, heavy rainfall, coastal and lowland flooding and beach erosion next week. Stay tuned....

Colder Values Poised To Sweep Across The Western States

The GFS and ECMWF ensemble packages hint at a chance for strong cooling for the U.S. in the medium range. Note that on the attached 500MB mean forecasts that a well-defined -AO and -NAO complex seems to suppress the shortwave sequence southward, culminating in a full-latitude trough over the Eastern Seaboard by September 28. If, as I suspect, this intrusion of cooler air combines with the aforementioned "possible" Nor'easter, yet another case of a cold front reaching Cuba and the Strait of Yucatan could be in play in about ten days.

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

After Initial Cooling, Milder Values May Return To Area East Of Rocky Mountains

The shift to a semizonal flow may occur midway through the extended period. Ensemble forecasts tend toward a flatter 500MB longwave pattern with reduced emphasis on higher latitude blocking signatures (although one such positive height anomaly may remain in the vicinity of Greenland). the dominant force in the prevailing conditions appears to be a high-velocity jet stream associated with a deep low in the Gulf of Alaska. As this wind maximum pushes inland, warmer values should develop downwind of the Great Plains, eventually settling into parts of the Midwest and Dixie. Keep in mind that this configuration is largely a dry one outside of the Pacific Northwest.

The La Nina Vs, El Nino Debate And What It Means For Autumn And Winter

While some forecasters have decided to "jump the gun" on a possible return to La Nina conditions, the actual evidence for a return to a negative ENSO event is marginal at best. Realize that all across the Atlantic Basin, mean wind vectors are WESTERLY (check out the satellite view for proof) and NOT from the east. This is why there is near-zero potential for a later September or October surprise from an African ITCZ wave. The September atmosphere has a very "El Nino" look to it. See also the abundant moisture fetch across Mexico and Central America, a signature often precedent to the development of a lower latitude storm track in late fall and winter.

Consider also that for all the chatter about "subsurface cooling", the equatorial Pacific Basin is still relatively warm with rises in marine temperatures very evident west of the International Dateline. If you use the argument that most stronger La Ninas (and last year was largely a moderate, not strong, episode) then it is possible that a weak -ENSO could occur. That said, the PDO signal is not a good indicator of cooler equatorial waters and it seems probable that a neutral to weak El Nino episode will take place during the 2008-2009 winter season. Bottom line: any way you cut it, temperature forecasts for the October - March period should have a COLDER slant east of the Rocky Mountains.

Prepared by Meteorologist LARRY COSGROVE on
Thursday Evening, September 18, 2008 at 8:45 P.M. CT

Copyright 2008 by Larry Cosgrove

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