WEATHERAmerica Newsletter, Sunday, October 21, 2007 at 2:50 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)
ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
E TX....LA....MS....AL....FL....GA....W SC....W NC....W VA....W WV....KY....TN....AR
SE MO....S IL....S IN....SW OH

STRONG WINDS
(Pressure Gradient Derived)
BC

STRONG WINDS
(Santa Ana)
S CA


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

Scattered Locations In
E TX....LA....MS....AL....FL....GA....W SC....W NC....W VA....W WV....KY....TN....AR
SE MO....S IL....S IN....SW OH
(QPF 1 - 4")

Isolated Locations In
E KS....W MO
(QPF 1 - 2")

Scattered Locations In
BC
(QPF 1 - 4")


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

Scattered Locations In
SW OR....CA....W AZ

Scattered Locations In
S MS....S AL....FL....S GA


SHORT RANGE OUTLOOK
(Through The Next 72 Hours)

Torrential Rain, Severe Thunderstorms In The Lower Mississippi Valley

The presence of a full-latitude trough complex (with a portion closing off into a deep 500MB low in KS and OK) and a tropical disturbance in the Gulf of Mexico indicates the potential for both heavy rains and severe weather in parts of Dixie and the Corn Belts late tonight through a part of Tuesday. Emphasis for the heaviest rainfall totals would seem to be from S LA into MS, AL, and the FL Panhandle (where microbursts and tornadoes may also occur). GA and the Carolinas may see some moisture. But with a looming subtropical high and huge moisture deficits (15 - 20 inches), it can be said with some degree of certainty that the drought in the Southeast will not be dented by this event.
More Near-Record Warmth For The Eastern States....

Before the strong cold front can reach the Eastern Seaboard, the presence of an unseasonably strong heat ridge will mean rapid warm advection from the Deep South into the New England states. So much so that on Monday afternoon, readings as high as 90 deg F may reach close to the Mason-Dixon line and the 80 deg F maximum isotherm may sneak into parts of NH and ME.

....While Cold Intrusion Comes To The Rescue By Midweek

The coldest temperatures seen so far in the autumn season will push across the Great Lakes and Northeast during the near term (while a pocket of the cP+mP regime closes off in the lower and central Great Plains). The 30 deg F nocturnal isotherm may reach as far south as an Interstate 90 line (Angola IN to Framingham MA) on Wednesday night before the chilly domain exits out into the Maritime Provinces and northern Atlantic Ocean.

Significant Storm In Aleutian Islands Will Target The Pacific Northwest

A broad and powerful storm complex over the Aleutian Islands and Gulf of Alaska will push steadily east-southeastward during the near term. While the main impacts of this feature will be over British Columbia, some of the torrential rainfall and strong winds should affect Washington and Idaho on Monday night and Tuesday. The disturbance should move along the Canadian border, over a small heat ridge on CA and AZ. Surface convergence across the Intermountain Region between the subtropical high and the cyclone may generate a "Santa Ana" wind in the "Southland" of the Golden State. The advance of the low center into the Prairie Provinces may bring a moderate, dried regime to central Canada and the Great Plains in the wake of the realm of colder values now in place across those regions.


MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Cutoff Low Over South Central U.S. Could Be A Huge Rainmaker....

The numerical models are in agreement in forecasting a rather strong, closed 500MB low to become situated over the south central U.S. by midweek. Until a strong shortwave in the polar westerlies acts to kick this feature northeastward late this week, the flow structure between the broad disturbance and (yet another) heat ridge over the Southeast will allow for the introduction of copious amounts of deep tropical moisture into the warm sector. When the storm finally ejects out into the Midwest (I favor the 12z Oct 21 GFS scenario over that of the concurrent GGEM version), there could be a tremendous severe weather event from the "Arklatex" vicinity into the Ohio Valley with heavy stratiform rains in parts of the upper Mississippi Valley and Great Lakes during a Thursday - Saturday time frame.

....But Will The Drought In The Southeast Ever Abate?

The ongoing harsh drought may be relieved slightly in the near term from a tropical disturbance merging with a frontal structure, but long term prospects for meaningful rainfall appear to be slight. The situation boils down to the recurrent heat ridge across the Southeast, a Great Smokies/Bermuda hybrid that is a close synoptic cousin to the subtropical high which plagued the Corn Belt, Old South, and lower Mid-Atlantic regions during August and September. Teleconnections on a forecast -EPO/-AO/-NAO 500MB configuration for the late medium range and extended period strongly support the emergence of a flat, subsident anticyclone across the Deep South into the Carolinas. And that type of 500MB longwave pattern is normally associated with warmth, and drought, over Dixie.

Watching The Ice And Snow Cover For Clues About The Upcoming Winter

While record heat has been an ongoing issue for parts of the lower 48 states, colder values have claimed a foothold over the Arctic regions. This drop in temperature at higher latitudes is critical to the development of a snow and ice shield, without which the formation of true cAk regimes would be compromised. Snow and ice is expanding southward at a fairly even pace and scope, with much of northern Canada and Russia now under an appreciable snow field.

Note, however, the ice-free condition of the Arctic Sea above Siberia and the still-open waters near Nunavut AR and Greenland. This condition seems to be tied to an ongoing tendency for negative displays in the Arctic and Northern Atlantic Oscillations (in other words the formation of high latitude positive height anomalies which may ultimately suppress the jet stream and storm tracks into lower latitudes. Bottom line: there is hope for a colder, snowier type of winter (despite the moderate La Nina episode in play) for some portions of the contiguous U.S. The complete winter season forecast will be submitted around Halloween Day (October 31, 2007).

The Recurring Heat Ridge

While there is some hope for important rainfall in the near term over the parched Southeast, the reality is that the numerical models and the variants of same are in agreement on resurrection of a heat ridge along the shoreline of GA and the Carolinas in the medium range. The subtropical high should act to enhance the deep moisture fetch associated with the ejection of the closed 500MB low over the lower Great Plains. So while cities such as Atlanta GA, Spartanburg SC, Charlotte NC and Norfolk VA remain thirsty (relatively speaking), severe weather and flood threats may emerge from the lower and middle Mississippi Valley into the Corn Belt and lower Great Lakes by Days 4 - 6.

What is especially troubling is that the operational and ensemble GFS suite at 12z Oct 21 suggest that a new flat ridge will become situated over Dixie after 240 hours. This alignment, should it verify, means that the potential for precipitation would be limited across the Old South during the first week of November.

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

Disturbance Over The Gulf Of Mexico

While the odds on warm-core cyclogenesis occurring in the Gulf of Mexico are relatively low, the disturbance which has now emerged north of the Strait of Yucatan could play an important role in bringing needed rainfall (as well as unwanted severe weather) to parts of Dixie during the near term. Now linked with the trough and frontal complex over the Great Plains, the core of the impulse will track northward into the Mississippi Delta tonight and tomorrow before merging with the surface front on Monday afternoon. Aside from the danger of microbursts and tornadoes, some locations in the Deep South may see as much as 4 inches of rainfall as the energy sweeps north and northeastward from LA into TN. As an added bonus, the Atlanta GA metro could actually see a period of torrential rainfall and thunderstorms on Monday night. This downpour will not really ease the drought in the Peach State, but ANY falling moisture is better than none!

Tropical Storm Kiko

The tiny tropical storm is situated perilously close to dry, stable air to its west and a shearing southwest flow to its near north. Kiko is over warm waters, and until the circulation moves into the cooled mP layer there is a chance for intensification (with rough seas) affecting parts of western Mexico and the southern tip of Baja CA.

Typhoon Kajiki


Typhoon Kajiki is now fully entrained within the polar westerlies, and in another 48 hours should be extratropical. Linked with the powerful storm over the Sea of Okhotsk, Kajiki may, like LingLing before it, bring gales to portions of the Aleutian Islands, Alaska and British Columbia at some point in the medium range.

Prepared by Meteorologist LARRY COSGROVE on
Sunday, October 21, 2007 at 2:50 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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