[Tropical] Tropical Weather Discussion

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AXNT20 KNHC 181723
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT FRI SEP 18 2009

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS
OF SOUTH AMERICA...AND ATLANTIC OCEAN TO THE AFRICAN COAST
FROM THE EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED
ON SATELLITE IMAGERY...METEOROLOGICAL ANALYSIS...WEATHER
OBSERVATIONS...AND RADAR.

BASED ON 1200 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
1630 UTC.

...SPECIAL FEATURES...
AN AREA OF SHOWERS/THUNDERSTORMS ASSOCIATED WITH A 1010 MB LOW
NEAR 13N39W CONTINUES TO BECOME BETTER ORGANIZED. SCATTERED
MODERATE/ISOLATED STRONG CONVECTION IS MAINLY TO THE E OF THE
LOW CENTER FROM 11N-16N BETWEEN 35W-38W. UPPER LEVEL WIND
CONDITIONS REMAIN ONLY MODERATELY FAVORABLE FOR FURTHER
DEVELOPMENT. THERE IS A MEDIUM CHANCE...30 TO 50 PERCENT...OF
THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HRS.

...TROPICAL WAVES...

TROPICAL WAVE IS ALONG 24W/25W S OF 18N MOVING W NEAR 15 KT. THE
WAVE COINCIDES WITH A DEEP LAYER MOISTURE MAXIMUM OBSERVED IN
TOTAL PRECIPITABLE WATER IMAGERY. THERE IS ALSO AN AREA OF 85O
MB VORTICITY NEAR THE WAVE AXIS DEPICTED BY THE CIMSS WAVETRAK
MODEL GUIDANCE. SCATTERED MODERATE/STRONG CONVECTION IS FROM
16N-19N BETWEEN 22W-27W.

TROPICAL WAVE IS ALONG 89W S OF 21N MOVING W NEAR 15 KT. THE
WAVE LIES WITHIN A BROAD AREA OF DEEP LAYER MOISTURE. DEEP
CONVECTION IS CONFINED ONLY TO THE S PORTION OF THE WAVE IN THE
E PACIFIC BASIN. HOWEVER...ISOLATED SHOWERS ARE TO THE E OF THE
WAVE AXIS FROM 16N-22N. THIS ACTIVITY MAY BE MORE INFLUENCED BY
UPPER LEVEL DIFFLUENCE.

...ITCZ...

THE ITCZ AXIS IS CENTERED ALONG 15N15W 16N23W 13N30W 9N43W
11N56W 9N63W. SCATTERED MODERATE/STRONG CONVECTION IS FROM
15N-18N BETWEEN 18W-22W. SCATTERED MODERATE/ISOLATED STRONG
CONVECTION IS FROM 15N-17N BETWEEN 28W-30W. ISOLATED MODERATE
CONVECTION IS FROM 9N-11N BETWEEN 56W-61W.

...DISCUSSION...

GULF OF MEXICO...
AS OF 1500 UTC...A SURFACE TROUGH EXTENDS FROM S MISSISSIPPI
NEAR 30N89W ALONG 27N92W 26N98W. SURFACE CONVERGENCE NEAR THE
TROUGH IS SUPPORTING ISOLATED SHOWERS/THUNDERSTORMS ALONG THE
TROUGH AXIS. A STATIONARY FRONT LINES THE NERN GULF EXTENDING
FROM OFF THE FLORIDA COAST NEAR 28N83W ALONG 30N86W 31N91W TO A
1010 MB SURFACE LOW OVER LOUISIANA NEAR 31N93W. SCATTERED
SHOWERS/THUNDERSTORMS ARE AHEAD OF THE FRONT LINING THE NW COAST
OF FLORIDA FROM 27N-31N BETWEEN 83W-85W. A FEW ISOLATED
SHOWERS/THUNDERSTORMS ARE IN THE SW GULF S OF 22N W OF 93W. THE
FRONT AND SURFACE TROUGH ARE SUPPORTED BY AN UPPER LEVEL TROUGH
ACROSS MUCH OF THE CENTRAL GULF WITH AXIS ALONG 90W. AN UPPER
LEVEL RIDGE CENTERED OVER MEXICO NEAR 20N102W COVERS THE FAR SW
GULF. UPPER LEVEL DIFFLUENCE BETWEEN THESE FEATURES IS ENHANCING
THE SHOWER ACTIVITY IN THE SW GULF. A SEPARATE UPPER LEVEL RIDGE
CENTERED OVER THE WRN ATLC NEAR 29N73W EXTENDS OVER THE FAR SE
GULF. UPPER LEVEL DIFFLUENCE BETWEEN THIS RIDGE AND THE UPPER
LEVEL TROUGH IS ENHANCING THE SHOWER ACTIVITY NEAR THE FRONT IN
THE NE GULF.

CARIBBEAN SEA...
ISOLATED SHOWERS/THUNDERSTORMS ARE ACROSS THE NW CARIBBEAN N OF
20N W OF 82W. THIS ACTIVITY IS ENHANCED BY UPPER LEVEL
DIFFLUENCE BETWEEN AN UPPER LEVEL LOW COVERING MUCH OF THE
CENTRAL AND W CARIBBEAN CENTERED NEAR 15N78W AND THE UPPER LEVEL
RIDGE OVER THE WRN ATLC THAT EXTENDS INTO THE SE GULF. THE UPPER
LEVEL LOW IS ALSO SUPPORTING ISOLATED SHOWERS/THUNDERSTORMS
ACROSS THE CENTRAL CARIBBEAN FROM 11N-20N BETWEEN 70W-82W.
ISOLATED SHOWERS/THUNDERSTORMS ARE ALSO ACROSS THE ERN CARIBBEAN
FROM 13N-16N BETWEEN 63W-67W. THESE SHOWERS ARE ENHANCED BY
MOIST SLY AND DIFFLUENT FLOW BETWEEN THE UPPER LEVEL LOW AND
UPPER LEVEL RIDGING OVER THE E CARIBBEAN. MODERATE TRADE WINDS
ARE ACROSS MUCH OF THE BASIN. EXPECT SHOWER ACTIVITY TO CONTINUE
ACROSS THE WRN CARIBBEAN AS THE UPPER LEVEL LOW SHIFTS W-SW.

ATLANTIC OCEAN...
A SURFACE TROUGH EXTENDS FROM NEAR 31N71W TO THE NW BAHAMAS NEAR
26N77W. SURFACE CONVERGENCE NEAR THE TROUGH IS SUPPORTING
ISOLATED SHOWERS/POSSIBLE THUNDERSTORMS ALONG THE TROUGH AXIS.
SCATTERED SHOWERS/THUNDERSTORMS ARE S OF THE TROUGH AXIS FROM
23N-26N BETWEEN 76W-79W. A 1012 MB SURFACE LOW...REMNANTS OF
FRED...IS CENTERED NEAR 25N66W. A SURFACE TROUGH EXTENDS THROUGH
THE LOW CENTER FROM 28N65W TO 24N66W. SCATTERED
SHOWERS/THUNDERSTORMS ARE MAINLY W OF THE LOW CENTER FROM
23N-26N BETWEEN 66W-69W. THE REMAINDER OF THE ATLC IS DOMINATED
BY A BROAD SURFACE RIDGE ANCHORED BY A 1025 MB HIGH NEAR 34N46W.
IN THE UPPER LEVELS...AN UPPER LEVEL RIDGE DOMINATES THE WRN
ATLC CENTERED NEAR 29N73W. AN UPPER LEVEL LOW LIES TO THE SE
CENTERED NEAR 21N58W. UPPER LEVEL DIFFLUENCE BETWEEN THESE TWO
FEATURES IS ENHANCING SHOWER ACTIVITY NEAR THE SURFACE LOW NEAR
25N66W. AN UPPER LEVEL TROUGH EXTENDS ACROSS THE NE ATLC WITH
AXIS ALONG 30N22W TO 22N37W. A BROAD UPPER LEVEL RIDGE COVERS
THE TROPICAL ATLC ENHANCING CONVECTION ALONG THE ITCZ.

$$
WALTON





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