[Tropical] Tropical Weather Discussion

[EMWIN Server]

AXNT20 KNHC 241759
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT WED SEP 24 2008

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
1715 UTC.

...SPECIAL FEATURES...
A BROAD AREA OF LOW PRESSURE IS OVER HISPANIOLA...PUERTO
RICO...AND THE ADJACENT WATERS...WITH AN ASSOCIATED 1006 MB LOW
NEAR 19N71W...OR OVER WESTERN PORTIONS OF THE DOMINICAN
REPUBLIC. CONVECTION HAS DECREASED IN AREAL COVERAGE AND
INTENSITY OVER THE LAST 24 HOURS. HOWEVER...SCATTERED MODERATE
CONVECTION LINGERS NORTHEAST OF THE DOMINICAN REPUBLIC AND
OVER THE MONA PASSAGE APPROACHING PUERTO RICO...OR FROM 16N-22N
BETWEEN 66W-70W. THIS SYSTEM HAS THE POTENTIAL TO DEVELOP INTO A
TROPICAL CYCLONE AS IT MOVES GENERALLY NORTHWARD...AND AWAY FROM
HISPANIOLA...TODAY AND TONIGHT. THE POTENTIAL FOR HEAVY RAINFALL
AND LIFE-THREATENING FLASH FLOODS AND MUD SLIDES OVER HISPANIOLA
WILL CONTINUE DURING THE NEXT DAY OR SO.

...TROPICAL WAVES...
AN ATLANTIC OCEAN TROPICAL WAVE IS ALONG 39W/40W S OF 17N MOVING
W NEAR 15 KT. A CLUSTER OF MODERATE CONVECTION HAS DEVELOPED
FROM 13N-15N BETWEEN 37W AND THE WAVE AXIS. COMPUTER MODEL
GUIDANCE DEPICTS WEAK CIRCULATION OVER THE NORTHERN PORTION OF
THE WAVE AXIS.

A CARIBBEAN SEA TROPICAL WAVE IS ALONG 64W S OF 18N MOVING W
NEAR 15 KT. CONFLUENT AND DRY FLOW ALOFT OVER THE NORTHERN
PORTION OF THE WAVE AXIS IS INHIBITING CONVECTION OVER THIS
PORTION OF THE WAVE. ISOLATED SHOWERS ARE WITHIN 90 NM OF THE
WAVE SOUTH OF 10N...WHERE SOME WEAK CYCLONIC CURVATURE HAS BEEN
NOTED FROM A RECENT ASCAT PASS.

A CARIBBEAN SEA TROPICAL WAVE IS ALONG 77W S OF 16N MOVING W
NEAR 15 KT. THIS WAVE HAD PREVIOUSLY PULLED WEST OF THE SYSTEM
DISCUSSED IN THE SPECIAL FEATURES SECTION. THIS WAVE HAS
CONSIDERABLY LOST DEFINITION...AS IT IS EMBEDDED WITHIN A
MINIMUM OF DEEP LAYER MOISTURE PER MIMIC TOTAL PRECIPITABLE
WATER IMAGERY...WHERE THE UPPER LEVEL FLOW IS RELATIVELY WEAK.
NEVERTHELESS...ISOLATED SHOWERS ARE OCCURRING WITHIN 120 NM OF
THE WAVE AXIS. THE GREATER PROPORTION OF SHOWERS IS ALONG THE
WAVE SOUTH OF 12N...WHERE WEAK DIFFLUENCE IN A LOW AMPLITUDE
UPPER RIDGE IS PROVIDING SOME UPPER SUPPORT.

A WESTERN CARIBBEAN SEA TROPICAL WAVE IS ALONG 89W/90W S OF 18N
MOVING W 10-15 KT. THE WAVE IS INLAND OVER CENTRAL AMERICA AND
IN THE E PACIFIC WITH ISOLATED SHOWERS 120 NM ON EITHER SIDE OF
THE WAVE AXIS N OF 12N...MAINLY AFFECTING GUATEMALA...
BELIZE...AND WESTERN HONDURAS.

...THE ITCZ...
ITCZ AXIS IS CENTERED ALONG 7N11W 6N24W 7N34W 9N46W 9N61W.
CLUSTERS OF SCATTERED MODERATE/STRONG CONVECTION ARE FROM 4N-8N
BETWEEN 12W-19W. ISOLATED MODERATE CONVECTION IS FROM 3N-10N
BETWEEN 22W-31W.

...DISCUSSION...

GULF OF MEXICO...
AN UPPER RIDGE EXTENDS EASTWARD OVER THE SOUTHERN GULF FROM AN
UPPER HIGH NEAR 22N103W...WITH NORTHWESTERLY UPPER FLOW
AFFECTING MUCH OF THE GULF. WEAK DIFFLUENCE IN THIS FLOW
COMBINED WITH DEEP LAYER MOISTURE IS GENERATING SCATTERED
MODERATE CONVECTION WITHIN 300 NM OF THE TEXAS AND MEXICAN
COASTLINE ALONG THE GULF. A STRONG UPPER TROUGH OVER THE EASTERN
CONUS CONTINUES DIGGING SOUTHWARD OVER THE EASTERN GULF. AS
UPPER LEVEL SUPPORT WITH THIS TROUGH INTERACTS WITH A DEEPENING
SURFACE LOW OVER THE SOUTHWEST ATLC...NORTHERLY SURFACE WINDS
NEAR 20 KT WILL PULL DRY AIR INTO THE NORTHEAST GULF.
NORTHWESTERLY UPPER FLOW WEST OF THE TROUGH AXIS WILL CONTINUE
BRINGING DRY AND STABLE AIR...RESULTING IN FAIR WEATHER
CONDITIONS...OVER THE CENTRAL AND EASTERN GULF.

CARIBBEAN SEA...
THE MAIN CONCERN CONTINUES TO BE THE AREA OF LOW PRESSURE
DESCRIBED IN THE SPECIAL FEATURES SECTION. THE UPPER RIDGE
PRESENTLY OVER THE GULF OF MEXICO COVERS THE W CARIBBEAN W OF
78W...WITH A NARROW UPPER TROUGH AXIS EXTENDING FROM EASTERN
CUBA TO COSTA RICA. TO THE EAST OF THIS UPPER TROUGH...AN UPPER
RIDGE IS AFFECTING THE EASTERN CARIBBEAN EAST OF 73W. DEEP LAYER
DRY AIR BEING PULLED INTO THE WESTERN AND CENTRAL CARIBBEAN FROM
THE NORTH ARE LIMITING CONVECTION THERE...WHERE WESTERLY SURFACE
FLOW IS PRESENT. THE ONLY SUBSTANTIAL CONVECTION OVER THE
CARIBBEAN IS OVER THE NORTHEAST CARIBBEAN DISCUSSED IN THE
SPECIAL FEATURES SECTION...WITH ISOLATED STRONG CONVECTION IN A
SMALL AREA OFF THE COSTA RICA COAST FROM 9N-12N BETWEEN 81W-83W.

ATLANTIC OCEAN...
ONE CONCERN FOR THE SW ATLC IS THE LOW PRESSURE AREA DESCRIBED
IN THE SPECIAL FEATURES ABOVE...AS IT IS FORECAST TO MOVE INTO
THE SW ATLC WATERS. ISOLATED MODERATE CONVECTION IN ASSOCIATION
WITH THIS SYSTEM IS AFFECTING AREAS OF THE SOUTHWEST ATLANTIC
FROM 20N-23N BETWEEN 66W-70W. ANOTHER CONCERN IS THE DEEPENING
SURFACE LOW NEAR 33N74W. UPPER SUPPORT FROM A DEEP UPPER TROUGH
NEAR THE EASTERN US COASTLINE CONTINUES TO STRENGTHEN THE
ATTENDANT SURFACE LOW. AS A RESULT...STRONG NORTHEASTERLY
SURFACE FLOW CONTINUES TO WRAP DRIER AND SLIGHTLY COOLER AIR
OVER THE SW ATLC AND FLORIDA PENINSULA BEHIND A COLD FRONT OVER
THE SW ATLC. THE COLD FRONT EXTENDS FROM 34N71W TO 31N72W TO
SOUTH FLORIDA NEAR 26N81W...WITH GALE FORCE NORTHEASTERLY
SURFACE WINDS BEHIND THE COLD FRONT. BANDS OF SHOWERS AND
ISOLATED THUNDERSTORMS CONTINUE TO DEVELOP ALONG THE FRONT EAST
OF 76W. FARTHER EAST...A BROAD UPPER RIDGE SEPARATES THE UPPER
TROUGH NEAR THE EASTERN US COASTLINE FROM A LARGE UPPER LOW
CENTERED NEAR 26N49W. THIS UPPER LOW IS BRINING CLOUDINESS AND
ISOLATED SHOWERS TO THE CENTRAL ATLANTIC FROM 20N-30N BETWEEN
39W-53W. ANOTHER UPPER LOW IS CENTERED NEAR 32N20W...WITH AN
ATTENDANT SURFACE LOW NEAR 34N23W. A SURFACE TROUGH TRAILS
SOUTHWARD FROM THIS LOW TO NEAR 28N23W TO NEAR 24N29W.
INSTABILITY AND LOW LEVEL SUPPORT FROM THESE FEATURES ARE
GENERATING ISOLATED SHOWERS OVER THE EASTERN ATLANTIC N OF 27N
EAST OF 26W. AN UPPER RIDGE EXTENDS WESTWARD FROM AN UPPER HIGH
NEAR 15N20W AND COVERS MUCH OF THE TROPICAL ATLANTIC...WITH
MAINLY DRY AND STABLE AIR UNDER THE RIDGE...RESULTING IN MAINLY
FAIR WEATHER. MOISTURE ON THE WESTERN PERIPHERY OF THE RIDGE IS
RESULTING IN SCATTERED MODERATE CONVECTION FROM 14N-18N BETWEEN
57W-60W.

$$
COHEN/DGS



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