[Tropical] Tropical Weather Discussion

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AXNT20 KNHC 251801
TWDAT

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT THU SEP 25 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 996 MB SURFACE LOW IS CENTERED NEAR 32N77W...OR ABOUT 170 NM
SE OF THE SOUTH CAROLINA-NORTH CAROLINA BORDER. THIS LOW IS
MOVING SLOWLY WESTWARD. THERE IS STILL A POSSIBILITY THAT THIS
SYSTEM COULD DEVELOP INTO A SUBTROPICAL OR TROPICAL CYCLONE
LATER TODAY. REGARDLESS OF WHETHER OR NOT THIS SYSTEM BECOMES A
SUBTROPICAL OR TROPICAL CYCLONE...STRONG WINDS...COASTAL
FLOODING...HIGH SURF...AND DANGEROUS RIP CURRENTS WILL CONTINUE
ALONG PORTIONS OF THE U.S. EAST COAST DURING THE NEXT COUPLE OF
DAYS. OUTER RAINBANDS AND GUSTY WINDS CONTINUE SPREADING ONSHORE
FROM SOUTH CAROLINA TO VIRGINIA.

A 1003 MB SURFACE LOW IS CENTERED NEAR 23N69W...OR ABOUT 150 NM
NE OF THE TURKS AND CAICOS ISLANDS...MOVING NORTHWARD NEAR 10
KT. MODERATE WEST-SOUTHWESTERLY FLOW ALOFT HAS SHEARED
CONVECTION AWAY FROM THE LOW LEVEL CIRCULATION...AND THIS SYSTEM
IS NOT WELL ORGANIZED. CLUSTERS OF MODERATE CONVECTION ARE
MAINLY OVER THE SOUTHWEST ATLANTIC WATERS FROM 20N-25N BETWEEN
62W-68W. UPPER-LEVEL WINDS COULD BRIEFLY BECOME MORE CONDUCIVE
FOR DEVELOPMENT...AND THIS SYSTEM COULD BECOME A TROPICAL
CYCLONE OVER THE NEXT DAY OR SO.

...TROPICAL WAVES...
A NEW TROPICAL WAVE HAS BEEN ADDED TO THE 1200 UTC MAP ALONG
25W/26W S OF 17N MOVING W NEAR 15 KT. A LOOP OF MIMIC TOTAL
PRECIPITABLE WATER PRODUCT FROM CIMSS DEPICTS A WELL-DEFINED
MAXIMUM IN DEEP LAYER MOISTURE...WHICH HAD PREVIOUSLY PUSHED
WEST OF THE AFRICAN COAST EARLY THIS WEEK. SCATTERED MODERATE
CONVECTION IS OCCURRING 90 NM EITHER SIDE OF THE WAVE AXIS N OF
12N...AND VISIBLE SATELLITE IMAGERY DEPICTS LOW LEVEL
CIRCULATION IN THE MOTION OF CLOUD ELEMENTS.

A TROPICAL WAVE IS ALONG 36W/37W S OF 17N MOVING W 10-15 KT.
THIS TROPICAL WAVE HAS BEEN RELOCATED EASTWARD FROM ITS 25/0600
UTC POSITION ALONG 42W/43W S OF 17N. SCATTERED MODERATE
CONVECTION HAS BEEN NOTED FROM 11N-17N BETWEEN 33W-40W...WHERE
TOTAL PRECIPITABLE WATER IS MAXIMIZED. VISIBLE SATELLITE IMAGERY
NOW DEPICTS A WELL-DEFINED INVERTED-V SIGNATURE IN THE
ASSOCIATED LOW LEVEL CLOUD ELEMENTS...AND COMPUTER MODEL
GUIDANCE CONFIRMS THE PRESENCE OF LOW LEVEL CIRCULATION IN THE
NEW LOCATION FOR THIS WAVE. ADDITIONALLY...EXTRAPOLATION BASED
ON WAVE PASSAGE INFERRED FROM UPPER AIR TIME SECTION AT DAKAR
SENEGAL SUPPORTS THIS NEW LOCATION.

TROPICAL WAVE IS ALONG 68W S OF 15N MOVING W 10-15 KT. WAVE
POSITION IS BASED ON CONTINUITY AS IT HAS ALMOST NO SATELLITE
SIGNATURE AND NO ASSOCIATED SHOWERS/CONVECTION.

TROPICAL WAVE IS ALONG 81W/82W S OF 16N MOVING W 10-15 KT. WAVE
IS EMBEDDED WITHIN SOMEWHAT DRY STABLE AIR GENERALLY LIMITING
CLOUD FORMATION AND SHOWERS...THOUGH ISOLATED SHOWERS ARE
OCCURRING 90 NM EITHER SIDE OF THE WAVE AXIS S OF 10N IN THE
PRESENCE OF AN UPPER TROUGH.

TROPICAL WAVE IS ALONG 92W/93W S OF 20N MOVING W NEAR 10 KT.
WAVE IS IN THE BAY OF CAMPECHE ACROSS S MEXICO INTO THE E
PACIFIC REGION. ISOLATED SHOWERS ARE OCCURRING 90 NM EITHER SIDE
OF THE WAVE AXIS IN THE BAY OF CAMPECHE AND OVER CENTRAL
AMERICA....WITH ISOLATED SHOWER ACTIVITY FARTHER SOUTH INTO THE
E PACIFIC.

...THE ITCZ...
ITCZ AXIS IS ANALYZED ALONG 10N13W 8N24W 9N40W 9N54W 9N60W.
CLUSTERS OF MODERATE WITH EMBEDDED ISOLATED STRONG CONVECTION
ARE FROM 4N-12N BETWEEN 10W-21W...NEAR THE W AFRICAN COAST.

...DISCUSSION...

GULF OF MEXICO...
AN UPPER RIDGE AXIS EXTENDS EASTWARD FROM AN UPPER HIGH OVER THE
W MEXICAN COAST ACROSS THE S GULF. AS A DEEP UPPER TROUGH OVER
THE E CONUS AND W ATLANTIC ATTEMPTS TO DIG SOUTHWARD...THE
WESTERLY UPPER FLOW OVER MUCH OF THE GULF CONTINUES TO
STRENGTHEN. UNUSUALLY DRY AIR IN THE TROUGH CONTINUES TO
PERCOLATE INTO THE NORTHERN AND CENTRAL GULF ON NORTHEASTERLY
SURFACE WINDS... AFFECTING ADJACENT COASTAL REGIONS AS WELL.
HOWEVER...UPPER LEVEL DIFFLUENT FLOW SOUTH OF THE UPPER RIDGE
AXIS...COMBINED WITH LOW LEVEL CONVERGENCE IN THE VICINITY OF A
SURFACE TROUGH EXTENDING ALONG THE EASTERN MEXICAN COAST...IS
SUPPORTING SCATTERED MODERATE CONVECTION FROM 18N-23N BETWEEN
89W-97W... AFFECTING WESTERN AREAS OF THE BAY OF CAMPECHE AND
THE YUCATAN PENINSULA.

CARIBBEAN SEA...
THE UPPER RIDGE OVER THE GULF EXTENDS EASTWARD INTO THE NW
CARIBBEAN N OF 13N AND W OF 80W. THE PERSISTENT SHARP UPPER
TROUGH OVER THE CENTRAL CARIBBEAN IS BECOMING COUPLED WITH THE
DEEP UPPER TROUGH OVER THE W ATLANTIC AND NORTHERN GULF. AS A
RESULT...DEEP LAYER DRY AIR FROM THE TROUGH FURTHER NORTH IS
BEING TRANSPORTED SOUTHWARD INTO THE CENTRAL CARIBBEAN. THIS DRY
AIR IS RESULTING IN FAIR WEATHER CONDITIONS ACROSS MUCH OF THE
GULF. TO THE EAST OF THIS SHARP UPPER TROUGH IS ANOTHER UPPER
RIDGE THAT IS ANCHORED BY AN UPPER HIGH OVER THE LESSER ANTILLES
NEAR 15N61W. WEAK FLOW ALOFT FOR THE MOST PART IS PREVENTING THE
DEVELOPMENT OF ANY SUBSTANTIAL CONVECTION ACROSS THE CARIBBEAN.

ATLANTIC OCEAN...
TWO MAIN CONCERNS FOR THE W ATLC ARE THE ACTIVITY DESCRIBED IN
THE SPECIAL FEATURES SECTION ABOVE. THE DEEP UPPER TROUGH
ASSOCIATED WITH THE FIRST SPECIAL FEATURE COVERS THE W ATLC W OF
70W AND EXTENDS INTO THE CENTRAL CARIBBEAN. STRONG VERTICAL
SUPPORT E OF THE TROUGH AXIS IS INTENSIFYING A 996 MB LOW NEAR
32N77W. IN THE SW ATLANTIC...DRIER AND SLIGHTLY COOLER AIR IS
WRAPPING AROUND THIS LOW BEHIND A COLD FRONT THAT EXTENDS FROM
32N72W TO NEAR 29N73W. FROM THIS POINT...A STATIONARY FRONT
EXTENDS TO THE SW TO NEAR 26N76W TO 24N80W. A WEAK SURFACE
TROUGH BEHIND THESE FRONTS ALSO TRAILS SOUTHWESTWARD FROM THE
LOW TO NEAR 28N76W AND TO NEAR 26N80W. WHILE THERE IS SOME
SURFACE CONVERGENCE IN THE VICINITY OF THESE FEATURES...DEEP
LAYER DRY AIR IN THE BASE OF THE UPPER TROUGH IS INHIBITING
CONVECTIVE DEVELOPMENT ACROSS THE SW ATLANTIC. TO THE WEST OF
THE SURFACE TROUGH...NORTHWESTERLY SURFACE WINDS OF 20-25 KT ARE
OCCURRING. THE SECOND SURFACE LOW DISCUSSED IN THE SPECIAL
FEATURES SECTION IS GENERATING GALE FORCE WINDS OVER PORTIONS OF
THE SW N ATLC...AND A GALE WARNING IS IN EFFECT FROM 21N-29N
BETWEEN 65W-69W. THIS SURFACE LOW IS BENEATH AN UPPER
RIDGE...WITH DIFFLUENCE IN THE UPPER FLOW ENHANCING THE
ASSOCIATED CONVECTION DISCUSSED IN THE SPECIAL FEATURES SECTION.
THIS UPPER RIDGE COVERS MUCH OF THE W ATLC...AND IS ANCHORED BY
THE UPPER HIGH NEAR 15N61W. AN UPPER LEVEL CLOSED LOW NEAR
25N46W CONTINUES TO WEAKEN AND IS BECOMING AN OPEN UPPER
TROUGH...WITH AN ATTENDANT 1013 MB SURFACE LOW NEAR 30N25W AND A
SURFACE TROUGH TRAILING SOUTHWARD FROM THE SURFACE LOW TO NEAR
25N25W. ISOLATED SHOWERS ARE OCCURRING FROM 22N-26N BETWEEN
46W-50W. DEEP LOW PRESSURE IS AFFECTING THE NE ATLC CENTERED
NEAR 31N22W...WITH ISOLATED SHOWERS FROM 29N-32N BETWEEN
20W-25W. THE TYPICAL E/W UPPER RIDGE THAT EXTENDS FROM AFRICA TO
NEAR 45W ALONG 13N CONTINUES TO BE SUPPRESSED TO THE S DUE TO
THE UPPER TROUGHS MENTIONED ABOVE. A WEAK SURFACE RIDGE REMAINS
OVER THE CENTRAL ATLC N OF 23N BETWEEN 30W-50W WITH A 1021 MB
HIGH NEAR 30N38W. THESE RIDGES AND SURFACE HIGH ARE RESULTING IN
GENERALLY FAIR WEATHER ACROSS THE REMAINDER OF THE ATLC.

$$
COHEN/HUFFMAN/KIMBERLAIN



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