[SMCARA] TROPICAL WEATHER DISCUSSION

[J D Delancy]

Special Weather Bulletin - National Hurricane Cenmter Analysis 


AXNT20 KNHC 121801
TWDAT 

TROPICAL WEATHER DISCUSSION
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
205 PM EDT WED SEP 12 2007

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL 
AMERICA...THE GULF OF MEXICO...THE CARIBBEAN SEA...NORTHERN 
SECTIONS OF SOUTH AMERICA...AND THE 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 
1745 UTC.

...SPECIAL FEATURES...

SEPTEMBER IS CLIMATOLOGICALLY THE MOST ACTIVE MONTH OF THE 
TROPICAL SEASON IN THE ATLANTIC BASIN. THIS MORNING...TWO 
TROPICAL DEPRESSIONS HAVE FORMED. ONE IN THE NORTHWESTERN GULF 
OF MEXICO AND THE OTHER ONE WELL EAST OF THE LESSER ANTILLES. AT 
12/1800 UTC TROPICAL DEPRESSION NINE IS UPGRADED TO TROPICAL 
STORM HUMBERTO...THE EIGHTH NAMED STORM OF THE 2007 ATLANTIC 
HURRICANE SEASON. 

THE AREA OF LOW PRESSURE LOCATED EAST OF THE LESSER ANTILLES HAS 
ACQUIRED ENOUGH ORGANIZATION TO BE DECLARED A TROPICAL 
DEPRESSION...THE EIGHTH OF THE ATLANTIC SEASON. AT 12/1500 UTC 
THIS SYSTEM WAS CENTERED NEAR 13.2N 44.6W...OR ABOUT 980 NM EAST 
OF THE LESSER ANTILLES. IT IS MOVING WEST-NORTHWEST AT 10 KT. 
MAXIMUM SUSTAINED WINDS SPEED IS 30 KT WITH GUSTS TO 40 KT. 
SOME STRENGTHENING IS FORECAST DURING THE NEXT 24 HOURS AND THE 
DEPRESSION COULD BECOME A TROPICAL STORM LATER TODAY OR TONIGHT.
ESTIMATED MINIMUM CENTRAL PRESSURE IS 1007 MB. SEE LATEST 
NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT3/WTNT23 
KNHC FOR MORE DETAILS. NUMEROUS MODERATE TO ISOLATED STRONG 
CONVECTION IS JUST WEST OF THE CENTER FROM 12N-14N BETWEEN 
44W-46W. 

RADAR...SATELLITE...AND BUOY OBSERVATIONS INDICATE THAT THE AREA 
OF LOW PRESSURE IN THE WESTERN GULF OF MEXICO HAS BECOME BETTER 
ORGANIZED THIS MORNING. LATEST DOPPLER RADAR OBSERVATIONS SHOW 
THAT THIS SYSTEM IS GETTING BETTER ORGANIZED...WITH MORE BANDING 
FEATURES.  AT 12/1500 THIS SYSTEM WAS UPGRADED TO A TROPICAL 
DEPRESSION STATUS. THREE HOURS LATER...IT WAS CLASSIFIED AS A 
TROPICAL STORM HUMBERTO. AT 12/1800 HUMBERTO WAS CENTERED NEAR 
28.3N 95.1W...OR ABOUT 60 NM SOUTH-SOUTHWEST OF GALVESTON TEXAS 
AND ABOUT 125 NM EAST-NORTHEAST OF CORPUS CHRISTI TEXAS. IT IS 
MOVING TOWARD THE NORTH AT 5 KT. MAXIMUM SUSTAINED WINDS SPEED 
IS 40 KT WITH HIGHER GUSTS. ESTIMATED MINIMUM CENTRAL PRESSURE 
IS 1005 MB. SEE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO 
HEADERS MIATCMAT4/WTNT24 KNHC FOR MORE DETAILS. BANDS OF SHOWERS 
WITH EMBEDDED TSTMS ASSOCIATED WITH THIS TROPICAL CYCLONE ARE 
ALREADY AFFECTING PARTS OF THE UPPER TEXAS COAST AND SW 
LOUISIANA. RAINFALL AMOUNTS OF 5 TO 10 INCHES ARE EXPECTED ALONG 
THE MIDDLE AND UPPER TEXAS COAST AND IN EXTREME SOUTHWESTERN 
LOUISIANA...WITH ISOLATED MAXIMUM ACCUMULATIONS OF 15 INCHES 
POSSIBLE.  

...TROPICAL WAVES...

A HIGH AMPLITUDE TROPICAL WAVE IS IN THE E ATLC ALONG 34W/35W S 
OF 24N MOVING W 15 KT. IN GENERAL...THIS WAVE HAS CHANGED LITTLE 
OVER THE PAST COUPLE OF DAYS. THE STRUCTURE REMAINS WELL DEFINED 
WITH A LARGE SHIELD OF CYCLONIC TURNING NOTED IN THE LOW TO MID 
LEVEL CLOUD FIELD. IN ADDITION...THE UW-CIMSS TPW ANIMATION 
REVEALS A LARGE WWD PROPAGATING SURGE OF MOISTENED AIR. DESPITE 
THE FAIR STRUCTURE...DEEP MOISTURE IS MINIMAL WITH WIDELY 
SCATTERED SHOWERS AND TSTMS FROM 17N-20N BETWEEN 31W-36W. 

TROPICAL WAVE IS IN THE CENTRAL CARIB ALONG 73W S OF 21N MOVING 
W 15-20 KT. A SURGE OF MOISTURE ASSOCIATED WITH THIS FEATURE IS 
WELL DEFINED ON THE TROPICAL PRECIPITABLE WATER ANIMATION FROM 
CIMSS. THE WAVE IS INTERACTING WITH AN UPPER LOW CURRENTLY 
LOCATED OVER E CUBA. THE WAVE IS HELPING TO INDUCE SOME SHOWER 
ACTIVITY OVER HISPANIOLA AND WESTERN VENEZUELA.   

...THE ITCZ...
ITCZ AXIS IS CENTERED ALONG 11N15W 14N32W 13N40W 10N47W 8N63W. 
WIDELY SCATTERED MODERATE CONVECTION IS 6N-14N EAST OF 25W... 
AND FROM 5N-11N BETWEEN 28W-35W. SCATTERED MODERATE TO ISOLATED 
STRONG CONVECTION IS OVER NE VENEZUELA AND NW GUYANA. SIMILAR 
CONVECTION IS ALSO SEEN APPROACHING THE WEST AFRICAN COAST  
LIKELY ASSOCIATED WITH THE NEXT TROPICAL WAVE. THE VERTICAL 
SOUNDING FROM DAKAR SHOWS THAT THE WAVE IS JUST THERE.  

DISCUSSION...

THE GULF OF MEXICO...
THE BIG WEATHER STORY OF THE DAY IS THE DEVELOPMENT OF TROPICAL 
DEPRESSION NINE OVER THE NW PORTION OF THE GULF. SEE SPECIAL 
FEATURES FOR MORE DETAILS. WATER VAPOR IMAGES AND THE CIMSS TPW 
PRODUCT SHOW A MOISTENED ATMOSPHERE ACROSS THE ENTIRE AREA. AN 
UPPER RIDGE ANCHORED ON A HIGH OVER CENTRAL MEXICO COVERS THE 
WESTERN HALF OF THE GULF AND THE N GULF WATERS WHILE THE BROAD 
CYCLONIC CIRCULATION OF AN UPPER LOW LOCATED OVER E CUBA 
DOMINATES THE REMAINDER OF THE GULF WITH N AND NE UPPER LEVEL 
FLOW. A STATIONARY FRONTAL BOUNDARY IS ANALYZED JUST INLAND 
ACROSS THE GULF COAST STATES. THIS BOUNDARY IS NOT EXPECTED TO 
PUSH VERY FAR...IF AT ALL...INTO THE OFFSHORE WATERS TODAY 
BEFORE RETREATING AND WEAKENING. A WEAK SURFACE TROUGH RUNS FROM 
27N84W TO 23N85W. MOSAIC DOPPLER RADAR FROM THE SE US...SHOWS 
SCATTERED SHOWERS AND ISOLATED TSTMS OVER THE EASTERN GULF... 
EAST OF 86W RELATED TO THE THIS TROUGH.

THE CARIBBEAN SEA...
THE BROAD CYCLONIC CIRCULATION ASSOCIATED WITH AN UPPER LOW 
CURRENTLY SPINNING OVER ERN CUBA COVERS MOST OF THE WEST AND 
CENTRAL CARIBBEAN WHILE AN UPPER HIGH IN THE TROPICAL ATLC 
DOMINATES THE EASTERN CARIBBEAN AND THE LESSER ANTILLES. THE 
EPAC ITCZ COUPLED WITH A DIFFLUENT PATTERN ALOFT IS ENHANCING 
CLUSTERS OF SCATTERED MODERATE TO ISOLATED STRONG CONVECTION 
OVER THE SW CARIBBEAN...INCLUDING ALSO PARTS OF PANAMA...COSTA 
RICA AND NICARAGUA. A TROPICAL WAVE APEX EXTENDS FROM HAITI TO 
COLOMBIA. SEE ABOVE. MODERATE TO STRONG TRADE WINDS ARE BLOWING 
ACROSS THE CARIBBEAN SEA WITH A SOLID ARE OF 20 TO 25 KT WINDS 
BETWEEN HISPANIOLA AND COLOMBIA. AN EARLIER QUIKSCAT PASS 
CONFIRMED THESE WINDS. 

THE ATLANTIC OCEAN...
NOT MUCH HAS CHANGED OVER THE PAST DAY OR TWO. THE SUBTROPICAL 
RIDGE CONTINUES TO BUILD WWD ACROSS FLORIDA AND THE EASTERN 
GULF. A 1023 MB HIGH CENTER IS LOCATED NEAR 32N57W. THE PRES 
GRADIENT IS TIGHTENED BETWEEN THIS HIGH AND THE SEMI-PERMANENT 
LOW LOCATED OVER N COLOMBIA. AS A RESULT...20-25 KT TRADE WINDS 
ARE BLOWING ACROSS THE CARIBBEAN SEA. ANOTHER 1022 MB HIGH IS 
SITUATED NEAR 32N28W. IN BETWEEN...A WEAK SFC TROUGH CLIPS THE 
FORECAST AREA NEAR 30N39W.  

ALOFT...A DIFFLUENT PATTERN EXISTS ACROSS THE W ATLC. THE 
CONTRIBUTING LARGE SCALE FEATURES CONSIST OF A NORTHWESTWARD 
MOVING UPPER LOW CENTERED OVER ERN CUBA...AN UPPER HIGH LOCATED 
NE OF BERMUDA AND AN ELONGATED UPPER TROUGH OVER THE CENTRAL 
ATLC. MUCH OF THE REMAINDER OF THE AREA IS DOMINATED BY AN E-W 
RIDGE COVERING THE AREA E OF THE LESSER ANTILLES WITH A MEAN 
AXIS ALONG 18N WITH SEVERAL EMBEDDED ANTICYCLONES. THIS RIDGE 
AND A CONFLUENT PATTERN...TO THE S OF A LARGE UPPER LOW NEAR SRN 
PORTUGAL...IS KEEPING WEATHER CONDITIONS FAIRLY TRANQUIL. 

$$
GR