000 AXNT20 KNHC 131749 TWDAT TROPICAL WEATHER DISCUSSION NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 805 AM EDT THU SEP 13 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 1715 UTC. ...SPECIAL FEATURES... HUMBERTO WAS DOWNGRADED TO A TROPICAL STORM AT 13/1500 UTC. AT THIS TIME...IT WAS LOCATED INLAND NEAR 30.6N 93.2W OR ABOUT 65 NM WEST-NORTHWEST OF LAFAYETTE LOUISIANA MOVING TOWARD THE NE AT 10 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 990 MB. MAXIMUM SUSTAINED WIND SPEED IS 55 KT WITH GUSTS TO 70 KT. SEE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT4/WTNT24 KNHC FOR MORE DETAILS. BASED ON OPERATIONAL ESTIMATES...HUMBERTO STRENGTHENED FROM A 30 KT DEPRESSION AT 15Z YESTERDAY TO A 75 KT HURRICANE AT 09Z THIS MORNING...AN INCREASE OF 45 KT IN 18 HOURS. TO PUT THIS DEVELOPMENT IN PERSPECTIVE...NO TROPICAL CYCLONE IN THE HISTORICAL RECORD HAS EVER REACHED THIS INTENSITY AT A FASTER RATE NEAR LANDFALL. CURRENTLY DOPPLER RADAR FROM LAKE CHARLES LOUISIANA IS SHOWING PARTS OF THE NORTHERN EYEWALL AND WELL DEFINED RAINBANDS ON THE E SEMICIRCLE OF THE STORM. RAINFALL AMOUNTS OF 4 TO 8 INCHES ARE EXPECTED ACROSS MUCH OF LOUISIANA AND MISSISSIPPI...AS WELL AS EXTREME SOUTHEASTERN ARKANSAS...WITH ISOLATED MAXIMUM AMOUNTS OF 12 INCHES POSSIBLE. DISORGANIZED TROPICAL DEPRESSION EIGHT CONTINUES TO MOVE WEST-NORTHWESTWARD ACROSS THE OPEN OCEAN. AT 13/1500 UTC...IT WAS CENTERED NEAR 13.9N 48.0W OR ABOUT 775 NM EAST OF THE LESSER ANTILLES MOVING WEST-NORTHWEST AT 7 KT. ESTIMATED MINIMUM CENTRAL PRESSURE IS 1006 MB. MAXIMUM SUSTAINED WIND SPEED REMAINS 30 KT WITH GUSTS TO 40 KT. SEE LATEST NHC FORECAST/ADVISORY UNDER AWIPS/WMO HEADERS MIATCMAT3/WTNT23 KNHC FOR MORE DETAILS. THIS DEPRESSION HAS GENERALLY CHANGED LITTLE THROUGHOUT THE MORNING HOURS. AN EARLIER QUIKSCAT PASS SUGGESTED THAT SYSTEM COULD HAVE BEEN A TROPICAL STORM FOR A FEW HOURS WITH A FEW UN-FLAGGED 35 KT WINDS JUST TO THE EAST AND SOUTH OF THE CENTER. THERE IS STILL A CHANCE THAT THE DEPRESSION WILL DEVELOP INTO A TROPICAL STORM DURING THE NEXT 24 HOURS...BUT AFTERWARD THE SYSTEM WILL ENCOUNTER A HIGHER SHEAR ENVIRONMENT ASSOCIATED WITH A DEEP LAYER MID-ATLANTIC TROUGH. ACCORDING TO THE NHC OFFICIAL FORECAST...T.D. EIGHT WILL BE ABOUT 200 NM EAST OF THE NORTHERN LEEWARD ISLANDS THIS TUE AS A WEAK TROPICAL STORM. ...TROPICAL WAVES... TROPICAL WAVE IS ALONG 20W S OF 17N MOVING W 10-15 KT. THIS WAVE CAME OFF OF THE AFRICAN COAST SOMETIME BETWEEN 12Z AND 23Z YESTERDAY. THE VERTICAL SOUNDING FROM DAKAR DEPICTED VERY WELL THE PASSAGE OF THE WAVE AXIS. THE WAVE IS FAIRLY WELL ORGANIZED WITH A WEAK AREA OF LOW PRESSURE ALONG THE WAVE AXIS NEAR 10N...ANALYZED 1010 MB. WIDELY SCATTERED MODERATE CONVECTION IS FROM 6N-16N WEST OF 25W. HIGH AMPLITUDE TROPICAL WAVE IS ALONG 38W S OF 21N MOVING SLOWLY WWD. THE NRN PART OF THE WAVE IS BEGINNING TO FRACTURE BASED ON THE POLEWARD MOVEMENT OF MOISTURE SEEN ON SATELLITE AND THE UW CIMSS TPW ANIMATION. A TROUGH WAS ANALYZED TO REFLECT THIS ALONG 37W/38W FROM 22N-26N. THE GFS MODEL SUGGESTS THAT A SFC LOW MAY FORM ALONG THE TROUGH AXIS DURING THE NEXT 24 HOURS. A CLUSTER OF MODERATE CONVECTION IS WITHIN 75 NM OF AXIS FROM 22N-24N. SIMILAR CONVECTION IS FOUND ON THE NORTHERN END OF THE WAVE AXIS. TROPICAL WAVE IS IN THE CENTRAL CARIBBEAN ALONG 79W S OF 22N MOVING W 15 KT. THE WAVE IS INTERACTING WITH AN UPPER UPPER LOW LOCATED NEAR THE CAYMAN ISLANDS. CLUSTERS OF MODERATE CONVECTION HAVE DEVELOPED JUST AHEAD OF THE WAVE AXIS...AFFECTING THE NW CARIBBEAN JUST NORTH OF HONDURAS AND THE EASTERN PLAINS OF NICARAGUA. THIS ACTIVITY IS BEEN ENHANCING BY A DIFFLUENT PATTERN ALOFT BETWEEN THE AFOREMENTIONED UPPER LOW AND AN UPPER RIDGE THAT COVERS THE CENTRAL CARIBBEAN. ...THE ITCZ... ITCZ AXIS IS CENTERED ALONG 11N17W 8N26W 12N35W 12N43W 11N50W 9N63W. BESIDES FOR THE CONVECTION ASSOCIATED WITH THE TROPICAL WAVES AND T.D. EIGHT...SCATTERED MODERATE/ISOLATED STRONG CONVECTION IS NEAR THE COAST OF SW AFRICA...AND FROM 6N-11N BETWEEN 57W-62W. THIS ACTIVITY IS AFFECTING THE SOUTHERN WINDWARD ISLANDS. DISCUSSION... THE GULF OF MEXICO... MOSAIC DOPPLER RADAR FROM THE SE U.S. SHOWS BANDS OF SHOWERS WITH EMBEDDED TSTMS ASSOCIATED WITH THE CYCLONIC CIRCULATION OF HUMBERTO OVER THE NW CORNER OF THE GULF...PARTICULARLY SOUTH OF LOUISIANA. THE MOIST ONSHORE FLOW BETWEEN HUMBERTO AND A SFC RIDGE THAT DOMINATES THE STATE OF FLORIDA AND THE EASTERN GULF IS ALSO GENERATING SOME SHOWER ACTIVITY OVER SOUTHERN ALABAMA AND THE FLORIDA PANHANDLE. ISOLATED SHOWERS ARE FOUND OVER THE REMAINDER OF THE GULF. A SFC TROUGH IS ANALYZED OVER THE NW GULF AND RUNS FROM HUMBERTO 25N97W. ALOFT...NELY UPPER LEVEL WINDS DOMINATES MOST OF THE REGION BETWEEN AN UPPER HIGH LOCATED OVER NORTHERN FLORIDA AND UPPER LOW NEAR THE CAYMAN ISLANDS. ABUNDANT MID/UPPER LEVEL MOISTURE IS SEEN ON WV IMAGERY. THE RIDGE IS FORECAST TO WEAKEN FRI AND SAT IN ADVANCE OF COLD FRONT MOVING S AND STALLING OVER NRN COASTAL WATERS SUN. THE CARIBBEAN SEA... A BROAD UPPER LOW HAS BEEN DRIFTING WWD ACROSS THE BASIN AND NOW IS CENTERED NEAR THE CAYMAN ISLANDS. A DIFFLUENT PATTERN ALOFT BETWEEN THIS LOW AND AN UPPER RIDGE THAT COVERS MOST OF THE CENTRAL AND EASTERN CARIBBEAN IS ENHANCING CLUSTERS OF SCATTERED MODERATE/ISOLATED STRONG CONVECTION OVER THE SW CARIBBEAN SOUTH OF 12N BETWEEN 75W-80W AND THE GULF OF PANAMA. THE ERN CARIB IS EXPERIENCING MOSTLY FAIR WEATHER AS DRIER AIR FROM THE TROPICAL ATLC HAS BEEN ADVECTED INTO THE AREA. HOWEVER...TYPICAL CLOUDS LINES OR STREAMERS HAVE DEVELOPMENT DOWNSTREAM THE LESSER ANTILLES UNDER AN ELY WIND FLOW. MODERATE TO STRONG TRADE WINDS ARE BLOWING ACROSS THE REGION...STRONGEST BETWEEN COLOMBIA AND HISPANIOLA WHERE QSCAT DEPICTED A SOLID AREA OF 20-25 KT WINDS. THE ATLANTIC OCEAN... TROPICAL DEPRESSION EIGHT IS THE MAIN FEATURE OF INTEREST ACROSS THE TROPICAL AND SUBTROPICAL ATLC DISCUSSION ZONE. FOR DETAILS AND REFERENCES REFER TO THE SPECIAL FEATURE SECTION ABOVE. OTHERWISE...THE ATLC BASIN IS FAIRLY QUIET EARLY THIS AFTERNOON UNDER THE INFLUENCE OF THE SUBTROPICAL RIDGE THAT HAS A PAIR OF HIGH CENTERS ALONG AND JUST N OF OUR BORDER IN THE CENTRAL AND W ATLC. A 1027 MB HIGH IS CENTERED IN THE E ATLC WELL N OF AREA WITH RIDGING EXTENDING MAINLY SOUTH TO 22N25W. A WEAK SFC TROUGH CLIPS THE FORECAST AREA NEAR 30N40W. ALOFT...A COMPLEX PATTERN IS IN PLACE ACROSS THE SW ATLC. THE CONTRIBUTING LARGE SCALE FEATURES CONSIST OF A WESTWARD MOVING UPPER LOW CENTERED NEAR THE CAYMAN ISLANDS...A RIDGE ANCHORED ON A HIGH OVER NORTHERN FLORIDA AND ANOTHER UPPER LOW CENTERED NEAR 26N64W. THIS FEATURE IS GENERATING SCATTERED SHOWERS FROM 22N-28N BETWEEN 60W-67W. AN UPPER TROUGH IS OVER THE CENTRAL ATLC N OF 23N. THE REMAINDER OF THE REGION IS DOMINATED BY AN E-W RIDGE COVERING THE AREA FROM THE LESSER ANTILLES EWD WITH A MEAN AXIS ALONG 17N WITH SEVERAL EMBEDDED ANTICYCLONES. $$ GR