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| #455633 (Received by flhurricane at: 11:48 AM 02.Sep.2011) TCDAT3 TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL132011 1000 AM CDT FRI SEP 02 2011 SATELLITE IMAGERY...SURFACE OBSERVATIONS...AND DOPPLER RADAR DATA INDICATE THE CONVECTIVE PATTERN ASSOCIATED WITH TROPICAL DEPRESSION THIRTEEN HAS IMPROVED DURING THE PAST 6 HOURS...WITH A LARGE AREA OF VERY COLD CLOUD TOPS OF -80C AND COLDER HAVING DEVELOPED IN THE EASTERN SEMICIRCLE. HOWEVER...THE RECENT INCREASE IN DEEP CONVECTION HAS YET TO TRANSLATE INTO AN INCREASE IN THE SURFACE WINDS. AN AIR FORCE RESERVE RECON AIRCRAFT INVESTIGATING HAS ONLY MANAGED TO FIND 850 MB FLIGHT-LEVEL OF 37-38 KT. OIL RIG OBSERVATIONS WELL ABOVE THE SURFACE ALSO SUGGEST THE SYSTEM IS STILL A DEPRESSION. THE DEPRESSION HAS BEEN ESSENTIALLY STATIONARY...TRAPPED IN A FLAT RIDGE PATTERN THAT EXTENDS SOUTHWARD FROM A 700-400 MB HEIGHT CENTER LOCATED OVER THE OHIO AND TENNESSEE VALLEYS. NARROW RIDGE AXES EXTEND SOUTHEASTWARD TO FLORIDA AND ALSO SOUTHWESTWARD TO SOUTH TEXAS...WHICH IS EFFECTIVELY BLOCKING ANY SIGNIFICANT MOTION. A DEEP TROUGH THAT IS AMPLIFYING OFF THE U.S. EAST COAST IS EXPECTED TO MAINTAIN THIS BLOCKING PATTERN FOR THE NEXT 36 HOURS OR SO. BY 48 HOURS...THE BLOCKING PATTERN IS EXPECTED TO SLOWLY BREAK DOWN AS A WEAK SHORTWAVE TROUGH MOVES EASTWARD ACROSS THE SOUTHERN AND CENTRAL PLAINS STATES. THIS WEAK PROGRESSIVE PATTERN SHOULD ALLOW FOR THE CYCLONE TO MOVE SLOWLY TO THE NORTH AND THEN TO THE NORTHEAST. HOWEVER...THE FORWARD SPEED IS ONLY EXPECTED TO BE AROUND 5 KT...WHICH IS GOING TO EXACERBATE THE HEAVY RAINFALL THREAT OVER LOUISIANA AND POINTS NORTHEASTWARD. THE FORECAST TRACK IS SLIGHTLY EAST OF THE PREVIOUS ADVISORY TRACK...MAINLY DUE TO THE MORE EASTWARD INITIAL POSITION...AND LIES JUST EAST OF THE NHC MODEL CONSENSUS. WATER VAPOR IMAGERY AND UPPER-AIR OBSERVATIONS INDICATE A SMALL UPPER-LEVEL LOW OVER SOUTHWESTERN LOUISIANA THAT HAS BEEN PRODUCING MODERATE TO STRONG SOUTHWESTERLY WIND SHEAR ACROSS THE CYCLONE HAS BEEN WEAKENING AND LIFTING OUT TO THE NORTHWEST. THIS HAS ALLOWED THE WIND SHEAR TO DECREASE AND ALSO TURN TO THE SOUTH AND SOUTHEAST. DISSIPATION OF THIS UPPER-LEVEL LOW AND ITS ATTENDANT DRY AIR INFLUENCE IS FORECAST TO OCCUR IN THE NEXT 24 HOURS...ALLOWING THE SYSTEM TO STRENGTHEN. GIVEN THE SLOW MOTION...LESSENING SHEAR...AND VERY WARM SSTS NEAR 30C THE CYCLONE COULD BE NEAR HURRICANE STRENGTH BEFORE LANDFALL OCCURS IN ABOUT 48 HOURS. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE NHC INTENSITY CONSENSUS MODEL...ICON...WITH THE STATISTICAL MODELS ABOVE AND THE DYNAMICAL MODELS BELOW THE OFFICIAL FORECAST. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 27.3N 91.5W 30 KT 35 MPH 12H 03/0000Z 27.6N 91.5W 35 KT 40 MPH 24H 03/1200Z 28.2N 91.8W 45 KT 50 MPH 36H 04/0000Z 28.8N 91.6W 55 KT 65 MPH 48H 04/1200Z 29.7N 91.3W 55 KT 65 MPH...INLAND 72H 05/1200Z 29.8N 90.4W 50 KT 60 MPH...INLAND 96H 06/1200Z 31.0N 88.5W 40 KT 45 MPH...INLAND 120H 07/1200Z 33.0N 87.0W 25 KT 30 MPH...POST-TROP/EXTRATROP $$ FORECASTER STEWART |
