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| #365507 (Received by flhurricane at: 4:42 PM 02.Aug.2010) TCDAT4 TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 2 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042010 500 PM EDT MON AUG 02 2010 TROPICAL DEPRESSION FOUR IS GRADUALLY GETTING BETTER ORGANIZED AND IS PROBABLY VERY NEAR TROPICAL STORM STRENGTH BASED ON THE RECENT DEVELOPMENT OF SHARPLY CURVED CONVECTIVE BANDS IN THE NORTHERN SEMICIRCLE. SATELLITE INTENSITY ESTIMATES WERE T2.0/30 KT FROM BOTH TAFB AND SAB...AND THAT IS THE INITIAL INTENSITY ESTIMATE USED FOR THIS ADVISORY. UPPER-LEVEL OUTFLOW IS SLIGHTLY RESTRICTED TO THE EAST...BUT HAS BEEN IMPROVING AND BECOMING MORE SYMMETRICAL IN THE REMAINING QUADRANTS DURING THE PAST 6 HOURS. THE INITIAL MOTION ESTIMATE IS 290/14. DESPITE THEIR POOR INITIALIZATION OF THE DEPRESSION AT 12Z...THE NHC MODEL GUIDANCE IS TIGHTLY CLUSTERED AND IN GOOD AGREEMENT ON A WEST-NORTHWESTWARD MOTION ACCOMPANIED BY A STEADY INCREASE IN FORWARD SPEED EXCEEDING 20 KT THROUGH 72 HOURS. AFTER THAT...HOWEVER...THE MODELS DIVERGE SIGNIFICANTLY WITH THE UKMET...ECMWF...AND GFDL MODELS TAKING A VERY WEAK CYCLONE QUICKLY NORTHWARD THROUGH A LARGE BREAK IN THE SUBTROPICAL RIDGE...WHEREAS THE HWRF...GFS...AND NOGAPS MODELS FAVOR A SLOWER AND MORE NORTHWESTWARD MOTION. THE MAIN DIFFERENCES LIE IN HOW THE MODELS HANDLE A MID-LEVEL TROUGH ALONG THE U.S. EAST COAST THAT IS FORECAST TO DIG SOUTHWARD OUT OF NORTHERN CANADA OVER THE NEXT 5 DAYS. THE UKMET...ECMWF...AND GFDL HAVE MORE AMPLIFIED TROUGH WHICH ERODES THE RIDGE OFF THE SOUTHEAST U.S. COAST...WHILE THE OTHER MODELS HAVE LESS AMPLITUDE AND MORE ZONAL FLOW ACROSS THE U.S. MID-ATLANTIC STATES. GIVEN THE CURRENT 5-WAVE PROGRESSIVE PATTERN...MY FEELING IS THAT SUCH SIGNIFICANT EROSION OF THE SUBTROPICAL RIDGE WOULD BE SOMEWHAT OVERDONE FOR THIS TIME OF THE YEAR. AS A RESULT...THE OFFICIAL FORECAST TRACK IS A LITTLE TO THE LEFT OF AND SLOWER THAN THE PREVIOUS ADVISORY...AND IS SIMILAR TO THE CONSENSUS MODELS...TVCN AND TVCC...ESPECIALLY AT DAYS 4 AND 5. THE UPPER-LEVEL ENVIRONMENT IS EXPECTED TO REMAIN FAVORABLE FOR SOME STRENGTHENING OVER THE NEXT COUPLE OF DAYS...ESPECIALLY IN 36 HOURS WHEN ALL OF THE MODELS ARE FORECASTING A SMALL ANTICYCLONE TO DEVELOP OVER THE CYCLONE. BY 48-60 HOURS...THE GFS-BASED SHIPS INTENSITY MODEL IS FORECASTING THE SHEAR TO INCREASE TO MORE THAN 30 KT...WHICH WOULD NORMALLY RESULT IN WEAKENING OF A TROPICAL CYCLONE. HOWEVER...THESE HIGH SHEAR VALUES MAY BE AN ARTIFACT OF THE THE GFS HAVING MUCH STRONGER UPPER-LEVEL WINDS AS COMPARED TO THE OTHER GLOBAL MODELS...AND ALSO DUE TO 500 KM RADIUS THAT THE SHIPS USES IN COMPUTING SHEAR VALUES. IF THE DEPRESSION REMAINS UNDERNEATH THE UPPER-LEVEL ANTICYCLONE RATHER THAN OUTRUNNING IT ...THEN THE CYCLONE WILL END UP STRONGER THAN THE OFFICIAL FORECAST INDICATES. FORECAST POSITIONS AND MAX WINDS INITIAL 02/2100Z 13.0N 42.5W 30 KT 12HR VT 03/0600Z 13.8N 45.2W 35 KT 24HR VT 03/1800Z 15.2N 49.9W 40 KT 36HR VT 04/0600Z 16.8N 54.4W 45 KT 48HR VT 04/1800Z 18.5N 57.8W 55 KT 72HR VT 05/1800Z 22.5N 64.0W 55 KT 96HR VT 06/1800Z 26.0N 67.5W 50 KT 120HR VT 07/1800Z 29.0N 69.5W 50 KT $$ FORECASTER STEWART |
