ZCZC MIATCDAT2 ALL TTAA00 KNHC DDHHMM TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER 1 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072006 1100 PM EDT SUN SEP 10 2006 THE SMALL AREA OF LOW PRESSURE...THAT HAS BEEN MOVING SLOWLY ACROSS THE CENTRAL ATLANTIC FOR THE PAST FEW DAYS...ESCAPED TODAY FROM THE NORTHERLY WIND SHEAR ASSOCIATED WITH THE UPPER-LEVEL ANTICYCLONE OVER HURRICANE FLORENCE. AS A RESULT...DURING THE PAST SEVERAL HOURS THE DEEP CONVECTION HAS BEEN PERSISTENT AND HAS BECOME MORE INVOLVED WITH THE LOW-LEVEL CENTER...AND ADVISORIES ARE INITIATED ON TROPICAL DEPRESSION SEVEN. THE INTENSITY IS SET TO 25 KT BASED ON THE 00 UTC DVORAK T-NUMBERS OF 1.5...AND ON AN OBSERVATION FROM SHIP MSJX8 OF WEST-SOUTHWEST WINDS OF 27 KT JUST SOUTH OF THE CENTER. COMPLETELY OPPOSITE TO WHAT FLORENCE HAS BEEN...THIS DEPRESSION IS A SMALL CYCLONE. THE CIRCULATION IS PERHAPS 300 NM WIDE AND THE DEEP CONVECTION IS CONFINED MOSTLY TO THE SOUTHERN SEMICIRCLE. MICROWAVE OVERPASSES BY QUIKSCAT AND SSMI...AND SEVERAL FORTUITOUS SHIP OBSERVATIONS HAVE MADE FINDING THE CENTER A LITTLE EASIER THAN USUAL FOR A DEPRESSION. IT HAD BEEN MOVING WEST-SOUTHWESTWARD DURING THE PAST 24 HOURS...BUT MORE RECENTLY IT APPEARS TO BE HEADING SLOWLY WESTWARD OR 270/5. STEERING CURRENTS ARE OBVIOUSLY RATHER WEAK...AND THE GLOBAL MODELS ESSENTIALLY DO NOT KNOW THE DEPRESSION EXISTS. THE GLOBAL MODEL FIELDS SUGGEST A STEERING PATTERN THAT WOULD TURN THE DEPRESSION SLOWLY NORTHWESTWARD...INTO THE WEAKNESS IN THE SUBTROPICAL RIDGE LEFT BEHIND BY FLORENCE. THE GFS-BASED BAM TRAJECTORIES SHOW RECURVATURE INTO THE MID-LATITUDE WESTERLIES BY DAY 4 OR 5. THE GFDL FORECASTS AN IMMEDIATE NORTHWESTWARD MOTION THAT LASTS FOR A COUPLE OF DAYS UNTIL THE MODEL CIRCULATION DISSIPATES. THE OFFICIAL FORECAST FOLLOWS THE PATH OF THE BAM MODELS...BUT NOT AS QUICKLY...SINCE GLOBAL MODELS OTHER THAN THE GFS SUGGEST THE TROPICAL CYCLONE COULD MEANDER WITHIN THE WEAKNESS IN THE RIDGE. THIS IS A HIGHLY UNCERTAIN FORECAST...AND THE NORTHWARD MOTION LATE IN THE FORECAST PERIOD CAN BE INTERPRETED TO MEAN WE ARE NOT YET SURE IF THIS CYCLONE WILL RECURVE. AS HAS BEEN THE CASE FOR MANY OF THE ATLANTIC TROPICAL CYCLONES THIS YEAR...THE UPPER-LEVEL PATTERN SURROUNDING THE DEPRESSION IS FAIRLY COMPLICATED. THE DEPRESSION APPEARS TO HAVE ABOUT 48 HOURS WITHIN AN UPPER-LEVEL ENVIRONMENT THAT COULD ALLOW FOR SOME GRADUAL STRENGTHENING. THE SHIPS GUIDANCE SUGGESTS THIS COULD BECOME A STRONG TROPICAL STORM...BUT THE CYCLONE MIGHT HAVE TO CONTEND WITH INCREASING EASTERLY SHEAR IN A COUPLE OF DAYS. THE OFFICIAL INTENSITY FORECAST IS QUITE A BIT MORE CONSERVATIVE THAN THE SHIPS GUIDANCE...AND IT IS CLOSER TO THE GFDL SOLUTION OF A WEAKER TROPICAL STORM THAT MIGHT NOT EVEN LAST FIVE DAYS. FORECAST POSITIONS AND MAX WINDS INITIAL 11/0300Z 20.2N 54.5W 25 KT 12HR VT 11/1200Z 20.3N 55.3W 30 KT 24HR VT 12/0000Z 20.7N 56.4W 35 KT 36HR VT 12/1200Z 21.4N 57.8W 40 KT 48HR VT 13/0000Z 22.2N 59.0W 45 KT 72HR VT 14/0000Z 24.5N 61.0W 45 KT 96HR VT 15/0000Z 27.0N 62.5W 45 KT 120HR VT 16/0000Z 29.5N 62.5W 45 KT $$ FORECASTER KNABB/BERG NNNN
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Page last modified: Monday, 11-Sep-2006 11:28:37 UTC