Tropical Depression SEVEN

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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
 
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