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Tropical Storm IRENE


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TROPICAL STORM IRENE DISCUSSION NUMBER  29
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT THU AUG 11 2005
 
THE OVERALL CONVECTIVE PATTERN OF IRENE HAS CHANGED LITTLE TODAY.
THE SYSTEM IS ELONGATED WEST-NORTHWEST TO EAST-SOUTHEAST AND THE
BEST ESTIMATE OF THE SURFACE CENTER LOCATION...BASED ON CURVED
LOW-LEVEL CLOUD LINES AND CONTINUITY...IS NEAR THE WESTERN EDGE OF
THE CLOUD SHIELD AND NOT FARTHER EAST INTO THE DEEP CONVECTION.
SATELLITE INTENSITY ESTIMATES ARE 35 KT...45 KT...AND 55 KT FROM
TAFB...SAB...AND AFWA...RESPECTIVELY. THE INITIAL INTENSITY IS
BEING MAINTAINED AT 45 KT FOR THIS ADVISORY...BUT IT COULD BE A
LITTLE HIGHER.
 
THE INITIAL MOTION ESTIMATE IS 300/13...BASED MAINLY ON CONTINUITY
AND AND PARTLY ON THE MODELS INDICATING A GRADUAL DECREASE IN
FORWARD SPEED. THERE HAS NOT BEEN MUCH CHANGE IN THE MODEL FORECAST
TRACKS OR THE MODELS' HANDLING OF THE SUBTROPICAL RIDGE TO THE
NORTH OF IRENE AND THE ALLEGED WEAKENING OF THE WESTERN PORTION OF
THE RIDGE AFTER 72 HOURS. HOWEVER...IT SHOULD BE NOTED THAT NONE OF
THE MODELS CREATE A WEAKNESS IN THE RIDGE DUE TO A SHORTWAVE TROUGH
MOVING INTO THE MEAN RIDGE POSITION. THE NOGAPS AND UKMET MODELS
ERODE THE RIDGE AS A RESULT OF AMPLIFYING IRENE TO A SIZE NEARLY
THE SAME AS THAT OF THE SYNOPTIC SCALE HIGH PRESSURE CENTER NEAR
BERMUDA...WHICH RESULTS IN IRENE TURNING SHARPLY NORTHWARD ALONG
70W LONGITUDE BY 72 HOURS. THAT SCENARIO SEEMS LESS LIKELY...GIVEN
THE CURRENT SIZE AND STRENGTH OF THE BERMUDA RIDGE AS NOTED IN
WATER VAPOR IMAGERY AND UPPER-AIR DATA. THE GFS HAS CONTINUED ITS
TREND OF DISSIPATING IRENE WITHIN 48 HOURS AND MAINTAINING AN EAST-
WEST ORIENTED MID-LEVEL RIDGE FROM BERMUDA WESTWARD TO THE
CAROLINAS. THE OFFICIAL FORECAST IS CLOSE TO THE GFDL AND GFDN
SOLUTIONS...WHICH MOVE IRENE FARTHER WEST AND CLOSER TO THE OUTER
BANKS OF NORTH CAROLINA...AND DECREASE THE FORWARD SPEED TO ONLY 5
KT AFTER 72 HOURS. THE OFFICIAL FORECAST IS NEARLY IDENTICAL TO THE
PREVIOUS TRACK AND REMAINS NEAR THE LEFT EDGE OF THE MODEL GUIDANCE
ENVELOPE. IT SHOULD BE POINTED OUT THAT WEAK STEERING CURRENTS LESS
THAN 10 KT OFTENTIMES RESULTS IN ERRATIC MOTION...SO A POSSIBLE
U.S. LANDFALL BY DAY 5 CAN NOT BE RULED OUT AT THIS TIME.

AN UPPER-LOW TO THE SOUTH OF IRENE HAS BEEN ADVECTING DRY MID-LEVEL
INTO THE SOUTHERN PORTION OF THE CYCLONE TODAY...WHICH HAS CAUSED
THE CONVECTION TO ERODE AND WEAKEN AT TIMES. HOWEVER...WATER VAPOR
IMAGERY INDICATES THE LOW HAS BEEN WEAKENING AND THAT THE MID- TO
UPPER-LEVEL FLOW IS GRADUALLY BECOMING SOUTHEASTERLY. THEREFORE...
DEEP CONVECTION HAS A BETTER CHANCE OF DEVELOPING LATER TONIGHT AND
IRENE COULD BECOME A HURRICANE BY THIS TIME TOMORROW. LATER IN THE
FORECAST PERIOD...DRY MID-LEVEL AIR REMAINS THE PRIMARY INHIBITING
FACTOR. THE SHIPS MODEL HAS NOW DECREASED THE PEAK INTENSITY TO
LESS THAN 70 KT DUE TO STRONG LOW-LEVEL ANTICYCLONIC FLOW
SURROUNDING THE CYCLONE AS IT MOVES INTO A STRONGER RIDGE. THE
OFFICIAL INTENSITY FORECAST REMAINS ABOVE THE SHIPS MODEL SINCE THE
GFS-BASED SHIPS MODEL DOES NOT HOLD ONTO IRENE AFTER 48 HOURS...
WHICH RESULTS IN AN ARTIFICIALLY STRONGER RIDGE IN THE GFS MODEL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      11/2100Z 25.7N  63.9W    45 KT
 12HR VT     12/0600Z 26.7N  65.6W    55 KT
 24HR VT     12/1800Z 27.8N  67.6W    60 KT
 36HR VT     13/0600Z 28.5N  69.1W    65 KT
 48HR VT     13/1800Z 29.1N  70.5W    70 KT
 72HR VT     14/1800Z 31.0N  72.5W    75 KT
 96HR VT     15/1800Z 33.0N  73.5W    75 KT
120HR VT     16/1800Z 35.0N  74.5W    75 KT
 
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