[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 090833
TCDAT4
TROPICAL DEPRESSION IRENE DISCUSSION NUMBER  19
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT TUE AUG 09 2005

THERE HAS BEEN SOME INCREASE IN THE DEEP CONVECTION NEAR THE CENTER
OF IRENE'S MOSTLY EXPOSED CIRCULATION.  WHILE THERE MAY WELL BE 35
KT WINDS IN THE SYSTEM AT THIS MOMENT...I WOULD PREFER TO SEE THIS
CONVECTION PERSIST NEAR THE CENTER BEFORE RE-UPGRADING IRENE TO A
TROPICAL STORM.  THE 6Z DVORAK INTENSITY ESTIMATES WERE 25-30
KT...AND THE INITIAL INTENSITY WILL BE HELD AT 30 KT FOR THIS
ADVISORY.

IRENE IS PRESENTLY MOVING PAST A BREAK IN THE MID-LEVEL SUBTROPICAL
RIDGE...WITH THE RIDGE CURRENTLY WELL ESTABLISHED AT MID- TO UPPER
LEVELS TO THE NORTHWEST OF THE CYCLONE.  THIS PATTERN IS IMPARTING
A NORTHERLY FLOW OF VERY DRY AIR AT UPPER LEVELS OVER IRENE THAT IS
HINDERING DEVELOPMENT.  GLOBAL MODELS INDICATE THAT THIS UPPER FLOW
SHOULD TRANSITION TO SOUTHEASTERLIES OVER THE NEXT COUPLE OF DAYS
AS IRENE MOVES CLOSER TO...AND TO THE NORTHEAST OF THE UPPER LOW
CURRENTLY NEAR 30N/73W.  SOME RESTRENGTHENING IS THEREFORE SHOWN IN
THE OFFICIAL FORECAST.  HOWEVER...THIS STRENGTHENING IS
CONSIDERABLY LESS THAN INDICATED BY THE SHIPS GUIDANCE...WHICH
BRINGS IRENE TO NEARLY HURRICANE STRENGTH IN FIVE DAYS.  AN
OPPOSING VIEW IS PROVIDED BY THE GFDL MODEL...WHICH DISSIPATES
IRENE WITHIN 24 HOURS.  AS IRENE HAS PERSERVERED THROUGH AN
ENVIRONMENT SEEMINGLY MORE HOSTILE THAN WHAT IT IS CURRENTLY
EMBEDDED IN...I AM GETTING LESS HOPEFUL ABOUT THE DISSIPATION
OPTION.

THE INITIAL MOTION IS 270/10.  THE MID-LEVEL SUBTROPICAL
RIDGE...CURRENTLY CENTERED NEAR BERMUDA...IS FORECAST BY THE
GFS...UKMET...NOGAPS...AND CANADIAN MODELS TO MOVE SOUTHWESTWARD
AND/OR SPLIT AND WEAKEN OVER THE NEXT THREE DAYS.  ALL OF THESE
MODELS SHOW A SHORT WAVE TROUGH IN THE WESTERLIES MOVING INTO THE
WESTERN ATLANTIC IN ABOUT THREE DAYS...AND THESE MODELS ALL LIFT
IRENE...OR ITS REMNANTS...NORTHWESTWARD WITH THE APPROACH OF THIS
TROUGH.  THE 00Z RUNS OF THE UKMET AND NOGAPS MODELS...HOWEVER...
LEAVE IRENE TO LINGER BEHIND AS THE TROUGH PROGRESSES EASTWARD.
THERE IS ALSO A CURIOUS SEPARATION BETWEEN THE SINGLE-LAYER BAM
MODELS AND THE MULTI-LEVEL GLOBAL MODELS...WITH THE BAM MODELS ALL
SHOWING A MORE STRAIGHT-LINE WEST-NORTHWESTERLY TRACK THROUGH THE
FIVE DAY PERIOD.  AMONG THE BAM MODELS...THE SHALLOW BAM HAS THE
MOST NORTHERLY TRACK...WHICH IS CONSISTENT WITH THE NORTHERLY
UPPER-LEVEL FLOW CURRENTLY OVER THE TROPICAL CYCLONE.  THE OFFICIAL
FORECAST HAS BEEN SHIFTED JUST A LITTLE TO THE WEST OF THE PREVIOUS
ADVISORY AND IS ALSO SLOWER AT DAYS 4 AND 5.  THE OFFICIAL FORECAST
LIES CLOSE TO THE LEFT EDGE OF THE GLOBAL MODEL GUIDANCE
ENVELOPE...AND IS ALSO CLOSE TO THE SHALLOW BAM FOR THE FIRST 72
HOURS.

FORECASTER FRANKLIN


FORECAST POSITIONS AND MAX WINDS

INITIAL      09/0900Z 22.5N  53.6W    30 KT
 12HR VT     09/1800Z 22.5N  55.2W    30 KT
 24HR VT     10/0600Z 23.0N  57.4W    35 KT
 36HR VT     10/1800Z 23.9N  59.4W    35 KT
 48HR VT     11/0600Z 25.2N  61.4W    40 KT
 72HR VT     12/0600Z 27.5N  64.5W    40 KT
 96HR VT     13/0600Z 29.0N  65.5W    45 KT
120HR VT     14/0600Z 30.0N  66.0W    45 KT


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