[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 070857
TCDAT1
TROPICAL STORM IDA DISCUSSION NUMBER  12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112009
400 AM EST SAT NOV 07 2009

THE SATELLITE PRESENTATION OF IDA HAS IMPROVED OVER THE PAST SEVERAL
HOURS...WITH AN INCREASE IN DEEP CONVECTION NEAR THE ESTIMATED
LOCATION OF THE LOW LEVEL CENTER.  THE INITIAL INTENSITY IS RAISED
TO 40 KT BASED ON A BLEND OF DVORAK ESTIMATES OF 35 KT FROM SAB AND
45 KT FROM TAFB.

THE INITIAL MOTION ESTIMATE IS 350/07...A LITTLE FASTER THAN BEFORE.
IDA IS MOVING NORTHWARD BETWEEN A MID-LEVEL ANTICYCLONE OVER THE
NORTH-CENTRAL CARIBBEAN SEA AND A DEEP-LAYER TROUGH OVER SOUTHERN
MEXICO. MOST OF THE TRACK GUIDANCE IS TIGHTLY CLUSTERED THROUGH 48
HOURS...AND THE OFFICIAL FORECAST THROUGH THAT TIME IS CLOSE TO THE
MODEL CONSENSUS BUT A LITTLE FASTER THAN THE PREVIOUS PACKAGE.
BEYOND 48 HOURS...THE GLOBAL MODELS SHOW A MID- TO UPPER-LEVEL
SHORTWAVE TROUGH DIGGING INTO THE NORTHERN GULF OF MEXICO. WHILE
THE DETAILS ON THE TIMING AND DEGREE OF THE INTERACTION BETWEEN IDA
AND THE TROUGH DIFFER...MOST OF THE MODELS...WITH THE EXCEPTION OF
THE NOGAPS AND GFDN...SHOW IDA TURNING NORTHWARD AND THEN
NORTHEASTWARD IN 3 OR 4 DAYS.  IDA IS EXPECTED TO BECOME
EXTRATROPICAL DURING THIS TIME AS IT BECOMES INVOLVED WITH A
LOW-LEVEL FRONTAL ZONE ALONG THE NORTHERN GULF COAST.  BY DAY 5
MOST OF THE GUIDANCE SHOWS THE FRONT...AND WHAT IS LEFT OF IDA...
TURNING SOUTHWARD OR SOUTHEASTWARD IN RESPONSE TO A LARGE SCALE MID-
TO UPPER-LEVEL TROUGH AMPLIFYING OVER EASTERN NORTH AMERICA.  AT
DAYS 3 THROUGH 5 THE NEW OFFICIAL THE FORECAST IS ADJUSTED A LITTLE
NORTHWARD TOWARD THE MODEL CONSENSUS.

WHILE IDA WILL BE MOVING OVER VERY WARM WATERS FOR THE NEXT COUPLE
OF DAYS THE ENVIRONMENT WILL NOT BE IDEAL FOR STRENGTHENING...AS
THE SHIPS MODEL SHOWS ABOUT 20 KT OF VERTICAL SHEAR OVER THE
CYCLONE. BEYOND THAT TIME...THE SHEAR IS FORECAST TO INCREASE
FURTHER AS IDA BEGINS TO INTERACT WITH THE MID- TO UPPER-LEVEL
SHORTWAVE.  THE OFFICIAL FORECAST IS ADJUSTED UPWARD A LITTLE GIVEN
THE OBSERVED INTENSITY TRENDS...AND SHOWS A PEAK OF 50 KT IN 36
HOURS. THIS IS IN BEST AGREEMENT WITH THE SHIPS MODEL AND CLOSE TO
THE ICON INTENSITY CONSENSUS.  GRADUAL WEAKENING IS EXPECTED TO
BEGIN ON DAY 2 AS IDA ENCOUNTERS THE STRONGER SHEAR AND BEGINS TO
UNDERGO EXTRATROPICAL TRANSITION.

MOST OF THE GLOBAL MODELS SHOW A LARGE PRESSURE GRADIENT DEVELOPING
BETWEEN A HIGH PRESSURE RIDGE OVER THE SOUTHEASTERN UNITED STATES
AND THE TROPICAL CYCLONE.  THIS WILL CONTRIBUTE TO A LARGE AREA OF
STRONG WINDS OVER THE GULF OF MEXICO NOT DIRECTLY ATTRIBUTABLE TO
IDA.

THE ISSUANCE OF A TROPICAL STORM WATCH IS REQUIRED AT THIS TIME FOR
PORTIONS OF THE YUCATAN PENINSULA AND WESTERN CUBA.

FORECAST POSITIONS AND MAX WINDS

INITIAL      07/0900Z 17.1N  84.1W    40 KT
 12HR VT     07/1800Z 18.1N  84.4W    45 KT
 24HR VT     08/0600Z 19.5N  85.0W    45 KT
 36HR VT     08/1800Z 21.0N  86.0W    50 KT
 48HR VT     09/0600Z 23.0N  87.1W    45 KT
 72HR VT     10/0600Z 26.5N  88.0W    45 KT
 96HR VT     11/0600Z 28.0N  86.5W    40 KT...EXTRATROPICAL
120HR VT     12/0600Z 27.0N  85.0W    35 KT...EXTRATROPICAL

$$
FORECASTER BRENNAN



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