Hurricane IDA Forecast Discussion

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WTNT41 KNHC 090841
TCDAT1
HURRICANE IDA DISCUSSION NUMBER  22
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112009
300 AM CST MON NOV 09 2009

DATA FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT SHOW THAT
VERTICAL SHEAR IS BEGINNING TO TAKE A TOLL ON IDA.  THE MINIMUM
PRESSURE IS ESTIMATED TO HAVE RISEN TO 988 MB BASED ON A DROPSONDE
MEASUREMENT OF 991 WITH ABOUT 30 KT OF WIND.  ALSO...THE FLIGHT-
LEVEL AND SFMR WIND DATA SUGGEST THAT THE LOW-LEVEL CENTER IS
BECOMING INCREASINGLY DISPLACED TO THE SOUTH OF THE FLIGHT-LEVEL
CENTER.  THIS IS CONSISTENT WITH TRENDS SEEN IN RECENT
GEOSTATIONARY AND MICROWAVE SATELLITE IMAGERY...AS THE COLDEST CLOUD
TOPS HAVE BEEN DISPLACED TO THE NORTH OF THE AIRCRAFT FIXES. THE
MAXIMUM 700-MB FLIGHT LEVEL WIND MEASURED BY THE AIRCRAFT SO FAR
WAS 74 KT.  BASED ON THESE DATA...THE INITIAL INTENSITY IS LOWERED
TO 80 KT FOR THIS ADVISORY...AND THIS COULD BE A LITTLE GENEROUS.

THE SHEAR OVER IDA IS CURRENTLY ANALYZED AT MORE THAN 30 KT BY
UW-CIMSS...AND IS FORECAST TO INCREASE MARKEDLY OVER THE NEXT 24
HOURS. THIS...IN COMBINATION WITH COOLER WATERS ALONG THE TRACK OF
IDA OVER THE NORTHERN GULF SHOULD RESULT IN A CONTINUED WEAKENING
TREND. THE OFFICIAL INTENSITY FORECAST HAS BEEN ADJUSTED DOWNWARD
...AND IS CLOSE TO OR A LITTLE ABOVE THE ICON CONSENSUS THROUGH 36
HOURS. AT 48 HOURS...THE OFFICIAL FORECAST LEANS TOWARD THE DECAY
SHIPS AND LGEM SINCE THE SYSTEM WILL BE WELL INLAND BY THAT TIME.
AS IDA MOVES INTO COOLER WATERS AND A MORE BAROCLINIC ENVIRONMENT
...EXTRATROPICAL TRANSITION WILL LIKELY BEGIN IN THE NEXT 24 HOURS.  
MOST OF THE GUIDANCE SUGGESTS IDA WILL BE EXTRATROPICAL IN 48 HOURS
AND THIS IS REFLECTED IN THE OFFICIAL FORECAST.

THE LATEST FIXES FROM THE AIRCRAFT SUGGEST THAT IDA IS MOVING ALONG
A HEADING OF 335/14...FASTER AND A LITTLE TO THE LEFT OF THE
PREVIOUS TRACK. IDA IS EXPECTED TO ACCELERATE NORTHWARD TODAY AHEAD
OF A MID- TO UPPER-LEVEL TROUGH MOVING EASTWARD ACROSS THE
SOUTH-CENTRAL UNITED STATES AND NORTHERN GULF OF MEXICO. THE TRACK
MODEL GUIDANCE CONTINUES TO BE IN GENERALLY GOOD AGREEMENT ON THIS
SCENARIO...HOWEVER...THE UKMET AND ECMWF REMAIN WESTERN OUTLIERS
WITH A TRACK CLOSER TO THE COAST OF SOUTHEASTERN LOUISIANA. THE
OFFICIAL FORECAST HAS BEEN ADJUSTED WESTWARD FROM THE PREVIOUS
PACKAGE THROUGH 24 HOURS TOWARD THE MULTI-MODEL CONSENSUS. AFTER
LANDFALL...IDA IS EXPECTED TO SLOW AND TURN SHARPLY TOWARD THE
EAST...AND THE NHC FORECAST CONTINUES TO FOLLOW THIS SCENARIO. MOST
OF THE GLOBAL MODEL GUIDANCE...ALONG WITH THE GFDL...SHOW WHAT IS
LEFT OF IDA DISSIPATING BY 72 HOURS AS IT IS ABSORBED INTO A FRONT
THAT IS MOVING THROUGH THE SOUTHEASTERN U.S. BEHIND A DEVELOPING
BAROCLINIC LOW OFF OF THE CAROLINAS.

FORECAST POSITIONS AND MAX WINDS

INITIAL      09/0900Z 25.1N  87.9W    80 KT
 12HR VT     09/1800Z 27.2N  88.2W    75 KT
 24HR VT     10/0600Z 29.6N  87.8W    70 KT
 36HR VT     10/1800Z 30.9N  86.7W    45 KT...INLAND
 48HR VT     11/0600Z 31.0N  85.0W    35 KT...EXTRATROPICAL INLAND
 72HR VT     12/0600Z...ABSORBED BY FRONT

$$
FORECASTER BRENNAN