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WTNT41 KNHC 082048
TCDAT1
HURRICANE IDA DISCUSSION NUMBER 20
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL112009
300 PM CST SUN NOV 08 2009
REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE
THAT THE CENTRAL PRESSURE OF IDA HAS FALLEN TO 976 MB. THE
AIRCRAFT REPORTED MAXIMUM 700 MB FLIGHT-LEVEL WINDS OF 94 KT...WITH
SFMR ESTIMATES OF 80-85 KT. BASED ON THIS...THE INITIAL INTENSITY
IS 85 KT. SATELLITE IMAGERY AND RADAR DATA FROM MEXICO AND CUBA
SUGGEST THAT THE EYEWALL HAS BECOME LESS ORGANIZED DURING THE PAST
FEW HOURS...WHICH MAY BE A SIGN THAT IDA IS BEING AFFECTED BY
ONGOING SOUTHWESTERLY VERTICAL WIND SHEAR.
THE INITIAL MOTION IS 330/9. IDA REMAINS BETWEEN A MID/UPPER-LEVEL
TROUGH OVER THE WESTERN GULF MEXICO AND ADJACENT LAND AREAS AND AN
MID/UPPER-LEVEL RIDGE OVER THE EASTERN GULF OF MEXICO AND THE
NORTHERN CARIBBEAN SEA. IN ADDITION...A STRONG LOW-LEVEL RIDGE
COVERS THE NORTHERN GULF OF MEXICO. THE DYNAMICAL MODELS FORECAST
THE TROUGH AND RIDGE TO MOVE EASTWARD...ALLOWING IDA TO TURN TOWARD
THE NORTH DURING THE NEXT 36 HR OR SO. ALTHOUGH THE TRACK GUIDANCE
AGREES WITH THIS SCENARIO...THERE IS CONSIDERABLE SPREAD IN BOTH
THE LOCATION AND TIMING OF THE MODELS FORECAST LANDFALLS ON THE
NORTHERN GULF COAST. THE ECMWF AND CANADIAN MODELS ARE ON THE
WESTERN EDGE OF THE ENVELOPE WITH A SOUTHEASTERN LOUISIANA
LANDFALL. THE HWRF IS ON THE EASTERN EDGE WITH A LANDFALL IN THE
WESTERN FLORIDA PANHANDLE. THE UKMET IS NOTABLE SLOWER THAN THE
OTHER GUIDANCE...CALLING FOR A LANDFALL AFTER 48 HR. THE NEW
FORECAST TRACK COMPROMISES BETWEEN THESE EXTREMES AND LIES NEAR THE
VARIOUS CONSENSUS MODELS. AFTER LANDFALL...THE MODEL SPREAD GETS
VERY LARGE DUE TO MAJOR MODEL DIFFERENCES ON THE EVOLUTION OF THE
WESTERLIES OVER THE SOUTHEASTERN UNITED STATES. THE NEW TRACK
CALLS FOR A SLOW EASTWARD MOTION...BUT THIS PART OF THE TRACK IS
LOW CONFIDENCE.
THE HURRICANE MAY BE ABLE TO STRENGTHEN A LITTLE MORE IN THE NEXT
6-12 HR. AFTER THAT...IDA WILL BE MOVING OVER COOLER SEA SURFACE
TEMPERATURES...INTO A COOLER AIR MASS...AND INTO STRONGER SHEAR.
WHILE EXTRATROPICAL TRANSITION SHOULD BEGIN BEFORE IDA MAKES
LANDFALL...IT IS UNCERTAIN IF THE CYCLONE WILL COMPLETELY LOSE ITS
TROPICAL CHARACTERISTICS BEFORE LANDFALL. IN ADDITION...MOST OF
THE INTENSITY GUIDANCE FORECAST IDA TO MAINTAIN HURRICANE STRENGTH
UNTIL LANDFALL ON THE NORTHERN GULF COAST. BASED ON THIS...THE NEW
INTENSITY FORECAST MAINTAINS IDA AS A WEAKENING HURRICANE THROUGH
LANDFALL...THEN FORECASTS EXTRATROPICAL TO FINISH AFTER LANDFALL.
IN THE 72-120 HR TIME FRAME...THE GFS...ECMWF...AND CANADIAN MODELS
FORECAST A LARGE BAROCLINIC LOW OFF THE U. S. EAST COAST. IT IS
NOT CLEAR WHETHER THIS IS THE REMAINS OF IDA OR A SECOND LOW THAT
ABSORBS IDA. THE FORECAST CONTINUES TO USE THE SCENARIO THAT
THIS IS A SECOND LOW THAT ABSORBS THE REMAINS OF IDA.
THE UNCERTAINTY OVER THE TIMING OF EXTRATROPICAL TRANSITION REQUIRES
A HURRICANE WATCH FOR PORTIONS OF THE NORTHERN GULF COAST.
REGARDLESS OF WHETHER IDA LOSES ITS TROPICAL CHARACTERISTICS BEFORE
LANDFALL...THE CYCLONE IS EXPECTED TO BRING STRONG WINDS...HEAVY
RAINS...AND STORM SURGES TO THAT AREA. OUTSIDE OF THE HURRICANE
WATCH AREA....POTENTIAL HAZARDS ASSOCIATED WITH IDA ARE BEING
HANDLED BY LOCAL NATIONAL WEATHER SERVICE OFFICES ALONG THE GULF
COAST IN THEIR PRODUCT SUITE WITH MARINE AND COASTAL FLOOD
WATCHES...WARNINGS...AND ADVISORIES.
FORECAST POSITIONS AND MAX WINDS
INITIAL 08/2100Z 22.2N 86.3W 85 KT
12HR VT 09/0600Z 23.9N 87.4W 85 KT
24HR VT 09/1800Z 26.7N 88.0W 85 KT
36HR VT 10/0600Z 29.2N 87.9W 75 KT
48HR VT 10/1800Z 30.7N 86.7W 55 KT...EXTRATROPICAL INLAND
72HR VT 11/1800Z 31.0N 83.0W 40 KT...EXTRATROPICAL INLAND
96HR VT 12/1800Z 30.0N 79.5W 40 KT...EXTRATROPICAL
120HR VT 13/1800Z...ABSORBED BY EXTRATROPICAL LOW
FORECASTER BEVEN
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