[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT42 KNHC 062048
TCDAT2

HURRICANE KATIA DISCUSSION NUMBER  35
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 PM AST TUE SEP 06 2011

A 1509 UTC TRMM PASS INDICATES THAT THE INNER EYEWALL OF KATIA IS
DECAYING WITHIN A LARGER EYEWALL...SUGGESTING THAT THE HURRICANE IS
NEARING THE END OF ITS EYEWALL REPLACEMENT.  AN AIR FORCE RESERVE
HURRICANE HUNTER AIRCRAFT MADE TWO PASSES THROUGH THE STORM AND
MEASURED A MAXIMUM 700-MB FLIGHT-LEVEL WIND OF 114 KT AND A PEAK
SURFACE WIND OF 81 KT FROM THE SFMR. THE PLANE ALSO REPORTED THAT
THE NEW RADIUS OF MAXIMUM WINDS IS ABOUT 55 N MI...AND THE 64- AND
50-KT RADII HAVE EXPANDED. BASED ON THE EXPANSION OF THE WIND
FIELD...IT IS ASSUMED THAT THE STANDARD FLIGHT-LEVEL TO SURFACE
WIND RELATIONSHIP MAY BE TOO HIGH...AND THE INITIAL INTENSITY IS
THEREFORE SET AT 90 KT.

KATIA CONTINUES TO MOVE ON TRACK WITH A MOTION OF 310/9 KT.  THE
HURRICANE WILL BE MOVING AROUND THE SOUTHWESTERN PERIPHERY OF A
CENTRAL ATLANTIC MID-LEVEL HIGH IN THE SHORT-TERM...AND IT SHOULD
BEGIN TO TURN TOWARD THE NORTH-NORTHWEST AND NORTH OVER THE NEXT 48
HOURS.  AFTER 48 HOURS...THE MODEL GUIDANCE CONTINUES TO TREND
TOWARD A DELAY IN PICKING UP THE HURRICANE BY THE FAST MID-LATITUDE
WESTERLIES...AND THIS HAS RESULTED IN ANOTHER SLIGHT NORTHWESTWARD
SHIFT OF THE GUIDANCE ENVELOPE AT 72 HOURS.  THE OFFICIAL FORECAST
HAS ALSO BEEN NUDGED IN THAT DIRECTION...ENDING UP ON THE
SOUTHEASTERN SIDE OF THE GUIDANCE.  THE MODEL SPREAD APPEARS TO BE
DECREASING AT THE 96- AND 120-HOUR POINTS AS ALL THE MODELS HAVE
KATIA ACCELERATING IN THE WESTERLIES...AND LITTLE CHANGE TO THE
OFFICIAL FORECAST WAS REQUIRED AT THOSE TIMES.

THE EXPANDING WIND FIELD OF KATIA SUGGESTS THAT LITTLE CHANGE IN
INTENSITY SHOULD OCCUR OVER THE NEXT DAY OR SO.  THE HURRICANE
COULD BE AFFECTED BY SOME SHEAR OVER THE NEXT FEW DAYS...AND IT IS
ALREADY MOVING OVER WATERS THAT ARE BECOMING GRADUALLY LESS
CONDUCIVE FOR RE-STRENGTHENING.  GRADUAL WEAKENING IS INDICATED IN
THE OFFICIAL FORECAST FROM 24 TO 72 HOURS.  THEREAFTER...THE
VARIOUS GLOBAL MODEL FIELDS ARE IN AGREEMENT SHOWING KATIA BECOMING
ENTANGLED WITHIN A FRONTAL ZONE BY 96 HOURS.  THE CYCLONE IS
THEREFORE FORECAST TO BE EXTRATROPICAL AT THAT TIME...BUT THERE
SHOULD BE ENOUGH BAROCLINIC FORCING TO MAINTAIN IT AS A POWERFUL
HURRICANE-FORCE LOW PRESSURE SYSTEM.  THE OFFICIAL FORECAST AT THE
END OF THE FORECAST PERIOD IS BASED ON GUIDANCE FROM THE OCEAN
PREDICTION CENTER.


FORECAST POSITIONS AND MAX WINDS

INIT  06/2100Z 27.6N  66.7W   90 KT 105 MPH
 12H  07/0600Z 28.5N  67.8W   90 KT 105 MPH
 24H  07/1800Z 29.9N  69.2W   90 KT 105 MPH
 36H  08/0600Z 31.7N  70.0W   85 KT 100 MPH
 48H  08/1800Z 34.0N  69.9W   85 KT 100 MPH
 72H  09/1800Z 39.0N  66.0W   75 KT  85 MPH
 96H  10/1800Z 44.0N  50.0W   70 KT  80 MPH...POST-TROP/EXTRATROP
120H  11/1800Z 53.0N  29.0W   70 KT  80 MPH...POST-TROP/EXTRATROP

$$
FORECASTER BERG/BRENNAN


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