Hurricane KATIA

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HURRICANE KATIA DISCUSSION NUMBER  29
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
500 AM AST MON SEP 05 2011
 
THE INNER CORE CONVECTIVE STRUCTURE OF KATIA APPEARS TO HAVE GONE
THROUGH ANOTHER ROUND OF CHANGES...WITH A NET DECREASE IN
ORGANIZATION NOTED DURING THE PAST SEVERAL HOURS.  AN EYE THAT WAS
EVIDENT EARLIER IS NO LONGER APPARENT...AND THERE ARE SIGNS THAT
DRY AIR HAS BEEN ROTATING AROUND THE CIRCULATION. UPPER-LEVEL
OUTFLOW IS GOOD TO EXCELLENT...WITH A FAVORABLE OUTFLOW SINK TO THE
NORTHEAST.  A BLEND OF DVORAK T AND CI NUMBERS FROM TAFB AND SAB IS
LOWER THAN 6 HOURS AGO...SO THE CURRENT INTENSITY ESTIMATE IS
LOWERED TO 85 KT.
 
THE CENTER OF KATIA HAS BEEN WOBBLING. SMOOTHING OF RECENT FIXES
YIELDS AN INITIAL MOTION ESTIMATE OF 305/10. LARGE-SCALE MODELS
SHOW KATIA ROUNDING THE SOUTHWEST PERIPHERY OF A MID-LEVEL RIDGE
EAST OF BERMUDA...HEADING TOWARD A WEAKNESS OVER THE WESTERN
ATLANTIC. THIS SYNOPTIC PATTERN SHOULD STEER THE HURRICANE ON A
GENERAL NORTHWESTWARD COURSE WITH SOME DECELERATION OVER THE NEXT
FEW DAYS. LATER IN THE FORECAST PERIOD...FURTHER WESTWARD PROGRESS
IS LIKELY TO BE BLOCKED AS KATIA IS CAUGHT UP IN DEEP-LAYER
SOUTHERLY FLOW IN BETWEEN A LARGE CUT-OFF LOW OVER THE EASTERN
UNITED STATES AND THE RIDGE TO THE EAST. ONCE KATIA GAINS ENOUGH
LATITUDE...GLOBAL MODELS SHOW THE HURRICANE BECOMING EMBEDDED IN 
STRONG MID-LATITUDE WESTERLY FLOW AND ACCELERATING NORTHEASTWARD.
THE SPREAD IN THE TRACK GUIDANCE HAS DECREASED SUBSTANTIALLY SINCE
YESTERDAY AND NOW FAVORS A RECURVATURE SCENARIO ROUGHLY ALONG
70W...WITH EVEN THE LEFT-LEANING UKMET/ECMWF MODELS NOW SHIFTING
SIGNIFICANTLY TO THE EAST. THE MAIN FORECAST CHALLENGE IS HOW
QUICKLY KATIA WILL TURN NORTHWARD AND THEN NORTHEASTWARD...WITH
MAJOR TIMING DIFFERENCES NOTED AMONGST THE VARIOUS MODELS. THE
OFFICIAL TRACK IS NEARLY IDENTICAL TO THE PREVIOUS ONE THROUGH 72
HOURS BUT IS ADJUSTED TO THE RIGHT AND IS MUCH FASTER TOWARD THE
END OF THE PERIOD BUT NOT AS FAST AS THE MULTI-MODEL CONSENSUS.
 
ACCORDING TO THE GLOBAL MODELS...KATIA WILL BE MOVING THROUGH AN
ENVIRONMENT CHARACTERIZED BY ANTICYCLONIC UPPER-LEVEL FLOW AND WEAK
VERTICAL SHEAR THROUGH AT LEAST 72 HOURS. THE PRIMARY NEGATIVE
FACTORS THAT COULD INHIBIT THE INTENSIFICATION PROCESS DURING THIS
TIME ARE LOW MID-LEVEL RELATIVE HUMIDITY VALUES...A LOWERING OF THE
OCEANIC HEAT CONTENT ALONG THE FORECAST TRACK...AND THE POSSIBILITY
OF A MORE STABLE ENVIRONMENT DUE TO WARMING ALOFT.  AFTER 96
HOURS...INCREASING SHEAR AND THE PROXIMITY OF KATIA TO A STRONG
GRADIENT OF SEA SURFACE TEMPERATURES WILL LIKELY CONTRIBUTE TO
WEAKENING. THE OFFICIAL INTENSITY FORECAST HAS BEEN LOWERED EARLY
IN THE FORECAST PERIOD BASED ON CURRENT TRENDS AND ALSO AT THE END
OF THE PERIOD AS A RESULT OF ADJUSTMENTS TO THE TRACK FORECAST. 

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  05/0900Z 23.9N  62.0W   85 KT 100 MPH
 12H  05/1800Z 25.0N  63.2W   90 KT 105 MPH
 24H  06/0600Z 26.3N  64.8W   95 KT 110 MPH
 36H  06/1800Z 27.5N  66.2W  100 KT 115 MPH
 48H  07/0600Z 28.6N  67.6W  105 KT 120 MPH
 72H  08/0600Z 31.4N  70.1W  100 KT 115 MPH
 96H  09/0600Z 35.5N  69.5W   90 KT 105 MPH
120H  10/0600Z 38.5N  64.0W   75 KT  85 MPH
 
$$
FORECASTER KIMBERLAIN
 
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