[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 031452
TCDAT2

HURRICANE KATIA DISCUSSION NUMBER  22
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
1100 AM AST SAT SEP 03 2011

SEVERAL MICROWAVE OVERPASSES FROM EARLIER THIS MORNING INDICATED
THAT KATIA IS TILTED...WITH A MID-LEVEL MICROWAVE EYE DISPLACED
ABOUT 20 N MI NORTH OF THE LOW-LEVEL CENTER. FINAL T-NUMBERS ARE
DOWN TO T3.5 FROM BOTH SAB AND TAFB...ALTHOUGH CI NUMBERS ARE STILL
HOLDING AT 4.0. UW-CIMSS ADT ESTIMATES HAVE BEEN DECREASING AND ARE
NOW T4.1. KATIA WILL BE KEPT AS A HURRICANE FOR NOW...BUT IF TRENDS
CONTINUE IT COULD ONCE AGAIN WEAKEN TO A TROPICAL STORM LATER
TODAY.

KATIA IS BEGINNING TO MOVE AROUND THE SOUTHWESTERN PERIPHERY OF A
MID-LEVEL HIGH LOCATED OVER THE CENTRAL ATLANTIC...AND TOWARDS A
DEEP-LAYER TROUGH OVER THE WESTERN ATLANTIC.  WITH THE EXCEPTION OF
THE UKMET...WHICH IS DISREGARDED IN THIS FORECAST...THE MODELS ARE
IN GOOD AGREEMENT ON MAINTAINING A MOTION OF ABOUT 305 DEGREES AT 9
TO 10 KT FOR THE NEXT 48 HOURS.  BEYOND 72 HOURS...THE TRACK
FORECAST BECOMES A LITTLE MORE COMPLEX AND PARTLY DEPENDS ON HOW
TROPICAL STORM LEE AND U.S. EAST COAST TROUGHING EVOLVE OVER THAT
TIME.  AS AN EXAMPLE...THE ECMWF DEVELOPS A DEEP-LAYER CLOSED LOW
OVER THE OHIO VALLEY BY DAY 5 WHILE THE GFS HAS A SHARP TROUGH
ALMOST READY TO CLOSE OFF OVER NEW YORK AND PENNSYLVANIA.  DESPITE
THESE DIFFERENCES...BOTH MODELS HAVE SHIFTED TO THE EAST...WITH THE
GFS BEING THE FASTEST DUE TO THE MORE PROGRESSIVE FLOW.  THE NHC
TRACK FORECAST IS ALSO SHIFTED TO THE EAST YET STILL LIES TO THE
WEST OF THOSE USUALLY RELIABLE MODELS.

STRONG SOUTHERLY SHEAR IS AFFECTING KATIA AND IS LIKELY ALLOWING DRY
AIR TO ERODE THE DEEP CONVECTION ON THE SOUTHERN SIDE.  THIS SHEAR
IS EXPECTED TO PERSIST FOR THE NEXT 24 HOURS OR SO...DURING WHICH
TIME NO STRENGTHENING IS INDICATED IN THE FORECAST.  THEREAFTER...
THE SHIPS MODEL SHOWS THAT UPPER-LEVEL WINDS COULD BECOME MORE
CONDUCIVE FOR STRENGTHENING...BUT THIS WILL ALL DEPEND ON THE
LOCATION OF KATIA RELATIVE TO THE DEEP-LAYER TROUGH OR CLOSED LOW
OVER THE EASTERN UNITED STATES.  THE NHC INTENSITY FORECAST WILL
CONTINUE TO INDICATE INTENSIFICATION BETWEEN DAYS 2 AND 4...AND IS
STILL ABOVE THE INTENSITY CONSENSUS AND STATISTICAL MODELS.

IT IS ADVISABLE NOT TO FOCUS ON SLIGHT FORECAST INTENSITY CHANGES
NEAR THE MAJOR HURRICANE THRESHOLD OF 100 KT SINCE THERE IS VERY
LITTLE DIFFERENCE BETWEEN A 95- AND 100-KT HURRICANE.  BASED ON THE
CURRENT FORECAST AND RECENT NHC INTENSITY ERRORS...AT THIS TIME
THERE IS ROUGHLY A 50/50 CHANCE OF KATIA STILL BECOMING A MAJOR
HURRICANE DURING THE NEXT 5 DAYS.

FORECAST POSITIONS AND MAX WINDS

INIT  03/1500Z 19.6N  55.8W   65 KT  75 MPH
 12H  04/0000Z 20.3N  57.0W   65 KT  75 MPH
 24H  04/1200Z 21.3N  58.7W   65 KT  75 MPH
 36H  05/0000Z 22.5N  60.4W   70 KT  80 MPH
 48H  05/1200Z 23.7N  62.1W   75 KT  85 MPH
 72H  06/1200Z 26.5N  65.0W   85 KT 100 MPH
 96H  07/1200Z 29.5N  67.5W   95 KT 110 MPH
120H  08/1200Z 32.0N  69.5W   90 KT 105 MPH

$$
FORECASTER BERG


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