This is one of the best discussions I've read in a long time, worth posting here IMO
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WTNT42 KNHC 270250
TCDAT2
TROPICAL STORM KAREN DISCUSSION NUMBER 9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122007
1100 PM EDT WED SEP 26 2007
A NOAA HURRICANE HUNTER PLANE HAS BEEN CONDUCTING A RESEARCH MISSION
THIS EVENING AND HAS FOUND AN UNUSUAL WIND STRUCTURE WITH KAREN.
SFMR DATA SHOW THAT THE STORM HAS A VERY LARGE WIND FIELD...WITH
ONE PEAK IN SURFACE WINDS ABOUT 105 NM AWAY FROM THE CENTER AND
ANOTHER ONE AT A MORE TYPICAL DISTANCE. IN ADDITION...DROPSONDE AND
SFMR DATA SUGGEST THAT THE FLIGHT LEVEL CENTER IS DISPLACED TO THE
NORTH AND EAST OF THE SURFACE CENTER...LIKELY DUE TO SOUTHWESTERLY
WIND SHEAR. THE MAXIMUM FLIGHT LEVEL WIND REPORTED WAS 69 KT...WITH
A PEAK SURFACE VALUE OF 62 KT REPORTED TWICE...NOT QUITE HURRICANE
STRENGTH. HOWEVER...GIVEN THE DEGRADED SATELLITE APPEARANCE FROM
EARLIER TODAY...KAREN WAS ALMOST CERTAINLY A HURRICANE THEN...AND
THIS LIKELIHOOD MAY BE REFLECTED IN THE FINAL BEST TRACK FOR THE
SYSTEM. AT THIS TIME...THE INITIAL INTENSITY IS HELD AT 60 KT FOR
THIS ADVISORY.
MODERATE TO STRONG SOUTHWESTERLY SHEAR FROM THE MID-OCEANIC TROUGH
IS AFFECTING KAREN AND THIS SHEAR IS FORECAST TO PERSIST OR EVEN
INTENSIFY OVER THE NEXT SEVERAL DAYS. IT IS HARD TO FORECAST
ANYTHING BUT SLOW WEAKENING GIVEN SUCH AN UNFAVORABLE ENVIRONMENT
AHEAD OF THE STORM...AND KAREN'S CHANCE TO INTENSIFY HAS PROBABLY
ENDED. INITIALLY...MOST MODELS DO NOT WEAKEN THE SYSTEM VERY
MUCH...PERHAPS DUE TO THE LARGE STORM BEING ABLE TO MAINTAIN ITSELF
DESPITE THE SHEAR. EVENTUALLY...MOST INTENSITY GUIDANCE WEAKEN THE
SYSTEM...AND THE OFFICIAL FORECAST IS CLOSER TO THE HWRF/GFDL
MODELS THAN THE SHIPS MODEL. IT IS OF NOTE THAT THE GFDL EVEN
MAKES THIS SYSTEM A TROPICAL DEPRESSION BY THE END OF THE FORECAST.
MICROWAVE AND AIRCRAFT DATA HAVE PROVIDED A GOOD ESTIMATE OF THE
INITIAL MOTION...ABOUT 300/12. THIS GENERAL MOTION SHOULD CONTINUE
FOR THE NEXT DAY OR SO AS WEAK RIDGING PERSISTS TO THE NORTH OF THE
STORM. STEERING CURRENTS THEN WEAKEN AS A MIDDLE TO UPPER-LEVEL
TROUGH CREATES A WEAKNESS IN THE RIDGE. THE MODELS THAT HAVE A
STRONG...VERTICALLY-STACKED REPRESENTATION OF KAREN...E.G. THE
ECMWF/GFDN/HWRF/NOGAPS SOLUTIONS..SHOW THE STORM MOVING MORE TO THE
NORTHWEST OR NORTH-NORTHWEST IN RESPONSE TO THE TROUGH. HOWEVER
THE GFDL/GFS/UKMET MODELS GENERALLY HAVE A WEAKER STORM AT THAT
TIME...AND SHOW LITTLE REACTION TO THE TROUGH AND JUST SLOW THE
STORM AS IT ENTERS THE AREA OF LIGHTER STEERING CURRENTS. THE
OFFICIAL FORECAST FAVORS THE LEFTWARD SOLUTION AT THIS TIME AND IS
ADJUSTED WESTWARD OF THE PREVIOUS FORECAST.
THE WIND RADII WERE EXPANDED THANKS TO SFMR DATA...QUIKSCAT...AND
NOAA BUOY 41041.
FORECAST POSITIONS AND MAX WINDS
INITIAL 27/0300Z 13.0N 45.1W 60 KT
12HR VT 27/1200Z 13.9N 46.7W 60 KT
24HR VT 28/0000Z 15.2N 48.7W 60 KT
36HR VT 28/1200Z 16.5N 50.5W 55 KT
48HR VT 29/0000Z 17.5N 51.9W 50 KT
72HR VT 30/0000Z 19.4N 54.7W 45 KT
96HR VT 01/0000Z 21.0N 57.0W 45 KT
120HR VT 02/0000Z 22.5N 58.5W 45 KT
$$
FORECASTER BLAKE
