[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT41 KNHC 280251
TCDAT1

TROPICAL DEPRESSION OPHELIA DISCUSSION NUMBER  22
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
1100 PM AST TUE SEP 27 2011

FROM THE BIG PICTURE...OPHELIA APPEARS TO BE REASONABLY WELL
ORGANIZED WITH AN EXPANDING OUTFLOW PATTERN AND A ROUND BALL OF
DEEP CONVECTION NEAR THE APPARENT CENTER.  HOWEVER...THE AIR FORCE
RESERVE HURRICANE HUNTER AIRCRAFT DATA SHOW A MUCH DIFFERENT
PERSPECTIVE WITH THE LOW-LEVEL CIRCULATION ELONGATED FROM
EAST-NORTHEAST TO WEST-SOUTHWEST...AND PEAK SFMR VALUES OF ONLY
ABOUT 29 KT.  THE INITIAL WINDS ARE SET TO 30 KT IN ACCORDANCE WITH
THE SFMR ESTIMATES.

THE INTENSITY FORECAST IS A CHALLENGING ONE THIS EVENING.  WHILE THE
CURRENT OUTFLOW PATTERN WOULD SEEM TO FAVOR SIGNIFICANT
INTENSIFICATION...WATER VAPOR IMAGES SHOW AN UPPER-LEVEL TROUGH
OVER THE WESTERN ATLANTIC MOVING EASTWARD TOWARD THE TROPICAL
CYCLONE.  THIS TROUGH SHOULD SOON RAISE THE SHEAR NEAR OPHELIA...
AND SINCE THE CYCLONE HAS A RATHER POOR INITIAL LOW-LEVEL
STRUCTURE...ONLY A SLOW INCREASE IN WINDS IS FORECAST FOR THE FIRST
DAY OR SO.  THEREAFTER...THE TROUGH SHOULD WEAKEN AS AN UPPER HIGH
BUILDS CLOSER TO THE TROPICAL CYCLONE...WHICH COULD SUPPORT A
FASTER INTENSIFICATION.  IT IS INTERESTING TO NOTE THAT THE
STATISTICAL GUIDANCE IS CONSIDERABLY LOWER THAN THE LAST
RUN...SHOWING OPHELIA ONLY BECOMING A MODERATE TROPICAL STORM
BECAUSE OF MUCH HIGHER FORECAST SHEAR.  SOME OF THIS CHANGE MIGHT
BE DUE TO THE INTERPOLATED OLD OFFICIAL FORECAST WHICH IS A GOOD
DEAL FASTER THAN THE GFS...AND IN THIS CASE INTRODUCES SOME
ARTIFICIAL SHEAR AS IT IS CLOSER TO THE STRONG UPPER-LEVEL WINDS.
I HAVE ELECTED NOT TO CHANGE THE PREVIOUS FORECAST PEAK INTENSITY
GIVEN THE GFDL/HWRF STILL SHOWING CATEGORY 2 HURRICANE STATUS...BUT
THE TREND IN THESE STATISTICAL MODELS SHOULD BE WATCHED.

MICROWAVE AND AIRCRAFT DATA SUGGEST THE CENTER...SUCH AS IT IS...HAS
REFORMED TO THE NORTHEAST...A BIT CLOSER TO THE MID-LEVEL CENTER.
A LONG-TERM MOTION ESTIMATE IS 305/3...BUT THIS IS HIGHLY
UNCERTAIN.  OPHELIA IS EXPECTED TO MOVE A BIT FASTER TO THE
NORTHWEST BY EARLY THURSDAY AS IT IS STEERED AROUND THE
SOUTHWESTERN PORTION OF A STRENGTHENING SUBTROPICAL RIDGE.   A TURN
TO THE NORTH AND NORTHEAST SHOULD OCCUR BY THE WEEKEND DUE TO A
LARGE MID-LATITUDE TROUGH DIVING OFFSHORE OF THE EASTERN UNITED
STATES.  THE MODEL GUIDANCE IS IN BETTER AGREEMENT THAN SIX HOURS
AGO...ALTHOUGH THE SPEED DIFFERENCES ARE SIGNIFICANT.  THE NHC
FORECAST IS ADJUSTED EAST OF THE PREVIOUS ONE FOR THE FIRST COUPLE
OF DAYS TO ACCOUNT FOR THE CENTER REPOSITIONING...AND IS VERY CLOSE
TO THE TRACK AFTER THAT TIME...A BIT FASTER THAN THE MULTI-MODEL
CONSENSUS.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 18.4N  59.5W   30 KT  35 MPH
 12H  28/1200Z 18.8N  59.8W   35 KT  40 MPH
 24H  29/0000Z 19.5N  60.2W   40 KT  45 MPH
 36H  29/1200Z 20.9N  60.9W   45 KT  50 MPH
 48H  30/0000Z 22.4N  61.6W   55 KT  65 MPH
 72H  01/0000Z 26.3N  62.7W   65 KT  75 MPH
 96H  02/0000Z 32.0N  61.5W   75 KT  85 MPH
120H  03/0000Z 40.0N  57.5W   70 KT  80 MPH

$$
FORECASTER BLAKE



This weather bulletin was brought to you by:
www.nashvilleweather.net