Tropical Storm RAFAEL

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TROPICAL STORM RAFAEL DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL172012
1100 AM AST SAT OCT 13 2012
 
THE SATELLITE PRESENTATION OF RAFAEL HAS BEGUN TO IMPROVE A
LITTLE...WITH DEEP CONVECTION MOVING A LITTLE CLOSER TO THE CENTER
AS THE UPPER-LEVEL LOW THAT HAS BEEN PRODUCING STRONG SOUTHWESTERLY
SHEAR OVER THE CYCLONE BEGINS TO LIFT NORTHWARD. DATA FROM THE
LATEST AIRCRAFT MISSION SUGGEST THAT THE PEAK WINDS ARE NO HIGHER
THAN 35 KT...WITH THE CENTRAL PRESSURE REMAINING 1006 MB. GLOBAL
MODELS AND THE SHIPS MODEL DIAGNOSTICS SUGGEST THAT THE SHEAR WILL
CONTINUE TO DECREASE OVER THE NEXT DAY OR SO...WHICH SHOULD ALLOW
THE CYCLONE TO BEGIN STRENGTHENING GRADUALLY. MOST OF THE GUIDANCE
SHOWS INTENSIFICATION CONTINUING THROUGH 72 HOURS DESPITE
SOUTHWESTERLY SHEAR INCREASING TO MODERATE VALUES BY THAT TIME...
AND THE OFFICIAL FORECAST SHOWS RAFAEL BECOMING A HURRICANE IN
ABOUT 3 DAYS. LATE IN THE PERIOD THE SHEAR INCREASES ABOVE 40
KT...WHICH SHOULD RESULT IN SOME WEAKENING AS THE CYCLONE BECOMES
EXTRATROPICAL. THE NEW NHC FORECAST IS A LITTLE HIGHER THAN THE
PREVIOUS ONE...CLOSE TO THE LATEST INTENSITY CONSENSUS AIDS THROUGH
48 HOURS...AND A LITTLE LOWER THAN THE CONSENSUS AFTER THAT TIME.
 
THE LATEST AIRCRAFT FIXES SUGGEST THAT THE CENTER IS LOCATED A
LITTLE TO THE EAST OF PREVIOUS ESTIMATES...AND THE INITIAL MOTION
ESTIMATE IS A SOMEWHAT UNCERTAIN 330/08. EARLY IN THE PERIOD THERE
IS SOME SIZEABLE SPREAD IN THE GUIDANCE. THE GFS SHOWS A DUE
NORTHWARD MOTION AS IT MAINTAINS THE UPPER-LOW WEST OF RAFAEL A
LITTLE LONGER WHILE THE ECMWF AND UKMET WEAKEN THE LOW FASTER AND
SHOW A MORE NORTHWESTWARD MOTION. THE OFFICIAL FORECAST TRACK LIES
IN BETWEEN...SHOWING A NORTH-NORTHWESTWARD MOTION IN THE FIRST 24
HOURS FOLLOWED BY A SLIGHT LEFT TURN ON DAY 2 AS A MID-LEVEL RIDGE
REBUILDS TO THE NORTHEAST OF RAFAEL. THE NHC FORECAST DURING THIS
TIME REMAINS CLOSE TO THE PREVIOUS TRACK AND IS A LITTLE TO THE
LEFT OF THE MIDDLE OF THE GUIDANCE ENVELOPE. LATE IN THE PERIOD THE
TRACK UNCERTAINTY GROWS LARGER. THE GFS REMAINS A FAST OUTLIER
COMPARED TO THE OTHER GLOBAL MODELS AND THE GEFS ENSEMBLE MEAN...
SHOWING RAFAEL ACCELERATING NORTHWARD AHEAD OF A CUTOFF LOW OVER THE
NORTHEASTERN UNITED STATES. THIS SCENARIO IS ALSO SHOWN BY THE GFDL
AND HWRF...WHOSE LARGE-SCALE FIELDS ARE DRIVEN BY THE GFS OUTPUT. ON
THE OTHER HAND BOTH THE ECMWF AND UKMET SHOW RAFAEL INTERACTING
WITH A SHORTWAVE TROUGH THAT MOVES OFF THE SOUTHEAST U.S. COAST AND
INDICATE A SLOWER NORTHEASTWARD MOTION. THE NHC FORECAST DURING
DAYS 3 THROUGH 5 REMAINS TO THE RIGHT OF AND SLOWER THAN MULTI-
MODEL CONSENSUS...GIVEN THAT THE GFS APPEARS TO BE A LEFT OUTLIER
COMPARED TO EVEN ITS OWN ENSEMBLE MEAN. NEEDLESS TO SAY...
CONFIDENCE IS QUITE LOW IN THE EXTENDED PORTION OF THE FORECAST
PERIOD...WITH THE 96-HOUR POSITIONS OF THE GFS AND ECMWF MORE THAN
800 MILES APART.
 
USERS ARE REMINDED NOT TO FOCUS ON THE EXACT TRACK OF THE CENTER
DURING THE NEXT DAY OR SO...SINCE HEAVY RAIN AND WIND IMPACTS
EXTEND WELL EAST AND SOUTHEAST OF THE CENTER.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  13/1500Z 15.9N  64.1W   35 KT  40 MPH
 12H  14/0000Z 17.4N  64.6W   40 KT  45 MPH
 24H  14/1200Z 19.2N  65.2W   50 KT  60 MPH
 36H  15/0000Z 21.0N  66.2W   55 KT  65 MPH
 48H  15/1200Z 22.6N  67.0W   60 KT  70 MPH
 72H  16/1200Z 28.0N  66.0W   65 KT  75 MPH
 96H  17/1200Z 38.0N  61.0W   55 KT  65 MPH
120H  18/1200Z 45.0N  54.0W   50 KT  60 MPH...POST-TROP/EXTRATROP
 
$$
FORECASTER BRENNAN
 
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