Tropical Storm HERNAN

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TROPICAL STORM HERNAN DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP092008
800 PM PDT THU AUG 07 2008
 
THE CENTER OF HERNAN IS SOMEWHAT DIFFICULT TO LOCATE THIS EVENING. 
RECENT MICROWAVE IMAGERY CONTINUES TO SHOW A MID-LEVEL EYE
FEATURE...BUT ALSO SHOWS THAT THIS IS DISPLACED TO THE WEST OR
SOUTHWEST OF THE LOW-LEVEL CENTER.  ANALYSES FROM CIMSS AT THE
UNIVERSITY OF WISCONSIN INDICATE 15-20 KT OF EAST-NORTHEASTERLY
VERTICAL WIND SHEAR OVER HERNAN...WHICH IS THE LIKELY CAUSE OF THIS
STRUCTURE.  SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN
55 KT...AND THAT REMAINS THE INITIAL INTENSITY.

THE MICROWAVE DATA SHOWS THAT HERNAN HAS MOVED WESTWARD DURING THE
PAST SEVERAL HOURS...WITH THE INITIAL MOTION A SOMEWHAT UNCERTAIN
275/11.  THE STORM IS CURRENTLY ON THE SOUTH SIDE OF A MID-LEVEL
RIDGE AND HEADING FOR A LARGE MID/UPPER-LEVEL TROUGH THAT COVERS
THE AREA NORTH OF 22N BETWEEN 130-150W.  THE LARGE-SCALE MODELS
FORECAST THE TROUGH TO LIFT OUT TO THE NORTH DURING THE FORECAST
PERIOD...WITH ENOUGH MID-LEVEL RIDGING DEVELOPING TO STEER HERNAN
GENERALLY WEST-NORTHWESTWARD FOR THE NEXT 48-72 HR.  AFTER THAT
TIME...HERNAN WILL BE WEAKENING OVER COOLER SEA SURFACE
TEMPERATURES...AND LOW-LEVEL FLOW SHOULD CAUSE THE CYCLONE TO TURN
WESTWARD AND EVENTUALLY WEST-SOUTHWESTWARD.  THE NEW FORECAST TRACK
IS SIMILAR TO...BUT SOUTH OF...THE PREVIOUS FORECAST BASED ON THE
CURRENT LOCATION AND MOTION...AND CALLS FOR HERNAN TO RESUME A
WEST-NORTHWEST MOTION DURING THE NEXT FEW HOURS.  THE NEW TRACK IS
DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE IN GOOD AGREEMENT WITH THE
MODEL CONSENSUS.

IT SHOULD BE NOTED THAT THE MICROWAVE IMAGERY HINTS THAT THE CENTER
OF HERNAN IS SOUTH OF THE ADVISORY POSITION.  IF SUBSEQUENT DATA
SHOW THIS IS INDEED THE CASE...AN ADDITIONAL ADJUSTMENT TO THE
FORECAST TRACK MAY BE NECESSARY ON THE NEXT ADVISORY.

ON THE FORECAST TRACK...THE CENTER OF HERNAN WOULD REACH THE 26C SEA
SURFACE TEMPERATURE ISOTHERM IN ABOUT 36 HR.  THE ECMWF...GFS...AND
NOGAPS FORECAST A GRADUAL DECREASE IN SHEAR DURING THIS TIME...
WHICH COULD ALLOW GRADUAL STRENGTHENING.  ON THE OTHER HAND...THE
UKMET SHOWS LITTLE CHANGE OR EVEN SOME INCREASE IN THE SHEAR.  THE
VARIOUS INTENSITY GUIDANCE MODELS CALL FOR SOME STRENGTHENING WITH
PEAK INTENSITIES IN THE 60-70 KT RANGE.  GIVEN THE CURRENT
STRUCTURE AND SHEAR...THE NEW INTENSITY FORECAST CALL FOR A SLOWER
INTENSIFICATION RATE THAN THE PREVIOUS FORECAST...WITH HERNAN
REACHING HURRICANE STRENGTH IN ABOUT 24 HR.  AFTER 36 HR...THE
FORECAST MOTION OVER COOLER WATER SHOULD CAUSE WEAKENING...WITH THE
CYCLONE EXPECTED TO DECAY TO A NON-CONVECTIVE LOW IN ABOUT 120 HR.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      08/0300Z 14.9N 117.8W    55 KT
 12HR VT     08/1200Z 15.3N 119.4W    60 KT
 24HR VT     09/0000Z 15.8N 121.2W    65 KT
 36HR VT     09/1200Z 16.2N 122.8W    65 KT
 48HR VT     10/0000Z 16.8N 124.5W    60 KT
 72HR VT     11/0000Z 18.0N 128.0W    45 KT
 96HR VT     12/0000Z 18.5N 130.5W    30 KT
120HR VT     13/0000Z 18.0N 134.0W    20 KT...REMNANT LOW
 
$$
FORECASTER BEVEN
 
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