Tropical Storm HAISHEN Advisory Thu Sep 03

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (HAISHEN) WARNING
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 11W (HAISHEN), LOCATED APPROXIMATELY 336 NM SOUTH OF
IWO TO, HAS TRACKED WESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS.
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS SPIRAL
BANDING WRAPPING INTO A LOW LEVEL CIRCULATION CENTER (LLCC) THAT
IS CURRENTLY OBSCURED BY AN 88 NM DIAMETER AREA OF CENTRAL DENSE
OVERCAST. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE
BASED ON THE MSI LOOP AND A MICROWAVE EYE FEATURE PRESENT IN RECENT
020645Z SSMIS 91 GHZ AND 37 GHZ IMAGERY. THE INITIAL INTENSITY OF
65 KTS IS PLACED WITH FAIR CONFIDENCE AND IS HEDGED ABOVE A PGTW
DVORAK CURRENT INTENSITY ESTIMATE OF T3.5 (55 KTS) BASED ON THE
WIND FIELD DATA IN A 020401Z AMSR2 PASS. THE SYSTEM IS TRACKING
ALONG THE COMBINED STEERING INFLUENCE OF A SUBTROPICAL RIDGE
(STR) TO THE WEST AND AN EXTENSION OF THE STR TO THE NORTHEAST
AND THROUGH A FAVORABLE ENVIRONMENT OF VERY WARM (29-30 CELSIUS)
SEA SURFACE TEMPERATURES (SST), LOW (10-15 KTS) VERTICAL WIND
SHEAR (VWS) AND ROBUST EQUATORWARD OUTFLOW ALOFT.
3. FORECAST REASONING.
   A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM
THE LAST PROGNOSTIC REASONING MESSAGE.
   B. A MIDLATITUDE SHORTWAVE TROUGH APPROACHING FROM THE NORTHWEST
HAS BEGUN TO ERODE THE STR TO THE WEST WHILE THE STR TO THE NORTHEAST
HAS BEGUN TO BUILD AND IS NOW TAKING OVER AS THE PRIMARY STEERING
MECHANISM FOR THE SYSTEM. THE INFLUENCE OF THIS STEERING RIDGE WILL
DRIVE THE SYSTEM WEST-NORTHWESTWARD THROUGH TAU 72. INITIALLY, HIGHLY
FAVORABLE ENVIRONMENTAL CONDITIONS OF LOW (5-15 KTS) VWS, VERY WARM
(30-31 CELSIUS) SST AND CONTINUED DIVERGENT FLOW ALOFT WILL ALLOW THE
SYSTEM TO INTENSIFY TO 110 KTS BY TAU 48. THERE IS A DISTINCT
POSSIBILITY OF RAPID INTENSIFICATION DURING THIS TIME DUE TO THE HIGHLY
FAVORABLE ENVIRONMENTAL CONDITIONS. THIS INTENSIFICATION TREND WILL
SLOW AFTER TAU 48 AS TY HAISHEN BEGINS A MORE NORTHWESTWARD TRACK DUE
TO INCREASING, MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS),
RESULTING IN A PEAK INTENSITY OF 120 KTS BY TAU 72. NUMERICAL MODEL
SOLUTIONS ARE IN FAIR AGREEMENT THROUGH TAU 72 WITH A MAXIMUM 96 NM
SPREAD DURING THIS TIME. DESPITE THIS TIGHT AGREEMENT IN MODEL
SOLUTIONS, THE MODEL TRACKS HAVE CONTINUED TO DRIVE WESTWARD WITH
SUCCESSIVE COMPUTATIONAL RUNS. THIS DRIFT IN MODEL SOLUTIONS OVER TIME
LENDS ONLY FAIR CONFIDENCE IN THE INITIAL PORTION OF THE JTWC FORECAST
TRACK.
   C. BEYOND TAU 72, TY HAISHEN WILL CONTINUE TO TRACK NORTHWESTWARD
ALONG THE PERIPHERY OF THE DOMINANT STR, NOW TO THE EAST. CONTINUED
FAVORABLE CONDITIONS WILL ALLOW THE SYSTEM TO MAINTAIN ITS PEAK
INTENSITY OF 120 KTS THROUGH TAU 96 AS IT BEGINS ITS APPROACH TOWARDS
THE KOREAN PENINSULA. BETWEEN TAU 96 AND TAU 120, TY HAISHEN WILL TRACK
WEST OF KYUSHU AND CROSS THE KOREAN STRAIT, ULTIMATELY MAKING LANDFALL
OVER SOUTH KOREA NEAR BUSAN BEFORE TAU 120. AT THIS TIME THE INTENSITY
WILL DROP RAPIDLY DUE TO THE FRICTIONAL EFFECTS OF THE UNDERLYING
TERRAIN AND THE SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION (ETT) AS IT
APPROACHES THE BAROCLINIC ZONE. NUMERICAL MODEL SOLUTIONS ARE IN TIGHT
AGREEMENT FOR THIS EXTENDED PORTION OF THE JTWC FORECAST, HOWEVER THE
AFOREMENTIONED DRIFT IN MODEL SOLUTIONS OVER THE PAST 48 HOURS OF
COMPUTATIONAL SIMULATIONS REDUCES THE CONFIDENCE IN TRACK GUIDANCE,
DESPITE THE HIGH DEGREE OF TRACK AGREEMENT. DUE TO THIS ANALYSIS, THERE
IS ONLY FAIR CONFIDENCE ASSESSED IN THE OVERALL JTWC FORECAST TRACK.
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