WDPN33 PGTW 080300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 31W (HAIYAN)
WARNING NR 20//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON 31W (HAIYAN), LOCATED APPROXIMATELY 310 NM 
SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 22 KNOTS 
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE 
IMAGERY (EIR) REVEALS THE NEARLY ANNULAR EYEWALL HAS PERSISTED BUT 
IS BEGINNING TO SHRINK SLIGHTLY OVER THE PAST FEW HOURS. THE EYE 
ITSELF HAS ALSO DECREASED IN SIZE AS THE SYSTEM HAS MOVED OVER THE 
ISLANDS IN THE EASTERN PHILIPPINES. DVORAK ESTIMATES HAVE DECREASED 
FOR ALL AGENCIES TO REFLECT THE WEAKENING OF THE DEEP CONVECTIVE 
STRUCTURE OF THE EYEWALL OBSERVED IN EIR. THE CURRENT INTENSITY HAS 
BEEN DECREASED TO 160 KNOTS BASED ON THE DVORAK ESTIMATES. UPPER 
LEVEL ANALYSIS INDICATES A STRONG RADIAL OUTFLOW PATTERN IS 
PROVIDING VERY FAVORABLE OUTFLOW TO THE SUBSTANTIAL LOW LEVEL 
INFLOW. VERTICAL WIND SHEAR REMAINS VERY LOW (LESS THAN 5 KNOTS), 
ALLOWING GOOD VERTICAL TRANSPORT AND IS SUPPORTING THE RECENT 
INTENSITY INCREASES OBSERVED IN EIR. STY 31W IS CURRENTLY TRACKING 
ALONG THE SOUTHERN PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE 
(STR) TO THE NORTH.
3. FORECAST REASONING.
   A. NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. STY 31W IS EXPECTED TO MAINTAIN A WESTWARD TRACK ALONG THE 
SOUTHERN EDGE OF THE STR, SHIFTING TO A NORTHWESTWARD TRACK BETWEEN 
TAU 48 AND 72. DUE TO THE EXTREMELY FAVORABLE ENVIRONMENTAL 
CONDITIONS, THE SYSTEM IS EXPECTED TO REMAIN AT SUPER TYPHOON 
INTENSITY OVER THE NEXT 24 HOURS. A SLIGHT INCREASE IN LOW LEVEL 
FRICTIONAL EFFECTS ASSOCIATED WITH THE PASSAGE OF THE SYSTEM OVER 
THE PHILIPPINES AND A WEAKENING TREND IN THE SEA SURFACE 
TEMPERATURES (SSTS) WILL LEAD TO INTENSITIES DECREASING FOR THE 
REMAINDER OF THE FORECAST. THE WEAKENING TREND WILL BE SLOW AT FIRST 
AS THE ENVIRONMENT REMAINS MARGINAL TO GOOD THROUGH TAU 48, BUT AS 
THE SYSTEM APPROACHES THE COAST OF VIETNAM, THE WEAKENING TREND WILL 
BE AMPLIFIED BY GREATER LAND INTERACTION AND LESS THAN FAVORABLE 
SSTS NEAR THE COAST. LANDFALL IS EXPECTED SHORTLY AFTER TAU 48, AND 
WILL CAUSE A MORE AGGRESSIVE WEAKENING OF THE LOW LEVEL STRUCTURE 
DUE TO THE FRICTIONAL LAND EFFECTS.
   C. IN THE EXTENDED TAUS, STY 31W IS FORECAST TO TRACK MORE 
NORTHWESTERLY INTO NORTHERN VIETNAM THROUGH TAU 96, SHIFTING TO A 
NORTHEASTERLY TRACK AS THE CIRCULATION BEGINS TO INTERACT WITH A MID-
LATITUDE TROUGH, CURRENTLY LOCATED OVER CENTRAL CHINA, AND MOVING TO 
THE NORTHWESTERN PERIPHERY OF THE STR. THE TRACK OVER LAND WILL 
CAUSE A RAPID WEAKENING AS FRICTIONAL LAND EFFECTS INCREASE AND 
GREATER INTERACTION WITH THE MID-LATITUDE WESTERLIES IMPACTS UPPER 
LEVEL SUPPORT, WITH A COMPLETE DISSIPATION OF THE SYSTEM BEYOND TAU 
120. BASED ON THE GOOD AGREEMENT OF DYNAMIC MODEL GUIDANCE, THERE IS 
HIGH CONFIDENCE IN THE FORECAST TRACK.//
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