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推薦大家都讀這份報文，解釋為何 JTWC 能給 170 節。
WDPN33 PGTW 072100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 31W (HAIYAN) WARNING NR
19//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUPER TYPHOON 31W (HAIYAN), LOCATED APPROXIMATELY 425 NM SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 21 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY (EIR) REVEALS A NEARLY ANNULAR EYEWALL HAS CONTINUED TO EXPAND UNDER THE INFLUENCE OF A VERY FAVORABLE ENVIRONMENT. THE CURRENT INTENSITY HAS BEEN INCREASED TO 170 KNOTS BASED ON THE 8.0/8.0 DVORAK ESTIMATES FROM PGTW, KNES, AND RJTD. THE WESTERN PERIPHERY OF THE EYEWALL HAS STARTED TO TRACK OVER THE EASTERN ISLANDS OF THE PHILIPPINES WITHIN THE PAST FEW HOURS, WITH THE STRONG SECONDARY FEEDER BAND ALONG THE WESTERN HALF OF THE SYSTEM MOVING INTO THE CENTRAL PHILIPPINES. THIS SECONDARY BAND OF DEEP CONVECTION HAS BEEN WEAKENING AS IT TRANSITS OVER THE PHILIPPINES, BUT IS STILL SUPPORTING FLOW INTO THE LOW LEVEL ALONG THE WESTERN PERIPHERY. 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 WEST-NORTHWESTWARD 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 AND RECENT INTENSIFICATION, THE SYSTEM IS EXPECTED TO REMAIN AT SUPER TYPHOON INTENSITY OVER THE NEXT 24 HOURS. LANDFALL WITH THE PHILIPPINES IS BEGINNING AS NOTED EARLIER WITH THE WESTERN PERIPHERY STARTING TO MOVE OVER THE EASTERN ISLANDS OF THE PHILIPPINES. 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 AROUND TAU 60, 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. 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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