WDPN32 PGTW 071500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (ETAU) WARNING
NR 04//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (ETAU), LOCATED APPROXIMATELY 666 NM 
SOUTH-SOUTHEAST OF IWAKUNI, JAPAN, HAS TRACKED NORTHWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. IN GENERAL, ANIMATED ENHANCED INFRARED 
SATELLITE IMAGERY INDICATES A SLOWLY CONSOLIDATING SYSTEM WITH 
PERSISTENT DEEP CONVECTION, HOWEVER, THE BULK OF THE DEEP 
CONVECTION CONTINUES TO SHEAR NORTHWARD OVER THE NORTHERN SEMI-
CIRCLE OF THE SYSTEM. A 071138Z METOP-B 89GHZ IMAGE SHOWS CURVED 
CONVECTIVE BANDING WRAPPING INTO A WELL-DEFINED CENTER, WHICH 
SUPPORTS THE INITIAL POSITION WITH GOOD CONFIDENCE. THE INITIAL 
INTENSITY IS NOW ASSESSED AT 35 KNOTS BASED ON DVORAK INTENSITY 
ESTIMATES OF 35 KNOTS (T2.5) AS WELL AS A 071138Z ASCAT IMAGE, 
WHICH SHOWED 35-KNOT WINDS OVER THE EASTERN QUADRANT. UPPER-LEVEL 
ANALYSIS INDICATES A GENERALLY FAVORABLE ENVIRONMENT WITH LOW 
VERTICAL WIND SHEAR AND ROBUST POLEWARD VENTING. TS 18W IS 
CURRENTLY TRACKING ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED 
SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY FROM
THE PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 18W WILL CONTINUE TRACKING NORTHWARD THROUGH TAU 36 ALONG 
THE WESTERN PERIPHERY OF THE LOW- TO MID-LEVEL SUBTROPICAL RIDGE. 
TS 18W SHOULD CONTINUE TO SLOWLY INTENSIFY THROUGH TAU 24 UNDER 
FAVORABLE CONDITIONS. HOWEVER, AFTER TAU 24 THE SYSTEM WILL 
ENCOUNTER STRONG VERTICAL WIND SHEAR ASSOCIATED WITH A DEEP 
SHORTWAVE TROUGH FORECAST TO DEEPEN OVER WESTERN JAPAN BY TAU 24. 
AFTER TAU 36, TS 18W WILL WEAKEN FURTHER AS IT MAKES LANDFALL OVER 
HONSHU AND IS EXPECTED TO DISSIPATE BY TAU 72. THE REMNANTS OF THE 
SYSTEM WILL LIKELY TRACK NORTHWESTWARD AS A LOW-LEVEL HIGH BUILDS 
TEMPORARILY OVER THE EAST SEA. DYNAMIC GUIDANCE IS NOW IN GOOD 
AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST 
TRACK. HOWEVER, THERE IS SOME UNCERTAINTY IN THE PEAK INTENSITY DUE 
TO THE NARROW 24-HOUR WINDOW OF FAVORABLE CONDITIONS. THUS FAR, THE 
SYSTEM HAS SLOWLY DEVELOPED BUT NOW APPEARS TO BE SHOWING SOME 
INITIAL SIGNS OF INTENSIFICATION. REGARDLESS OF PEAK INTENSITY, 
ENVIRONMENTAL CONDITIONS NORTH OF 30 DEGREES NORTH LATITUDE ARE 
EXPECTED TO BE HOSTILE THEREFORE RAPID WEAKENING IS ANTICIPATED 
PRIOR TO LANDFALL.//  
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