WDPN31 PGTW 210300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 18W (ROKE) WARNING NR 39//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TYPHOON (TY) 18W (ROKE), LOCATED APPROXIMATELY 230 NM 
SOUTHWEST OF YOKOSUKA, JAPAN, HAS TRACKED NORTHEASTWARD AT 18 KNOTS 
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) SHOWS TY 18W ROARING PAST THE SOUTHERN TIP OF WAKAYAMA 
PREFECTURE ON A STEADY NORTHEASTERLY BEARING. DESPITE THE DOWNWARD 
INTENSITY TREND, THE MSI REVEALS A STRONG EYEWALL WITH TIGHTLY 
CURVED BANDING WRAPPING INTO THE CORE. A 202036Z SSMIS IMAGE REVEALS 
THE CONTINUING PRESENCE OF A MICROWAVE EYE AS WELL AS THICK 
CONCENTRIC BANDING AROUND THE EYEWALL. ANIMATED WATER VAPOR IMAGERY 
REVEALS PRONOUNCED STRETCHING IN THE UPPER CLOUD SHIELD OVER THE 
NORTHEAST QUADRANT, INDICATING TY 18W IS ENTERING A ZONE OF 
INCREASING VERTICAL WIND SHEAR. AT THIS TIME, HOWEVER, THE BENEFITS 
OF ENHANCED POLEWARD OUTFLOW MAY BE MITIGATING THE EFFECTS OF 
INCREASING SHEAR. THE CURRENT INTENSITY IS BASED ON A COMBINATION OF 
DVORAK ESTIMATES RANGING FROM 80 TO 100 KNOTS FROM PGTW, RJTD, AND 
KNES, PLUS A SHIP REPORT OF 80 KTS FROM VERY CLOSE TO THE CENTER OF 
THE STORM NEAR 202100Z. THE APPROACH OF TY 18W TOWARDS THE EAST 
COAST OF HONSHU IS PUMPING WARM AIR INTO THE WESTERN FLANK OF THE 
SUBTROPICAL RIDGE, AMPLIFYING THE LONG WAVE PATTERN. CONSECUTIVE 
SOUNDINGS FROM SHIONOMISAKI (47778) AND HACHIOJIMA (47678) SHOW 
HEIGHT RISES AND THE ESTABLISHMENT OF FIRM SOUTHERLY FLOW INTO THE 
MID-TROPOSPHERE, INDICATING THE ESTABLISHMENT OF A FIRM RIDGE THAT 
WILL PREVENT SHARP EASTWARD MOVEMENT OF THE SYSTEM. SURFACE ANALYSIS 
INDICATES THAT A BAROCLINIC BOUNDARY IS NORTH OF THE KANTO REGION 
AND THAT THE WARM AND SATURATED AIR ASSOCIATED WITH TY 18W IS ADDING 
VOLATILITY TO THE BOUNDARY. AN 850 MB ISOTHERM ANALYSIS CONFIRMS 
THAT THE TIGHTEST ISOTHERMAL PACKING IS WELL NORTH OF THE KANTO 
PLAIN AND STRONGER ON THE WINDWARD SIDE OF THE JAPANESE ALPS. 
ASSUMING CONSTANT BEARING AND SPEED, THE CENTER OF TY 18W IS HEADING 
FOR HAMAMATSU WITH AN ETA NEAR 210400Z. 
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS 
PROGNOSTIC REASONING BULLETIN.
   B. AT THE SAME TIME IT ENCOUNTERS THE RUGGED TERRAIN OF SOUTH-
CENTRAL HONSHU, TY 18W WILL MOVE INTO A ZONE OF SHARPLY INCREASED 
VERTICAL WIND SHEAR. IT WILL ALSO ENCOUNTER BOUNDARY LAYER 
IMPEDIMENTS DUE TO INTERACTION WITH THE BAROCLINIC ZONE. THOSE THREE 
FACTORS WILL COMBINE TO EXERT A TREMENDOUS WEAKENING FORCE ON TY 
18W, BUT ITS RAPID MOVEMENT THROUGH THE SHIZUOKA PREFECTURE AND THEN 
THE KANTO PLAIN WILL ALLOW THE STORM TO MAINTAIN ENOUGH STRUCTURAL 
INTEGRITY TO REGENERATE ALMOST IMMEDIATELY AFTER IT MOVES OFF-SHORE 
OF THE TOHOKU REGION. AT THAT POINT, THE EXTRA-TROPICAL TRANSITION 
PROCESS WILL BE NEARLY COMPLETE AND TY 18W WILL HAVE TRANSITIONED 
INTO A STORM FORCE MID-LATITUDE LOW. MODEL GUIDANCE IS IN EXCELLENT 
AGREEMENT BUT DOES NOT PROPERLY ACCOUNT FOR THE STEERING 
INFLUENCE THE JAPANESE ALPS WILL HAVE ON THE LOW LEVEL CIRCULATION 
CENTER. ADDITIONALLY, THERE EXISTS SOME WEST BIAS IN THE UPPER 
LEVELS DUE TO A MODEL PROPENSITY TO STEER TOWARD THE UPPER LEVEL 
HEIGHT FALLS. FOR THOSE REASONS, THE JTWC FORECAST STAYS 
EAST OF CONSENSUS.//
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