WDPN33 PGTW 080300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (NANGKA) WARNING NR 19//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 11W (NANGKA), LOCATED APPROXIMATELY 284 NM EAST OF
SAIPAN, HAS TRACKED WESTWARD AT 15 KNOTS OVER THE PAST SIX HOURS.
ANIMATED WATER VAPOR IMAGERY DEPICTS A TUTT CELL TO THE NORTHWEST OF 
THE SYSTEM WITH UPPER LEVEL TROUGHING EXTENDING FURTHER EQUATORWARD 
ALONG THE WESTERN PERIPHERY OF NANGKA. THE PRESENCE OF THIS UPPER 
LEVEL FEATURE IS RESTRICTING CONVECTIVE DEVELOPMENT ALONG THE 
WESTERN SIDE OF THE SYSTEM. ANIMATED MULTISPECTRAL SATELLITE IMAGERY 
FURTHER SHOWS THAT, WHILE STILL EVIDENT, THE ONCE CLEAR AND DEFINED 
EYE HAS BECOME CLOUD FILLED. A 072349Z AMSU-B MICROWAVE IMAGE 
REVEALS THAT THE DEEP CONVECTIVE RING SURROUNDING THE MICROWAVE EYE 
FEATURE HAS BROKEN ALTHOUGH SPIRAL CONVECTIVE BANDING STILL 
PERSISTS. THE CURRENT INTENSITY HAS MAINTAINED AT 120 KNOTS BASED ON 
AN AVERAGE ASSESSMENT OF DVORAK INTENSITY ESTIMATES FROM ALL 
AGENCIES. DESPITE THE CLOSE PROXIMITY OF THE TUTT CELL, UPPER LEVEL 
ANALYSIS SHOWS ROBUST OUTFLOW TO THE EAST AND EQUATORWARD AS WELL AS 
MODERATE (10-20 KNOT) VERTICAL WIND SHEAR (VWS). TY NANGKA IS 
TRACKING ALONG THE SOUTHERN PERIPHERY OF AN EXTENSION OF THE DEEP 
LAYERED SUB-TROPICAL RIDGE (STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE 
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 11W WILL CONTINUE TO TRACK ON A NORTHWESTWARD TRAJECTORY 
THROUGH TAU 72 UNDER THE DOMINANT STEERING STR. UPPER LEVEL 
ENVIRONMENTAL CONDITIONS HAVE DEGRADED OVER THE PAST SIX HOURS 
RESULTING IN A POTENTIAL WEAKENING PHASE. HOWEVER, CONDITIONS ARE 
EXPECTED TO IMPROVE AS THE TUTT FEATURE MOVES AWAY FROM THE SYSTEM. 
ONCE THE UPPER LEVELS RETURN TO MORE FAVORABLE CONDITIONS, BETWEEN 
TAU 24 AND 36, THE SYSTEM IS FORECASTED TO SLIGHTLY INTENSIFY BEFORE 
TAU 48. TY NANGKA WILL ENCOUNTER MODERATE (20-25 KNOT) VWS NEAR TAU 
48, WHICH WILL BEGIN A SLOW WEAKENING TREND.
   C. IN THE EXTENDED TAUS, TY 11W WILL BEGIN TO SLOW AS AN 
APPROACHING MID-LATITUDE TROUGH WEAKENS THE EXTENSION OF THE STR 
CAUSING A WEAK STEERING ENVIRONMENT AND ALLOWING THE SYSTEM TO BEGIN 
MOVING POLEWARD. THE POSITIVE TILT TO THE TROUGH AND A BUILDING 
UPPER LEVEL RIDGE TO THE WEST WILL CAUSE INCREASED VWS (30+ KNOTS) 
AND REDUCED OUTFLOW ALOFT, INCREASING THE WEAKENING RATE. DYNAMIC 
MODEL GUIDANCE IS IN VERY TIGHT AGREEMENT THROUGHOUT THE FORECAST 
PERIOD, WITH THE EXCEPTION OF HWRF AND EGRR, WHICH TRACKS THE SYSTEM 
WESTWARD, ABSENT OF ANY INFLUENCE OF THE DEEP MID-LATITUDE TROUGH.  
THE JTWC FORECAST TRACK SPEEDS CONTINUE TO SLOW IN THE EXTENDED TAUS 
AS THE MODELS ADJUST TO THE TIMING AND IMPACT OF THE APPROACHING 
TROUGH. BASED ON THE TIGHT MODEL GROUPING AND CONTINUED STABILITY OF 
CONSENSUS, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST.//
NNNN