Tropical Storm NANGKA Advisory 08.07.

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.//
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