Tropical Storm KROSA Advisory Thu Aug 08

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (KROSA) WARNING
NR 007//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 11W (KROSA), LOCATED APPROXIMATELY 216 NM SOUTH OF
IWO TO, HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX
HOURS. THE CURRENT POSITION IS BASED ON RECENT SATELLITE FIXES FROM
PGTW AND RJTD AND 070336Z GMI 37 GHZ IMAGERY SHOWING A CLEAR CENTER
FEATURE. THE INITIAL INTENSITY IS SET AT 60 KNOTS BASED ON A
CONSENSUS OF SUBJECTIVE DVORAK AND AUTOMATED INTENSITY ESTIMATES FROM
MULTIPLE SOURCES. TS 11W HAS CONTINUED TO STEADILY INTENSIFY OVER THE
PAST SIX HOURS DUE TO PASSAGE OVER VERY WARM WATER (29-30C), AN
IMPROVING POLEWARD AND EASTWARD OUTFLOW PATTERN ALOFT AND LIGHT TO
MODERATE (15-20 KNOTS) VERTICAL WIND SHEAR. RECENT MICROWAVE
SATELLITE IMAGERY, INCLUDING THE AFOREMENTIONED GMI PASS, INDICATE A
CONTRACTING LOW LEVEL CIRCULATION CENTER, CONSISTENT WITH THE RECENT
INTENSIFICATION TREND. TS 11W IS TRACKING NORTH-NORTHWESTWARD ALONG
THE PERIPHERY OF SUBTROPICAL RIDGING SITUATED TO THE NORTH AND EAST.
3. FORECAST REASONING.
   A. THE EXTENDED FORECAST TRACK HAS SHIFTED A BIT WESTWARD RELATIVE
TO THE PREVIOUS FORECAST IN ANTICIPATION OF A STEERING RIDGE BUILDING
TO THE NORTH OF THE SYSTEM AFTER TAU 72.
   B. TS 11W IS EXPECTED TO CURVE FROM A NORTHWESTWARD ORIENTED TRACK
TOWARD A NORTHEASTWARD ORIENTED TRACK OVER THE NEXT 72 HOURS AS
RIDGING BUILDING TO THE SOUTH OF THE SYSTEM EXERTS AN INCREASING
INFLUENCE ON STORM MOTION. FORWARD TRACK SPEED WILL REMAIN FAIRLY
SLOW THROUGH AT LEAST TAU 48 AS RIDGE PATTERN REORIENTS AND THE
ASSOCIATED STEERING INFLUENCES SHIFT. PASSAGE OVER WARM WATER AND THE
FAVORABLE OUTFLOW PATTERN WILL RESULT IN FURTHER INTENSIFICATION OVER
THE NEXT 48 HOURS OR SO. THEREAFTER, A SLIGHT INCREASE IN VERTICAL
WIND SHEAR AND INTERACTION WITH A MORE STABLE AIR MASS TO THE NORTH
ARE EXPECTED TO HINDER ADDITIONAL INTENSIFICATION. ALTHOUGH THERE ARE
SUBTLE DIFFERENCES IN THE ORIENTATION OF THE STEERING RIDGES DURING
THIS PERIOD, NUMERICAL MODEL GUIDANCE IS IN GOOD OVERALL AGREEMENT
REGARDING THE TRACK SCENARIO. THE CURRENT TRACK FORECAST IS PLACED
NEAR THE MULTI-MODEL CONSENSUS AND PREVIOUS JTWC TRACK FORECASTS
THROUGH TAU 72, WITH HIGH CONFIDENCE.
   C. MODEL GUIDANCE DIVERGES IN THE EXTENDED FORECAST PERIOD, WITH A
GROUPING CONSISTING OF THE GFS, NAVGEM AND JGSM MODELS DEPICTING A
NORTH-NORTHWESTWARD TO POLEWARD TRACK AND A SECOND GROUPING
CONSISTING OF THE ECMWF, ECMWF ENSEMBLE AND UKMET MODEL DEPICTING A
STEADY NORTHWESTWARD TRACK. IN BOTH MODEL SCENARIOS, RIDGING TO THE
NORTH OF THE SYSTEM WEAKENS IN THE NEAR TERM IN RESPONSE TO A
DEVELOPING MIDLATITUDE TROUGH. THE ECMWF AND UKMET MODELS INDICATE
THAT A RIDGE WILL BUILD TO THE NORTH OF TS 11W, IN THE WAKE OF THIS
TROUGH, AND TAKE OVER AS THE DOMINANT STEERING MECHANISM. THIS RIDGE
WILL SENDING THE SYSTEM ON A TRAJECTORY TO THE WEST OF THE OTHER
MODEL GROUP. THIS SCENARIO APPEARS INCREASINGLY PROBABLE GIVEN AN
ANTICIPATED ZONAL MIDLATITUDE FLOW PATTERN. THUS THE CURRENT JTWC
TRACK FORECAST LIES TO THE LEFT OF THE MULTI-MODEL CONSENSUS IN FAVOR
OF THE ECMWF, UKMET GROUPING. ADDITIONAL ADJUSTMENTS MAY BE NECESSARY
IN SUBSEQUENT FORECASTS. GIVEN THE NOTED BIFURCATION IN THE MODEL
GUIDANCE, CONFIDENCE IN THE JTWC TRACK FORECAST IS LOW FOR THE
EXTENDED PERIOD. RESTRICTED OUTFLOW AND ENTRAINMENT OF MORE STABLE
AIR ARE EXPECTED TO REDUCE INTENSITY A BIT IN THE EXTENDED RANGE.//
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