Tropical Storm KAMMURI Advisory Wed Sep 24

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (KAMMURI)
WARNING NR 01//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 17W (KAMMURI), LOCATED APPROXIMATELY 320
NM NORTHEAST OF SAIPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 04 KNOTS
OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS
A PARTIALLY-EXPOSED, ELONGATED LOW-LEVEL CIRCULATION CENTER (LLCC)
LOCATED ON THE WESTERN EDGE OF A LARGE SWATH OF DEEP CONVECTION. A
241135Z METOP-B 89GHZ COMPOSITE IMAGE ALSO SHOWS EXPOSED LLCC AND
BROAD NATURE OF THE SYSTEM AND SUPPORTS THE INITIAL POSITION WITH
FAIR CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED AT 30 KNOTS BASED
ON DVORAK INTENSITY ESTIMATES RANGING FROM 25 TO 30 KNOTS AND RECENT
ASCAT IMAGERY SHOWING 25 TO 30 KNOT WINDS OVER THE EASTERN SEMI-
CIRCLE. UPPER-LEVEL ANALYSIS INDICATES A GENERALLY FAVORABLE
ENVIRONMENT WITH LOW VERTICAL WIND SHEAR AND GOOD POLEWARD OUTFLOW,
ENHANCED BY A DEEP MIDLATITUDE TROUGH TO THE NORTHEAST. TD 17W IS
TRACKING SLOWLY ALONG THE SOUTHERN PERIPHERY OF A WEAK EXTENSION OF
THE LOW-LEVEL SUBTROPICAL RIDGE (STR) TO THE NORTHEAST AND NORTH.
 3. FORECAST REASONING.
   A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE AND SETS THE
FORECAST PHILOSOPHY.
   B. TD 17W IS TRACKING SLOWLY UNDER THE STEERING INFLUENCE OF A
WEAK EXTENSION OF THE LOW-LEVEL STR, WHICH IS REINFORCED BY A
MIDLATITUDE SHORTWAVE RIDGE. THE SYSTEM IS EXPECTED TO REMAIN WITHIN
THIS WEAK STEERING ENVIRONMENT THROUGH TAU 36 AND SHOULD CONTINUE TO
TRACK SLOWLY NORTHWESTWARD. BY TAU 48, TD 17W SHOULD ACCELERATE AS
IT TURNS MORE POLEWARD TOWARD A DEVELOPING BREAK IN THE STR PRODUCED
BY AN APPROACHING MIDLATITUDE SHORTWAVE TROUGH. DYNAMIC GUIDANCE IS
SPLIT INTO TWO CLUSTERS WITH JGSM, JENS, GFDN AND HWRF INDICATING A
WEST-NORTHWESTWARD TRACK WHILE GFS, AEMN, COTC, NVGM AND ECMF DEPICT
A RE-CURVE SCENARIO. THE MODEL SPREAD AT TAU 72 IS ABOUT 320 NM,
INDICATIVE OF THE MODEL UNCERTAINTY IN THE CURRENT FORECAST
SCENARIO. THE JTWC FORECAST FAVORS THE RE-CURVE SCENARIO, WHICH
MAKES MORE SENSE BASED ON MIDLATITUDE WESTERLY FLOW FORECAST TO
STRENGTHEN SOUTH OF JAPAN. TD 17W IS FORECAST TO INTENSIFY SLOWLY
THROUGH TAU 24 BUT SHOULD RAPIDLY INTENSIFY TO A PEAK INTENSITY OF
80 KNOTS BY TAU 72 DUE TO INCREASED POLEWARD OUTFLOW.
   C. IN THE EXTENDED TAUS, TD 17W SHOULD RE-CURVE POLEWARD OF THE
STR AXIS INTO THE MIDLATITUDE WESTERLIES AND BEGIN EXTRA-TROPICAL
TRANSITION (ETT) AND WEAKEN AS IT INTERACTS WITH THE BAROCLINIC
ZONE. OVERALL, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST BASED ON
THE AFOREMENTIONED MODEL UNCERTAINTY AND POSITIONING UNCERTAINTY DUE
TO THE BROAD NATURE OF THE SYSTEM.//
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