Tropical Storm CHABA Advisory Thu Sep 29

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (CHABA)
WARNING NR 02//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 21W (CHABA), LOCATED APPROXIMATELY 127
NM EAST-NORTHEAST OF ANDERSEN AFB, GUAM HAS TRACKED WESTWARD
AT 14 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY DEPICTS A STRUGGLING CONVECTIVE STRUCTURE WITH THE
BULK OF DEEP CONVECTION SHEARED TO THE WEST OF A SLOWLY
CONSOLIDATING LOW LEVEL CIRCULATION CENTER (LLCC). A 280731Z GPM
89GHZ MICROWAVE IMAGE SHOWS DEEP CONVECTION TO THE WEST OF A DEFINED
BUT BROAD LLCC AND SUPPORTS THE CURRENT POSITION WITH GOOD
CONFIDENCE. THE INITIAL INTENSITY HAS BEEN HELD AT 30 KNOTS, BELOW
DVORAK INTENSITY ESTIMATES RANGING FROM 35-45 KNOTS, DUE TO A PAIR
OF ASCAT PASSES NEAR 280000Z SHOWING 25 TO 30 KNOT WINDS.
ENVIRONMENTAL ANALYSIS INDICATES A MARGINAL ENVIRONMENT WITH LOW TO
MODERATE EASTERLY VERTICAL WIND SHEAR (VWS) AND WEAK DIVERGENCE
ALOFT. HOWEVER, SEA SURFACE TEMPERATURES NEAR 29 CELSIUS ARE
FAVORABLE AND WILL HELP TO SUSTAIN THE CURRENT CONVECTION. TD 21W IS
TRACKING WESTWARD UNDER THE STEERING INFLUENCE OF A DEEP LAYERED SUB-
TROPICAL RIDGE (STR) TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. THE STR LOCATED TO THE NORTH WILL REMAIN THE DOMINANT STEERING
MECHANISM THROUGHOUT THE FORECAST PERIOD, DRIVING THE SYSTEM
WESTWARD AND THEN NORTHWESTWARD. MARGINAL ENVIRONMENTAL CONDITIONS
WILL SUPPORT A SLOW INTENSIFICATION OVER THE NEXT TWO DAYS.
AFTERWARDS, IMPROVED UPPER LEVEL OUTFLOW AND LOW VERTICAL WIND SHEAR
WILL CAUSE AN INCREASED RATE OF INTENSIFICATION, ALLOWING TD CHABA
TO REACH TYPHOON INTENSITY BY TAU 72. DYNAMIC MODEL GUIDANCE BEGINS
TO SPREAD BEYOND TAU 24, AS THEY DEPICT DIFFERENCES IN THE STRENGTH
OF THE STEERING STR AND THE STRENGTH OF THE STORM. THE CURRENT
FORECAST TRACK IS PLACED JUST WEST OF THE MULTI-MODEL CONSENSUS, IN
FAVOR OF THE ECMWF MODEL SOLUTION, SHOWING A STRONGER STEERING RIDGE
AND A LATER TURN TO THE NORTH.
   C. IN THE EXTENDED PERIOD, TD 21W WILL CONTINUE TO TRACK
NORTHWESTWARD AS IT APPROACHES THE WESTERN EXTENT OF THE STEERING
STR. BY THE END OF THE FORECAST PERIOD THE SYSTEM WILL BEGIN TO
ROUND THE SOUTHWESTERN PERIPHERY AND START THE POLEWARD TURN.
INTENSIFICATION IS EXPECTED TO CONTINUE AS A MID-LATITUDE TROUGH
APPROACHES FROM THE WEST, CREATING A STRONG POLEWARD OUTFLOW
CHANNEL. THERE IS A POTENTIAL, WITH WARMER SSTS AND INCREASED
OUTFLOW, THAT THE CYCLONE WILL UNDERGO A PERIOD OF RAPID
INTENSIFICATION DURING THIS PORTION OF THE FORECAST. DYNAMIC MODEL
GUIDANCE CONTINUES TO DIVERGE THROUGH THE ENTIRE FORECAST, SHOWING A
500NM SPREAD AT TAU 120. THIS LARGE SPREAD IN MODEL GUIDANCE LEADS
TO A LOW CONFIDENCE IN THE JTWC TRACK FORECAST.//
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