Tropical Storm CHABA Advisory czw., 29.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (CHABA)
WARNING NR 05//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 21W (CHABA), LOCATED APPROXIMATELY 137 NM
WEST-NORTHWEST OF ANDERSEN AFB, HAS TRACKED WESTWARD AT 13 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY DEPICTS FLARING CONVECTION BRIEFLY PROPAGATING TOWARD THE
SOUTHWEST IN THE LAST SEVERAL HOURS, WITH 282113Z AND 282201Z SSMIS
91GHZ MICROWAVE IMAGERY SUGGESTING THAT THE LOW-LEVEL CIRCULATION
CENTER (LLCC) IS LOCATED NEAR THE NORTHERN EDGE OF THE DEEPEST
CONVECTION. THIS IS CORROBORATED BY THE LOW-LEVEL CLOUD LINES
EVIDENT IN ANIMATED VISIBLE SATELLITE IMAGERY. ONLY A BROAD
COUNTERCLOCKWISE CIRCULATION IS EVIDENT IN THE GUAM RADAR
REFLECTIVITY, BUT RADAR BEAM HEIGHTS ARE INCREASING AS TD CHABA
MOVES AWAY FROM THE RADAR SITE. THE CURRENT POSITION IS PLACED WITH
LOW CONFIDENCE BY AID OF THE AFOREMENTIONED IMAGERY. DVORAK
INTENSITY ESTIMATES HAVE FINALLY COME DOWN SINCE THE LAST SEVERAL
WARNING CYCLONES, RANGING FROM T2.0 (KNES) TO T2.5 (PGTW AND RJTD),
WHICH BETTER SUPPORTS THE WIND VECTORS OBSERVED IN SCATTEROMETER
DATA. A PARTIAL 282029Z WINDSAT PASS DEPICTS 20 TO 25 KNOT WINDS
PREDOMINANTLY LOCATED ON THE NORTHERN SEMICIRCLE OF TD CHABA. BASED
ON A BLEND OF THE ABOVE DATA, THE CURRENT INTENSITY IS MAINTAINED AT
30 KNOTS. TD CHABA CONTINUES TO BE LOCATED IN A MARGINAL ENVIRONMENT
WITH WARM SSTS AND LOW VERTICAL WIND SHEAR, BUT UNDER RESTRICTED
POLEWARD OUTFLOW DUE TO A WEAK MID-LEVEL TUTT CELL PASS TO ITS
NORTH. TD CHABA IS TRACKING WEST-NORTHWESTWARD UNDER THE STEERING
INFLUENCE OF A SYNOPTIC SCALE, 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 WILL CONTINUE TO STEER TD CHABA TOWARD THE WEST-
NORTHWEST WITH A GRADUAL TURN TOWARD THE NORTHWEST. SLOW
INTENSIFICATION IS FORECAST IN THE NEAR TERM, FOLLOWED BY MORE
STEADY INTENSIFICATION BEYOND TAU 24 WHEN THE TUTT CELL CURRENTLY TO
THE NORTH OF TD CHABA WEAKENS, SLIDES WESTWARD, AND OPENS UP A
POLEWARD OUTFLOW CHANNEL. WITH CONTINUED LOW VERTICAL WIND SHEAR AND
IMPROVED OUTFLOW, TD CHABA IS FORECAST TO REACH TYPHOON INTENSITY IN
THE 48 TO 72 HOUR TIME FRAME. DYNAMIC MODEL GUIDANCE REMAINS IN GOOD
AGREEMENT WITH A GENERAL WEST-NORTHWEST TO NORTHWESTWARD TRACK
THROUGH TAU 72, AND THE OFFICIAL JTWC FORECAST TRACK REMAINS NEAR
THE MULTIMODEL CONSENSUS.
   C. IN THE EXTENDED PERIOD, DYNAMIC MODEL GUIDANCE DIVERGES
GREATLY BEYOND TAU 72, WITH THE GFS AND GFS/JGSM ENSEMBLES REMAINING
ON THE RIGHT SIDE OF THE MODEL GUIDANCE ENVELOPE. THE OFFICIAL
FORECAST TRACK REMAINS TO THE WEST OF THE MULTIMODEL CONSENSUS AT
TAU 96 AND 120, BUT THESE LATER POSITIONS MAY NEED TO BE SHIFTED
EASTWARD IN FUTURE WARNINGS IF THE MODEL SUITE FOLLOWS THE EASTWARD
SHIFT THAT WAS EXHIBITED BY ECMWF EARLIER TODAY. THE TIMING OF THE
TURN TO THE NORTH AND EVENTUAL RECURVE TOWARD THE NORTHEAST WILL
DEPEND ON THE STRENGTH OF THE MID-LATITUDE TROUGH THAT IS FORECAST
TO ERODE THE WESTERN PORTION OF THE STR. VERTICAL WIND SHEAR IS
FORECAST TO REMAIN LOW AND ACCOMPANIED BY FAVORABLE OUTFLOW THROUGH
MOST OF THE FORECAST PERIOD, SO CONTINUED INTENSIFICATION IS
FORECAST. HOWEVER, A TRACK TO THE WEST OF OKINAWA WOULD TAKE TD
CHABA OVER COOLER SSTS, WHICH MAY HINDER INTENSIFICATION AT THE
LATER TAUS. THE OFFICIAL INTENSITY FORECAST LIES CLOSE TO THE
INTENSITY CONSENSUS, BUT THE ACTUAL INTENSIFICATION RATE OF TD CHABA
WILL STRONGLY DEPEND ON ITS EVENTUAL TRACK. THIS UNCERTAINTY LEADS
TO A LOW CONFIDENCE IN BOTH THE JTWC TRACK AND INTENSITY FORECASTS
BEYOND TAU 72.//
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