Tropical Storm SAOLA Advisory Sa, 21.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 27W (TWENTYSEVEN)
WARNING NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 27W (TWENTYSEVEN), LOCATED APPROXIMATELY
284 NM EAST-SOUTHEAST OF ROTA, NORTHERN MARIANA ISLANDS, HAS TRACKED
WESTWARD AT 03 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) DEPICTS AN EXPOSED, WELL-DEFINED LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH ISOLATED DEEP CONVECTION FLARING OVER
THE SOUTH QUADRANT OF THE LLCC AND BROADER FRAGMENTED BANDING OVER
THE NORTHERN SEMI-CIRCLE. MSI SUPPORTS THE INITIAL POSITION WITH
GOOD CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED AT 30 KNOTS BASED
ON DVORAK CURRENT INTENSITY ESTIMATES OF T2.0 (30 KNOTS) FROM PGTW
AND RECENT OSCAT DATA. UPPER-LEVEL ANALYSIS INDICATES MODERATE (15
TO 20 KNOTS) SOUTHEASTERLY VERTICAL WIND SHEAR OFFSET BY DIFFLUENCE
ALOFT, ENHANCED BY A WEAKENING TUTT CELL NEAR 26N 154E. TD 27W IS
TRACKING SLOWLY WESTWARD WITHIN A COMPETING STEERING ENVIRONMENT.
3. FORECAST REASONING.
   A. NO CHANGE IN THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 27W IS POSITIONED WITHIN A COMPETING STEERING ENVIRONMENT
WITH A LOW-LEVEL (700MB) SUBTROPICAL RIDGE TO THE NORTH AND
NORTHEAST, AND A NEAR-EQUATORIAL RIDGE (NER) TO THE WEST AND
SOUTHWEST. A RECENT GUAM UPPER-AIR SOUNDING SUPPORTS THE PRESENCE OF
THE NER POSITIONED WEST OF GUAM WITH NORTH TO NORTHWEST WINDS AT 850-
500MB, WHICH MAINTAINS RESULTING IN A SLOW WESTWARD TO WEST-
SOUTHWESTWARD TRACK THROUGH TAU 72. THE TUTT CELL TO THE NORTH,
WHICH IS CURRENTLY AIDING OUTFLOW, IS FORECAST TO WEAKEN WITH WEAK
EASTERLY DIFFLUENT FLOW AND INCREASING VERTICAL WIND SHEAR
PERSISTING OVER THE SYSTEM THROUGH TAU 48 TO 72. THIS WILL LIMIT
INTENSIFICATION. DYNAMIC AIDS ARE IN FAIR AGREEMENT WITH SOLUTIONS
TRACKING 27W SLOWLY WEST-NORTHWESTWARD TO SOUTHWESTWARD.
   C. IN THE EXTENDED PERIOD, TD 27W, IF IT SURVIVES THE MARGINAL
UPPER-LEVEL CONDITIONS, IS FORECAST TO ACCELERATE WEST-NORTHWESTWARD
AS THE NER WEAKENS AND RECEDES EQUATORWARD, AND THE SUBTROPICAL
STEERING RIDGE BECOMES THE DOMINANT STEERING INFLUENCE. TD 27W IS
EXPECTED TO SLOWLY INTENSIFY AS UPPER-LEVEL DIFFLUENCE IMPROVES.
OVERALL, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST DUE TO
UNCERTAINTY IN THE MODEL GUIDANCE AND COMPLEX STEERING ENVIRONMENT.//
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