Tropical Storm SAOLA Advisory Wed Oct 25

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (SAOLA)
WARNING NR 22//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
TROPICAL STORM 27W (SAOLA), LOCATED APPROXIMATELY 326 NM NORTH-
NORTHWEST OF YAP, HAS TRACKED WEST-NORTHWESTWARD AT 20 KNOTS OVER
THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS
A SLOWLY CONSOLIDATING SYSTEM WITH FLARING DEEP CONVECTION NEAR THE
POORLY ORGANIZED LOW LEVEL CIRCULATION CENTER (LLCC). THE FLARE UP
OF CONVECTION NEAR THE LLCC HAS PERSISTED OVER THE PAST SIX HOURS,
INDICATING INCREASING ORGANIZATION, BUT THE BULK OF THE CONVECTIVE
ACTIVITY REMAINS CONFINED TO THE SOUTHWESTERN QUADRANT OF THE
SYSTEM. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON
THE AVAILABLE VISIBLE IMAGERY AND SUPPORTED BY A 250010Z ASCAT-B
BULLSEYE PASS, CONFIRMING THE LLCC TOWARDS THE NORTHERN EDGE OF THE
CONVECTIVE BALL. A 242237Z AMSU MICROWAVE IMAGE SHOWED A BROAD LLCC,
WITH WEAK CONVECTIVE ACTIVITY ALONG THE SOUTHERN PERIPHERY OF THE
LLCC. THE INITIAL INTENSITY HAS BEEN INCREASED TO 40 KNOTS BASED ON
A COMBINATION OF DVORAK CURRENT INTENSITIES OF 2.5 (35 TO 45 KTS)
FROM PGTW AND RJTD, AND AUTOMATED DVORAK INTENSITIES OF 2.8 (43
KTS), AND SUPPORTED BY THE ASCAT DATA, SHOWING A BROAD SWATH OF 35-
40 KNOT WINDS ON THE SOUTHERN PORTION OF THE SYSTEM, THOUGH THESE
ARE IN THE AREA OF DEEP CONVECTION. ENVIRONMENTAL CONDITIONS ARE
MARGINALLY SUPPORTIVE OF FURTHER DEVELOPMENT, WITH MODERATE (15-20
KNOTS) NORTHEASTERLY VERTICAL WIND SHEAR (VWS) AND HIGH SEA SURFACE
TEMPERATURES (31 DEG CELSIUS). THE MAIN LIMITING FACTOR CONTINUES TO
BE THE WEAK WESTWARD AND EQUATORWARD OUTFLOW, WHICH IS BEING
HINDERED BY AN UPPER-LEVEL INVERTED TROUGH JUST EAST OF THE SYSTEM.
TS SAOLA IS TRACK WEST-NORTHWESTWARD ALONG THE SOUTHWESTERN
PERIPHERY OF A DEEP SUBTROPICAL RIDGE (STR) LOCATED TO THE
NORTHEAST. THE TRACK HAS BECOME MORE NORTHWESTWARD OVER THE PAST SIX
HOURS, AFTER THE PREVIOUS MORE WESTWARD TRAJECTORY, AS THE STR HAS
BEGUN TO REORIENT.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE IN THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TS 27W WILL CONTINUE TO TRACK TO THE WEST-NORTHWEST FOR THE
NEXT 12 HOURS ALONG THE SOUTHWESTERN PERIPHERY OF THE STR. A BREAK
IN THE RIDGE PATTERN IS FORECAST TO DEVELOP ALONG 13OE LONGITUDE BY
AROUND TAU 24, ALLOWING SAOLA TO TURN NORTHWESTWARD AFTER TAU 24.
TRACK SPEEDS WILL SLOW FROM TAU 24 THROUGH 72, AS THE SYSTEM
ENCOUNTERS A WEAK STEERING ENVIRONMENT AHEAD OF AN APPROACHING UPPER-
LEVEL MID-LATITUDE TROUGHS. THE STRENGTH AND TIMING OF THIS TROUGH
IS KEY TO JUST HOW FAR WEST TS 27W WILL TRACK BEFORE ROUNDING THE
RIDGE AXIS NEAR TAU 72 IN THE VICINITY OF OKINAWA. ONCE THE SYSTEM
ROUNDS THE RIDGE AXIS, IT WILL ACCELERATE RAPIDLY NORTHEASTWARD
ALONG THE SOUTH COAST OF JAPAN. THE SYSTEM IS FORECAST TO MAINTAIN
INTENSITY FOR 12 HOURS AS IT CONTINUES TO CONSOLIDATE AND PERSISTENT
CONVERGENT AND WEAK OUTFLOW CONDITIONS CONTINUE. SLOW
INTENSIFICATION RESUMES THROUGH TAU 72, AS THE SYSTEM MOVES INTO A
BETTER OUTFLOW ENVIRONMENT AND ESTABLISHES A POLEWARD CHANNEL,
PEAKING AT 70 KNOTS BY TAU 72 AS IT ROUNDS THE RIDGE. DYNAMIC MODEL
GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 48 WITH INCREASING
UNCERTAINTY THROUGH TAU 72, DUE TO DIFFERENCES IN THE TIMING AND
SHARPNESS OF THE TURN TO THE NORTHEAST. ECMWF AND HWRF ARE THE
WESTERN OUTLIERS, INDICATING A VERY CLOSE CPA, OR DIRECT HIT, ON
OKINAWA AROUND TAU 72. NAVGEM AND COTC ARE THE EASTERN OUTLIERS.
WITH THE HIGH DEGREE OF UNCERTAINTY BEYOND TAU 48, THERE IS MODERATE
CONFIDENCE IN THIS PORTION OF THE JTWC TRACK FORECAST.
   C. AFTER TAU 72, TS 27W WILL ROUND THE STR AXIS, ACCELERATING
RAPIDLY TO THE NORTHEAST THROUGH THE END OF THE FORECAST PERIOD. AS
THE SYSTEM BEGINS TO INTERACT WITH AN APPROACHING MID-LATITUDE
TROUGH, INCREASING VWS AND DECREASING SSTS, IT WILL BEGIN TO UNDERGO
EXTRATROPICAL TRANSITION (ET) BY TAU 96 AND COMPLETE THE TRANSITION
BEFORE THE TAU 120 POINT. DYNAMIC MODEL GUIDANCE HAS SIGNIFICANT
UNCERTAINTY IN THE LATER PERIOD OF THE FORECAST, BOTH IN TRACK AND
FORWARD SPEED. THE JTWC TRACK IS NEAR THE CONSENSUS TRACK, BUT
SIGNIFICANTLY FASTER IN ALONG TRACK SPEED. THE PRINCIPLE OUTLIERS
ARE GFS AND ECMWF, BOTH INDICATING A SHARPER TURN AND LYING TO THE
SOUTH OF THE JTWC TRACK, WHILE UKMET AND THE ECMWF ENSEMBLE
INDICATES A SHALLOWER TURN TRACKING THE SYSTEM OVER TOKYO. DUE TO
THE VARIABILITY IN BOTH TRACK AND SPEED, THERE IS LOW CONFIDENCE IN
THIS PORTION OF THE JTWC OFFICIAL FORECAST.//
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