Subject: Pacific-NW: Subj/prognostic Reasoning For Typhoon 15w (megi) Warning Nr 25//

From: SIU Weather Processor <[email-masked]>
Date: Mon, 18 Oct 2010 21:25:11 -0500

WDPN31 PGTW 190300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (MEGI) WARNING NR 25//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TYPHOON (TY) 15W (MEGI), LOCATED APPROXIMATELY 455 NM SOUTHEAST
OF HONG KONG, CHINA, HAS TRACKED WESTWARD AT 07 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY INDICATES THAT
TY 15W HAS CONTINUED TO RE-CONSOLIDATE AND HAS DEVELOPED A 10-NM
DIAMETER IRREGULAR EYE OVER THE PAST 06 HOURS. A 182321Z TRMM 37H
IMAGE SHOWS A WELL-DEFINED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH
A MICROWAVE EYE FEATURE AND STRONG CORE CONVECTION OVER ALL
QUADRANTS.
ANIMATED WATER VAPOR IMAGERY DEPICTS EXCELLENT RADIAL OUTFLOW
ESPECIALLY POLEWARD OUTFLOW, ENHANCED BY STRONG WESTERLY FLOW ACROSS
CENTRAL CHINA. THERE IS GOOD CONFIDENCE IN THE CURRENT POSITION
BASED ON THE TRMM IMAGE AND EYE. THE CURRENT INTENSITY OF 90 KNOTS
IS BASED ON DVORAK ESTIMATES OF 90 KNOTS FROM PGTW AND KNES.
3. FORECAST REASONING.
   A. NO CHANGE IN FORECAST PHILOSOPHY, HOWEVER, THE TRACK HAS
BEEN ADJUSTED BETWEEN TAU 24 AND 72 TO ACCOUNT FOR A MORE POLEWARD
TRACK.
   B. TY 15W IS TRACKING WESTWARD ALONG THE SOUTHWEST PERIPHERY
OF THE MID-LEVEL SUBTROPICAL RIDGE (STR). THE FORECAST TRACK HAS
BEEN ADJUSTED SLIGHTLY BETWEEN TAU 24-72 BASED ON OBSERVED WEAKENING
OF THE WESTERN STR, WHICH HAS SHIFTED WEST OVER THE PAST 24 HOURS,
AND EVIDENCE OF A DEEPENING SHORTWAVE TROUGH OVER SOUTHEAST CHINA.
MODEL GUIDANCE (INCLUDING THE MAJORITY OF ENSEMBLE DATA) HAS
SUBSEQUENTLY TRENDED TOWARD A STAIR-STEP PATTERN. ALL AVAILABLE
DYNAMIC MODELS (ECMWF, UKMO, GFS, JGSM, NOGAPS, GFDN) PLUS WBAR
NOW SUPPORT THIS TRACK AND ARE IN GOOD AGREEMENT WITH THE EXCEPTION
OF THE GFS. THE GFS INDICATES A SHARP NORTH-NORTHEASTWARD TRACK
THROUGH TAU 72 BEFORE TURNING NORTH-NORTHWESTWARD. ANALYSIS OF
THE GFS SLP FIELDS INDICATES POSSIBLE EXCESSIVE INTERACTION WITH A
WEAK SYSTEM FORMING NORTHEAST OF LUZON; A LOBE OF LOWER PRESSURES
EXTENDING NORTHEASTWARD OUT OF TY 15W IS EVIDENT AS WELL AS THE
INTERACTION. THIS TRACK IS DEEMED AS UNLIKELY CONSIDERING THE
STRENGTH AND POSITION OF THE STEERING STR POSITIONED TO THE NORTH.
TY 15W IS EXPECTED TO TURN WEST-NORTHWESTWARD TO NORTHWESTWARD
IN THE NEXT 12-24 HOURS IN RESPONSE TO THE WEAKENING WESTERN STR
ASSOCIATED WITH A SHORTWAVE TROUGH PROPAGATING THROUGH SOUTHEAST
CHINA, AND SHOULD TRACK NORTHWESTWARD THROUGH TAU 72. TY 15W IS
FORECAST TO INTENSIFY TO A PEAK INTENSITY OF 115 KNOTS BY TAU
48 DUE TO THE FAVORABLE ENVIRONMENT.
   C. THE SYSTEM IS FORECAST TO RESUME A WEST-NORTHWESTWARD TRACK
AS THE STR RE-BUILDS NORTH NEAR TAU 72. TY 15W IS EXPECTED TO
WEAKEN AFTER TAU 72 DUE TO INCREASING VWS AND LAND INTERACTION
NEAR TAU 96.32//

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