Subject: Pacific-NW: Subj/prognostic Reasoning For Typhoon 15w (megi) Warning Nr 29//

From: SIU Weather Processor <[email-masked]>
Date: Tue, 19 Oct 2010 22:04:04 -0500

WDPN31 PGTW 200300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (MEGI) WARNING NR 29//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TYPHOON (TY) 15W (MEGI), LOCATED APPROXIMATELY 350 NM SOUTH-
SOUTHEAST OF HONG KONG, CHINA HAS TRACKED NORTHWESTWARD AT 03 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
SHOWS A BANDING EYE WITH A DECREASE IN DEEP CONVECTIVE BANDING ON
THE NORTHERN PERIPHERY OF THE SYSTEM. HOWEVER, THE OVERALL SYSTEM
SIZE HAS EXPANDED WITH INCREASED CONVECTION IN THE SOUTHERN
PERIPHERY. THE CURRENT POSITION IS CENTERED ON THE BANDING EYE
FEATURE AS DEPICTED IN THE MULTISPECTRAL IMAGERY AND THE CURRENT
INTENSITY IS BASED ON A PGTW DVORAK ESTIMATE OF 115 KNOTS. WATER
VAPOR ANIMATION DEPICTS EXCELLENT RADIAL OUTFLOW. TY 15W IS IN A
WEAK STEERING ENVIRONMENT, SLOWLY TRACKING ALONG THE SOUTHWESTERN
PERIPHERY OF A MID-LEVEL SUBTROPICAL RIDGE WEAKENED BY A DEEPENING
MID-LATITUDE SHORTWAVE TROUGH MOVING OVER SOUTHEAST CHINA.
3. FORECAST REASONING.
   A. THE FORECAST TRACK HAS BEEN ADJUSTED POLEWARD DUE TO A MID-
LATITUDE TROUGH, WHICH WEAKENED THE SUBTROPICAL STEERING RIDGE MORE
THAN PREVIOUSLY EXPECTED.
   B. TY 15W IS FORECAST TO TRACK NORTH-NORTHWESTWARD WITHIN THE
NEXT 12 HOURS AS THE DEEPENING MID-LATITUDE TROUGH CONTINUES TO
WEAKEN AND REORIENT THE STEERING RIDGE. POLEWARD OUTFLOW IS EXPECTED
TO ENHANCE INTO THE MID-LATITUDE TROUGH AND ALLOW FURTHER
INTENSIFICATION WITHIN THE NEXT 24 HOURS. AFTER TAU 24, TY MEGI
SHOULD BEGIN WEAKENING AS IT CONTINUES TO MOVE NORTHWARD INTO AN
AREA OF INCREASED VERTICAL WIND SHEAR AND DECREASED OCEAN HEAT
CONTENT.
   C. BY TAU 72, TY 15W IS FORECAST TO MAKE LANDFALL EAST OF HONG
KONG. THE SYSTEM SHOULD CONTINUE TO TRACK OVER LAND AND BEGIN TO
DISSIPATE AS A SIGNIFICANT TROPICAL CYCLONE BY TAU 120. MODEL
GUIDANCE IS SOMEWHAT SPREAD WITH GFS, UKMO, JGSI TRACKING THE SYSTEM
MORE POLEWARD AND STAIR-STEPPING IT BACK TO THE NORTHWEST AS THE
SUBTROPICAL RIDGE TO THE NORTH RE-BUILDS. ECMWF TRACKS THE SYSTEM
MORE TO THE NORTHWEST, TOWARDS HONG KONG. THIS FORECAST IS IN
AGREEMENT WITH MODEL CONSENSUS AND TRENDS TO THE LEFT OF MODEL
CONSENSUS.//

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