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

From: SIU Weather Processor <[email-masked]>
Date: Mon, 18 Oct 2010 09:29:11 -0500

WDPN31 PGTW 180300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (MEGI) WARNING NR
23//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TYPHOON (TY) 15W (MEGI), LOCATED APPROXIMATELY 130 NM NORTH-
NORTHWEST OF MANILA, PHILIPPINES, HAS TRACKED WEST-SOUTHWESTWARD AT
13 KNOTS OVER THE PAST 06 HOURS. THOUGH STRIPPED OF ITS SUPER
TYPHOON STATUS WHILE TRANSITING THE HIGH PEAKS OF THE SIERRA MADRE
AFTER MAKING LANDFALL AROUND 0330Z, MEGI REMAINS A FORMIDABLE
TROPICAL CYCLONE. THE DEEPEST CENTRAL CONVECTION HAS WEAKENED AND
BECOME FRAGMENTED DUE TO CONTINUED LAND INTERACTION. HOWEVER, MEGI
IS POSITIONED TO ENTER BACK OVER FAVORABLE WATERS WITHIN THE NEXT
FEW HOURS AND RE-INTENSIFY. POLEWARD OUTFLOW REMAINS ROBUST AND
VERTICAL WIND SHEAR IS LOW. THE INITIAL INTENSITY FOR THE PREVIOUS
TWO FORECASTS HAS BEEN DIFFICULT TO ACCESS DUE TO A LACK OF
OBSERVATIONAL DATA FROM THE PHILIPPINES AND UNREPRESENTATIVE DVORAK
ESTIMATES. POSITION CONFIDENCE IS FAIR BASED ON SPIRAL BANDS RINGING
THE CENTER PART OF THE CIRCULATION IN INFRARED SATELLITE IMAGERY.
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. ONCE MEGI MOVES INTO THE SOUTH CHINA SEA WITHIN THE NEXT FEW
HOURS AN ADEQUATE INTENSITY ESTIMATE CAN BE DETERMINED WITH THE
DVORAK METHOD. UNTIL THEN THE INTENSITY FORECAST IS HINGED ON AN
INITIAL INTENSITY OF 95 KNOTS. NONETHELESS THE SYSTEM WILL LIKELY
RAMP UP QUICKLY UNDER THE INFLUENCE OF HIGH SEA SURFACE
TEMPERATURES, STRONG VORTICITY, LOW VERTICAL WIND SHEAR, AND A
POLEWARD OUTFLOW MECHANISM. THE SYSTEM WILL TRACK DUE WEST FOR THE
FIRST 24 HOURS, THEN TURN MORE NORTH OF WEST TOWARDS A (SHORT WAVE)
TROUGH-INDUCED BREAK IN THE STEERING RIDGE. GFS, WBAR, AND NOGAPS
APPEAR TO EXCESSIVELY WEAKEN THE STEERING RIDGE AND STALL THE SYSTEM
IN THE EASTERN SOUTH CHINA SEA FOR 1 TO 1.5 DAYS THEN PULL THE
SYSTEM UNREALISTICALLY NORTH TO THE EAST OF HONG KONG. ECMWF WEAKENS
THE RIDGE SIGNIFICANTLY LESS, CONSISTENT WITH THE MOSTLY ZONAL FLOW
PROGGED IN THE MID-LATITUDES, AND BUILDS THE RIDGE BACK IN TO THE
WEST AFTER THE TROUGH PASSES AROUND TAU 72. THIS FORECAST LIES ON
THE ECMWF SIDE OF THE MODEL CONSENSUS, WHICH TO DATE HAS PERFORMED
THE BEST AMONG THE CONSENSUS MEMBERS.
   C. THERE IS SIGNIFICANT SPREAD AMONG THE MODELS IN THE LATER
FORECAST PERIOD. A SLIGHT INCREASE IN VERTICAL WIND SHEAR WILL
LIKELY MODERATE INTENSIFICATION OR EVEN INCITE SLIGHT WEAKENING AS
MEGI NEARS CHINA.//

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