Subject: Pacific-NW: Subj/prognostic Reasoning For Tropical Storm 13w (malakas) Warning

From: SIU Weather Processor <[email-masked]>
Date: Tue, 21 Sep 2010 09:42:20 -0500

WDPN32 PGTW 211500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (MALAKAS) WARNING
NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. TROPICAL STORM (TS) 13W (MALAKAS), LOCATED APPROXIMATELY 230
NM NORTH-NORTHWEST OF SAIPAN, HAS TRACKED GENERALLY WEST-
NORTHWESTWARD OVER THE PAST 12 HOURS UNDER THE INFLUENCE OF A WEAK
LOW- TO MID-LEVEL SUBTROPICAL STEERING RIDGE ANCHORED TO THE NORTH.
THE 211200Z POSITION WAS REPOSITIONED SLIGHTLY TO THE NORTH OF THE
PREVIOUS POSITION BASED ON ANALYSIS OF RECENT MICROWAVE DATA
(210654Z SSMI, 210744Z SSMIS, 210852Z SSMIS and 211044z SSMIS) AND A
PARTIALLY-EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) EVIDENT IN
RECENT ANIMATED INFRARED SATELLITE IMAGERY. IT APPEARS THE UPPER-
LEVEL ANTICYCLONE PROVIDING GOOD RADIAL OUTFLOW HAS SHIFTED A BIT TO
THE NORTHEAST OF THE SYSTEM, TEMPORARILY INCREASING THE VERTICAL
WIND SHEAR OVER THE CYCLONE AND CAUSING THE PARTIALLY-EXPOSED LLCC.
THE CURRENT INTENSITY OF 40 KNOTS IS CONSISTENT WITH DVORAK
ESTIMATES FROM PGTW, KNES (BOTH T2.5) AND RJTD (T3.0).
3. FORECAST REASONING
    A. THE FORECAST PHILOSOPHY HAS NOT CHANGED SIGNIFICANTLY SINCE
THE LAST PROGNOSTIC REASONING BULLETIN.
    B. TS 13W WILL CONTINUE TO TRACK GENERALLY WESTWARD OVER THE
NEXT 36-48 HOURS BEFORE A DIGGING MID-LATITUDE TROUGH STARTS TO
WEAKEN THE STEERING RIDGE, ENABLING THE CYCLONE TO ROUND THE RIDGE
AXIS BY TAU 72. AS TS 13W APPROACHES THE RIDGE AXIS, THE SYSTEM IS
EXPECTED TO INTENSIFY AS IT TRACKS OVER HIGH OCEAN HEAT CONTENT WITH
GOOD POLEWARD OUTFLOW INTO A TUTT CELL LOCATED TO THE EAST OF THE
CYCLONE. MALAKAS SHOULD REACH MAXIMUM INTENSITY AROUND TAU 72 DUE TO
ENHANCED POLEWARD OUTFLOW INTO THE MID-LATITUDE WESTERLIES AHEAD OF
THE APPROACHING TROUGH.
    C. AFTER CRESTING THE SUBTROPICAL RIDGE AXIS, TS 13W WILL
ACCELERATE POLEWARD BY TAU 96 AND UNDERGO EXTRATROPICAL TRANSITION
IN THE GRADIENT BETWEEN THE MID-LATITUDE TROUGH TO THE WEST AND A
SUBTROPICAL RIDGE TO THE EAST. DURING EXTRATROPICAL TRANSITION, THE
CYCLONE WILL REMAIN QUITE INTENSE AS STRONG UPPER LEVEL DIFFLUENCE
AND A SHARP TEMPERATURE GRADIENT ASSOCIATED WITH THE APPROACHING
TROUGH PROVIDE A STRONG BAROCLINIC DEVELOPMENT MECHANISM. THE
AVAILABLE MODEL GUIDANCE IS IN VERY GOOD AGREEMENT WITH THIS TRACK
PHILOSOPHY THOUGH THE GFS TRACKER CONTINUES TO REFLECT AN EARLIER
POLEWARD TURN INDICATIVE OF AN UNREALISTICALLY SHALLOW SYSTEM. FOR
INTENSITY, THE FORECAST IS ABOVE THE GUIDANCE IN CONSIDERATION OF
THE VERY FAVORABLE UPPER-LEVEL ENVIRONMENT.//

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