Subject: Pacific-NW: Subj/prognostic Reasoning For Tropical Storm 13w Warning Nr 10//

From: SIU Weather Processor <[email-masked]>
Date: Wed, 22 Sep 2010 09:35:05 -0500

WDPN32 PGTW 221500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W WARNING NR 10//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 13W (MALAKAS), LOCATED APPROXIMATELY 300
NM SOUTH OF IWO TO, JAPAN, HAS TRACKED NORTHWESTWARD AT 06 KNOTS
OVER THE PAST SIX HOURS. THE SYSTEM CONTINUES TO HAVE A PARTIALLY-
EXPOSED, WELL-DEFINED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH
SHALLOW CONVECTIVE BANDING WRAPPING TIGHTLY INTO THE SYSTEM CENTER.
THE BULK OF THE DEEP CONVECTION IS STILL BEING DISPLACED EQUATORWARD
FROM THE LLCC DUE TO NORTHERLY VERTICAL WIND SHEAR ASSOCIATED WITH
UPPER-LEVEL CONFLUENT FLOW FORMED BY THE MOVEMENT OF A TUTT CELL
NORTH OF THE SYSTEM. MALAKAS HAS TRACKED STEADILY WEST-NORTHWESTWARD
ALONG THE SOUTHERN FLANK OF THE SUBTROPICAL STEERING RIDGE, WHICH IS
BEGINNING TO WEAKEN AS A DEEPENING MID-LATITUDE TROUGH MOVES THROUGH
THE YELLOW SEA. THE CURRENT INTENSITY IS BASED ON DVORAK ESTIMATES
OF 35 KNOTS FROM PGTW, KNES AND RJTD AS WELL AS FROM RECENT SATCON
ESTIMATES OF APPROXIMATELY 45 KNOTS. THE CURRENT POSITION IS
SUPPORTED WITH HIGH CONFIDENCE BY A 221128Z TRMM MICROWAVE IMAGE.
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. ALTHOUGH TS MALAKAS IS CURRENTLY BEING SUPPRESSED BY UPPER
LEVEL CONVERGENCE ON ITS NORTHERN FLANK, IT IS IN A FAVORABLE
ENVIRONMENT FOR DEVELOPMENT WITH GOOD POLEWARD OUTFLOW INTO A SERIES
OF TUTT CELLS TO THE NORTH AND EAST, HIGH OCEAN HEAT CONTENT AND LOW
VERTICAL WIND SHEAR. ACCORDINGLY, TS 13W IS EXPECTED TO INTENSIFY
STEADILY AS IT APPROACHES THE SUBTROPICAL RIDGE AXIS OVER THE NEXT
24-36 HOURS, REACHING TYPHOON STRENGTH BEFORE 24/00Z AS POLEWARD
OUTFLOW INCREASES SIGNIFICANTLY AHEAD OF THE APPROACHING MID-
LATITUDE TROUGH. NUMERICAL MODEL GUIDANCE IS IN VERY TIGHT AGREEMENT
WITH THIS SCENARIO. THIS FORECAST LIES JUST WEST OF THE JTWC
CONSENSUS SINCE THE GFS TRACKER CONTINUES TO REFLECT AN IMPROBABLY
SHARP POLEWARD TURN BY TAU 12.
   C. THE APPROACH OF THE TROUGH WILL ALSO ACT TO FURTHER WEAKEN THE
STEERING RIDGE, GIVING THE SYSTEM AN ALLEY TO RECURVE INTO THE MID-
LATITUDES. BY TAU 48, EXTRA-TROPICAL TRANSITION WILL BEGIN AS THE
SYSTEM NEARS 30 DEGREES NORTH LATITUDE AND STARTS COUPLING WITH THE
MID-LATITUDE WESTERLIES. SURFACE AND GRADIENT-LEVEL WIND FIELDS
AROUND THE STORM WILL EXPAND AS IT INTERACTS WITH A STRONG BARO-
CLINIC ZONE AND COMES UNDER THE INFLUENCE OF A DEEP, NEGATIVELY
TILTED TROUGH. THE SYSTEM WILL REMAIN VIGOROUS IN ITS WARM SECTOR AS
SEA SURFACE TEMPERATURES REMAIN HIGH THROUGH 35 DEGREES NORTH. THE
SYSTEM WILL OCCLUDE AND COMPLETE TRANSITION TO A STORM FORCE EXTRA-
TROPICAL LOW BEFORE CROSSING 40 DEGREES NORTH AROUND TAU 72.//

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