Subject: Pacific-NW: Subj/prognostic Reasoning For Tropical Storm 13w Warning Nr 08//

From: SIU Weather Processor <[email-masked]>
Date: Tue, 21 Sep 2010 21:09:05 -0500

WDPN32 PGTW 220300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W WARNING NR 08//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 13W (MALAKAS), LOCATED APPROXIMATELY
300 NM SOUTH-SOPUTHEAST OF IWO TO, JAPAN HAS TRACKED WEST-NORTHWEST
AT 06 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY, A 211629Z AMSR-E 89GHZ IMAGE, AND A 2321Z VISUAL
IMAGE ALL COMBINE TO SIGNIFICANTLY INCREASE POSITION CONFIDENCE. THE
SYSTEM HAS A FULLY-EXPOSED, WELL-DEFINED LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH TIGHTLY-WRAPPED SHALLOW CONVECTIVE BANDING
WRAPPING INTO A WELL-DEFINED CENTER. THE BULK OF THE DEEP CONVECTION
IS BEING DISPLACED EQUATORWARD FROM THE LLCC. ANIMATED WATER VAPOR
IMAGERY SHOWS OUTFLOW OVER THE POLEWARD QUADRANTS IS BEING
SUPPRESSED BY CONFLUENT FLOW ALOFT, RESULTING IN THE STORM
DEVELOPING AT A LOWER-THAN-CLIMATOLOGICAL RATE. THE SYSTEM IS
TRACKING STEADILY WEST-NORTHWESTWARD ALONG THE SOUTHERN FLANK OF THE
SUB-TROPICAL RIDGE, WHICH IS BEGINNING TO WEAKEN AS A DEEPENING MID-
LATITUDE TROUGH MOVES INTO THE YELLOW SEA. THE CURRENT INTENSITY IS
BASED ON AN AVERAGE OF DVORAK ESTIMATES OF 35 AND 45 KNOTS FROM PGTW
AND RJTD.
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. TS 13W IS EXPECTED TO INTENSIFY SLOWLY OVER THE NEXT 24-36
HOURS AS IT TRACKS ALONG THE SOUTHERN PERIPHERY OF THE STEERING
RIDGE. ALTHOUGH THE SYSTEM IS BEING SUPPRESSED BY UPPER LEVEL
CONVERGENCE ON ITS NORTHERN FLANK, IT IS IN A FAVORABLE ENVIRONMENT
FOR DEVELOPMENT, WITH HIGH OCEAN HEAT CONTENT AND LOW VERTICAL WIND
SHEAR. THE APPROACH OF AN UPSTREAM MID-LATITUDE TROUGH WILL ACT TO
WEAKEN THE RIDGE ALONG THE 140TH MERIDIAN, GIVING THE SYSTEM AN
ALLEY TO RECURVE INTO THE MID-LATITUDES. AS TS 13W APPROACHES THE
AXIS OF THE RIDGE AND SLOWS BEFORE RECURVATURE, POLEWARD OUTFLOW
WILL INCREASE AND THE SYSTEM WILL INTENSIFY SIGNIFICANTLY, REACHING
TYPHOON STRENGTH BY 22/06Z. NUMERICAL MODEL GUIDANCE IS IN STRONG
AGREEMENT WITH THE TRACK INCLUDING THE AREA OF RECURVATURE, WITH
SOME VARIATION IN TIMING. ECMWF, TYPICALLY A STRONG PERFORMER,
PROJECTS THE SLOWEST AND LATEST TURN AROUND THE RIDGE.  THE FORECAST
ADJUSTS CONSENSUS SLIGHTLY SLOWER IN COGNIZANCE OF THIS TENDENCY BUT
OTHERWISE ADHERES CLOSELY TO CONSENSUS DUE TO STRONG AGREEMENT AND
COHERENT SCENARIO PRESENTED BY THE DYNAMIC AIDS.
   C.  EXTRA-TROPICAL TRANSITION WILL BEGIN AS THE SYSTEM CROSSES 30
DEGREES NORTH LATITUDE AND BEGINS COUPLING WITH THE MID-LATITUDE
WESTERLIES.  SURFACE AND GRADIENT-LEVEL WINDFIELDS AROUND THE STORM
WILL EXPAND AS IT INTERACTS WITH A STRONGLY BAROCLINIC 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.
//

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
To unsubscribe from WX-TROPL send e-mail to [email-masked] with
"unsub wx-tropl" in the body of your message. For help with WX-TROPL
write to [email-masked]. For more information on tropical weather products, see the NHC Homepage at http://www.nhc.noaa.gov.