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

From: SIU Weather Processor <[email-masked]>
Date: Wed, 13 Oct 2010 21:27:03 -0500

WDPN31 PGTW 140300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (MEGI) WARNING NR
05//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 15W (MEGI) LOCATED APPROXIMATELY 175 NM
NORTH-NORTHEAST OF YAP, HAS TRACKED WEST-NORTHWESTWARD AT 07 KNOTS
OVER THE PAST 06 HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
SHOWS FLARING CONVECTION IN THE CENTER OF THE STORM WITH DEEP
CONVECTIVE BANDING ALONG THE EASTERN AND SOUTHERN SIDE OF THE
SYSTEM. UPPER-LEVEL OUTFLOW IS EVIDENT EQUATORWARD AND TO THE EAST
INTO A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT). UPPER-LEVEL
ANALYSIS SHOWS THE SYSTEM UNDER THE UPPER-LEVEL ANTICYCLONE WITH AN
UPPER-LEVEL TROUGH TO THE NORTHWEST.  THE TROUGH AND ASSOCIATED
SUBSIDENCE IS PUTTING SIGNIFICANT PRESSURE ON THE WESTERN SIDE OF
THE SYSTEM AS EVIDENT BY THE SUPPRESSED CONVECTION WEST OF THE
CIRCULATION CENTER AND LACK OF RADIAL OUTFLOW. THE SYSTEM IS
CURRENTLY TRACKING WESTWARD ALONG THE SOUTHWESTERN PERIPHERY OF THE
SUBTROPICAL RIDGE (STR) TO THE NORTH. THE CURRENT INTENSITY IS BASED
ON THE DVORAK ESTIMATE FROM PGTW.
3. FORECAST REASONING.
   A. THE FORECAST REASONING HAS NOT CHANGED SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. IN THE NEXT 12 TO 24 HOURS THE UPPER-LEVEL TROUGH TO THE
NORTHWEST OF TS 15W WILL FILL AND RELIEVE THE PRESSURE ON THE
WESTERN SIDE OF THE SYSTEM.  UNTIL THIS HAPPENS, TS 12W WILL BE SLOW
TO INTENSIFY. AFTER TAU 24, A PASSING MID-LATITUDE TROUGH WILL
REORIENT THE STR TO THE EAST AND ALLOW FOR THE SYSTEM TO ACCELERATE
SLIGHTLY TO THE NORTHWEST. DURING THIS PERIOD, POLEWARD OUTFLOW IS
EXPECTED TO BECOME ENHANCED INTO THE DIFFLUENT REGION AHEAD OF THE
TROUGH. THIS, COUPLED WITH HIGH SEA SURFACE TEMPERATURES, LARGE
AMOUNTS OF PRECIPITABLE WATER IN THE AIR COLUMN, AND EASTWARD
OUTFLOW INTO THE TUTT, WILL LIKELY CAUSE THE SYSTEM TO UNDERGO RAPID
INTENSIFICATION (RI).  ADDITIONALLY, GFDN AND COAMPS-TC ARE BOTH
INDICATING RI BETWEEN TAU 24 AND 48. AFTER TAU 48, THE RIDGE IS
EXPECTED TO RE-BUILD WESTWARD FOLLOWING THE PASSAGE OF THE MID-
LATITUDE TROUGH FORCING THE STORM TRACK TO A MORE WESTWARD
TRAJECTORY.
   C. AFTER TAU 72, TS 15W IS FORECAST TO CONTINUE WESTWARD INTO THE
VICINITY OF LUZON. THE ENVIRONMENT REMAINS FAVORABLE FOR CONTINUED
INTENSIFICATION FOLLOWING THE PERIOD OF RI, AND THE SYSTEM IS
EXPECTED TO DEEPEN FURTHER BEFORE MAKING LANDFALL. NUMERICAL MODEL
GUIDANCE SUPPORTS THIS FORECAST TRACK WITH GFDN, GFS, JGSM AND ECMWF
ALL IN VERY CLOSE AGREEMENT. UKMO IS SLIGHTLY SOUTH AND FAST AND
NOGAPS IS NORTH AND SLOW.//

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