Subject: Pacific-NW: Subj/prognostic Reasoning For Tropical Depression 18w Warning Nr 02//

From: SIU Weather Processor <[email-masked]>
Date: Fri, 12 Nov 2010 21:01:05 -0600

WDPN31 PGTW 130300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W WARNING NR 02//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. TROPICAL DEPRESSION (TD) 18W (EIGHTEEN), LOCATED
APPROXIMATELY 280 NM SOUTHEAST OF HUE, VIETNAM, HAS TRACKED
NORTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. THE CURRENT
POSITION IS BASED ON A 122330Z PGTW SATELLITE FIX AND A 122359Z
SSMIS MICROWAVE IMAGE. THE CURRENT INTENSITY ESTIMATE OF 25 KNOTS IS
BASED ON DVORAK T-NUMBER VALUES OF 1.5 FROM PGTW AS WELL AS
STRUCTURE EVIDENT IN MICROWAVE SATELLITE IMAGERY. RECENT ANIMATED
MULTISPECTRAL SATELLITE IMAGERY INDICATES THAT SOUTH-SOUTHWESTERLY
VERTICAL WIND SHEAR IS DISPLACING THE DEEPEST CONVECTION ASSOCIATED
WITH TD 18W TO THE NORTHEAST OF THE LOW LEVEL CIRCULATION CENTER
(LLCC).
3. FORECAST REASONING
    A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE FOR TD 18W.
    B. TD 18W IS TRACKING NORTHWESTWARD UNDER THE INFLUENCE OF LOW
TO MID-LEVEL SUBTROPICAL RIDGING TO THE NORTH. GENERALLY
NORTHWESTWARD MOTION IS EXPECTED TO CONTINUE THROUGH THE FORECAST
PERIOD AS THE SUBTROPICAL RIDGE TO THE NORTH REMAINS THE DOMINANT
STEERING INFLUENCE. THIS TRACK SCENARIO IS SUPPORTED BY THE
AVAILABLE NUMERICAL GUIDANCE. TD 18W LIES NORTHWEST OF AN UPPER
LEVEL ANTICYCLONE CENTER IN AN AREA OF MODERATE SOUTHWESTERLY
VERTICAL WIND SHEAR, WHICH IS INCREASING IN MAGNITUDE AHEAD OF AN
APPROACHING UPPER LEVEL TROUGH TO THE NORTHWEST. THE CYCLONE IS
EXPECTED TO MAINTAIN INTENSITY OR WEAKEN UNDER THE COMPETING
INFLUENCES OF INCREASING VERTICAL WIND SHEAR AND FAVORABLE POLEWARD
OUTFLOW AHEAD OF THE APPROACHING TROUGH. BY TAU 24, THE CYCLONE WILL
MOVE INLAND AND DISSIPATE BELOW THE WARNING THRESHOLD INTENSITY OF
25 KNOTS. HOWEVER, DUE TO EFFECTS OF SUSTAINED VERTICAL WIND SHEAR,
DISSIPATION MAY OCCUR PRIOR TO LANDFALL.//

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