Subject: Pacific-NW: Subj/prognostic Reasoning For Tropical Depression 17w (seventeen)

From: SIU Weather Processor <[email-masked]>
Date: Thu, 21 Oct 2010 22:12:04 -0500

WDPN32 PGTW 220300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN)
WARNING NR 02//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. TROPICAL DEPRESSION (TD) 17W (SEVENTEEN), LOCATED
APPROXIMATELY 760 NM NORTHEAST OF HAGATNA, GUAM, HAS TRACKED WEST-
NORTHWESTWARD AT 06 KNOTS OVER THE PAST SIX HOURS. THE CURRENT
POSITION IS BASED ON A 212221Z SSMI PASS AND A 212330Z PGTW
SATELLITE FIX. THE CURRENT INTENSITY ESTIMATE OF 30 KNOTS IS
CONSISTENT WITH A DVORAK ESTIMATE OF 30 KNOTS FROM KNES AT 212057Z
AND STRUCTURE EVIDENT IN THE SSMI IMAGE. DEEP CONVECTION IS CONFINED
TO THE SOUTH AND WESTERN PORTIONS OF THE PARTIALLY-EXPOSED LOW LEVEL
CIRCULATION CENTER (LLCC) DUE TO MODERATE NORTHEASTERLY VERTICAL
WIND SHEAR INDUCED BY THE FLOW BETWEEN AN UPPER LEVEL RIDGE TO THE
NORTH AND A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL TO THE
SOUTH. THE LLCC IS TRACKING WEST-NORTHWESTWARD UNDER THE INFLUENCE
OF A DEEP SUBTROPICAL STEERING RIDGE TO THE NORTH.
3. FORECAST REASONING.
    A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE FOR THIS
SYSTEM.
    B. TD 17W WILL CONTINUE TO TRACK NORTHWESTWARD ALONG THE
SOUTHERN PERIPHERY OF A DEEP SUBTROPICAL RIDGE TO THE NORTH FOR THE
NEXT 12 HOURS. THEREAFTER, A DEVELOPING MIDLATITUDE TROUGH TO THE
NORTH IS EXPECTED TO WEAKEN THE STEERING RIDGE AND INDUCE A POLEWARD
TURN INTO A WEAK STEERING ENVIRONMENT. THE AVAILABLE NUMERICAL
GUIDANCE IS IN GOOD GENERAL AGREEMENT WITH THIS SCENARIO, ALTHOUGH
THERE IS SOME UNCERTAINTY IN THE EXACT TRACK GIVEN THE ANTICIPATED
WEAK STEERING SETUP. UNFAVORABLE ENVIRONMENTAL FACTORS, INCLUDING
PERSISTENT UPPER LEVEL CONVERGENCE, EASTERLY VERTICAL WIND SHEAR,
AND THE INTRODUCTION OF DRIER AIR TOWARD THE CENTER OF TD 17W FROM
THE SOUTHEAST, SHOULD PREVENT THE SYSTEM FROM INTENSIFYING
SIGNIFICANTLY AND RESULT IN DISSIPATION BELOW THE 25 KNOT WARNING
THRESHOLD INTENSITY BY TAU 36.//

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