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Tropical Storm MARTY


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TROPICAL STORM MARTY DISCUSSION NUMBER   8
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP162009
800 PM PDT THU SEP 17 2009
 
ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN INDICATE THAT
MARTY IS EXPERIENCING 10-15 KT OF SOUTHERLY VERTICAL WIND SHEAR
THIS EVENING.  THIS IS CAUSING THE CYCLONE TO HAVE A SOMEWHAT RAGGED
APPEARANCE EVEN THOUGH THE CLOUD TOPS OF THE CENTRAL CONVECTION ARE
COLDER THAN -80C.  THE INITIAL INTENSITY REMAINS 40 KT FOR THIS
ADVISORY...WHICH IS IN AGREEMENT WITH AN AMSU INTENSITY ESTIMATE AT
17/2031 UTC AND SLIGHTLY ABOVE THE SATELLITE INTENSITY ESTIMATES
FROM TAFB AND SAB.

THE FORECAST TRACK TAKES MARTY OVER GRADUALLY DECREASING SEA SURFACE
TEMPERATURES AND INTO AN AREA OF STRONG SOUTHWESTERLY SHEAR.  IN
ADDITION...TOTAL PRECIPITABLE WATER DATA SHOWS VERY DRY AIR NOT FAR
TO THE WEST OF THE STORM.  THIS COMBINATION SHOULD CAUSE MARTY TO
WEAKEN DURING THE PERIOD...AND THE INTENSITY FORECAST CALLS FOR IT
TO DECAY TO A REMNANT LOW IN ABOUT 48 HR IN AGREEMENT WITH THE
PREVIOUS FORECAST.  MOST OF THE GLOBAL MODELS SHOW THE REMNANT LOW
BREAKING OPEN INTO A TROUGH BEFORE 96 HR...SO THE NEW 96 HR
FORECAST CALLS FOR DISSIPATION.
 
MARTY IS CURRENTLY IN AN AREA OF LIGHT STEERING CURRENTS BETWEEN A
MID/UPPER-LEVEL RIDGE OVER MEXICO AND A SECOND MID/UPPER-LEVEL
RIDGE WEST OF THE CYCLONE.  THE DYNAMICAL MODELS FORECAST THE RIDGE
OVER MEXICO TO STEER MARTY SLOWLY TOWARD THE NORTHWEST FOR THE NEXT
24-36 HR.  AFTER THAT TIME...MARTY SHOULD GRADUALLY TURN WESTWARD
AS THE SYSTEM WEAKENS AND A LOW-LEVEL RIDGE TO THE NORTHWEST
BECOMES THE MAIN INFLUENCE ON THE MOTION.  THE NEW FORECAST TRACK
IS AN UPDATE OF THE PREVIOUS TRACK.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0300Z 20.0N 113.6W    40 KT
 12HR VT     18/1200Z 20.5N 114.2W    35 KT
 24HR VT     19/0000Z 21.4N 115.2W    35 KT
 36HR VT     19/1200Z 22.2N 116.2W    30 KT
 48HR VT     20/0000Z 22.6N 117.4W    25 KT...REMNANT LOW
 72HR VT     21/0000Z 22.5N 119.5W    20 KT...REMNANT LOW
 96HR VT     22/0000Z...DISSIPATED
 
$$
FORECASTER BEVEN
 
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