Tropical Storm ISELLE

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TROPICAL STORM ISELLE DISCUSSION NUMBER   4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP102008
200 AM PDT THU AUG 14 2008
 
THE CONVECTIVE PATTERN HAS BECOME SLIGHTLY MORE SYMMETRIC...BUT
MICROWAVE IMAGERY INDICATES THE LOW-LEVEL CENTER IS STILL BENEATH
THE NORTHEASTERN PORTION OF THE DEEP CONVECTION.  THE DVORAK
INTENSITY ESTIMATES ARE UNCHANGED FROM THE PREVIOUS CYCLE AT 35-45
KT...BUT MANUAL ANALYSIS OF THE AMBIGUITIES FROM A QUIKSCAT PASS AT
AROUND 01Z SUGGESTS MAXIMUM WINDS OF ABOUT 40 KT...SO THE ADVISORY
INTENSITY IS NUDGED UPWARD TO 40 KT.  THE CYCLONE IS STRUGGLING IN
THE FACE OF EASTERLY SHEAR...AND DYNAMICAL MODELS FORECAST THE
SHEAR TO PERSIST THROUGHOUT THE FIVE-DAY FORECAST PERIOD...WITH
PERHAPS EVEN A SLIGHT INCREASE IN SHEAR IN THE SHORT TERM.  AS A
RESULT...DESPITE WARM WATERS ALONG THE FORECAST TRACK FOR THE NEXT
TWO TO THREE DAYS...NONE OF THE OBJECTIVE INTENSITY MODELS FORECAST
MUCH STRENGTHENING IF AT ALL.  THE OFFICIAL FORECAST CLOSELY
FOLLOWS THE SHIPS AND LGEM GUIDANCE IN CALLING FOR VERY LITTLE
CHANGE IN STRENGTH THROUGH ABOUT 72 HOURS...FOLLOWED BY WEAKENING
OVER COOLER WATERS.  GIVEN THE PERSISTENT SHEAR...AND THE
LESS-THAN-IMPRESSIVE DEPICTIONS OF THE CYCLONE IN THE DYNAMICAL
MODEL FIELDS...IT REMAINS POSSIBLE THAT ISELLE WILL NOT LAST THE
ENTIRE FIVE-DAY PERIOD.

THE 01Z QUIKSCAT PASS...AND PASSIVE MICROWAVE IMAGERY FROM SSMIS AND
TRMM AT AROUND THAT SAME TIME...INDICATE THAT THE CENTER OF ISELLE
IS MOVING A TINY BIT SOUTH OF THE PREVIOUS TRACK...AND THE INITIAL
MOTION ESTIMATE IS ABOUT 290/9.  ISELLE IS BEING STEERED BY A MID-
TO UPPER-LEVEL RIDGE OVER NORTHERN MEXICO...AND ALL OF THE
DYNAMICAL MODELS FORECAST THE CYCLONE TO CONTINUE MOVING GENERALLY
WEST-NORTHWESTWARD OR NORTHWESTWARD FOR THE NEXT TWO TO THREE DAYS
ALONG THE SOUTHWESTERN PERIPHERY OF THAT RIDGE.  THE SPREAD IN THE
TRACK GUIDANCE IS SIGNIFICANT THROUGH 72 HOURS AND BECOMES
PARTICULARLY LARGE BEYOND THAT TIME...SEEMINGLY IN PART BECAUSE THE
MODELS DO NOT AGREE ON THE STRENGTH AND DEPTH OF ISELLE.  SOME
MODELS INCLUDING THE GFS AND UKMET FORECAST THE CYCLONE TO HAVE
DISSIPATED BY DAY 3.  PRESUMING A WEAKENED ISELLE DOES LAST INTO
THE 3-5 DAY TIME FRAME...IT WILL PROBABLY NOT BE MOVING VERY FAST
SINCE IT WILL LIKELY BE TOO FAR EAST TO GET CAUGHT UP IN THE
LOW-LEVEL EASTERLY TRADES.  THE NEW OFFICIAL TRACK FORECAST IS VERY
SIMILAR TO THE PREVIOUS ONE...AND IS CLOSE TO THE CONSENSUS THROUGH
ABOUT 48 HOURS.
 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      14/0900Z 17.6N 108.6W    40 KT
 12HR VT     14/1800Z 18.3N 109.8W    45 KT
 24HR VT     15/0600Z 19.1N 110.9W    45 KT
 36HR VT     15/1800Z 19.9N 112.0W    45 KT
 48HR VT     16/0600Z 20.5N 113.1W    45 KT
 72HR VT     17/0600Z 21.0N 114.5W    40 KT
 96HR VT     18/0600Z 21.5N 116.0W    30 KT
120HR VT     19/0600Z 21.5N 117.5W    25 KT
 
$$
FORECASTER KNABB
 
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