Tropical Storm LOWELL

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TROPICAL STORM LOWELL DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP132008
800 PM PDT MON SEP 08 2008
 
THE OBLIGATORY EVENING CONVECTIVE BURST HAS ONCE AGAIN DEVELOPED
OVER THE LOW-LEVEL CENTER LIKE IT HAS THIS TIME THE PAST 2 DAYS...
EXCEPT THIS EVENING A CURVED BAND HAS FORMED IN THE WESTERN
SEMICIRCLE. THIS CURVED BAND PATTERN HAS YIELDED AN OFFICIAL DVORAK
INTENSITY ESTIMATE OF T3.0/45 KT FROM BOTH TAFB AND SAB...WHEREAS A
SHEAR PATTERN YIELDS A DATA T-NUMBER OF T3.5/55 KT. THE INITIAL
INTENSITY OF 50 KT IS A BLEND OF THESE TWO T-NUMBER ESTIMATES.
 
THE INITIAL MOTION ESTIMATE IS 310/07. IT APPEARS THAT THE FORWARD
SPEED OF LOWELL HAS DECREASED DURING THE PAST 6 HOURS...WHICH IS IN
LINE WITH WHAT THE GLOBAL AND REGIONAL MODELS HAVE BEEN ADVERTISING
THE PAST 2 DAYS. A MID- TO UPPER-LEVEL TROUGH CONTINUES TO DROP
SOUTHWARD ALONG THE U.S. CALIFORNIA COAST...AND THIS FEATURE IS
EXPECTED TO GRADUALLY ERODE THE SUBTROPICAL RIDGE TO THE NORTH OF
LOWELL BY 24-36 HOURS. A WEAKNESS IS EXPECTED TO DEVELOP IN THE
RIDGE AXIS ALLOWING LOWELL TO MOVE NORTHWARD THROUGH THE RIDGE AND
EVENTUALLY RECURVE NORTHEASTWARD ACROSS BAJA CALIFORNIA AND
NORTHWESTERN MEXICO. NHC MODEL GUIDANCE IS IN REASONABLE AGREEMENT
ON THIS TRACK SCENARIO EVOLVING THROUGH 72 HOURS...BUT THEN DIVERGE
IN BOTH TRACK AND FORWARD SPEED AFTER THAT. IF LOWELL IS AS WEAK AS
FORECAST BY THE TIME IT REACHES BAJA CALIFORNIA...THE LOW-LEVEL
CIRCULATION WILL LIKELY GET STRIPPED AWAY BY THE MOUNTAINOUS
TERRAIN AND BE LEFT BEHIND OVER THE PACIFIC. AS A MINIMUM...
HOWEVER...THE SIERRA MADRE MOUNTAINS OVER NORTHWESTERN MEXICO WILL
KEEP THE LOW-LEVEL CENTER FROM RACING NORTHEASTWARD TO SOUTHERN
MINNESOTA LIKE THE HWRF MODEL IS PREDICTING. THE OFFICIAL FORECAST
IS JUST AN UPDATE OF THE PREVIOUS TRACK...AND IS CLOSE TO THE NHC
MODEL CONSENSUS.
 
SIMILAR TO THE PAST TWO EVENINGS...A LARGE CONVECTIVE BURST WITH
CLOUD TOPS AS COLD AS -85C HAS DEVELOPED OVER AND SOUTHWEST OF THE
LOW-LEVEL CENTER. LAST NIGHT'S INTENSIFICATION PROCESS WAS
DISRUPTED BY ENHANCED VERTICAL SHEAR CREATED BY EASTERLY OUTFLOW
EMANATING FROM A LARGE MESOSCALE CONVECTIVE SYSTEM THAT DEVELOPED
ALONG THE SIERRA MADRE MOUNTAINS TO THE EAST OF LOWELL. THE FEATURE
THAT TRIGGERED THAT MCS HAS SINCE MOVED NORTHWARD OVER THE GULF OF
CALIFORNIA AND SHOULD NOT BE A FACTOR TONIGHT. HOWEVER...DESPITE
WEAKENING SHEAR CONDITIONS FORECAST BY ALL THE MODELS...ALONG WITH
VERY MOIST MID-TROPOSPHERIC CONDITIONS...THE STATISTICAL SHIPS
INTENSITY MODEL AND THE VARIOUS DYNAMICAL MODELS ARE ALL PREDICTING
LOWELL WILL WEAKEN THROUGHOUT THE FORECAST PERIOD. THE OFFICIAL
INTENSITY FORECAST FOLLOWS THE PREVIOUS FORECAST TREND...BUT
REMAINS ABOVE THE SHIPS INTENSITY FORECAST...WHICH DISSIPATES
LOWELL IN ABOUT 72 HOURS.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      09/0300Z 20.2N 112.5W    50 KT
 12HR VT     09/1200Z 20.9N 113.1W    45 KT
 24HR VT     10/0000Z 21.7N 113.4W    45 KT
 36HR VT     10/1200Z 22.7N 113.2W    40 KT
 48HR VT     11/0000Z 23.7N 112.7W    35 KT
 72HR VT     12/0000Z 24.9N 111.8W    35 KT...INLAND OVER SRN BAJA
 96HR VT     13/0000Z 26.0N 111.0W    30 KT...OVER GULF OF CALIF
120HR VT     14/0000Z 27.3N 109.9W    30 KT...INLAND OVER MEXICO
 
$$
FORECASTER STEWART
 
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