Tropical Storm LOWELL

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TROPICAL STORM LOWELL DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP132008
800 AM PDT MON SEP 08 2008
 
MORNING VISIBLE SATELLITE IMAGERY AND A RECENT TRMM MICROWAVE
OVERPASS SHOW THAT THE CENTER OF LOWELL REMAINS EXPOSED TO THE
NORTHEAST OF A CONVECTIVE BURST.  ANALYSES FROM CIMSS AT THE
UNIVERSITY OF WISCONSIN INDICATE THAT THE STORM IS UNDERGOING ABOUT
20 KT OF EASTERLY VERTICAL WIND SHEAR.  SATELLITE INTENSITY
ESTIMATES FROM TAFB AND SAB ARE 35 AND 45 KT RESPECTIVELY.  IN
ADDITION...JUST-RECEIVED QUIKSCAT DATA SHOW 40 KT WINDS...AND THERE
WAS AN AMSU INTENSITY ESTIMATES OF 43 KT AT 0926Z.  BASED ON THE
ABOVE...THE INITIAL INTENSITY IS REDUCED TO 45 KT.

THE INITIAL MOTION IS 305/8.  LOWELL IS CURRENTLY BEING STEERED BY A
MID-LEVEL RIDGE OVER NORTHERN MEXICO.  ALL OF THE DYNAMICAL MODELS
FORECAST A MID/UPPER-LEVEL TROUGH TO AMPLIFY SOUTHWARD ALONG THE
WEST COAST OF THE UNITED STATES DURING THE NEXT COUPLE OF
DAYS...WHICH SHOULD TURN LOWELL TOWARD THE NORTH AND EVENTUALLY
NORTHEAST.  WHILE THE GUIDANCE AGREES ON THIS SCENARIO...THERE IS
MAJOR DISAGREEMENT ON HOW FAST LOWELL WILL MOVE AFTER RECURVATURE. 
THE HWRF...BAMD...AND LBAR ALL MOVE THE THE STORM VERY RAPIDLY...
WITH 120 HR POSITIONS OVER THE CENTRAL UNITED STATES.  ON THE OTHER
HAND...THE GFS...UKMET...AND BAMS KEEP THE LOW-LEVEL CENTER WEST
AND SOUTHWEST OF BAJA CALIFORNIA THROUGH 120 HR.  THE DIFFERENCE
BETWEEN THE BAMS AND BAMD MODELS IS STRIKING AND SUGGESTS THE
POSSIBILITY THAT LOWELL COULD SHEAR APART AFTER 48 HR.  THE NEW
FORECAST TRACK IS A LITTLE FASTER THAN THE SLOWEST MODELS...CALLING
FOR LOWELL TO REACH BAJA CALIFORNIA IN ABOUT 4 DAYS AND THEN MOVE
INTO THE GULF OF CALIFORNIA.  IT IS A LITTLE TO THE RIGHT OF THE
PREVIOUS TRACK BASED ON THE INITIAL POSITION AND MOTION...AND IT IS
IN THE BEST AGREEMENT WITH THE GFDL AND ECMWF.

THE DYNAMICAL MODELS FORECAST THE CURRENT SHEAR TO DIMINISH IN ABOUT
24 HOUR.  HOWEVER...LOWELL IS FORECAST TO MOVE TOWARD COOLER SEA
SURFACE TEMPERATURES DURING THE NEXT 48 HR...AND THIS SHOULD REDUCE
THE CHANCE OF INTENSIFICATION EVEN WHEN THE SHEAR DECREASES.  AFTER
RECURVATURE...LOWELL MAY FIND WARMER WATER...BUT IS ALSO EXPECTED
TO ENCOUNTER INCREASING SOUTHWESTERLY SHEAR..  THE INTENSITY
GUIDANCE IS IN GOOD AGREEMENT ON A SLOW WEAKENING DURING THE
FORECAST PERIOD.  THUS...THE NEW INTENSITY FORECAST ALSO CALLS FOR
A GRADUAL WEAKENING...WITH THE NEW INTENSITIES A LITTLE ABOVE ALL
THE GUIDANCE.  AN ALTERNATIVE SCENARIO WOULD BE FOR LOWELL TO SHEAR
APART AFTER 48 HR...WHICH WOULD LEAD TO DISSIPATION BEFORE THE END
OF THE FORECAST PERIOD.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      08/1500Z 19.0N 111.2W    45 KT
 12HR VT     09/0000Z 19.8N 112.0W    45 KT
 24HR VT     09/1200Z 20.7N 113.0W    45 KT
 36HR VT     10/0000Z 21.5N 113.5W    40 KT
 48HR VT     10/1200Z 22.4N 113.5W    40 KT
 72HR VT     11/1200Z 23.5N 112.5W    35 KT
 96HR VT     12/1200Z 25.0N 111.0W    30 KT...INLAND
120HR VT     13/1200Z 26.0N 110.0W    30 KT
 
$$
FORECASTER BEVEN
 
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