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Tropical Storm NORBERT


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TROPICAL STORM NORBERT DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP152008
800 AM PDT SUN OCT 05 2008
 
NORBERT'S CLOUD PATTERN CONTINUES TO SHOW AN INCREASE IN
ORGANIZATION WITH THE CENTER NOW SITUATED BENEATH A CDO FEATURE. 
DVORAK SATELLITE INTENSITY ESTIMATES HAVE INCREASED TO 45 KT FROM
BOTH TAFB AND SAB BUT THE INITIAL INTENSITY IS SET SLIGHTLY
HIGHER...AT 50 KT...AS A COMPROMISE BETWEEN THE AFOREMENTIONED
SUBJECTIVE DVORAK ESTIMATES AND OBJECTIVE ESTIMATES FROM THE
UNIVERSITY OF WISCONSIN.  NORBERT IS EXPECTED TO BE OVER WARM
WATERS AND WITHIN A RELATIVELY LOW VERTICAL SHEAR ENVIRONMENT FOR
THE NEXT COUPLE OF DAYS SO FURTHER INTENSIFICATION IS ANTICIPATED. 
HOWEVER...THE AMOUNT OF INTENSIFICATION REMAINS UNCLEAR WITH A
SPREAD OF NEARLY 55 KT IN THE VARIOUS INTENSITY MODELS AT DAY 3. 
FURTHER COMPLICATING THE INTENSITY FORECAST IS THE UNCERTAINTY IN
FORWARD SPEED THAT ARISES AT THE EXTENDED PERIODS AS NORBERT
APPROACHES COOLERS WATERS AND A STABLE AIRMASS NORTH OF 20 DEGREES
NORTH LATITUDE.  GIVEN THE PRESENT TREND IN SATELLITE IMAGERY...THE
OFFICIAL FORECAST SHOWS NORBERT REACHING PEAK INTENSITY SOONER THAN
THE PREVIOUS ADVISORY AND IS IN CLOSEST AGREEMENT WITH THE LGEM AND
GFDL MODELS.   
 
THERE HAVE BEEN NO RECENT MICROWAVE PASSES RESULTING IN A SOMEWHAT
UNCERTAIN CENTER POSITION ESTIMATE.  SINCE THE OVERALL CLOUD MASS
APPEARS TO BE MOVING MORE TOWARDS THE WEST-NORTHWEST...THE INITIAL
MOTION IS NOW ESTIMATED TO BE 290/7.  A LARGE DEEP-LAYER TROUGH IS
CURRENTLY MOVING EASTWARD OVER THE WESTERN HALF OF THE U.S. WITH A
NARROW SLIVER OF A MID-LEVEL RIDGE EXTENDING WESTWARD FROM MEXICO
JUST TO NORTH OF NORBERT.  THE SHORT-TERM TRACK WILL BE DICTATED BY
WHETHER OR NOT THIS NARROW RIDGE REMAINS IN PLACE AS THE LARGER
TROUGH PASSES TO THE NORTH.  MOST OF THE DYNAMICAL MODELS MAINTAIN
SUFFICIENT RIDGING TO THE NORTH TO KEEP THE CYCLONE ON A GENERAL
WEST-NORTHWESTWARD HEADING FOR 2-3 DAYS.  ONLY THE UKMET MODEL
SHOWS THE RIDGE BREAKING DOWN ALLOWING NORBERT TO TRACK MORE
TOWARDS THE NORTH...CLOSER TO THE COAST OF MEXICO.  BY DAYS 4 AND
5...GLOBAL MODELS ALL FORECAST A MIDDLE TO UPPER-LEVEL CUTOFF LOW
TO FORM SEVERAL HUNDRED MILES WEST OF THE BAJA PENINSULA...
RESULTING IN THE CYCLONE TURNING TOWARD THE NORTHWEST OR NORTH. 
HOWEVER...THE MODELS HAVE CONSIDERABLE SPREAD IN FORWARD SPEED AT
THESE EXTENDED RANGES. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      05/1500Z 14.6N 101.9W    50 KT
 12HR VT     06/0000Z 14.8N 102.8W    60 KT
 24HR VT     06/1200Z 15.2N 104.0W    65 KT
 36HR VT     07/0000Z 15.6N 105.1W    70 KT
 48HR VT     07/1200Z 16.1N 106.3W    75 KT
 72HR VT     08/1200Z 17.0N 108.5W    75 KT
 96HR VT     09/1200Z 19.0N 111.0W    70 KT
120HR VT     10/1200Z 21.5N 112.5W    65 KT
 
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