000
WTPZ42 KNHC 240237
TCDEP2
TROPICAL STORM NORA DISCUSSION NUMBER   5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP172009
800 PM PDT WED SEP 23 2009
 
THE CONVECTIVE STRUCTURE OF NORA HAS CHANGED SUBSTANTIALLY OVER THE
PAST SIX HOURS.  THE PROMINENT BAND OF CONVECTION THAT WAS OVER THE
WESTERN PART OF THE CIRCULATION HAS DISSIPATED...LEAVING A CENTRAL
DENSE OVERCAST THAT HAS INCREASED A LITTLE IN SIZE.  UPPER-LEVEL
CIRRUS MOTIONS AROUND NORA SUGGEST THAT SOME WEST-SOUTHWESTERLY
SHEAR HAS BEGUN TO IMPACT THE SYSTEM...AND THE MID- AND LOW-LEVEL
CIRCULATIONS MAY BE A BIT SEPARATED.  NONETHELESS...DVORAK
ESTIMATES FROM TAFB AND SAB ARE 45 KT AND 55 KT...RESPECTIVELY...
AND THE OBJECTIVE ESTIMATE FROM THE UW-CIMSS ADT IS 55 KT.  A BLEND
OF THESE ESTIMATES SUPPORTS THE INITIAL INTENSITY OF 50 KT.
ALTHOUGH THE SHEAR APPEARS TO BE INCREASING...THE INTENSITY MODELS
INDICATE THAT SOME ADDITIONAL STRENGTHENING IS POSSIBLE.
HOWEVER...NONE OF THE GUIDANCE...NOW INCLUDING THE GFDL AND HWRF...
SHOWS NORA BECOMING A HURRICANE.  THE STORM IS LOCATED BENEATH A
NARROW UPPER-LEVEL RIDGE BUT IS PRECARIOUSLY CLOSE TO STRONG UPPER
LEVEL WESTERLIES TO ITS NORTH AND NORTHWEST.  THESE WINDS SHOULD
ULTIMATELY CAUSE STEADY WEAKENING TO OCCUR AFTER 24 HOURS AS THE
SYSTEM MOVES WESTWARD.  THE INTENSITY MODELS ARE IN GOOD AGREEMENT
FOR MOST OF THE FORECAST PERIOD...AND THE NHC FORECAST IS UNCHANGED
FROM THE PREVIOUS ADVISORY.
 
THE INITIAL MOTION OF 295/6 IS A LITTLE UNCERTAIN SINCE IT IS NOT
READILY APPARENT HOW MUCH THE LOW-LEVEL CENTER IS SEPARATED FROM
THE MID-LEVEL CENTER.  NOW THAT THE GFDL AND HWRF NO LONGER
INDICATE AS MUCH STRENGTHENING...THEIR TRACKS DO NOT SHOW A
PRONOUNCED NORTHWESTWARD MOTION AND ARE CLOSER TO THE REST OF THE
MODEL GUIDANCE.  AS A RESULT...THE MODEL CONSENSUS TVCN HAS SHIFTED
SIGNIFICANTLY TO THE SOUTH...WITH MOST OF THE MODELS NOW AGREEING
ON A WEST OR EVEN WEST-SOUTHWESTWARD MOTION DURING THE NEXT 72
HOURS.  THE NEW OFFICIAL FORECAST HAS ALSO BEEN SHIFTED A LITTLE
FARTHER SOUTH...BUT IT IS LOCATED ALONG THE NORTHERN EDGE OF THE
GUIDANCE ENVELOPE.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      24/0300Z 17.0N 119.1W    50 KT
 12HR VT     24/1200Z 17.3N 120.0W    55 KT
 24HR VT     25/0000Z 17.4N 121.1W    55 KT
 36HR VT     25/1200Z 17.3N 122.1W    50 KT
 48HR VT     26/0000Z 17.2N 123.3W    40 KT
 72HR VT     27/0000Z 17.5N 126.0W    30 KT
 96HR VT     28/0000Z 19.0N 128.5W    25 KT...REMNANT LOW
120HR VT     29/0000Z 20.0N 131.0W    20 KT...REMNANT LOW
 
$$
FORECASTER BERG