Tropical Storm NORA

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TROPICAL STORM NORA DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP172009
200 AM PDT WED SEP 23 2009
 
DURING THE PAST SEVERAL HOURS...THE SYSTEM HAS BEEN TRANSITIONING
FROM A BLOB OF CENTRAL CONVECTION ON SATELLITE IMAGERY TO A SPIRAL
BANDING PATTERN...RESULTING IN A MORE ORGANIZED APPEARANCE.  A
QUIKSCAT PASS AT 0218 UTC SUGGESTED THE MAXIMUM WINDS WERE AT LEAST
30 KT AND AN ASCAT PASS AT 0537 UTC GENERALLY SHOWED STRONGER WINDS
THROUGHOUT THE CIRCULATION THAN THE QUIKSCAT...BUT STILL SHOWED A
PEAK WIND SPEED OF ABOUT 30 KT.  DVORAK SATELLITE ESTIMATES FROM
TAFB/SAB WERE 35 AND 30 KT...RESPECTIVELY...AT 0600 UTC...WITH AMSU
ESTIMATES OF ABOUT 40-45 KT.  AFTER CONSIDERING THE IMPROVED
SATELLITE APPEARANCE AND GENERAL LOW BIAS OF ASCAT...THE INITIAL
INTENSITY WILL BE SET TO 35 KT.
 
NORA APPEARS TO HAVE ABOUT 24-36 HOURS TO STRENGTHEN BEFORE
SHEAR INCREASES FROM THE SOUTHWEST OR WEST DUE TO A LARGE EASTERN
PACIFIC TROUGH.  THE REGIONAL HURRICANE MODELS...HWRF/GFDL...
CONTINUE TO INTENSIFY THE CYCLONE MORE THAN THE STATISTICAL
GUIDANCE...WITH THE HWRF MAKING NORA A HURRICANE BY TOMORROW.   
IT IS WORTH POINTING OUT THAT THE SHIPS RAPID INTENSIFICATION INDEX
DOES SHOW A 25 PERCENT CHANCE OF A 30 KT INCREASE DURING THE NEXT
24 HOURS...WHICH IS ABOUT 3 TIMES THE SAMPLE MEAN.  THE OFFICIAL
FORECAST IS INCREASED FROM THE PREVIOUS FORECAST BUT REMAINS CLOSER
TO THE STATISTICAL GUIDANCE AT THIS TIME.   IN THE LONGER-TERM...
THE HWRF/GFDL PROBABLY SHOW TOO STRONG OF A SYSTEM IN A SHEARED
ENVIRONMENT...A COMMON BIAS OF THOSE MODELS.  COOLER WATERS AND
STRONG SHEAR SHOULD PROMOTE WEAKENING OF NORA INTO A REMNANT LOW IN
ABOUT 3 DAYS.
 
A SERIES OF MICROWAVE PASSES OVERNIGHT HAS HELPED SET THE INITIAL
MOTION TO 310/7...THOUGH THE RECENT ASCAT DATA SUGGEST THE CENTER
COULD BE MOVING A LITTLE MORE TO THE LEFT.  A MIDDLE TO UPPER-LEVEL
RIDGE TO THE NORTH IS STEERING THE CYCLONE FOR THE TIME BEING. 
THERE IS A FAIR AMOUNT OF MODEL DISAGREEMENT ON WHETHER THE RIDGE
BUILDS WESTWARD...CAUSING A MORE WESTWARD TRACK OF NORA...OR IF THE
MID- TO UPPER-LEVEL TROUGH CURRENTLY OVER THE EASTERN PACIFIC
CAUSES A BREAK IN THE RIDGE...WHICH WOULD ALLOW THE CURRENT
NORTHWESTWARD MOTION TO CONTINUE.  THE GFS/GFDL/ECMWF/HWRF
GENERALLY FAVOR A WEST OR WEST-NORTHWEST TRACK FOR THE NEXT FEW
DAYS... WHILE THE NOGAPS AND CANADIAN MODELS SUPPORT A NORTHWESTWARD
MOTION.  MY INCLINATION IS TO LEAN TOWARD THE MORE EQUATORWARD
SOLUTION AS THIS SYSTEM SEEMS UNLIKELY TO GET STRONG ENOUGH TO FEEL
THE FULL EFFECTS OF THAT TROUGH.  THE OFFICIAL FORECAST IS SHIFTED
TO THE LEFT...BUT IS NOT AS FAR LEFT AS THE MODEL CONSENSUS DUE TO
CONTINUITY CONSIDERATIONS.  IT WOULD NOT BE SURPRISING IF THE
FORECAST SHIFTED FARTHER WESTWARD LATER TODAY. IN A FEW DAYS...A
MORE SIGNIFICANT BREAK IN THE RIDGE COULD ALLOW THE SYSTEM TO GAIN
LATITUDE.  HOWEVER...IT IS EXPECTED THAT THE CYCLONE WILL BE QUITE
WEAK BY THAT TIME AND UNABLE TO RESPOND TO THAT STEERING FLOW.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      23/0900Z 16.1N 116.7W    35 KT
 12HR VT     23/1800Z 16.6N 117.5W    45 KT
 24HR VT     24/0600Z 17.0N 118.4W    50 KT
 36HR VT     24/1800Z 17.2N 119.4W    45 KT
 48HR VT     25/0600Z 17.4N 120.6W    35 KT
 72HR VT     26/0600Z 17.7N 123.0W    25 KT...REMNANT LOW
 96HR VT     27/0600Z 18.0N 125.5W    20 KT...REMNANT LOW
120HR VT     28/0600Z...DISSIPATED
 
$$
FORECASTER BLAKE
 
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