Tropical Storm DANNY

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TROPICAL STORM DANNY DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL052009
1100 AM EDT WED AUG 26 2009
 
SATELLITE IMAGERY SHOWS THAT THE AREA OF DISTURBED WEATHER EAST OF
THE BAHAMAS HAS DEVELOPED A WELL-DEFINED LOW-LEVEL CIRCULATION...
WITH REPORTS FROM A NOAA HURRICANE HUNTER AIRCRAFT INDICATING THE
SYSTEM WAS BEST DEVELOPED BELOW 12000 FT.  QUIKSCAT DATA AND A FEW
OBSERVATIONS FROM THE STEPPED FREQUENCY MICROWAVE RADIOMETER SHOW
TROPICAL STORM-FORCE WINDS OCCURRING NORTH AND NORTHEAST OF THE
CENTER...WHICH IS THE BASIS FOR THE INITIAL INTENSITY OF 40 KT. 
THE STORM HAS BEEN TANGLED UP WITH AN UPPER-LEVEL LOW FOR THE PAST
24 HR...AND THE STRUCTURE AS MUCH RESEMBLES A SUBTROPICAL CYCLONE
AS A TROPICAL CYCLONE.  HOWEVER...CYCLONE PHASE ANALYSES FROM
FLORIDA STATE UNIVERSITY SUGGEST THAT DANNY IS MARGINALLY MORE
TROPICAL THAN SUBTROPICAL...HENCE THE DESIGNATION OF TROPICAL
STORM.

THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 295/16.  DANNY IS ON THE
NORTH SIDE OF A MID/UPPER-LEVEL LOW CENTERED OVER HISPANIOLA AND
SOUTHWEST OF A MID/UPPER-LEVEL RIDGE CENTER EAST OF BERMUDA.
THIS PATTERN SHOULD STEER DANNY GENERALLY NORTHWESTWARD WITH
DECELERATION FOR THE NEXT 24-36 HR.  AFTER THAT...THE LARGE SCALE
MODELS AGREE THAT A MID/UPPER-LEVEL TROUGH SHOULD DEVELOP OVER THE
GREAT LAKES...WITH A MID/UPPER-LEVEL LOW OVER THE GULF COAST STATES
MOVING EASTWARD OR NORTHEASTWARD.  THIS EVOLUTION SHOULD CAUSE
DANNY TO TURN NORTHWARD AND THEN RECURVE INTO THE WESTERLIES. 
WHILE THE MODELS AGREE ON THE SCENARIO...THERE IS ENOUGH SPREAD IN
THE TRACK GUIDANCE TO MAKE IT UNCERTAIN WHAT LAND AREA DANNY MAY
AFFECT.  THE NAM AND THE CANADIAN MODELS FORECAST DANNY TO MAKE
LANDFALL IN EASTERN NORTH CAROLINA IN ABOUT 72 HR AND MOVE UP THE
U. S. EASTERN SEABOARD...WHILE THE GFS...HWRF... AND GFDL SHOW A
MORE EASTERLY TRACK THAT KEEPS DANNY OFFSHORE UNTIL IT REACHES THE
CANADIAN MARITIMES.  THE FORECAST SPLITS THE DIFFERENCE BETWEEN
THESE EXTREMES AND IS A LITTLE TO THE LEFT OF THE MODEL CONSENSUS.
THE FORECAST TRACK IS ROUGHLY PARALLEL TO THE U. S. EAST COAST...AND
ANY DEVIATION FROM THE TRACK COULD MAKE A LARGE DIFFERENCE IN WHAT
AREAS GET IMPACTED BY DANNY.  THEREFORE...IT IS IMPORTANT NOT TO
FOCUS TOO MUCH ON THE EXACT FORECAST TRACK.
 
THE INTENSITY FORECAST IS PROBLEMATIC.  WATER VAPOR IMAGERY SHOWS
UPPER-LEVEL LOWS NORTH AND SOUTH OF DANNY CONNECTED BY A CYCLONIC
SHEAR AXIS.  THIS IS NOT A IDEAL PATTERN FOR STRENGTHENING...AND
THE LARGE-SCALE MODELS SUGGEST IT MAY TAKE 24-36 HR FOR THE
ENVIRONMENT TO BECOME MORE FAVORABLE.  BASED ON THIS AND THE
CURRENT ORGANIZATION OF THE STORM...THE INTENSITY FORECAST CALLS
FOR SLOW STRENGTHENING FOR THE FIRST 36 HR.  IT THEN SHOWS A FASTER
STRENGTHENING FROM 36-48 HR AS CONDITIONS BECOME MORE FAVORABLE. 
AFTER 48 HR...STRONG UPPER-LEVEL SOUTHWESTERLY WINDS ASSOCIATED
WITH THE EASTERN U. S. TROUGHS ARE EXPECTED TO CAUSE INCREASED
SHEAR AND THE BEGINNING OF EXTRATROPICAL TRANSITION.  THE INTENSITY
FORECAST THUS CALLS FOR DANNY TO REACH 65 KT IN 72 HR...THEN BECOME
EXTRATROPICAL BY 96 HR.  THE INTENSITY FORECAST IS IN BEST
AGREEMENT WITH THE SHIPS AND LGEM MODELS.  HOWEVER...IT IS WEAKER
THAN THE FORECASTS OF THE GFDL AND HWRF.


FORECAST POSITIONS AND MAX WINDS
 
INITIAL      26/1500Z 24.9N  70.3W    40 KT
 12HR VT     27/0000Z 25.8N  72.0W    40 KT
 24HR VT     27/1200Z 26.8N  73.6W    45 KT
 36HR VT     28/0000Z 28.1N  74.5W    50 KT
 48HR VT     28/1200Z 30.1N  75.2W    60 KT
 72HR VT     29/1200Z 36.0N  73.5W    65 KT
 96HR VT     30/1200Z 45.0N  67.0W    65 KT...EXTRATROPICAL
120HR VT     31/1200Z 52.0N  53.0W    60 KT...EXTRATROPICAL
 
$$
FORECASTER BEVEN
 
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