[AARA] TROPICAL STORM ISAAC DISCUSSION NUMBER 3

Herman Campbell kn5grk at lusfiber.net
Tue Aug 21 20:35:19 CDT 2012


*Sent:* Tuesday, August 21, 2012 6:59 PM
*Subject:* TROPICAL STORM ISAAC DISCUSSION NUMBER 3
Model runs courtesy of Weather Underground (www.wunderground.com 
<http://www.wunderground.com>)
WTNT44 KNHC 212051

TROPICAL STORM ISAAC DISCUSSION NUMBER3
NWS NATIONAL HURRICANE CENTER MIAMI FLAL092012
500 PM AST TUE AUG 21 2012

AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING THE
DEPRESSION REPORTED 44-KT FLIGHT-LEVEL WINDS AT 1000 FT TO THE
NORTHEAST OF THE CENTER...AS WELL AS BIAS-CORRECTED SFMR SURFACE
WINDS ESTIMATES OF 35 KT.THE MINIMUM CENTRAL PRESSURE WAS NEAR
1005 MB.BASED ON THESE DATA...THE DEPRESSION IS UPGRADED TO
TROPICAL STORM ISAAC.DESPITE THE UPGRADE...THE CYCLONE REMAINS
SOMEWHAT DISORGANIZED...WITH THE LOW-LEVEL CENTER PARTLY EXPOSED TO
THE NORTH OF THE MAIN CONVECTIVE MASS.

THE LOW-LEVEL CENTER HAS NUDGED NORTHWARD DURING THE AFTERNOON...BUT
THE LONGER TERM MOTION IS 275/15.ISAAC REMAINS SOUTH OF A
DEEP-LAYER SUBTROPICAL RIDGE THAT IS FORECAST TO REMAIN INTACT
FOR THE NEXT 72 HR...WHICH SHOULD KEEP THE CYCLONE MOVING GENERALLY
WESTWARD AS SHOWN BY THE TIGHTLY CLUSTERED TRACK MODEL GUIDANCE.
AFTER THAT...THE GLOBAL MODELS CONTINUE TO FORECAST A SHORTWAVE
TROUGH TO DIG SOUTHWARD OVER THE SOUTHEASTERN U.S. AND THE GULF OF
MEXICO...WHICH IS EXPECTED TO WEAKEN THE RIDGE ACROSS FLORIDA AND
THE BAHAMAS.THIS SHOULD ALLOW THE CYCLONE TO TURN WEST-
NORTHWESTWARD AND SLOW DOWN OVER THE NORTH-CENTRAL CARIBBEAN SEA AND
THE GREATER ANTILLES.THE GUIDANCE HAS BECOME LESS CONSISTENT
SINCE THE PREVIOUS ADVISORY.WHILE THE CENTER OF THE GUIDANCE
ENVELOPE HAS CHANGED LITTLE...THE ECMWF HAS SHIFTED WELL TO THE
LEFT OF ITS PREVIOUS FORECAST AND NOW CALLS FOR A WESTWARD MOTION
AFTER ISAAC PASSES HISPANIOLA.THE GFS ENSEMBLES HAVE ALSO SHIFTED
WESTWARD...ALTHOUGH THE ENSEMBLE MEAN STILL LIES NEAR THE FORECAST
TRACK.THE NEW TRACK IS NUDGED A LITTLE TO THE NORTH OF THE
PREVIOUS TRACK BEFORE 72 HR...AND THEN IS NUDGED A LITTLE TO THE
RIGHT THEREAFTER.THE TRACK FORECAST AFTER 72 HR LIES NEAR THE
LEFT EDGE OF THE GUIDANCE ENVELOPE...WITH ONLY THE ECMWF WELL TO
THE LEFT OF THE CURRENT FORECAST.USERS ARE CAUTIONED THAT FIVE-DAY
TRACK ERRORS AVERAGE ABOUT 225 N MI.

ISAAC CONTINUES TO EXPERIENCE SOME LIGHT NORTHEASTERLY VERTICAL WIND
SHEAR...AND IT MAY NOT YET HAVE FULLY MIXED OUT THE DRY AIR THAT
HAS BEEN AFFECTING IT.THE SHEAR IS FORECAST TO DIMINISH IN 12-24
HR...WHICH SHOULD ALLOW SOME STRENGTHENING.THAT BEING SAID...THE
GFDL AND HWRF MODELS HAVE BACKED OFF ON THEIR FORECASTS OF
INTENSIFICATION...AND THE SHIPS AND LGEM MODELS ARE ALSO LESS
BULLISH.BASED ON THIS...THE NEW INTENSITY FORECAST SHOWS A LITTLE
LESS STRENGTHENING THAN THE PREVIOUS ADVISORY.THE INTENSITY
FORECAST AFTER 72 HR HAS AGAIN BEEN REDUCED DUE TO THE FORECAST
TRACK SHOWING INCREASED LAND INTERACTION WITH HISPANIOLA AND CUBA.

FORECAST POSITIONS AND MAX WINDS

INIT21/2100Z 15.4N53.9W35 KT40 MPH
12H22/0600Z 15.6N56.4W40 KT45 MPH
24H22/1800Z 15.9N59.8W45 KT50 MPH
36H23/0600Z 16.3N63.1W50 KT60 MPH
48H23/1800Z 16.6N66.3W65 KT75 MPH
72H24/1800Z 17.0N71.0W75 KT85 MPH
96H25/1800Z 19.5N75.0W75 KT85 MPH
120H26/1800Z 21.5N78.0W65 KT75 MPH...INLAND

$$

FORECASTER BEVEN
WWWW

*Paul T. Rappmundt**
St. Mary Parish OEP **
Deputy Director**
985-395-7428 Office
***

*985-312-9183 Cell*






[Image of 5-day forecast and coastal areas under a warning or a watch]




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