000
WTNT44 KNHC 210252
TCDAT4
 
TROPICAL STORM IRENE DISCUSSION NUMBER   2
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
1100 PM AST SAT AUG 20 2011

SATELLITE IMAGES INDICATE THAT IRENE IS MAINTAINING A LARGE AREA OF
CURVED CONVECTIVE BANDING ON THE NORTHERN SIDE OF THE SURFACE
CIRCULATION.  THE GUADELOUPE RADAR CONTINUES TO SHOW THIS BANDING
ALONG WITH A DISTINCT MID-LEVEL VORTEX...WHICH IS ALSO VISIBLE IN
MICROWAVE IMAGERY.  THE INITIAL WIND SPEED IS KEPT AT 45 KT AND THE
NEXT RECONNAISSANCE AIRCRAFT IS SCHEDULED TO FLY THROUGH IRENE
AROUND 1200 UTC.  
 
THE INITIAL MOTION ESTIMATE IS ABOUT 280/19.  A MID-LEVEL RIDGE OVER
THE WESTERN ATLANTIC OCEAN IS EXPECTED TO SLOWLY WEAKEN OVER THE
NEXT COUPLE OF DAYS.  THIS PATTERN SHOULD STEER IRENE TOWARD THE
WEST-NORTHWEST WITH A GRADUAL DECELERATION.  AFTER THAT TIME...A
MID-LATITUDE TROUGH MOVING OFFSHORE OF THE SOUTHEASTERN UNITED
STATES SHOULD CAUSE A WEAKNESS IN THE SUBTROPICAL RIDGE.  HOW MUCH
OF A WEAKNESS IS STILL UP FOR DEBATE...HOWEVER...WITH A VARIETY OF
MODEL SOLUTIONS SHOWING LARGE TRACK SPREAD AT THE END OF THE
FORECAST PERIOD.  IN ADDITION...A FEW OF THE MODELS...SUCH AS THE
UKMET...SHOW MORE OF AN EMPHASIS ON A SECOND TROUGH MOVING INTO THE
SOUTHERN UNITED STATES AND DO NOT SHOW THE FIRST TROUGH AFFECTING
IRENE MUCH.  THE GUIDANCE HAS SHIFTED SOMEWHAT TO THE WEST IN THE
LATER PERIODS...AND IT SEEMS REASONABLE TO SHIFT THE NHC FORECAST
IN THAT DIRECTION.  GIVEN THE LARGE SPREAD IN THE GUIDANCE...THIS IS
A GOOD TIME TO REMIND USERS THAT THE AVERAGE DAY 4 AND 5 NHC TRACK
FORECAST ERRORS ARE 200 TO 250 MILES.  

WITH THE IMPROVED STRUCTURE ON MICROWAVE...STRENGTHENING IS LIKELY
AS THE STORM TRAVERSES VERY WARM WATERS WITH LIGHT SHEAR
CONDITIONS.  ALMOST ALL OF THE GUIDANCE SHOWS IRENE BECOMING A
HURRICANE IN A DAY OR TWO...AND THE NHC FORECAST HAS BEEN INCREASED
DURING THE FIRST 48 HOURS. AFTER THAT TIME THE INTENSITY FORECAST
IS HIGHLY DEPENDENT ON HOW MUCH LAND INTERACTION OCCURS WITH
HISPANIOLA AND CUBA. THE CURRENT TRACK TAKES THE SYSTEM OVER THE
SPINE OF CUBA BETWEEN 72 AND 96 HOURS...SO LITTLE INTENSIFICATION
IS EXPECTED DURING THAT TIME...WITH SOME STRENGTHENING EXPECTED
NEAR THE END OF THE PERIOD AFTER IRENE MOVES AWAY FROM LAND. IT IS
WORTH NOTING THAT THE MODELS THAT SHOW LESS LAND INTERACTION...SUCH
AS THE GFDL OR HWRF...HAVE IRENE BECOMING A MAJOR HURRICANE...AND
THAT IS NOT OUT OF THE QUESTION ESPECIALLY IF THE STORM MOVES ON
THE FAR LEFT OR RIGHT SIDE OF THE FORECAST CONE AND AVOIDS
SIGNIFICANT LAND INTERACTION. OVERALL...GIVEN THE TRACK FORECAST
UNCERTAINTY AND THE POTENTIAL FOR LAND INTERACTION...THE INTENSITY
FORECAST LATE IN THE PERIOD IS EVEN MORE UNCERTAIN THAN USUAL. 

A HURRICANE WATCH COULD BE REQUIRED FOR PUERTO RICO TOMORROW IF
THERE IS A SIGNIFICANT NORTHWARD ADJUSTMENT TO THE TRACK FORECAST.

OF NOTE...THIS IS THE THIRD EARLIEST TO HAVE NINE NAMED STORMS FORM
IN THE ATLANTIC BASIN...BEHIND 2005 AND 1936.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  21/0300Z 15.3N  59.9W   45 KT  50 MPH
 12H  21/1200Z 16.0N  62.7W   50 KT  60 MPH
 24H  22/0000Z 16.6N  65.9W   55 KT  65 MPH
 36H  22/1200Z 17.2N  68.4W   65 KT  75 MPH
 48H  23/0000Z 18.0N  70.7W   75 KT  85 MPH
 72H  24/0000Z 19.5N  75.0W   50 KT  60 MPH
 96H  25/0000Z 21.5N  78.0W   50 KT  60 MPH...INLAND
120H  26/0000Z 24.5N  80.5W   60 KT  70 MPH...OVER WATER
 
$$
FORECASTER BLAKE/BRENNAN