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| #451736 (Received by flhurricane at: 10:55 PM 20.Aug.2011) 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 |
