| HURRICANE IRENE DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011 1100 PM AST MON AUG 22 2011 AFTER DEEPENING QUICKLY EARLIER THIS EVENING...THE INTENSITY OF IRENE APPEARS TO HAVE LEVELED OFF A BIT OVER THE PAST HOUR OR TWO. THE CENTRAL PRESSURE HAS DECREASED TO 980 MB ...AND MAXIMUM WINDS HAVE NOT INCREASED SINCE THE EARLIER MAXIMUM OF 106 KT AT 850 MB. THEREFORE...THE INTENSITY WILL REMAIN 85 KT FOR THIS ADVISORY. OVERALL THE STRUCTURE OF THE CYCLONE HAS CONTINUED TO IMPROVE... WITH A LARGE CDO FORMING NEAR AND NORTH OF THE CENTER AND THE AIRCRAFT REPORTING A CLOSED EYEWALL. SOUTHERLY SHEAR OF ABOUT 10 KT IS PRESENT OVER IRENE ACCORDING TO SATELLITE ANALYSIS FROM UW-CIMSS AND THE SHIPS MODEL...BUT OTHERWISE THE ENVIRONMENT APPEARS CONDUCIVE FOR CONTINUED STRENGTHENING. THE NHC INTENSITY FORECAST IS UNCHANGED FROM THE PREVIOUS SPECIAL ADVISORY AND SHOWS IRENE BECOMING A MAJOR HURRICANE WITHIN 24 HOURS AND REACHING A PEAK OF 115 KT IN 72 HOURS...FOLLOWED BY SLOW WEAKENING LATE IN THE PERIOD. THE NHC FORECAST IS HIGHER THAN MOST OF THE GUIDANCE THROUGH 48 HOURS AND IS CLOSE TO A BLEND OF THE GFDL AND HWRF MODELS AFTER THAT TIME. AIRCRAFT FIXES INDICATE THAT THE CENTER WAS LOCATED A LITTLE TO THE RIGHT OF THE PREVIOUS TRACK AND THAT THE FORWARD MOTION HAS SLOWED A BIT...WITH AN INITIAL MOTION ESTIMATE OF 295/10. OVERALL THE TRACK FORECAST SCENARIO HAS NOT CHANGED...AS IRENE WILL CONTINUE MOVING WEST-NORTHWESTWARD FOR THE NEXT DAY OR SO ON THE SOUTHWEST PERIPHERY OF A WEAKENING MID-LEVEL RIDGE. A MID- TO UPPER-LEVEL TROUGH LOCATED EAST OF NORTH AMERICA WILL LIFT OUT WITHIN 24 HOURS...LEAVING A PRONOUNCED WEAKNESS IN THE RIDGE OVER THE BAHAMAS. THIS SHOULD ALLOW IRENE TO GRADUALLY TURN NORTHWESTWARD AND NORTH-NORTHWESTWARD BY DAYS 2 AND 3. AFTER THAT TIME...THE TRACK OF IRENE APPEARS TO BE SENSITIVE TO THE TIMING AND AMPLITUDE OF SEVERAL SHORTWAVE TROUGHS MOVING EASTWARD ACROSS THE UNITED STATES/CANADIAN BORDER. THE MODELS ARE SHOWING SOME RUN-TO-RUN VARIABILITY IN HOW MUCH THESE SHORTWAVES WILL AMPLIFY AND DEEPEN THE MEAN TROUGH OVER THE NORTHEASTERN UNITED STATES...WHICH WILL BE CRITICAL TO HOW SOON IRENE TURNS NORTHWARD OR EVEN EAST OF DUE NORTH AT DAYS 4 AND 5. OVERALL...THE SPREAD OF THE GUIDANCE SUITE HAS DECREASED THIS CYCLE...AND THE COMBINATION OF THE SLIGHT RIGHTWARD ADJUSTMENT OF THE INITIAL POSITION AND A SLIGHT WESTWARD SHIFT IN THE MODEL CONSENSUS RESULTS IN ONLY MINOR ADJUSTMENTS TO THE NEW NHC TRACK FORECAST. THE BIGGEST CHANGE IS A SLOWER FORWARD SPEED AT THE END OF THE PERIOD FOLLOWING THE BULK OF THE GUIDANCE THAT KEEPS THE CYCLONE OFFSHORE AT DAY 5. IT IS IMPORTANT TO REMIND USERS NOT TO FOCUS ON THE EXACT FORECAST TRACK...ESPECIALLY AT DAYS 4 AND 5...SINCE THE MOST RECENT 5-YEAR AVERAGE ERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES... RESPECTIVELY. OF NOTE...SYNOPTIC SURVEILLANCE FLIGHTS FROM THE NOAA G-IV AND AN AIR FORCE RESERVE C-130 AIRCRAFT PROVIDED NUMEROUS DROPSONDES FOR ASSIMILATION INTO THE 0000 UTC MODEL CYCLE. THIS SHOULD IMPROVE THE ANALYSIS OF THE ENVIRONMENT AROUND AND UPSTREAM OF IRENE. |