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| #536037 (Received by flhurricane at: 4:53 AM 22.Aug.2012) TCDAT4 TROPICAL STORM ISAAC DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012 500 AM AST WED AUG 22 2012 SATELLITE IMAGERY INDICATES A LARGE CENTRAL DENSE OVERCAST FEATURE HAS DEVELOPED OVER THE LOW-LEVEL CENTER...WHICH INCLUDES SOME COLD CLOUD TOPS OF -82 TO -86C VERY NEAR THE CENTER. HOWEVER...THE SHARP INCREASE IN INNER-CORE CONVECTION HAS NOT YET TRANSLATED INTO ANY SIGNIFICANT INTENSIFICATION. AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INVESTIGATING ISAAC THIS MORNING HAS ONLY FOUND THE CENTRAL PRESSURE TO BE 1003 MB AND 850 MB FLIGHT-LEVEL WINDS OF 47 KT...WHICH EQUATES TO ABOUT A 40-KT INTENSITY ESTIMATE. SFMR WINDS IN VERY HEAVY RAIN HAVE BEEN AS HIGH AS 61 KT. THE BIAS-ADJUSTED SFMR WINDS HAVE BEEN RUNNING AS HIGH AS 54 KT...BUT EVEN THOSE VALUES APPEAR TO BE INFLATED. UNTIL THE AIRCRAFT COMPLETES ITS MISSION... THE INITIAL INTENSITY WILL REMAIN AT 40 KT BASED ON THE 1003 MB PRESSURE-WIND RELATIONSHIP. THE INITIAL MOTION ESTIMATE IS 270/15 KT. THE INITIAL POSITION IS A BLEND OF THE RECON FIXES AND SATELLITE POSITIONS...AND CONTINUITY WITH PREVIOUS FORECAST TRACK. THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. THE EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG 30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. THROUGH 72 HOURS OR SO. AFTER THAT...A BROAD MID- TO UPPER-LEVEL TROUGH CURRENTLY LOCATED OVER THE CENTRAL U.S. FROM THE GREAT LAKES SOUTHWARD TO THE CENTRAL GULF COAST IS EXPECTED TO DIG SOUTHWARD OVER THE NEXT 24 HOURS BEFORE MOVING EASTWARD. THIS SHOULD RESULT IN AN EROSION OF THE RIDGE ACROSS THE EASTERN GULF OF MEXICO AND THE SOUTHEASTERN U.S. INCLUDING FLORIDA AND THE NORTHWESTERN BAHAMAS. THIS DEVELOPING WEAKNESS IN THE RIDGE SHOULD ALLOW ISAAC TO TURN NORTHWESTWARD ACROSS EASTERN AND CENTRAL CUBA BY DAYS 4 AND 5. THE NHC MODEL GUIDANCE IS IN EXCELLENT AGREEMENT ON THIS EVOLVING SCENARIO WITH THE ECMWF BEING THE LEFTMOST OF THE MODELS...AND THE GFDL AND HWRF BEING THE RIGHTMOST OF THE MODELS. GIVEN THE RUN-TO-RUN CONSISTENCY OF THE GFS AND UKMET MODELS...MORE WEIGHT HAS BEEN PLACED ON THOSE SOLUTIONS...ESPECIALLY SINCE THEY HAVE A STRONGER AND VERTICALLY DEEPER CYCLONE DEPICTED IN THE MODELS. SHEAR ANALYSES FROM THE GFS-BASED SHIPS INTENSITY MODEL AND UW-CIMSS INDICATE BOTH THE DEEP-LAYER AND MID-LEVEL SHEAR VALUES HAVE DROPPED BELOW 10 KT...WHICH HAS FINALLY ALLOWED DEEP CONVECTION TO DEVELOP AND PERSIST OVER THE LOW-LEVEL CENTER. THE SHEAR IS FORECAST TO GRADUALLY DECREASE TO VALUES BELOW 5 KT BY 24 HOURS... WHICH SHOULD ALLOW ISAAC TO STRENGTHEN TO A HURRICANE BY 48 HOURS. AT 72 AND 96 HOURS...THE INTENSITY HAS BEEN DECREASED DUE TO EXPECTED LAND INTERACTION WITH HAITI AND EASTERN CUBA. AFTER 96 HOURS...HOWEVER...ISAAC SHOULD BE MOVING BACK OVER THE VERY WARM WATER BETWEEN CUBA AND FLORIDA. WITH SEA-SURFACE TEMPERATURES EXPECTED TO BE NEAR 30C AND THE GFS AND UKMET MODELS DEPICTING A EXTENSIVE UPPER-LEVEL OUTFLOW REGIME COMPLETE WITH DUAL OUTFLOW CHANNELS TO THE NORTHWEST AND SOUTHEAST...STRENGTHENING APPEARS LIKELY. THE OFFICIAL INTENSITY REMAINS ON THE CONSERVATIVE SIDE DUE TO UNCERTAINTY IN HOW LONG AND HOW MUCH ISAAC WILL INTERACT WITH HISPANIOLA AND CUBA...AND IS CLOSE TO A BLEND OF THE SHIPS... DECAY-SHIPS...AND LGEM STATISTICAL-DYNAMICAL INTENSITY MODELS. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 15.5N 57.3W 40 KT 45 MPH 12H 22/1800Z 15.9N 59.7W 45 KT 50 MPH 24H 23/0600Z 16.3N 62.8W 55 KT 65 MPH 36H 23/1800Z 16.8N 65.7W 65 KT 75 MPH 48H 24/0600Z 17.3N 68.4W 75 KT 85 MPH 72H 25/0600Z 18.8N 73.2W 65 KT 75 MPH...OVER WATER 96H 26/0600Z 21.8N 77.1W 65 KT 75 MPH...OVER WATER 120H 27/0600Z 24.8N 80.0W 80 KT 90 MPH $$ FORECASTER STEWART |
