000WTNT44 KNHC 220259TCDAT4TROPICAL STORM IRENE DISCUSSION NUMBER 6NWS NATIONAL HURRICANE CENTER MIAMI FL AL0920111100 PM AST SUN AUG 21 2011IRENE HAS CONTINUED TO BECOME BETTER ORGANIZED OVER THE PAST FEWHOURS...AS THE CIRCULATION HAS BECOME BETTER DEFINED AND RADIALVELOCITIES HAVE INCREASED ON THE SAN JUAN DOPPLER RADAR. THERE HASALSO BEEN A MARKED INCREASE IN DEEP CONVECTION NEAR THE CENTER ONSATELLITE IMAGERY. DATA FROM THE AIR FORCE RESERVE HURRICANEHUNTER AIRCRAFT INDICATE THAT THE CENTRAL PRESSURE HAS FALLEN TO993 MB. THE INITIAL INTENSITY IS SET AT 60 KT BASED ON A PEAKBELIEVABLE SFMR VALUE OF 61 KT AND AN EXPANDING AREA OF 65 TO 70 KTDOPPLER VELOCITIES AT 5500 TO 6000 FEET ON THE SAN JUAN RADAR.UPPER-LEVEL OUTFLOW IS WELL ESTABLISHED IN ALL QUADRANTS EXCEPT THESOUTHEAST AND ATMOSPHERIC AND OCEANIC CONDITIONS APPEAR FAVORABLEFOR INTENSIFICATION...SO THE INTENSITY OF IRENE WILL LARGELY BEDETERMINED BY HOW MUCH THE CYCLONE INTERACTS WITH LAND IN THE NEXT24 TO 36 HOURS. SOME INTENSIFICATION IS STILL EXPECTED IN THE NEXTFEW HOURS...AND IRENE COULD BECOME A HURRICANE BEFORE MOVING OVERPUERTO RICO. THE CYCLONE IS THEN EXPECTED TO MOVE ACROSS THENORTHERN PORTION OF HISPANIOLA BETWEEN 24 AND 36 HOURS...AND SOMEWEAKENING IS INDICATED DURING THAT TIME. HOWEVER...IF IRENE MOVESNORTH OF HISPANIOLA...IT COULD BE STRONGER THAN INDICATED HERE IN36 TO 48 HOURS...OR WEAKER THAN FORECAST IF IT MOVES OVER MORE OFTHE HIGH TERRAIN OF THAT ISLAND. BEYOND 48 HOURS GRADUALSTRENGTHENING IS EXPECTED AS IRENE MOVES OVER WARM WATERS NEAR THEBAHAMAS...BUT THE UNCERTAINTY REMAINS VERY HIGH LATE IN THE PERIODDUE TO THE POTENTIAL FOR LAND INTERACTION IN THE SHORT TERM.WHILE THE FORWARD MOTION OF THE CENTER APPEARED TO HAVE SLOWED ONRADAR IMAGERY AS THE CYCLONE ORGANIZED DURING THE EVENING HOURS...THE LONG TERM INITIAL MOTION ESTIMATE IS TOWARD 285 DEGREES AT ABOUT13 KNOTS. THE SUBTROPICAL RIDGE CURRENTLY SITUATED NORTH OF THECYCLONE WILL BREAK DOWN OVER THE NEXT DAY OR SO AS A LONGWAVEMID-LEVEL TROUGH AMPLIFIES OVER THE WESTERN ATLANTIC. THE TROUGHBEGINS TO LIFT OUT AFTER ABOUT 36 HOURS OR SO...BUT LEAVES APRONOUNCED WEAKNESS IN THE RIDGE OVER THE SOUTHWESTERN ATLANTIC ANDTHE BAHAMAS. MOST OF THE GUIDANCE...WITH THE EXCEPTION OF THE GFDLAND UKMET...ARE IN RELATIVELY GOOD AGREEMENT ON TURNING IRENENORTHWESTWARD INTO THE BREAK IN THE RIDGE IN 2 OR 3 DAYS. HOWEVER...SIGNIFICANT SPREAD REMAINS IN THE SHARPNESS OF THIS TURNIN THE MODEL SUITE. FOR THIS CYCLE THE CONSENSUS TRACK MODELS HAVESHIFTED BACK A LITTLE TO THE LEFT...WITH THE GFS...ECMWF...AND HWRFMODELS ON THE EAST SIDE OF THE GUIDANCE ENVELOPE AND THEAFOREMENTIONED UKMET AND GFDL FARTHER WEST. THE OFFICIAL FORECASTHAS BEEN NUDGED TO THE RIGHT...TOWARD BUT STILL TO THE LEFT OF THEMODEL CONSENSUS...AND LIES CLOSE TO THE FIM MODEL PROVIDED BY THEHURRICANE FORECAST IMPROVEMENT PROJECT. IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK...ESPECIALLY AT DAYS 4 TO 5...SINCE THE MOST RECENT 5-YEAR AVERAGEERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES...RESPECTIVELY.FORECAST POSITIONS AND MAX WINDSINIT 22/0300Z 17.9N 65.5W 60 KT 70 MPH 12H 22/1200Z 18.6N 67.7W 60 KT 70 MPH 24H 23/0000Z 19.4N 70.1W 60 KT 70 MPH...INLAND 36H 23/1200Z 20.1N 72.1W 50 KT 60 MPH...OVER WATER 48H 24/0000Z 20.9N 73.9W 55 KT 65 MPH 72H 25/0000Z 23.5N 77.0W 65 KT 75 MPH 96H 26/0000Z 26.5N 79.5W 80 KT 90 MPH120H 27/0000Z 30.0N 81.5W 75 KT 85 MPH...INLAND$$FORECASTER BRENNAN
000WTNT44 KNHC 220859TCDAT4HURRICANE IRENE DISCUSSION NUMBER 7NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011500 AM AST MON AUG 22 2011IRENE HAS BECOME MUCH BETTER ORGANIZED DURING THE PAST 6 HOURSDESPITE ITS INTERACTION WITH THE MOUNTAINOUS TERRAIN OF PUERTORICO...AND A RAGGED EYE HAS BECOME APPARENT IN IMAGERY FROM THE FAATERMINAL DOPPLER WEATHER RADAR. ALSO...LARGE PATCHES OF DOPPLERVELOCITY VALUES AS HIGH AS 75-82 KT AT 1000-1500 FT AND 85-91 KT AT 2000-3000 FT HAVE BEEN INDICATED NORTHWEST AND NORTHEAST OF THECENTER OVER WATER FOR THE PAST 2 HOURS...WHICH EQUATES TO AT LEAST65-KT SURFACE WINDS. SOME DOPPLER VELOCITIES AS HIGH AS 72 KT HAVEALSO BEEN DETECTED AT 500-600 FT OVER WATER. AS A RESULT OF THESEDATA...IRENE HAS BEEN UPGRADED TO THE FIRST HURRICANE OF THE 2011ATLANTIC HURRICANE SEASON.TRENDING THROUGH ALL OF THE WOBBLES IN THE TRACK OVER THE PAST 6HOURS YIELDS AN AVERAGE MOTION OF ABOUT 290/10 KT. UPPER-AIR DATAAT 00Z INDICATE 500 MB HEIGHTS HAVE NOT CHANGED AT BERMUDA IN THEPAST 24 HOURS...WHICH SUGGESTS THAT THE SUBTROPICAL RIDGE TO THENORTH OF IRENE REMAINS QUITE STRONG. THE RIDGE IS EXPECTED TOCHANGE LITTLE FOR THE NEXT 48 HOURS OR SO...WHICH WOULD ACT TO KEEPIRENE MOVING IN A GENERAL WEST-NORTHWESTWARD DIRECTION OFF THENORTH COAST OF PUERTO RICO THIS MORNING AND JUST SKIRTING THENORTHERN COAST OF HISPANIOLA LATER TODAY AND ON TUESDAY. AFTERTHAT...THE GLOBAL MODELS DIVERGE ON THE SPECIFICS OF HOW LARGE ABREAK OR WEAKNESS IN THE SUBTROPICAL RIDGE DEVELOPS OVER FLORIDAAND THE SOUTHEASTERN U.S. AT 72 HOUR AND BEYOND...WHICH WILL ALLOWIRENE TO MOVE NORTHWESTWARD. THE ECMWF...NOGAPS...AND GFDN MODELSARE THE RIGHTMOST OF THE NHC MODEL GUIDANCE SUITE AND KEEP IRENEWELL EAST OF FLORIDA...WHEREAS THE UKMET AND THE GFDL MODELS ARETHE LEFTMOST OF THE MODELS AND TAKE IRENE NEAR THE WEST COAST OFFLORIDA. THE GFS AND THE REST OF THE NHC MODEL GUIDANCE IS TIGHTLYPACKED BETWEEN THESE TWO EXTREMES AND TAKE IRENE THROUGH THEBAHAMAS AND JUST OFF THE FLORIDA EAST COAST. WHAT IS NOTEWORTHY ISTHAT ALL OF THE GLOBAL AND REGIONAL MODELS AGREE THAT THE OUTFLOWOF IRENE WILL CONVERGE WITH WESTERLY TO NORTHWESTERLY UPPER-LEVELMID-LATITUDE FLOW OVER THE MID-ATLANTIC STATES BY 96 HOURS ANDBEYOND. THIS UPPER-LEVEL MASS CONFLUENCE COULD MAINTAIN THE MID- TOLOW-LEVEL RIDGE A LITTLE BIT STRONGER AND LONGER THAN WHAT NOGAPSAND THE ECMWF ARE FORECASTING...WHICH WOULD IN TURN KEEP IRENE ALITTLE CLOSER TO THE FLORIDA EAST COAST. AS A RESULT...THE OFFICIALFORECAST TRACK WAS ONLY SHIFTED SLIGHTLY TO THE EAST OF THEPREVIOUS TRACK AND IS NEAR THE CONSENSUS MODELS TVCN AND TVCA.IT NOW APPEARS THAT IRENE WILL NOT INTERACT WITH HISPANIOLA AS MUCHOR AS LONG AS PREVIOUSLY EXPECTED. THIS WILL ALSO RESULT IN MORESTRENGTHENING THAN PREVIOUSLY EXPECTED. IRENE IS FORECAST BY ALL OFTHE MODELS TO HAVE A VERY IMPRESSIVE OUTFLOW PATTERN THROUGHOUT THEFORECAST PERIOD...INCLUDING A LARGE UPPER-LEVEL TROUGH/LOW ABOUT1200 NMI EAST OF THE CYCLONE ACTING AS A MASS SINK. WITH IRENE ALSOEXPECTED TO BE OVER SSTS NEAR 30C AFTER 48 HOURS...SIGNIFICANTSTRENGTHENING COULD OCCUR. HOWEVER...THE OFFICIAL INTENSITY WILLREMAIN ON THE CONSERVATIVE SIDE DUE TO THE UNCERTAINTY IN HOW MUCHIRENE WILL INTERACT WITH THE MOUNTAINOUS TERRAIN OF HISPANIOLA FORTHE NEXT 24-36 HOURS. THE OFFICIAL INTENSITY FORECAST IS A BLEND OFTHE SHIPS/LGEM MODELS AND THE HWRF/GFDL MODELS. HOWEVER...GIVEN THEIMPRESSIVE UPPER-LEVEL FLOW PATTERN EXPECTED ACROSS IRENE...ITWOULD NOT SURPRISE ME IF THIS CYCLONE BECAME A MAJOR HURRICANE ATSOME TIME DURING ITS LIFETIME LIKE THE GFDL AND HWRF MODELS AREFORECASTING.IT IS IMPORTANT NOT TO FOCUS ON THE EXACT FORECAST TRACK...ESPECIALLY AT DAYS 4 TO 5...SINCE THE MOST RECENT 5-YEAR AVERAGEERRORS AT THOSE FORECAST TIMES ARE 200 AND 250 MILES...RESPECTIVELY.FORECAST POSITIONS AND MAX WINDSINIT 22/0900Z 18.4N 66.4W 65 KT 75 MPH 12H 22/1800Z 18.9N 68.2W 70 KT 80 MPH 24H 23/0600Z 19.7N 70.5W 70 KT 80 MPH...INLAND 36H 23/1800Z 20.4N 72.4W 70 KT 80 MPH...OVER WATER 48H 24/0600Z 21.3N 74.1W 75 KT 85 MPH 72H 25/0600Z 24.0N 77.1W 80 KT 90 MPH 96H 26/0600Z 27.0N 79.5W 85 KT 100 MPH120H 27/0600Z 30.9N 81.5W 95 KT 110 MPH$$FORECASTER STEWART