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Tropical Storm GABRIELLE Forecast Discussion Number 14

2013-09-11 22:36:43| National Hurricane Center (Atlantic)

Issued at 500 PM AST WED SEP 11 2013 000 WTNT42 KNHC 112036 TCDAT2 TROPICAL STORM GABRIELLE DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 500 PM AST WED SEP 11 2013 GABRIELLE CONTINUES TO MAINTAIN A SMALL AREA OF DEEP CONVECTION TO THE NORTHEAST OF THE CENTER IN AN ENVIRONMENT OF MODERATE SOUTHWESTERLY SHEAR. THE INITIAL INTENSITY IS LOWERED TO 35 KT BASED ON THE LATEST DVORAK DATA-T NUMBER FROM TAFB. LITTLE CHANGE IN STRENGTH IS FORECAST DURING THE NEXT 24 HOURS OR SO AS MODERATE SHEAR CONTINUES...BUT SOME INTENSIFICATION IS SHOWN FROM 36 TO 48 HOURS AS AN UPPER-LEVEL TROUGH MAY PROVIDE SOME BAROCLINIC SUPPORT FOR INTENSIFICATION. GABRIELLE SHOULD BECOME EXTRATROPICAL BY 72 HOURS...AND MAY BE DISSIPATED ENTIRELY BY THEN...HOWEVER A 72-HOUR POINT WAS PROVIDED IN THIS FORECAST FOR CONTINUITY. ALTERNATIVELY...IF DEEP CONVECTION DISSIPATES ENTIRELY...GABRIELLE COULD BECOME A POST-TROPICAL CYCLONE AT ANY TIME DURING THE FORECAST PERIOD. GABRIELLE HAS MOVED SLOWLY WEST-NORTHWESTWARD THIS AFTERNOON...AND THE INITIAL MOTION ESTIMATE IS 295/04. THIS MOTION IS PROBABLY DUE TO THE SHALLOW CYCLONE NOT FEELING THE INFLUENCE OF THE UPPER-LEVEL LOW OFF THE SOUTHEAST U.S. COAST AND THE DOMINANT STEERING FLOW COMING FROM A WEAK LOW-LEVEL RIDGE TO THE NORTH AND EAST. THE MODEL GUIDANCE CONTINUES TO INSIST ON A NORTHWARD TURN SOON...WITH A MOTION TOWARD THE NORTHWEST EXPECTED TOMORROW FOLLOWED BY A NORTH-NORTHEASTWARD ACCELERATION ON FRIDAY AHEAD OF AN EASTWARD-MOVING UPPER-LEVEL TROUGH. DUE TO THE WESTWARD MOTION TODAY...AND A WESTWARD SHIFT IN THE GUIDANCE DURING THE FIRST COUPLE OF DAYS...THE NHC FORECAST HAS BEEN ADJUSTED WESTWARD TOWARD THE NEW MULTI-MODEL CONSENSUS AND LIES ABOUT A DEGREE WEST OF THE PREVIOUS FORECAST BY 48 HOURS. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 32.8N 66.4W 35 KT 40 MPH 12H 12/0600Z 33.2N 67.1W 35 KT 40 MPH 24H 12/1800Z 34.4N 67.7W 35 KT 40 MPH 36H 13/0600Z 37.1N 67.2W 40 KT 45 MPH 48H 13/1800Z 41.5N 65.0W 45 KT 50 MPH 72H 14/1800Z 51.5N 55.5W 40 KT 45 MPH...POST-TROP/EXTRATROP 96H 15/1800Z...DISSIPATED $$ FORECASTER BRENNAN

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Tropical Storm GABRIELLE Forecast Discussion Number 13

2013-09-11 16:36:11| National Hurricane Center (Atlantic)

Issued at 1100 AM AST WED SEP 11 2013 000 WTNT42 KNHC 111436 TCDAT2 TROPICAL STORM GABRIELLE DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 1100 AM AST WED SEP 11 2013 GABRIELLE IS BARELY HANGING ON AS A TROPICAL CYCLONE...WITH A SMALL AREA OF DEEP CONVECTION TO THE EAST OF THE EXPOSED LOW-LEVEL CENTER. THIS IS LIKELY DUE TO THE 15-20 KT OF SOUTHWESTERLY SHEAR OVER THE CYCLONE. THE INTITAL INTENSITY HAS BEEN CONSERVATIVELY LOWERED TO 40 KT FOR THIS ADVISORY. LITTLE CHANGE IN STRENGTH IS FORECAST DURING THE NEXT 36 HOURS OR SO AS MODERATE SHEAR CONTINUES...BUT SOME INTENSIFICATION IS SHOWN AT 48 HOURS AS AN UPPER-LEVEL TROUGH MAY PROVIDE SOME BAROCLINIC SUPPORT FOR INTENSIFICATION. BY 72 HOURS...GABRIELLE SHOULD BE EXTRATROPICAL AND BE ABSORBED OR DISSIPATE SHORTLY THEREAFTER. ALTERNATIVELY...IF DEEP CONVECTION DISSIPATES ENTIRELY...GABRIELLE COULD BECOME A POST-TROPICAL CYCLONE AT ANY TIME DURING THE FORECAST PERIOD. THE CYCLONE HAS BEEN MOVING LITTLE OVER THE PAST FEW HOURS...AND IS NOW DRIFTING WESTWARD AT ABOUT 3 KT. HOWEVER THE MODEL GUIDANCE IS IN GOOD AGREEMENT ON A SLOW NORTHWESTWARD MOTION DEVELOPING DURING THE NEXT 12 TO 24 HOURS DUE TO AN UPPER-LEVEL LOW TO THE SOUTHWEST OF GABRIELLE AND A LOW/MID-LEVEL RIDGE TO THE NORTH AND NORTHEAST. RECURVATURE IS EXPECTED BY 48 HOURS AS THE CYCLONE ACCELERATES NORTH-NORTHEASTWARD AHEAD OF AN ADVANCING MID/UPPER-LEVEL TROUGH. THE TRACK MODEL GUIDANCE HAS TRENDED A LITTLE SLOWER AND TO THE LEFT THIS CYCLE AND SO HAS THE NHC TRACK WHICH NOW LIES ALONG...BUT IS A LITTLE FASTER THAN...THE TVCA MULTI-MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 32.5N 65.9W 40 KT 45 MPH 12H 12/0000Z 33.1N 66.5W 40 KT 45 MPH 24H 12/1200Z 33.8N 67.1W 40 KT 45 MPH 36H 13/0000Z 35.5N 67.2W 40 KT 45 MPH 48H 13/1200Z 38.6N 65.9W 45 KT 50 MPH 72H 14/1200Z 48.5N 58.0W 40 KT 45 MPH...POST-TROP/EXTRATROP 96H 15/1200Z...DISSIPATED $$ FORECASTER BRENNAN

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Hurricane HUMBERTO Forecast Discussion Number 12

2013-09-11 16:35:39| National Hurricane Center (Atlantic)

Issued at 1100 AM AST WED SEP 11 2013 000 WTNT44 KNHC 111435 TCDAT4 HURRICANE HUMBERTO DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 1100 AM AST WED SEP 11 2013 SATELLITE IMAGERY HAS BEEN REVEALING AN INTERMITTENT RAGGED EYE... WHICH IS ALSO DEPICTED AT THE MID-LEVELS ON A SSMI/S PASS EARLY THIS MORNING. THE EYE HAS BEEN APPARENT WITHIN A SMALL CENTRAL DENSE OVERCAST WHICH IS SURROUNDED BY A COUPLE OF DISTINCT CYCLONICALLY-CURVED CONVECTIVE BANDS. SATELLITE INTENSITY ESTIMATES ARE 4.0 AND 4.5 FROM TAFB AND SAB...AND THE OBJECTIVE NUMBERS FROM THE UNIVERSITY OF WISCONSIN HAVE REACHED 4.4 ON THE DVORAK SCALE. ON THIS BASIS..THE INITIAL INTENSITY AS BEEN SET AT 70 KNOTS. HUMBERTO HAS THE OPPORTUNITY TO STRENGTHEN A LITTLE MORE BEFORE IT REACHES COOLER WATER AND INCREASING SHEAR IN A COUPLE OF DAYS...AS DIAGNOSED BY THE SHIPS MODEL AND MOST OF THE INTENSITY GUIDANCE. GRADUAL WEAKENING SHOULD THEN BEGIN AS INDICATED IN THE NHC FORECAST. AS PREVIOUSLY FORECAST...HUMBERTO HAS MADE THE EXPECTED SHARP TURN TO THE NORTH...AND IS NOW MOVING TOWARD 350 DEGREES AT 8 KNOTS. THE CYCLONE IS BEEN STEERED BY THE SOUTHERLY FLOW BETWEEN A MID-LEVEL LOW OVER THE CENTRAL ATLANTIC AND A RIDGE OVER AFRICA. IN A COUPLE OF DAYS...GLOBAL MODELS FORECAST THAT THE LOW WILL WEAKEN AND HUMBERTO WILL ENCOUNTER STRONG HIGH PRESSURE TO THE NORTH. THIS PATTERN WOULD FORCE THE CYCLONE TO TURN TOWARD THE WEST OR WEST-NORTHWEST AND THIS IS THE GENERAL SOLUTION UNANIMOUSLY PROVIDED BY THE TRACK GUIDANCE. THE NHC FORECAST FOLLOWS THAT SCENARIO AND IS VERY CLOSE TO THE MULTI-MODEL CONSENSUS. THIS TRACK WILL KEEP HUMBERTO MOVING OVER THE NORTH ATLANTIC WATERS FOR SEVERAL DAYS. FORECAST POSITIONS AND MAX WINDS INIT 11/1500Z 16.7N 29.1W 70 KT 80 MPH 12H 12/0000Z 18.2N 29.4W 75 KT 85 MPH 24H 12/1200Z 20.0N 29.6W 70 KT 80 MPH 36H 13/0000Z 21.5N 30.0W 70 KT 80 MPH 48H 13/1200Z 23.0N 31.0W 65 KT 75 MPH 72H 14/1200Z 24.0N 35.5W 50 KT 60 MPH 96H 15/1200Z 24.5N 40.5W 45 KT 50 MPH 120H 16/1200Z 26.0N 45.5W 40 KT 45 MPH $$ FORECASTER AVILA

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Hurricane HUMBERTO Forecast Discussion Number 11

2013-09-11 10:53:06| National Hurricane Center (Atlantic)

Issued at 500 AM AST WED SEP 11 2013 000 WTNT44 KNHC 110853 TCDAT4 HURRICANE HUMBERTO DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 500 AM AST WED SEP 11 2013 CENTRAL CONVECTION HAS BEEN ON THE INCREASE WITH HUMBERTO DURING THE LAST FEW HOURS...WITH A LARGE CURVED BAND WRAPPING AROUND THE CYCLONE ALSO BECOMING BETTER DEFINED. THE DVORAK ESTIMATES AT 6Z SUPPORTED ANYTHING FROM 55-65 KT...AND GIVEN THE NOTABLE STRENGTHENING OF THE CENTRAL DEEP CONVECTION AND OVERALL INCREASE IN ORGANIZATION...THE INITIAL WINDS ARE RAISED TO 65 KT. HUMBERTO HAS ABOUT A DAY TO STRENGTHEN BEFORE AN INCREASE IN SHEAR AND COOLER WATERS SHOULD CAUSE A WEAKENING TREND. IN A FEW DAYS... ALTHOUGH WATER TEMPERATURES BEGIN TO RISE...STRONG WESTERLY SHEAR AND ENTRAINMENT OF DRIER AIR WILL PROBABLY KEEP THE CYCLONE GRADUALLY WEAKENING. THE INTENSITY GUIDANCE SEEMS TO BE WELL CLUSTERED...SO THE LATEST NHC WIND SPEED PREDICTION IS CLOSE TO THE PREVIOUS FORECAST AND THE MODEL CONSENSUS. HUMBERTO APPEARS TO BE TURNING TO THE RIGHT...NOW MOVING 330/7. THE CYCLONE SHOULD TURN TO THE NORTH SOON AND ACCELERATE SOME WHILE IT MOVES BETWEEN A RIDGE OVER NORTHWESTERN AFRICA AND A TROUGH OVER THE CENTRAL ATLANTIC OCEAN. IN A COUPLE OF DAYS...HUMBERTO IS LIKELY TO TAKE A HARD LEFT TURN DUE TO IT RUNNING AGAINST A RATHER STOUT RIDGE IN THE SUBTROPICAL EASTERN ATLANTIC. WITH HUMBERTO BECOMING A MORE SHALLOW SYSTEM IN THE LONGER RANGE...IT MAKES SENSE TO STAY ON THE SOUTHERN SIDE OF THE GUIDANCE ENVELOPE GIVEN THE STRONG LOW-LEVEL RIDGE FORECAST. THE LATEST NHC FORECAST IS BASICALLY AN UPDATE OF THE PREVIOUS ONE...STAYING EQUATORWARD OF THE MODEL CONSENSUS IN THE MEDIUM RANGE. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 16.0N 28.9W 65 KT 75 MPH 12H 11/1800Z 17.2N 29.2W 70 KT 80 MPH 24H 12/0600Z 19.0N 29.3W 75 KT 85 MPH 36H 12/1800Z 20.9N 29.6W 70 KT 80 MPH 48H 13/0600Z 22.5N 30.3W 65 KT 75 MPH 72H 14/0600Z 23.8N 33.9W 50 KT 60 MPH 96H 15/0600Z 24.2N 39.1W 45 KT 50 MPH 120H 16/0600Z 25.5N 44.0W 40 KT 45 MPH $$ FORECASTER BLAKE

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Tropical Storm GABRIELLE Forecast Discussion Number 12

2013-09-11 10:51:16| National Hurricane Center (Atlantic)

Issued at 500 AM AST WED SEP 11 2013 000 WTNT42 KNHC 110851 TCDAT2 TROPICAL STORM GABRIELLE DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 500 AM AST WED SEP 11 2013 SATELLITE IMAGERY INDICATES THAT GABRIELLE IS COMPRISED OF A SWIRL OF LOW/MID-LEVEL CLOUDS WITH NO DEEP CONVECTION NEAR THE CENTER AT THIS TIME. THIS IS LIKELY DUE TO THE ONGOING AFFECT OF 15-20 KT OF SOUTHWESTERLY VERTICAL WIND SHEAR. AN EARLIER ASCAT OVERPASS... RECENT REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT...AND OBSERVATIONS FROM BERMUDA SHOW THAT THE WINDS HAVE DECREASED DURING THE PAST 12 HOURS. THE INITIAL INTENSITY IS REDUCED TO 45 KT...AND THIS COULD BE GENEROUS. THE CYCLONE HAS TURNED TO THE LEFT SINCE THE PREVIOUS ADVISORY...AND THE INITIAL MOTION IS NOW 325/7. THIS IS PARTLY DUE TO AN UPPER- LEVEL LOW TO THE SOUTHWEST OF GABRIELLE AND A STRENGTHENING LOW/MID-LEVEL RIDGE TO THE NORTH. THIS COMBINATION SHOULD STEER GABRIELLE SLOWLY TOWARD THE NORTHWEST FOR THE NEXT 24 HOURS OR SO. AFTER THAT...THE CYCLONE IS LIKELY TO RECURVE ON THE EASTERN SIDE OF A LARGE DEEP-LAYER TROUGH MOVING INTO THE EASTERN UNITED STATES AND THE WESTERN ATLANTIC. WHILE THE TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT WITH THIS SCENARIO...IT HAS SHIFTED QUITE A BIT TO THE LEFT/WEST DURING THE FIRST 48 HOURS. THE NEW FORECAST TRACK IS ALSO SHIFTED TO THE LEFT DURING THAT TIME...BUT STILL LIES TO THE RIGHT OF THE CONSENSUS MODELS AND THE MIDDLE OF THE GUIDANCE ENVELOPE. GABRIELLE IS FORECAST TO REMAIN OVER WARM SEA SURACE TEMPERATURES FOR THE NEXT 48 HOURS. HOWEVER...THE DYNAMICAL MODELS CALL FOR CONTINUED STRONG SHEAR...WITH THE UKMET...NAVGEM...AND ECMWF MODELS FORECASTING THE CYCLONE TO DECAY TO A TROUGH DURING THIS TIME. AFTER THAT...WHATEVER IS LEFT OF THE CYCLONE IS EXPECTED TO MERGE WITH A FRONTAL SYSTEM BETWEEN 48-72 HOURS. THE INTENSITY FORECAST FOLLOWS THE GFS SCENARIO THAT GABRIELLE WILL REMAIN A TROPICAL CYCLONE UNTIL IT MERGES WITH THE FRONT...WITH THE NEW FORECAST A LITTLE WEAKER THAN THE PREVIOUS FORECAST. HOWEVER...AN ALTERNATIVE SCENARIO IS THAT THE CYCLONE DECAYS TO A REMNANT LOW OR TROUGH DURING THE NEXT 48 HOURS DUE TO A LACK OF CONVECTION. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 32.5N 65.7W 45 KT 50 MPH 12H 11/1800Z 33.1N 66.2W 45 KT 50 MPH 24H 12/0600Z 33.7N 66.8W 45 KT 50 MPH 36H 12/1800Z 34.7N 67.3W 40 KT 45 MPH 48H 13/0600Z 37.2N 66.5W 40 KT 45 MPH 72H 14/0600Z 46.5N 59.5W 40 KT 45 MPH...POST-TROP/EXTRATROP 96H 15/0600Z...DISSIPATED $$ FORECASTER BEVEN

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