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Hurricane HUMBERTO Forecast Discussion Number 15

2013-09-12 10:49:19| National Hurricane Center (Atlantic)

Issued at 500 AM AST THU SEP 12 2013 000 WTNT44 KNHC 120849 TCDAT4 HURRICANE HUMBERTO DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 500 AM AST THU SEP 12 2013 THE CLOUD PATTERN OF HUMBERTO REMAINS WELL ORGANIZED...WITH THE CYCLONE MAINTAINING A WELL-DEFINED CENTRAL DENSE OVERCAST COMPRISED OF COLD CLOUD TOPS AND OCCASIONAL HINTS OF A WARM SPOT OR EYE. THE OUTER CIRCULATION OF THE CYCLONE CONSISTS OF SEVERAL WELL-DEFINED BANDS...AND THE OUTFLOW REMAINS GOOD TO EXCELLENT IN ALL QUADRANTS. HOWEVER...SATELLITE DATA IN THE LAST FEW HOURS SUGGEST THAT THE STRUCTURE OF THE INNER CORE CONVECTION HAS BECOME LESS SYMMETRIC AND THERE HAS BEEN SOME DETERIORATION OF THE EYEWALL JUST RECENTLY. THESE DEVELOPMENTS SUGGEST THAT HUMBERTO MAY HAVE PEAKED IN INTENSITY. DVORAK INTENSITY ESTIMATES ARE A CONSENSUS T4.5 FROM TAFB AND SAB...AND ADT VALUES ARE RUNNING AROUND 4.5. ON THIS BASIS...THE INITIAL INTENSITY IS KEPT AT 75 KT. HUMBERTO HAS BEEN MOVING STEADILY NORTHWARD...005/12....AND A NORTHWARD OR NORTH-NORTHWESTWARD MOTION INTO A SIGNIFICANT WEAKNESS IN THE SUBTROPICAL RIDGE SHOULD CONTINUE FOR ANOTHER DAY OR SO. AFTER THAT...A WEAKENING HUMBERTO SHOULD TURN NORTHWESTWARD AND THEN WEST-NORTHWESTWARD IN BETWEEN A BLOCKING LOW- TO MID-LEVEL RIDGE OVER THE NORTHEAST ATLANTIC AND A MID- TO UPPER-LEVEL LOW TO THE WEST UNTIL ABOUT DAY 4. BY DAY 5...HUMBERTO SHOULD REACH A REGION OF WEAK STEERING AND SLOW DOWN WHILE IT TURNS NORTHWARD OR PERHAPS EVEN NORTHEASTWARD AHEAD OF A STRONG MID-LATITUDE TROUGH ADVANCING ACROSS ATLANTIC CANADA. THE GUIDANCE HAS SHIFTED TO THE RIGHT THIS CYCLE...ESPECIALLY BEYOND 48 HOURS...AND THE OFFICIAL FORECAST IS ADJUSTED IN THAT DIRECTION. HOW FAST AND IN WHAT DIRECTION THE CYCLONE IS STEERED LATE IN THE PERIOD WILL LIKELY BE DEPENDENT ON THE STRENGTH OF THE CYCLONE...WHICH IS MORE UNCERTAIN THAN NORMAL AT THE MOMENT. EVEN THOUGH HUMBERTO IS ALREADY OVER WATERS LESS THAN 26C... SUFFICIENT MID-LEVEL MOISTURE AND INSTABILITY HAVE ALLOWED THE CYCLONE TO MAINTAIN ITSELF. HOWEVER...WEAKENING IS EXPECTED SOON AS A RESULT OF INCREASING SHEAR ASSOCIATED WITH A LARGE AMPLITUDE UPPER-LEVEL TROUGH OVER THE EAST-CENTRAL ATLANTIC AND THE ENTRAINMENT OF DRIER AIR. SHIPS MODEL OUTPUT SHOWS THE WEST- SOUTHWESTERLY SHEAR INCREASING TO GREATER THAN 40 KT IN 2-3 DAYS WHILE HUMBERTO IS STILL OVER COOL WATERS...WHICH COULD RESULT IN THE CYCLONE DEGENERATING INTO A REMNANT LOW. HOWEVER...GLOBAL MODELS SHOW A RESILIENT HUMBERTO...PERHAPS BECAUSE OF THE LARGE SIZE OF ITS CIRCULATION...SURVIVING TO REACH WARMER WATERS BY DAY 4. SOME DECREASE IN THE SHEAR LATE IN THE PERIOD AND POTENTIAL INTERACTION WITH THE APPROACHING MID-LATITUDE TROUGH MIGHT ALLOW FOR SOME RE-STRENGTHENING...ALTHOUGH THIS IS NOT EXPLICITLY SHOWN IN THE FORECAST. THE OFFICIAL INTENSITY FORECAST IS ADJUSTED DOWNWARD AND LIES IN BETWEEN THE LOWER SHIPS MODEL GUIDANCE AND THE INTENSITY CONSENSUS AIDS AND FLORIDA STATE SUPERENSMBLE OUTPUT. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 20.5N 28.7W 75 KT 85 MPH 12H 12/1800Z 22.0N 29.4W 70 KT 80 MPH 24H 13/0600Z 23.5N 30.3W 65 KT 75 MPH 36H 13/1800Z 24.5N 31.9W 55 KT 65 MPH 48H 14/0600Z 25.1N 33.7W 45 KT 50 MPH 72H 15/0600Z 26.1N 38.5W 35 KT 40 MPH 96H 16/0600Z 27.3N 42.5W 30 KT 35 MPH 120H 17/0600Z 28.5N 45.0W 30 KT 35 MPH $$ FORECASTER KIMBERLAIN

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Tropical Depression GABRIELLE Forecast Discussion Number 16

2013-09-12 10:47:43| National Hurricane Center (Atlantic)

Issued at 500 AM AST THU SEP 12 2013 000 WTNT42 KNHC 120847 TCDAT2 TROPICAL DEPRESSION GABRIELLE DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 500 AM AST THU SEP 12 2013 A NEW BURST OF CONVECTION HAS FORMED JUST EAST OF THE CENTER OF GABRIELLE DURING THE PAST FEW HOURS. THE SATELLITE INTENSITY ESTIMATE FROM TAFB IS 30 KT...AND AN EARLIER OSCAT OVERPASS SHOWED 25-30 KT WINDS NORTHEAST OF THE CENTER. BASED ON THESE DATA...THE INITIAL INTENSITY REMAINS 30 KT. ALTHOUGH GABRIELLE REMAINS SHEARED...ANALYSES FROM CIMSS INDICATE THAT THE SHEAR HAS DECREASED TO 10-15 KT OVER THE PAST 24 HOURS. THE INITIAL MOTION IS NOW 305/6. WATER VAPOR IMAGERY AND SURFACE OBSERVATIONS SHOW A DEEP-LAYER TROUGH AND ASSOCIATED SURFACE COLD FRONT MOVING INTO THE NEW ENGLAND STATES. THE DYNAMICAL MODELS FORECAST A LOW TO FORM ON THE FRONT DURING THE NEXT 48 HOURS. THIS EVOLUTION SHOULD CREATE A BREAK IN THE SUBTROPICAL RIDGE OVER THE WESTERN ATLANTIC AND ALLOW GABRIELLE TO RECURVE INTO THE WESTERLIES. THE NEW FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK...CALLING FOR A TURN TOWARD THE NORTH DURING THE NEXT 24 HOURS FOLLOWED BY A NORTH-NORTHEASTWARD ACCELERATION TOWARD ATLANTIC CANADA. THE TRACK GUIDANCE CONTINUES TO AGREE ON THIS SCENARIO...AND THE NEW TRACK IS AGAIN AN UPDATE OF THE PREVIOUS ADVISORY. THE INTENSITY FORECAST IS A LITTLE TRICKY. ON ONE SIDE...THE DECREASED SHEAR...24 HOURS BEFORE CROSSING THE NORTH WALL OF THE GULF STREAM...AND POSSIBLE INTERACTION WITH A BAROCLINIC TROUGH SUGGEST SOME POTENTIAL FOR STRENGTHENING. ON THE OTHER HAND...THE CYCLONE IS LIKELY TO MERGE WITH THE NON-TROPICAL SYSTEM IN 36-48 HOURS...AND THE DYNAMICAL MODELS ARE IN GOOD AGREEMENT IN THE CYCLONE DECAYING TO A TROUGH IN 36 HOURS OR LESS. ON THE PREMISE THAT THE PROS WILL OUTWEIGH THE CONS...THE INTENSITY FORECAST SHOWS MODEST STRENGTHENING BEFORE EXTRATROPICAL TRANSITION. THE ALTERNATIVE SCENARIO REMAINS THAT GABRIELLE COULD DECAY TO A REMNANT LOW OR TROUGH AT ANY TIME DURING THE NEXT 48 HOURS. AFTER EXTRATROPICAL TRANSITION...THE CYCLONE OR ITS REMNANTS WILL QUICKLY BE ABSORBED BY THE LARGER NON-TROPICAL LOW. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 33.5N 67.7W 30 KT 35 MPH 12H 12/1800Z 34.4N 67.9W 30 KT 35 MPH 24H 13/0600Z 36.9N 67.5W 35 KT 40 MPH 36H 13/1800Z 41.2N 65.4W 35 KT 40 MPH 48H 14/0600Z 46.7N 61.8W 35 KT 40 MPH...POST-TROP/EXTRATROP 72H 15/0600Z...DISSIPATED $$ FORECASTER BEVEN

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Hurricane HUMBERTO Forecast Discussion Number 14

2013-09-12 04:51:11| National Hurricane Center (Atlantic)

Issued at 1100 PM AST WED SEP 11 2013 000 WTNT44 KNHC 120250 TCDAT4 HURRICANE HUMBERTO DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 1100 PM AST WED SEP 11 2013 THE LARGE CENTRAL DENSE OVERCAST OF HUMBERTO IS EXHIBITING QUITE COLD CLOUD TOPS WITH A SMALL EYE INTERMITTENTLY APPEARING. DVORAK SUBJECTIVE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN AT ABOUT 75 KT...WHILE ADT IS SLIGHTLY HIGHER AND CIMSS AMSU SLIGHTLY LOWER. THE INTENSITY ANALYSIS REMAINS AT 75 KT. A DIRECT HIT BY AN ASCAT SCATTEROMETER SATELLITE AT 2250Z PROVIDED OBSERVATIONS SHOWING THAT HUMBERTO IS LARGER IN ITS TROPICAL STORM AND 50-KT WIND RADII THAN EARLIER ESTIMATED. THIS IS CONSISTENT WITH THE LARGE CDO AND INDEPENDENT ANALYSES FROM CIRA AMSU SIZE VALUES. CURRENTLY...HUMBERTO HAS BEEN TRAVERSING OVER WATERS OF 26C AND THROUGH AN ATMOSPHERE OF WEAK VERTICAL WIND SHEAR...AS WELL AS MOIST AND UNSTABLE AIR. IT APPEARS...HOWEVER...THAT HUMBERTO MAY BE AT ITS PEAK INTENSITY. AS HUMBERTO TRACKS NORTHWARD THEN WESTWARD DURING THE NEXT FEW DAYS...THE CYCLONE WILL APPROACH A STRONG TROPICAL UPPER TROPOSPHERIC TROUGH. THIS TROUGH WILL INDUCE MODERATE TO STRONG SOUTHWESTERLY VERTICAL SHEAR BEGINNING IN ABOUT A DAY AND CONTINUING THROUGH THE END OF THE FORECAST PERIOD. THE HOSTILE SHEAR AND COOLER SSTS SHOULD INDUCE SUBSTANTIAL WEAKENING. THE INTENSITY PREDICTION IS SLIGHTLY LOWER THAN THAT FROM THE PREVIOUS ADVISORY AND CLOSEST TO THE IVCN INTENSITY CONSENSUS THROUGH 48 HOURS AND CLOSEST TO THE DECAY-SHIPS STATISTICAL MODEL THEREAFTER. THERE ARE SUGGESTIONS BY THE GLOBAL MODELS THAT HUMBERTO MAY ATTEMPT A COMEBACK STARTING AROUND DAY 5...BUT IT IS QUITE UNCERTAIN HOW MUCH OF A TROPICAL CYCLONE WILL BE REMAINING AT THAT TIME AFTER EXPERIENCING SUBSTANTIAL VERTICAL SHEAR FOR NEARLY FOUR DAYS. HUMBERTO IS MOVING DUE NORTH AT 11 KT...PRIMARILY DUE TO THE STEERING INDUCED BY A MID-LEVEL RIDGE OVER WEST AFRICA TO ITS EAST. IN ABOUT TWO DAYS...A STRONG DEEP-LAYER RIDGE WILL DEVELOP NEAR THE AZORES AND FORCE HUMBERTO TOWARD THE WEST FOR THE NEXT FEW DAYS. THE TRACK GUIDANCE IS TIGHTLY CLUSTERED AND THE TRACK PREDICTION IS NEARLY UNCHANGED FROM THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 19.1N 29.0W 75 KT 85 MPH 12H 12/1200Z 20.7N 29.2W 75 KT 85 MPH 24H 13/0000Z 22.3N 29.9W 70 KT 80 MPH 36H 13/1200Z 23.5N 31.0W 60 KT 70 MPH 48H 14/0000Z 24.2N 32.8W 50 KT 60 MPH 72H 15/0000Z 25.2N 37.6W 40 KT 45 MPH 96H 16/0000Z 26.0N 42.0W 35 KT 40 MPH 120H 17/0000Z 27.5N 45.0W 30 KT 35 MPH $$ FORECASTER LANDSEA

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Tropical Depression GABRIELLE Forecast Discussion Number 15

2013-09-12 04:47:42| National Hurricane Center (Atlantic)

Issued at 1100 PM AST WED SEP 11 2013 000 WTNT42 KNHC 120247 TCDAT2 TROPICAL DEPRESSION GABRIELLE DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013 1100 PM AST WED SEP 11 2013 DEEP CONVECTION ASSOCIATED WITH GABRIELLE HAS DISSIPATED AGAIN THIS EVENING...LEAVING THE CYCLONE AS A SWIRL OF LOW-LEVEL CLOUDS...EXCEPT FOR ONE SMALL THUNDERSTORM THAT HAS RECENTLY DEVELOPED WELL EAST OF THE CENTER. SUBJECTIVE AND OBJECTIVE DVORAK INTENSITY ESTIMATES HAVE DECREASED BELOW TROPICAL STORM STRENGTH... AND BASED ON THESE DATA THE SYSTEM HAS BEEN DOWNGRADED TO A 30-KT TROPICAL DEPRESSION. LITTLE CHANGE IN STRENGTH IS EXPECTED DURING THE NEXT 24 HOURS AS THE CYCLONE REMAINS WITHIN AN AREA OF MODERATE WESTERLY WIND SHEAR AND DRY MID-LEVEL AIR. SOME INTENSIFICATION IS POSSIBLE AS GABRIELLE INTERACTS WITH A MID- TO UPPER-LEVEL TROUGH IN 36 TO 48 HOURS. THE TROUGH IS EXPECTED TO PROVIDE SOME BAROCLINIC FORCING AND SHOULD ALSO CAUSE THE CYCLONE TO BECOME EXTRATROPICAL IN ABOUT 2 DAYS. THE SYSTEM IS LIKELY TO BE ABSORBED BY A FRONT OVER NOVA SCOTIA SHORTLY THEREAFTER...HOWEVER A 72-HOUR POINT WAS PROVIDED ONCE AGAIN FOR FORECAST CONTINUITY. WITH THE LACK OF DEEP CONVECTION THIS EVENING...AN ALTERNATIVE SCENARIO IS THAT GABRIELLE COULD DEGENERATE INTO A REMNANT LOW ON THURSDAY OR BECOME AN OPEN TROUGH AS IT BEGINS TO ACCELERATE NORTH- NORTHEASTWARD IN A DAY OR SO. GABRIELLE CONTINUES MOVING SLOWLY WEST-NORTHWESTWARD. THE CYCLONE SHOULD TURN NORTHWARD ON THURSDAY AS IT IS STEERED AROUND THE WESTERN PORTION OF A LOW- TO MID-LEVEL RIDGE OVER THE WEST-CENTRAL ATLANTIC. AFTER 24 HOURS...GABRIELLE IS FORECAST TO ACCELERATE NORTH-NORTHEASTWARD AHEAD OF THE TROUGH MOVING INTO THE NORTHEASTERN UNITED STATES. THE TRACK GUIDANCE AGREES ON THIS SCENARIO AND THE UPDATED NHC TRACK IS ESSENTIALLY AN UPDATE OF THE PREVIOUS ADVISORY. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 33.0N 67.0W 30 KT 35 MPH 12H 12/1200Z 33.7N 67.6W 30 KT 35 MPH 24H 13/0000Z 35.7N 67.6W 30 KT 35 MPH 36H 13/1200Z 39.4N 66.1W 35 KT 40 MPH 48H 14/0000Z 44.4N 62.8W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 15/0000Z 54.5N 50.0W 40 KT 45 MPH...POST-TROP/EXTRATROP 96H 16/0000Z...ABSORBED BY FRONT $$ FORECASTER BROWN

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Hurricane HUMBERTO Forecast Discussion Number 13

2013-09-11 22:40:12| National Hurricane Center (Atlantic)

Issued at 500 PM AST WED SEP 11 2013 000 WTNT44 KNHC 112040 TCDAT4 HURRICANE HUMBERTO DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 500 PM AST WED SEP 11 2013 THE CLOUD PATTERN HAS BECOME A LITTLE BETTER ORGANIZED THIS AFTERNOON IN BOTH MICROWAVE AND CONVENTIONAL IMAGERY. THE EYE...ALTHOUGH RAGGED AND INTERMITTENT...IS EMBEDDED WITHIN A SMALL CENTRAL DENSE OVERCAST WITH CONVECTIVE BANDS WRAPPING AROUND THE CENTER PRIMARILY IN THE SOUTHERN SEMICIRCLE. SATELLITE INTENSITY ESTIMATES ARE UNANIMOUSLY 4.5 ON THE DVORAK SCALE AND ON THIS BASIS...THE INITIAL INTENSITY HAS BEEN ADJUSTED UPWARD TO 75 KNOTS. HUMBERTO HAS THE OPPORTUNITY TO STRENGTHEN A LITTLE MORE BEFORE IT REACHES COOLER WATERS AND INCREASING SHEAR IN ABOUT A DAY. AFTER THAT TIME...GRADUAL WEAKENING SHOULD BEGIN. SATELLITE FIXES SUGGEST THAT HUMBERTO IS MOVING TOWARD THE NORTH OR 350 DEGREES AT 10 KNOTS. THE CYCLONE CONTINUES TO BE STEERED BY THE SOUTHERLY FLOW BETWEEN A MID-LEVEL LOW OVER THE CENTRAL ATLANTIC AND A RIDGE OVER AFRICA. IN A COUPLE OF DAYS...THE GLOBAL MODELS FORECAST THE LOW TO WEAKEN AND HUMBERTO WILL ENCOUNTER STRONG HIGH PRESSURE TO THE NORTH. THIS PATTERN WOULD FORCE HUMBERTO TO MAKE A HARD LEFT TURN AND THIS SCENARIO IS UNANIMOUSLY SHOWN BY THE TRACK GUIDANCE. THE NHC FORECAST IS VERY CLOSE TO THE MULTI-MODEL CONSENSUS AND IS IN THE MIDDLE OF THE TRACK GUIDANCE ENVELOPE. THE GLOBAL MODELS INDICATE THAT HUMBERTO WILL LINGER OVER THE EASTERN ATLANTIC FOR SEVERAL DAYS. FORECAST POSITIONS AND MAX WINDS INIT 11/2100Z 18.0N 29.0W 75 KT 85 MPH 12H 12/0600Z 19.5N 29.4W 80 KT 90 MPH 24H 12/1800Z 21.3N 29.9W 75 KT 85 MPH 36H 13/0600Z 22.8N 30.6W 65 KT 75 MPH 48H 13/1800Z 24.0N 32.0W 60 KT 70 MPH 72H 14/1800Z 24.5N 36.5W 45 KT 50 MPH 96H 15/1800Z 25.5N 41.5W 40 KT 45 MPH 120H 16/1800Z 27.0N 46.0W 35 KT 40 MPH $$ FORECASTER AVILA

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