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Tropical Storm Sebastien Forecast Discussion Number 12

2019-11-22 09:34:20| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Nov 22 2019 000 WTNT45 KNHC 220834 TCDAT5 Tropical Storm Sebastien Discussion Number 12 NWS National Hurricane Center Miami FL AL202019 500 AM AST Fri Nov 22 2019 Sebastien has become less organized this morning. Strong southwesterly vertical wind shear of about 40 kt and dry air have caused the low-level center of the storm to become exposed to the southwest of the main area of deep convection. The initial intensity remains 50 kt, based on the earlier ASCAT data, but this is a little above the latest Dvorak estimates and could be generous. The center of Sebastien has turned more to the right than expected, with the initial motion estimated to be 065/13 kt. The models have changed significantly this cycle, and now show a much slower forward motion to the northeast during the next few days. This change appears to be connected to a more vertically shallow system that moves in the lower-level flow. The NHC track forecast has been adjusted considerably slower and a little to the right of the previous one, but it still is faster than nearly all of the typically reliable models. Future adjustments will likely be made if the model trend continues. It no longer appears that Sebastien will strengthen. In fact, all of the intensity guidance now suggests that the storm will gradually weaken during the next few days as Sebastien remains in strong wind shear conditions and moves over progressively cooler waters. In addition, the model guidance has finally come into agreement that Sebastien will not merge with the nearby cold front. Instead, the cyclone is expected to move ahead of the weakening front and become a non-convective post-tropical cyclone in about 2 days, or less, when it moves over SSTs below 22 C. As mentioned in the previous discussion, NHC and the models have had a challenging time figuring out when Sebastien would lose its tropical characteristics, and we have held the transition timing steady at 48 hours for a while. Although there is still uncertainty, the models are in better agreement in the timing of this transition. This intensity forecast is a little lower than the previous one to come into better agreement with the latest models. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 24.8N 57.0W 50 KT 60 MPH 12H 22/1800Z 26.0N 54.3W 50 KT 60 MPH 24H 23/0600Z 27.2N 51.7W 45 KT 50 MPH 36H 23/1800Z 28.7N 49.1W 40 KT 45 MPH 48H 24/0600Z 30.0N 46.2W 35 KT 40 MPH...POST-TROPICAL 72H 25/0600Z 33.3N 38.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 26/0600Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Sebastien Wind Speed Probabilities Number 12

2019-11-22 09:34:20| National Hurricane Center (Atlantic)

Issued at 0900 UTC FRI NOV 22 2019 000 FONT15 KNHC 220834 PWSAT5 TROPICAL STORM SEBASTIEN WIND SPEED PROBABILITIES NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL202019 0900 UTC FRI NOV 22 2019 AT 0900Z THE CENTER OF TROPICAL STORM SEBASTIEN WAS LOCATED NEAR LATITUDE 24.8 NORTH...LONGITUDE 57.0 WEST WITH MAXIMUM SUSTAINED WINDS NEAR 50 KTS...60 MPH...95 KM/H. Z INDICATES COORDINATED UNIVERSAL TIME (GREENWICH) ATLANTIC STANDARD TIME (AST)...SUBTRACT 4 HOURS FROM Z TIME EASTERN DAYLIGHT TIME (EDT)...SUBTRACT 4 HOURS FROM Z TIME CENTRAL DAYLIGHT TIME (CDT)...SUBTRACT 5 HOURS FROM Z TIME WIND SPEED PROBABILITY TABLE FOR SPECIFIC LOCATIONS CHANCES OF SUSTAINED (1-MINUTE AVERAGE) WIND SPEEDS OF AT LEAST ...34 KT (39 MPH... 63 KM/H)... ...50 KT (58 MPH... 93 KM/H)... ...64 KT (74 MPH...119 KM/H)... FOR LOCATIONS AND TIME PERIODS DURING THE NEXT 5 DAYS PROBABILITIES FOR LOCATIONS ARE GIVEN AS OP(CP) WHERE OP IS THE PROBABILITY OF THE EVENT BEGINNING DURING AN INDIVIDUAL TIME PERIOD (ONSET PROBABILITY) (CP) IS THE PROBABILITY OF THE EVENT OCCURRING BETWEEN 06Z FRI AND THE FORECAST HOUR (CUMULATIVE PROBABILITY) PROBABILITIES ARE GIVEN IN PERCENT X INDICATES PROBABILITIES LESS THAN 1 PERCENT PROBABILITIES FOR 34 KT AND 50 KT ARE SHOWN AT A GIVEN LOCATION WHEN THE 5-DAY CUMULATIVE PROBABILITY IS AT LEAST 3 PERCENT. PROBABILITIES FOR 34...50...64 KT SHOWN WHEN THE 5-DAY 64-KT CUMULATIVE PROBABILITY IS AT LEAST 1 PERCENT. - - - - WIND SPEED PROBABILITIES FOR SELECTED LOCATIONS - - - - FROM FROM FROM FROM FROM FROM FROM TIME 06Z FRI 18Z FRI 06Z SAT 18Z SAT 06Z SUN 06Z MON 06Z TUE PERIODS TO TO TO TO TO TO TO 18Z FRI 06Z SAT 18Z SAT 06Z SUN 06Z MON 06Z TUE 06Z WED FORECAST HOUR (12) (24) (36) (48) (72) (96) (120) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - LOCATION KT $$ FORECASTER CANGIALOSI

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Tropical Storm Sebastien Forecast Advisory Number 12

2019-11-22 09:33:52| National Hurricane Center (Atlantic)

Issued at 0900 UTC FRI NOV 22 2019 000 WTNT25 KNHC 220833 TCMAT5 TROPICAL STORM SEBASTIEN FORECAST/ADVISORY NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL202019 0900 UTC FRI NOV 22 2019 THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. TROPICAL STORM CENTER LOCATED NEAR 24.8N 57.0W AT 22/0900Z POSITION ACCURATE WITHIN 30 NM PRESENT MOVEMENT TOWARD THE EAST-NORTHEAST OR 65 DEGREES AT 13 KT ESTIMATED MINIMUM CENTRAL PRESSURE 999 MB MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT. 50 KT....... 0NE 60SE 0SW 0NW. 34 KT.......120NE 140SE 50SW 70NW. 12 FT SEAS..180NE 150SE 120SW 150NW. WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT. REPEAT...CENTER LOCATED NEAR 24.8N 57.0W AT 22/0900Z AT 22/0600Z CENTER WAS LOCATED NEAR 24.6N 57.3W FORECAST VALID 22/1800Z 26.0N 54.3W MAX WIND 50 KT...GUSTS 60 KT. 50 KT... 0NE 60SE 0SW 0NW. 34 KT...110NE 130SE 60SW 70NW. FORECAST VALID 23/0600Z 27.2N 51.7W MAX WIND 45 KT...GUSTS 55 KT. 34 KT... 90NE 110SE 60SW 60NW. FORECAST VALID 23/1800Z 28.7N 49.1W MAX WIND 40 KT...GUSTS 50 KT. 34 KT... 90NE 110SE 60SW 60NW. FORECAST VALID 24/0600Z 30.0N 46.2W...POST-TROPICAL MAX WIND 35 KT...GUSTS 45 KT. 34 KT... 70NE 80SE 0SW 0NW. FORECAST VALID 25/0600Z 33.3N 38.4W...POST-TROP/REMNT LOW MAX WIND 25 KT...GUSTS 35 KT. EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 150 NM ON DAY 4 AND 175 NM ON DAY 5...AND FOR INTENSITY NEAR 15 KT EACH DAY OUTLOOK VALID 26/0600Z...DISSIPATED REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 24.8N 57.0W NEXT ADVISORY AT 22/1500Z $$ FORECASTER CANGIALOSI

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Tropical Storm Sebastien Forecast Discussion Number 11

2019-11-22 03:47:15| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Nov 21 2019 000 WTNT45 KNHC 220247 TCDAT5 Tropical Storm Sebastien Discussion Number 11 NWS National Hurricane Center Miami FL AL202019 1100 PM AST Thu Nov 21 2019 Sebastien continues to produce a central dense overcast and a large band on the southeastern side of the circulation. Recent scatterometer data indicate that the maximum winds are at least 45 kt and, since there has been little overall change in the satellite presentation within the past several hours, the initial wind speed will stay 50 kt. Two notable changes have been made on this forecast. As foreshadowed in the previous forecast, the models are no longer rapidly accelerating the storm to the northeast ahead of a mid-latitude trough. Instead, there is better agreement tonight on a steadier motion to the northeast due to the trough in the short term. Sebastien will then continue moving northeastward embedded in the mid-latitude southwesterly flow over the weekend. The new forecast is substantially slower than the last one, but is still on the faster side of the guidance due to continuity concerns. The second significant change is that Sebastien is no longer expected to become a hurricane. The cyclone will soon be moving over sub-26C waters with strong shear, and it seems unlikely to intensify much in these conditions. Model intensity guidance continues to decrease, and the NHC forecast follows that trend. Weakening should start by the weekend over even cooler waters in a high-shear environment, and these factors are forecast to cause Sebastien to transition into a non-convective low or extratropical cyclone in 36-48 h. Careful watchers of the forecast will note that NHC has predicted extratropical transition at 48 h since Sebastien was born over 48 h ago, and this timing has been pushed back primarily due to the slower-than-expected motion. Thus, this timing can't be considered a particularly confident prediction until the track forecast becomes more consistent. FORECAST POSITIONS AND MAX WINDS INIT 22/0300Z 24.7N 57.8W 50 KT 60 MPH 12H 22/1200Z 26.0N 55.5W 55 KT 65 MPH 24H 23/0000Z 28.0N 51.7W 55 KT 65 MPH 36H 23/1200Z 30.3N 47.7W 45 KT 50 MPH 48H 24/0000Z 32.7N 43.4W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 25/0000Z 37.5N 34.0W 35 KT 40 MPH...POST-TROP/EXTRATROP 96H 26/0000Z...DISSIPATED $$ Forecaster Blake

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