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Tropical Storm Sam Forecast Discussion Number 4
2021-09-23 17:00:20| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 23 2021 000 WTNT43 KNHC 231500 TCDAT3 Tropical Storm Sam Discussion Number 4 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Thu Sep 23 2021 Overnight and this morning, the satellite structure of the tropical cyclone has been improving, with prominent curved bands, and obvious low-level cloud motions that indicate the circulation is becoming established within the convection. In particular, an SSMIS microwave pass at 0653 UTC indicated a substantial improvement in the convective structure, with a well-defined curved band wrapping three-quarters of the way around the center in both the 91- and 37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated a tight, well-defined circulation had formed, with peak winds of 44-kt on the north side of the vortex. Subjective Dvorak satellite intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from TAFB. In addition, the latest objective intensity estimates from UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given the recent scatterometer data, the intensity has been set to 45-kt for this advisory. Thus, Tropical Depression 18 has been upgraded to Tropical Storm Sam. It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the 2005 hurricane season, and only trailing last year. Sam continues to move to the west-northwest with an estimated motion at 280/14 kt. A prominent mid-level ridge remains entrenched to the north of the cyclone, and this feature should continue to steer Sam to the west-northwest over the next 2-3 days. Over this period, the ridge orientation actually shifts more westward ahead of the cyclone as a deep-layer trough digs in well northeast of Sam. This gradual change in the synoptic pattern should also result in a slowdown in Sam's forward motion over the next 72 hours. While the track guidance remains in good agreement over this period, larger spread begins to take shape beyond the day 3 forecast. Similar to yesterday, the ECMWF model and its ensemble mean flanks the southwest side of the guidance envelop, while the GFS is on the northeast side by day 5. These differences appear to be partially related to the mid-level ridge intensity on the western end, which could begin to be eroded by a mid-latitude trough off the eastern United States in 120-h. In addition, differences in both the size and intensity of Sam may also play a role in its ultimate track evolution. For this advisory, the latest NHC track forecast remains close to the consensus aids TCVN and TCVE, which represents a slight shift westward and a bit faster motion compared to the previous forecast. Because Sam is forecast to slow down, it remains too early to determine what impacts, direct or indirect, could be felt by the Lesser Antilles by this cyclone. The previously mentioned SSMIS microwave imagery also indicated that the low-level center has become better aligned with the mid-level vortex, perhaps a bit earlier than anticipated given the scatterometer data from last night. This improvement in structure, combined with favorable low vertical wind shear under 10 kt and warm 28-29 C sea-surface temperatures, argues for significant intensification. In fact, the GFS-SHIPS rapid intensification index now give Sam a 39 percent chance of a 65-kt increase over the next 72 hours, which is more than 7 times the climatological value. Thus, the NHC intensity forecast has been raised quite a bit from the prior one, especially in the short-term. The latest forecast now makes Sam a hurricane in 36 hours, and a major hurricane in 72 hours. This intensity forecast is just a shade under the latest HCCA consensus aid. Thereafter, a more gradual intensification rate is forecast. By this period it is possible that Sam could undergo hard to predict inner-core changes such as eyewall replacement cycles. In addition, the ECMWF-SHIPS suggests that the shear may also increase a little in days 4-5 which also argues for a bit slower rate of intensification for this period. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 10.9N 38.1W 45 KT 50 MPH 12H 24/0000Z 11.3N 40.3W 50 KT 60 MPH 24H 24/1200Z 11.9N 42.9W 60 KT 70 MPH 36H 25/0000Z 12.3N 45.0W 70 KT 80 MPH 48H 25/1200Z 12.7N 46.7W 80 KT 90 MPH 60H 26/0000Z 13.2N 48.2W 90 KT 105 MPH 72H 26/1200Z 13.7N 49.4W 100 KT 115 MPH 96H 27/1200Z 15.3N 52.1W 105 KT 120 MPH 120H 28/1200Z 17.4N 55.2W 110 KT 125 MPH $$ Forecaster Papin
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Tropical Storm Sam Graphics
2021-09-23 16:56:52| National Hurricane Center (Atlantic)
5-Day Uncertainty Track last updated Thu, 23 Sep 2021 14:56:52 GMT Wind Speed Probabilities last updated Thu, 23 Sep 2021 14:56:52 GMT
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Tropical Storm Sam Wind Speed Probabilities Number 4
2021-09-23 16:56:19| National Hurricane Center (Atlantic)
Issued at 1500 UTC THU SEP 23 2021 000 FONT13 KNHC 231456 PWSAT3 TROPICAL STORM SAM WIND SPEED PROBABILITIES NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 1500 UTC THU SEP 23 2021 AT 1500Z THE CENTER OF TROPICAL STORM SAM WAS LOCATED NEAR LATITUDE 10.9 NORTH...LONGITUDE 38.1 WEST WITH MAXIMUM SUSTAINED WINDS NEAR 45 KTS...50 MPH...85 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 12Z THU 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 12Z THU 00Z FRI 12Z FRI 00Z SAT 12Z SAT 12Z SUN 12Z MON PERIODS TO TO TO TO TO TO TO 00Z FRI 12Z FRI 00Z SAT 12Z SAT 12Z SUN 12Z MON 12Z TUE FORECAST HOUR (12) (24) (36) (48) (72) (96) (120) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - LOCATION KT BARBUDA 34 X X( X) X( X) X( X) X( X) X( X) 4( 4) ANTIGUA 34 X X( X) X( X) X( X) X( X) X( X) 3( 3) GUADELOUPE 34 X X( X) X( X) X( X) X( X) X( X) 3( 3) DOMINICA 34 X X( X) X( X) X( X) X( X) X( X) 3( 3) MARTINIQUE 34 X X( X) X( X) X( X) X( X) X( X) 3( 3) BARBADOS 34 X X( X) X( X) X( X) X( X) X( X) 3( 3) $$ FORECASTER PAPIN
Summary for Tropical Storm Sam (AT3/AL182021)
2021-09-23 16:55:49| National Hurricane Center (Atlantic)
...DEPRESSION STRENGTHENS INTO TROPICAL STORM SAM, THE 18TH NAMED STORM OF THE SEASON... ...NOW FORECAST TO BECOME A HURRICANE TOMORROW... As of 11:00 AM AST Thu Sep 23 the center of Sam was located near 10.9, -38.1 with movement W at 16 mph. The minimum central pressure was 1003 mb with maximum sustained winds of about 50 mph.
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Tropical Storm Sam Public Advisory Number 4
2021-09-23 16:55:49| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 23 2021 000 WTNT33 KNHC 231455 TCPAT3 BULLETIN Tropical Storm Sam Advisory Number 4 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Thu Sep 23 2021 ...DEPRESSION STRENGTHENS INTO TROPICAL STORM SAM, THE 18TH NAMED STORM OF THE SEASON... ...NOW FORECAST TO BECOME A HURRICANE TOMORROW... SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION ----------------------------------------------- LOCATION...10.9N 38.1W ABOUT 1745 MI...2805 KM ESE OF THE NORTHERN LEEWARD ISLANDS MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H PRESENT MOVEMENT...W OR 280 DEGREES AT 16 MPH...26 KM/H MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES WATCHES AND WARNINGS -------------------- There are no coastal watches or warnings in effect. DISCUSSION AND OUTLOOK ---------------------- At 1100 AM AST (1500 UTC), the center of Tropical Storm Sam was located near latitude 10.9 North, longitude 38.1 West. Sam is moving toward the west near 16 mph (26 km/h) and this general motion is expected to continue over the next several days, but with a gradual slowdown in forward motion. Recent sallite wind data indicates maximum sustained winds have increased to near 50 mph (85 km/h) with higher gusts. Additional strengthening is forecast over the next several days, and Sam is now forecast to become a hurricane tomorrow and be near major hurricane intensity by the end of the weekend. Tropical-storm-force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 1003 mb (29.62 inches). HAZARDS AFFECTING LAND ---------------------- None. NEXT ADVISORY ------------- Next complete advisory at 500 PM AST. $$ Forecaster Papin
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