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Tropical Storm Sam Forecast Advisory Number 6
2021-09-24 04:39:40| National Hurricane Center (Atlantic)
Issued at 0300 UTC FRI SEP 24 2021 660 WTNT23 KNHC 240239 TCMAT3 TROPICAL STORM SAM FORECAST/ADVISORY NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 0300 UTC FRI SEP 24 2021 THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. TROPICAL STORM CENTER LOCATED NEAR 11.2N 40.9W AT 24/0300Z POSITION ACCURATE WITHIN 15 NM PRESENT MOVEMENT TOWARD THE WEST OR 280 DEGREES AT 13 KT ESTIMATED MINIMUM CENTRAL PRESSURE 997 MB MAX SUSTAINED WINDS 60 KT WITH GUSTS TO 75 KT. 50 KT....... 20NE 0SE 0SW 10NW. 34 KT....... 40NE 30SE 20SW 30NW. 12 FT SEAS.. 90NE 0SE 0SW 60NW. 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 11.2N 40.9W AT 24/0300Z AT 24/0000Z CENTER WAS LOCATED NEAR 11.1N 40.3W FORECAST VALID 24/1200Z 11.4N 42.9W MAX WIND 75 KT...GUSTS 90 KT. 64 KT... 15NE 0SE 0SW 10NW. 50 KT... 30NE 20SE 10SW 20NW. 34 KT... 50NE 40SE 20SW 40NW. FORECAST VALID 25/0000Z 11.8N 45.0W MAX WIND 90 KT...GUSTS 110 KT. 64 KT... 15NE 10SE 0SW 10NW. 50 KT... 30NE 20SE 10SW 20NW. 34 KT... 60NE 50SE 30SW 50NW. FORECAST VALID 25/1200Z 12.1N 46.6W MAX WIND 105 KT...GUSTS 130 KT. 64 KT... 20NE 15SE 10SW 15NW. 50 KT... 40NE 30SE 20SW 30NW. 34 KT... 70NE 60SE 40SW 60NW. FORECAST VALID 26/0000Z 12.5N 47.8W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 20NE 15SE 10SW 15NW. 50 KT... 40NE 30SE 20SW 30NW. 34 KT... 80NE 70SE 50SW 70NW. FORECAST VALID 26/1200Z 13.0N 49.1W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 80SE 60SW 70NW. FORECAST VALID 27/0000Z 13.5N 50.4W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 50NE 40SE 30SW 40NW. 34 KT...100NE 80SE 60SW 80NW. 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 28/0000Z 15.3N 53.2W MAX WIND 110 KT...GUSTS 135 KT. OUTLOOK VALID 29/0000Z 17.3N 56.1W MAX WIND 110 KT...GUSTS 135 KT. REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 11.2N 40.9W NEXT ADVISORY AT 24/0900Z $$ FORECASTER BERG
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Tropical Storm Sam Forecast Discussion Number 5
2021-09-23 22:59:56| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 23 2021 000 WTNT43 KNHC 232059 TCDAT3 Tropical Storm Sam Discussion Number 5 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 23 2021 Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory. Sam's estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual. With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 11.1N 39.7W 50 KT 60 MPH 12H 24/0600Z 11.5N 41.9W 60 KT 70 MPH 24H 24/1800Z 11.9N 44.2W 75 KT 85 MPH 36H 25/0600Z 12.3N 46.0W 90 KT 105 MPH 48H 25/1800Z 12.6N 47.4W 100 KT 115 MPH 60H 26/0600Z 13.1N 48.7W 105 KT 120 MPH 72H 26/1800Z 13.7N 50.0W 110 KT 125 MPH 96H 27/1800Z 15.2N 52.5W 110 KT 125 MPH 120H 28/1800Z 17.5N 55.5W 110 KT 125 MPH $$ Forecaster Papin
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Tropical Storm Sam Graphics
2021-09-23 22:52:55| National Hurricane Center (Atlantic)
5-Day Uncertainty Track last updated Thu, 23 Sep 2021 20:52:55 GMT Wind Speed Probabilities last updated Thu, 23 Sep 2021 20:52:55 GMT
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Summary for Tropical Storm Sam (AT3/AL182021)
2021-09-23 22:51:58| National Hurricane Center (Atlantic)
...SAM CONTINUES TO STRENGTHEN... ...RAPID INTENSIFICATION POSSIBLE OVER THE NEXT SEVERAL DAYS... As of 5:00 PM AST Thu Sep 23 the center of Sam was located near 11.1, -39.7 with movement W at 16 mph. The minimum central pressure was 1000 mb with maximum sustained winds of about 60 mph.
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Tropical Storm Sam Public Advisory Number 5
2021-09-23 22:51:58| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 23 2021 000 WTNT33 KNHC 232051 TCPAT3 BULLETIN Tropical Storm Sam Advisory Number 5 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 23 2021 ...SAM CONTINUES TO STRENGTHEN... ...RAPID INTENSIFICATION POSSIBLE OVER THE NEXT SEVERAL DAYS... SUMMARY OF 500 PM AST...2100 UTC...INFORMATION ---------------------------------------------- LOCATION...11.1N 39.7W ABOUT 1635 MI...2635 KM ESE OF THE NORTHERN LEEWARD ISLANDS MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H PRESENT MOVEMENT...W OR 280 DEGREES AT 16 MPH...26 KM/H MINIMUM CENTRAL PRESSURE...1000 MB...29.53 INCHES WATCHES AND WARNINGS -------------------- There are no coastal watches or warnings in effect. DISCUSSION AND OUTLOOK ---------------------- At 500 PM AST (2100 UTC), the center of Tropical Storm Sam was located near latitude 11.1 North, longitude 39.7 West. Sam is moving toward the west near 16 mph (26 km/h) and a general west to west-northwest motion is expected to continue over the next several days, with a gradual slowdown in forward motion. Maximum sustained winds have increased to near 60 mph (95 km/h) with higher gusts. Rapid intensification is forecast during the next several days. Sam is forecast to become a hurricane tomorrow morning and could become a major hurricane by Saturday. Tropical-storm-force winds extend outward up to 60 miles (95 km) from the center. The estimated minimum central pressure is 1000 mb (29.53 inches). HAZARDS AFFECTING LAND ---------------------- None. NEXT ADVISORY ------------- Next complete advisory at 1100 PM AST. $$ Forecaster Papin
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