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Hurricane Sam Forecast Discussion Number 15
2021-09-26 10:47:43| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Sep 26 2021 000 WTNT43 KNHC 260847 TCDAT3 Hurricane Sam Discussion Number 15 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sun Sep 26 2021 Sam remains a small but intense hurricane this morning, with a well-defined 10 n mi-wide eye. A 0500 UTC AMSR-2 microwave pass reveals Sam has a compact inner core of deep convection surrounding its eye, with curved convective bands primarily extending around the northern and eastern portions of the circulation. Infrared cloud top temperatures briefly warmed overnight in portions of Sam's central dense overcast, perhaps due to some intrusions of drier mid-level air. The 06z objective and subjective satellite estimates ranged from 113-127 kt, but Sam's satellite presentation has improved in recent infrared imagery as cloud top temperatures are cooling around the eye. Thus, the initial intensity is held at 125 kt for this advisory. NOAA aircraft are scheduled to investigate Sam later this afternoon, which should provide helpful information about the structure and intensity of the hurricane. Sam is moving toward the west-northwest, or 295/7 kt. A deep-layer subtropical ridge to the north and northeast of Sam should generally steer the hurricane west-northwestward to northwestward for the next several days. By midweek, an amplifying upper-level trough will move off the east coast of the U.S. and dig southward over the western Atlantic, which will erode the western extent of the steering ridge. The deep-layer southerly flow ahead of the trough should cause Sam to begin recurving, with a somewhat faster north-northwestward motion forecast by day 5. The latest NHC track forecast is very similar to the previous one and generally lies between the HFIP corrected consensus approach (HCCA) and TVCA aids. At days 4-5, there is more spread in the track guidance, with the ECMWF on the far left side of the guidance envelope and the GFS on the far right. The NHC forecast track has been nudged just slightly to the left at 96 and 120 h, but it still lies to the right of HCCA. The hurricane will likely experience some fluctuations in intensity during the next day or two. On the one hand, Sam remains over warm sea-surface temperatures of around 28.5 deg C, with vertical wind shear less than 10 kt as diagnosed from the SHIPS guidance. However, GOES-16 water vapor imagery shows some drier mid-level air in the surrounding environment that may periodically entrain into the inner core of Sam. Additionally, the onset of an eyewall replacement cycle remains a distinct possibility at this stage of Sam's life cycle, although the latest microwave data does not suggest one is imminent. The official NHC intensity forecast lies on the high end of the guidance for the first 36-48 h of the forecast period. Thereafter, the NHC forecast trends closer to the HCCA and IVCN consensus aids. Some gradual weakening is forecast later in the period as the southwesterly vertical wind shear begins to increase, but Sam is still forecast to remain a major hurricane through the 5-day period. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 13.8N 49.7W 125 KT 145 MPH 12H 26/1800Z 14.3N 50.6W 125 KT 145 MPH 24H 27/0600Z 15.0N 51.8W 120 KT 140 MPH 36H 27/1800Z 15.8N 52.9W 120 KT 140 MPH 48H 28/0600Z 16.6N 53.9W 115 KT 130 MPH 60H 28/1800Z 17.5N 55.0W 115 KT 130 MPH 72H 29/0600Z 18.4N 56.3W 115 KT 130 MPH 96H 30/0600Z 20.8N 59.3W 110 KT 125 MPH 120H 01/0600Z 24.5N 62.0W 110 KT 125 MPH $$ Forecaster Reinhart
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Hurricane Sam Public Advisory Number 15
2021-09-26 10:47:14| National Hurricane Center (Atlantic)
Issued at 500 AM AST Sun Sep 26 2021 000 WTNT33 KNHC 260847 TCPAT3 BULLETIN Hurricane Sam Advisory Number 15 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sun Sep 26 2021 ...SAM REMAINS A CATEGORY FOUR HURRICANE... ...FLUCTUATIONS IN INTENSITY LIKELY DURING THE NEXT DAY OR SO... SUMMARY OF 500 AM AST...0900 UTC...INFORMATION ---------------------------------------------- LOCATION...13.8N 49.7W ABOUT 940 MI...1510 KM ESE OF THE NORTHERN LEEWARD ISLANDS MAXIMUM SUSTAINED WINDS...145 MPH...230 KM/H PRESENT MOVEMENT...WNW OR 295 DEGREES AT 8 MPH...13 KM/H MINIMUM CENTRAL PRESSURE...943 MB...27.85 INCHES WATCHES AND WARNINGS -------------------- There are no coastal watches or warnings in effect. DISCUSSION AND OUTLOOK ---------------------- At 500 AM AST (0900 UTC), the center of Hurricane Sam was located near latitude 13.8 North, longitude 49.7 West. Sam is moving toward the west-northwest near 8 mph (13 km/h). This general motion is expected to continue today, followed by a turn toward the northwest on Monday. Then, a northwestward motion is forecast to continue through midweek. Maximum sustained winds are near 145 mph (230 km/h) with higher gusts. Sam is a category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Some fluctuations in intensity are expected during the next day or so. Thereafter, some slow weakening is forecast. Sam is a small hurricane. Hurricane-force winds extend outward up to 30 miles (45 km) from the center and tropical-storm-force winds extend outward up to 90 miles (150 km). The estimated minimum central pressure is 943 mb (27.85 inches). HAZARDS AFFECTING LAND ---------------------- SURF: Swells generated by Sam are forecast to reach the Lesser Antilles early this week. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. NEXT ADVISORY ------------- Next complete advisory at 1100 AM AST. $$ Forecaster Reinhart
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Hurricane Sam Wind Speed Probabilities Number 15
2021-09-26 10:47:14| National Hurricane Center (Atlantic)
Issued at 0900 UTC SUN SEP 26 2021 000 FONT13 KNHC 260847 PWSAT3 HURRICANE SAM WIND SPEED PROBABILITIES NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 0900 UTC SUN SEP 26 2021 AT 0900Z THE CENTER OF HURRICANE SAM WAS LOCATED NEAR LATITUDE 13.8 NORTH...LONGITUDE 49.7 WEST WITH MAXIMUM SUSTAINED WINDS NEAR 125 KTS...145 MPH...230 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 SUN 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 SUN 18Z SUN 06Z MON 18Z MON 06Z TUE 06Z WED 06Z THU PERIODS TO TO TO TO TO TO TO 18Z SUN 06Z MON 18Z MON 06Z TUE 06Z WED 06Z THU 06Z FRI FORECAST HOUR (12) (24) (36) (48) (72) (96) (120) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - LOCATION KT VIEQUES PR 34 X X( X) X( X) X( X) X( X) 2( 2) 1( 3) SAINT THOMAS 34 X X( X) X( X) X( X) X( X) 2( 2) 2( 4) SAINT CROIX 34 X X( X) X( X) X( X) X( X) 2( 2) 1( 3) SAINT MAARTEN 34 X X( X) X( X) X( X) 1( 1) 6( 7) 4(11) SAINT MAARTEN 50 X X( X) X( X) X( X) X( X) 2( 2) 1( 3) SABA 34 X X( X) X( X) X( X) 1( 1) 4( 5) 2( 7) ST EUSTATIUS 34 X X( X) X( X) X( X) 1( 1) 5( 6) 2( 8) ST KITTS-NEVIS 34 X X( X) X( X) X( X) 1( 1) 6( 7) 1( 8) BARBUDA 34 X X( X) X( X) X( X) 1( 1) 10(11) 3(14) BARBUDA 50 X X( X) X( X) X( X) X( X) 3( 3) X( 3) BARBUDA 64 X X( X) X( X) X( X) X( X) 1( 1) X( 1) ANTIGUA 34 X X( X) X( X) X( X) 1( 1) 9(10) 1(11) ANTIGUA 50 X X( X) X( X) X( X) X( X) 2( 2) 1( 3) ANTIGUA 64 X X( X) X( X) X( X) X( X) 1( 1) X( 1) GUADELOUPE 34 X X( X) X( X) X( X) 2( 2) 5( 7) 1( 8) DOMINICA 34 X X( X) X( X) X( X) 1( 1) 4( 5) X( 5) MARTINIQUE 34 X X( X) X( X) X( X) 1( 1) 2( 3) X( 3) $$ FORECASTER REINHART
Hurricane Sam Forecast Advisory Number 15
2021-09-26 10:46:43| National Hurricane Center (Atlantic)
Issued at 0900 UTC SUN SEP 26 2021 207 WTNT23 KNHC 260846 TCMAT3 HURRICANE SAM FORECAST/ADVISORY NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 0900 UTC SUN SEP 26 2021 THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. HURRICANE CENTER LOCATED NEAR 13.8N 49.7W AT 26/0900Z POSITION ACCURATE WITHIN 15 NM PRESENT MOVEMENT TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT 7 KT ESTIMATED MINIMUM CENTRAL PRESSURE 943 MB EYE DIAMETER 10 NM MAX SUSTAINED WINDS 125 KT WITH GUSTS TO 150 KT. 64 KT....... 25NE 20SE 15SW 20NW. 50 KT....... 40NE 30SE 20SW 30NW. 34 KT....... 80NE 80SE 40SW 60NW. 12 FT SEAS..150NE 90SE 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 13.8N 49.7W AT 26/0900Z AT 26/0600Z CENTER WAS LOCATED NEAR 13.7N 49.4W FORECAST VALID 26/1800Z 14.3N 50.6W MAX WIND 125 KT...GUSTS 150 KT. 64 KT... 25NE 20SE 15SW 20NW. 50 KT... 40NE 30SE 20SW 30NW. 34 KT... 90NE 80SE 40SW 70NW. FORECAST VALID 27/0600Z 15.0N 51.8W MAX WIND 120 KT...GUSTS 145 KT. 64 KT... 25NE 20SE 15SW 20NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 80SE 50SW 70NW. FORECAST VALID 27/1800Z 15.8N 52.9W MAX WIND 120 KT...GUSTS 145 KT. 64 KT... 30NE 25SE 20SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 28/0600Z 16.6N 53.9W MAX WIND 115 KT...GUSTS 140 KT. 64 KT... 35NE 30SE 25SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 70SW 80NW. FORECAST VALID 28/1800Z 17.5N 55.0W MAX WIND 115 KT...GUSTS 140 KT. 50 KT... 50NE 50SE 40SW 40NW. 34 KT...100NE 90SE 70SW 80NW. FORECAST VALID 29/0600Z 18.4N 56.3W MAX WIND 115 KT...GUSTS 140 KT. 50 KT... 60NE 50SE 40SW 40NW. 34 KT...100NE 90SE 70SW 90NW. 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 30/0600Z 20.8N 59.3W MAX WIND 110 KT...GUSTS 135 KT. OUTLOOK VALID 01/0600Z 24.5N 62.0W MAX WIND 110 KT...GUSTS 135 KT. REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 13.8N 49.7W NEXT ADVISORY AT 26/1500Z $$ FORECASTER REINHART
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Hurricane Sam Forecast Discussion Number 14
2021-09-26 04:51:08| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sat Sep 25 2021 000 WTNT43 KNHC 260251 TCDAT3 Hurricane Sam Discussion Number 14 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Sat Sep 25 2021 It's been an interesting evening with regards to analyzing the various data from a NOAA Hurricane Hunter research flight into Hurricane Sam. Dropsonde data in both the southeast and northwest quadrants indicate that small eyewall mesovortices and possibly even tornado-scale vortices were present based on the wind profiles showing sharply opposite-direction winds from what would normally be expected in those regions of the hurricane. Some dropsonde surface winds have been as high as 162 kt, which is more representative of a gust, while SFMR surface winds have been as high as 133 kt. However, the strongest 700-mb flight-level winds have been 138-139 kt in the northeastern quadrant, which equates to about 125-kt tangential surface winds. Three dropsondes released in the eye indicate that the pressure had remained steady at 943-944 mb during the duration of the aircraft reconnoiter. That pressure equates to about 125 kt based on various pressure-wind relationships. Based on that estimate and the 700-mb flight-level to surface-wind conversion, the advisory intensity is 125 kt, which is representative of the mean tangential winds and no localized wind perturbations. The initial motion is toward the west-northwest, or 295/07 kt. Not to sound like a broken record, but no significant changes were made to the previous track forecast and reasoning. Sam is expected to move slowly west-northwestward and northwestward over the next few days around the southwestern periphery of a deep-layer subtropical ridge that is situated to the north and northeast of the small hurricane. On days 4 and 5, an upper-level trough/low is forecast to dig southward and amplify off the U.S. east coast and extend all the way to the Bahamas. This feature should act to lift Sam northward at a faster forward speed. The latest NHC model guidance based on 12Z and 18Z model runs has shifted noticeably to the east of the previous runs, and the new NHC track forecast has been nudged in that direction as well. However, since the NOAA G-IV jet aircraft has been out there sampling the environment around Sam, it's best to remain conservative and not shift the track any farther to the east until the new 00Z model runs with that new aircraft data come in for the next advisory package at 0600Z. The new NHC track forecast lies about halfway between the previous advisory track on the left and the tightly packed consensus track models on the right. The radar images from the reconnaissance aircraft indicated that the eyewall was thin in many locations due to dry-air intrusions, and the latest SHIPS intensity output indicates that Sam will remain within a fairly dry mid-level environment. Also, the depth of the warm water beneath the hurricane isn't overly deep, which could result in cold upwelling owing to Sam's slow forward motion of only 5-7 kt during the next couple of days. Eyewall replacement cycles are also likely now due to the hurricane's small size and strong intensity. Thus, fluctuations in intensity seem likely for the next couple of days even though the vertical wind shear is expected to remain quite low at only 5-10 kt. On days 3-5, however, the shear is forecast to increase to 15-20 kt from the southwest, which is expected to induce a slow weakening trend. However, it is likely that Sam will remain a major hurricane through 120 h, even on days 3-5 due to the cyclone moving over warmer and deeper water during that 3-day period. The new official intensity forecast is essentially the same as the previous advisory, and remains above the consensus model and is near the higher end of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 13.5N 49.0W 125 KT 145 MPH 12H 26/1200Z 14.0N 49.9W 130 KT 150 MPH 24H 27/0000Z 14.7N 51.0W 125 KT 145 MPH 36H 27/1200Z 15.4N 52.1W 120 KT 140 MPH 48H 28/0000Z 16.3N 53.2W 120 KT 140 MPH 60H 28/1200Z 17.1N 54.3W 115 KT 130 MPH 72H 29/0000Z 18.0N 55.6W 115 KT 130 MPH 96H 30/0000Z 20.1N 58.5W 110 KT 125 MPH 120H 01/0000Z 23.7N 61.4W 110 KT 125 MPH $$ Forecaster Stewart
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