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Hurricane Sam Forecast Discussion Number 21
2021-09-27 22:51:13| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Sep 27 2021 000 WTNT43 KNHC 272051 TCDAT3 Hurricane Sam Discussion Number 21 NWS National Hurricane Center Miami FL AL182021 500 PM AST Mon Sep 27 2021 Sam's weakening trend ended earlier today, with the intensity likely bottoming out between the issuance of the previous advisory and now. Over the past few hours, microwave data and satellite imagery indicate that the inner core and eyewall of the hurricane are making a comeback. There is now a clear eye in visible imagery, convective cloud tops colder than -65 degrees C wrap completely around the center, and the convective mass is becoming more circular once again. An Air Force Reserve Hurricane Hunter aircraft currently investigating Sam indicates the central pressure has dropped to 957 mb, with peak 700-mb flight-level winds of 106 kt, SFMR values of 100 kt, and dropsonde surface winds of 102 kt. Given possible minor undersampling and the recent dropping of the hurricane's pressure, the initial advisory intensity is set at 105 kt. Sam continues to move northwest, or 315/8 kt. The hurricane is expected to continue this northwestward motion around a subtropical ridge over the next few days. By late Thursday or early Friday a turn toward the north is anticipated as Sam rounds the western periphery of the ridge. The cyclone should begin to gradually accelerate during that time in the increasing flow ahead of a mid- to upper-level trough emerging off the U.S. east coast later this week. Model guidance remains in very good agreement on this scenario and the latest NHC forecast track is little changed from the previous one, and lies near tightly clustered consensus track guidance. The southwesterly winds that brought in some dry air and disrupted Sam's impressive satellite appearance last night appear to have abated. As long as environmental winds surrounding the hurricane remain light, then the dry air surrounding the cyclone should not have as much of an impact on the hurricane's strength over the next few days. Based on the improving inner-core structure appearance of Sam, re-strengthening is forecast over the next 12 h. Thereafter, mainly minor fluctuations in intensity are indicated through 72 h. Beyond that time, increasing vertical wind shear and decreasing sea-surface temperatures should cause Sam to weaken. The latest NHC intensity forecast was nudged slightly upward through 36 h and is on the high end of the guidance. Thereafter, no changes were made, and that portion of the forecast is close to the various multimodel consensus solutions. Key Messages: 1. Large swells generated by Sam are affecting the Leeward Islands and will spread to portions of the Virgin Islands, Puerto Rico, Hispaniola, the Bahamas, and Bermuda by Thursday or Friday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by the weekend. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming weekend. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 16.3N 52.7W 105 KT 120 MPH 12H 28/0600Z 17.0N 53.5W 115 KT 130 MPH 24H 28/1800Z 17.8N 54.6W 115 KT 130 MPH 36H 29/0600Z 18.8N 55.8W 115 KT 130 MPH 48H 29/1800Z 19.8N 57.3W 110 KT 125 MPH 60H 30/0600Z 21.2N 59.0W 110 KT 125 MPH 72H 30/1800Z 22.9N 60.5W 110 KT 125 MPH 96H 01/1800Z 27.8N 62.0W 105 KT 120 MPH 120H 02/1800Z 34.7N 59.6W 100 KT 115 MPH $$ Forecaster Latto
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Hurricane Sam Forecast Advisory Number 21
2021-09-27 22:48:43| National Hurricane Center (Atlantic)
Issued at 2100 UTC MON SEP 27 2021 000 WTNT23 KNHC 272048 TCMAT3 HURRICANE SAM FORECAST/ADVISORY NUMBER 21 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 2100 UTC MON SEP 27 2021 THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. HURRICANE CENTER LOCATED NEAR 16.3N 52.7W AT 27/2100Z POSITION ACCURATE WITHIN 15 NM PRESENT MOVEMENT TOWARD THE NORTHWEST OR 315 DEGREES AT 8 KT ESTIMATED MINIMUM CENTRAL PRESSURE 957 MB EYE DIAMETER 15 NM MAX SUSTAINED WINDS 105 KT WITH GUSTS TO 130 KT. 64 KT....... 25NE 20SE 20SW 20NW. 50 KT....... 40NE 30SE 30SW 35NW. 34 KT....... 90NE 90SE 60SW 70NW. 12 FT SEAS..120NE 90SE 90SW 120NW. 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 16.3N 52.7W AT 27/2100Z AT 27/1800Z CENTER WAS LOCATED NEAR 16.0N 52.4W FORECAST VALID 28/0600Z 17.0N 53.5W MAX WIND 115 KT...GUSTS 140 KT. 64 KT... 25NE 20SE 20SW 20NW. 50 KT... 40NE 30SE 30SW 35NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 28/1800Z 17.8N 54.6W MAX WIND 115 KT...GUSTS 140 KT. 64 KT... 30NE 25SE 20SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 29/0600Z 18.8N 55.8W MAX WIND 115 KT...GUSTS 140 KT. 64 KT... 30NE 25SE 20SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 29/1800Z 19.8N 57.3W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 40NE 30SE 25SW 30NW. 50 KT... 60NE 50SE 40SW 50NW. 34 KT...100NE 90SE 70SW 90NW. FORECAST VALID 30/0600Z 21.2N 59.0W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 60NE 50SE 40SW 50NW. 34 KT...110NE 100SE 70SW 90NW. FORECAST VALID 30/1800Z 22.9N 60.5W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 70NE 60SE 50SW 50NW. 34 KT...120NE 110SE 80SW 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 01/1800Z 27.8N 62.0W MAX WIND 105 KT...GUSTS 130 KT. OUTLOOK VALID 02/1800Z 34.7N 59.6W MAX WIND 100 KT...GUSTS 120 KT. REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 16.3N 52.7W NEXT ADVISORY AT 28/0300Z $$ FORECASTER LATTO
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Hurricane Sam Forecast Discussion Number 20
2021-09-27 16:59:08| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Mon Sep 27 2021 759 WTNT43 KNHC 271459 TCDAT3 Hurricane Sam Discussion Number 20 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Mon Sep 27 2021 Sam appeared weaker on satellite images earlier this morning, and the eye was not apparent in visible or infrared satellite imagery. However, a 27/0741 UTC SSMIS pass still showed a well-defined eye. Recent visible and infrared images from the past couple of hours however, show that the structure is becoming better organized again, with the eye again becoming apparent on visible satellite. A NOAA Hurricane Hunter aircraft has made one pass through the hurricane so far and found that the central pressure has risen to about 966 mb. The current intensity estimate is reduced slightly to 110 kt. The aircraft has not yet sampled the northeast quadrant, where the strongest winds are likely occurring. The initial motion is estimated to be northwestward, or 310/7. A subtropical ridge to the north and northeast of Sam will remain the primary steering mechanism over the next several days, and Sam is expected to continue on a similar heading through the next 72 hours or so. A mid- to upper-level trough is forecast to emerge off the U.S. Northeast and Mid-Atlantic coast and dig southward over the western Atlantic late this week. The deep-layer southerly flow ahead of this feature should cause Sam to accelerate to the north-northwest by late Thursday and then turn northward on Friday. The NHC forecast track is largely unchanged from the previous, except it is shifted ever so slightly westward at the hour 72 and 96 points, about midway between the previous NHC forecast and the latest HCCA consensus. The confidence in the track forecast is medium to high through 3 to 4 days. Beyond that time, confidence is about average, as there is some increase in model spread noted. Regarding the intensity forecast, Sam is forecast to travel over warm ocean temperatures while it remains in an environment of low vertical wind shear through the next 4 days. Some mid-level dry air noted on water vapor channels to the west of Sam could play a role in limiting Sam's intensity through hour 48. However, the environment could become more moist again after that time. Due to these competing factors, I opted to hold the intensity steady for the next several days, although fluctuations in intensity can be expected. The NHC intensity forecast is above all model guidance through hour 60, and is in agreement with the model consensus at hour 72-96. After that time, increasing wind shear induced by the approaching mid- to upper-level trough to the northwest of the cyclone should cause weakening. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 15.7N 52.1W 110 KT 125 MPH 12H 28/0000Z 16.4N 52.8W 110 KT 125 MPH 24H 28/1200Z 17.3N 53.9W 110 KT 125 MPH 36H 29/0000Z 18.1N 55.1W 110 KT 125 MPH 48H 29/1200Z 19.1N 56.5W 110 KT 125 MPH 60H 30/0000Z 20.2N 58.1W 110 KT 125 MPH 72H 30/1200Z 21.7N 59.8W 110 KT 125 MPH 96H 01/1200Z 25.9N 62.2W 110 KT 125 MPH 120H 02/1200Z 32.0N 61.3W 100 KT 115 MPH $$ Forecaster Hagen/Pasch
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Hurricane Sam Forecast Advisory Number 20
2021-09-27 16:52:06| National Hurricane Center (Atlantic)
Issued at 1500 UTC MON SEP 27 2021 000 WTNT23 KNHC 271452 TCMAT3 HURRICANE SAM FORECAST/ADVISORY NUMBER 20 NWS NATIONAL HURRICANE CENTER MIAMI FL AL182021 1500 UTC MON SEP 27 2021 THERE ARE NO COASTAL WATCHES OR WARNINGS IN EFFECT. HURRICANE CENTER LOCATED NEAR 15.7N 52.1W AT 27/1500Z POSITION ACCURATE WITHIN 15 NM PRESENT MOVEMENT TOWARD THE NORTHWEST OR 310 DEGREES AT 7 KT ESTIMATED MINIMUM CENTRAL PRESSURE 966 MB MAX SUSTAINED WINDS 110 KT WITH GUSTS TO 135 KT. 64 KT....... 25NE 20SE 20SW 20NW. 50 KT....... 40NE 30SE 30SW 35NW. 34 KT....... 90NE 90SE 60SW 80NW. 12 FT SEAS..120NE 90SE 120SW 120NW. 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 15.7N 52.1W AT 27/1500Z AT 27/1200Z CENTER WAS LOCATED NEAR 15.4N 51.7W FORECAST VALID 28/0000Z 16.4N 52.8W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 25NE 20SE 20SW 20NW. 50 KT... 40NE 30SE 30SW 35NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 28/1200Z 17.3N 53.9W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 30NE 25SE 20SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 29/0000Z 18.1N 55.1W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 30NE 25SE 20SW 25NW. 50 KT... 50NE 40SE 30SW 40NW. 34 KT... 90NE 90SE 60SW 80NW. FORECAST VALID 29/1200Z 19.1N 56.5W MAX WIND 110 KT...GUSTS 135 KT. 64 KT... 40NE 30SE 25SW 30NW. 50 KT... 60NE 50SE 40SW 50NW. 34 KT...100NE 90SE 70SW 80NW. FORECAST VALID 30/0000Z 20.2N 58.1W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 60NE 50SE 40SW 50NW. 34 KT...110NE 100SE 70SW 80NW. FORECAST VALID 30/1200Z 21.7N 59.8W MAX WIND 110 KT...GUSTS 135 KT. 50 KT... 60NE 60SE 50SW 50NW. 34 KT...110NE 110SE 80SW 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 01/1200Z 25.9N 62.2W MAX WIND 110 KT...GUSTS 135 KT. OUTLOOK VALID 02/1200Z 32.0N 61.3W MAX WIND 100 KT...GUSTS 120 KT. REQUEST FOR 3 HOURLY SHIP REPORTS WITHIN 300 MILES OF 15.7N 52.1W NEXT ADVISORY AT 27/2100Z $$ FORECASTER HAGEN/PASCH
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Hurricane Sam Forecast Discussion Number 19
2021-09-27 10:49:06| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Sep 27 2021 000 WTNT43 KNHC 270848 TCDAT3 Hurricane Sam Discussion Number 19 NWS National Hurricane Center Miami FL AL182021 500 AM AST Mon Sep 27 2021 The satellite presentation of Sam deteriorated overnight, as GOES-16 infrared imagery showed periodic disruptions to Sam's inner core convection. The eye has not been readily apparent in conventional satellite imagery for much of the night, although very recent imagery suggests an eye could be re-emerging. Data from the reconnaissance aircraft last night indicated that an eyewall replacement cycle (ERC) was underway, and some dry mid-level air impinging on the western side of the circulation could also be negatively impacting Sam. No recent high-resolution microwave data is available, but an earlier scatterometer pass revealed that the tropical-storm-force wind radii had expanded just a bit in the eastern semicircle of the hurricane. The initial intensity is lowered to 115 kt for this advisory based on a blend of the objective and subjective Dvorak current intensity estimates. A NOAA hurricane hunter aircraft is scheduled to investigate Sam later this morning, which will provide crucial data to assess changes in Sam's structure and intensity. The initial motion is estimated to be northwestward, or 315/7 kt. A subtropical ridge to the north and northeast of Sam will remain the primary steering mechanism over the next several days, and Sam is expected to maintain a northwestward heading through midweek. Then, a mid- to upper-level trough is forecast to dig southward over the western Atlantic late this week. The deep-layer southerly flow ahead of this feature should cause Sam to move faster toward the north-northwest by 96 h, followed by a northward acceleration thereafter. The NHC forecast track is shifted slightly to the right of the previous one through the first 48-60 h of the forecast period, which accounts for the more northwestward initial motion observed during the past 12 h. Otherwise, the NHC track is virtually unchanged as the guidance has remained fairly consistent. Sam's intensity has likely peaked now that an ERC has commenced, but environmental conditions should allow Sam to persist as a major hurricane for the next several days. Sea-surface temperatures are expected to be 28 deg C or greater along Sam's track through 120 h, and the vertical wind shear is forecast to remain weak (< 10 kt) through at least midweek. Thus, the official NHC intensity forecast only shows very gradual weakening during the next several days, generally following the HCCA and IVCN consensus aids. Given the lower initial intensity of Sam, the latest NHC forecast is once again lower than the previous one. By 120 h, southerly shear associated with the deep-layer trough is forecast to increase over Sam, which should induce more weakening by this weekend. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 15.2N 51.4W 115 KT 130 MPH 12H 27/1800Z 15.9N 52.2W 115 KT 130 MPH 24H 28/0600Z 16.8N 53.3W 115 KT 130 MPH 36H 28/1800Z 17.7N 54.4W 115 KT 130 MPH 48H 29/0600Z 18.6N 55.6W 110 KT 125 MPH 60H 29/1800Z 19.6N 57.1W 110 KT 125 MPH 72H 30/0600Z 21.0N 58.8W 110 KT 125 MPH 96H 01/0600Z 24.9N 61.6W 110 KT 125 MPH 120H 02/0600Z 30.5N 62.0W 105 KT 120 MPH $$ Forecaster Reinhart
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