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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 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 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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Hurricane Sam Forecast Discussion Number 18
2021-09-27 04:49:28| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 26 2021 431 WTNT43 KNHC 270249 TCDAT3 Hurricane Sam Discussion Number 18 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Sun Sep 26 2021 A NOAA Hurricane Hunter aircraft investigating Sam this afternoon and evening found that the major hurricane likely peaked in intensity at around 135 kt with a central pressure of about 929 mb between 1900-2200 UTC when the eye contracted down to about 7 nmi in diameter. Since that time, however, wind and radar data from the aircraft reveal that Sam has been undergoing an eyewall replacement cycle (ERC), including significant erosion of the eyewall convection in the eastern semicircle, which has resulted in the central pressure increasing by at least 14 mb in only a few hours. The eye diameter has also doubled in size now. The ERC is also confirmed by the ragged inner-core convection and cloud-filled eye that have developed in satellite imagery. The advisory intensity of 125 kt is based on a blend of the available reliable SFMR surface wind data and flight-level wind data, and also using the pressure-wind relationship for a 943-mb central pressure. The initial motion estimate is now 315/06 kt. A strong subtropical ridge to the north and northeast of Sam is forecast by the global and regional models to remain entrenched across the central and eastern Atlantic for the next several days, resulting in the hurricane moving northwestward for the 72 hours. Thereafter, a mid- to upper-level trough is forecast to dig southward just off the U.S. east coast, creating a deep southerly flow regime that will act to accelerate Sam northward on days 4 and 5. The new NHC track forecast is similar the previous advisory, and lies close to the tightly packed consensus models through 72 hours. In the 96-120 hour period, the models diverge significantly on when and where Sam will begin moving out toward the north, and the NHC track forecast favors the more eastward GFS solution since that model has been performing quite well with Sam thus far. Now that Sam is undergoing an ERC, fluctuations in intensity could occur for the next day or two while the vertical wind shear remains low at least than 10 kt. Thereafter, a slow weakening trend is expected due to a modest increase in the wind shear from the southwest and occasional intrusions of drier mid-level air. The NHC intensity forecast is a little lower than the previous advisory, mainly due to the lower initial intensity, and closely follows the HCCA and IVCN consensus model intensity forecasts. FORECAST POSITIONS AND MAX WINDS INIT 27/0300Z 14.7N 50.8W 125 KT 145 MPH 12H 27/1200Z 15.3N 51.6W 125 KT 145 MPH 24H 28/0000Z 16.2N 52.8W 120 KT 140 MPH 36H 28/1200Z 17.0N 53.9W 120 KT 140 MPH 48H 29/0000Z 17.8N 55.0W 120 KT 140 MPH 60H 29/1200Z 18.7N 56.3W 115 KT 130 MPH 72H 30/0000Z 20.0N 58.0W 115 KT 130 MPH 96H 01/0000Z 23.4N 61.0W 110 KT 125 MPH 120H 02/0000Z 28.4N 62.6W 105 KT 120 MPH $$ Forecaster Stewart
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Hurricane Sam Forecast Discussion Number 17
2021-09-26 22:35:23| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 26 2021 000 WTNT43 KNHC 262035 TCDAT3 Hurricane Sam Discussion Number 17 NWS National Hurricane Center Miami FL AL182021 500 PM AST Sun Sep 26 2021 Sam has found a way to strengthen some more today. The ring of intense convection surrounding the 15-n-mi-diameter eye has expanded in size over the past several hours, while cloud tops colder than -70C still completely surround the center. An earlier GMI microwave overpass revealed very strong eyewall convection tightly wrapped around the small eye, a feature oftentimes seen with intense hurricanes. The latest Dvorak intensity estimates from TAFB and SAB remain T6.5/127 kt. However, the UW-CIMSS ADT estimate has recently increased to T6.6/130 kt. Based on the expansion of the ring of convection and a blend of these Dvorak estimates, the initial intensity is raised to 130 kt for this advisory. Sam remains compact, with winds of tropical storm force extending only 70 n mi from the center. A NOAA Hurricane Hunter aircraft is en route to investigate Sam and will provide more detailed intensity data. It would not take much further expansion of the convection and cooling of the cloud tops over the inner-most core of Sam for it to become a rare Category 5 hurricane on the Saffir-Simpson hurricane wind scale. Although plausible given the ideal environmental conditions over the next couple of days, this strengthening is not explicitly forecast to occur. The only factors within the next 2-3 days that could cause Sam to weaken would be an eyewall replacement cycle or the upwelling of cooler waters due to the hurricane's relatively slow forward motion. By 72 h, vertical wind shear is forecast to increase, which should cause gradual weakening. However, given the current strength of Sam, the new forecast calls for it to remain a major hurricane throughout the 5-day forecast period. The latest NHC intensity forecast was increased by 5 kt through 60 h due to the change in the initial intensity and remains on the high end of the guidance through that time. At 72 h and beyond, the intensity forecast is near the HFIP corrected consensus approach (HCCA) model. Sam has been wobbling a little north-northwestward and has slowed down over the past few hours, but the 12-h-average motion is about 305/06 kt. A subtropical ridge centered to the northeast of the hurricane should steer it northwestward for the next 3-4 days. By late Thursday, Sam is expected to reach the western periphery of the ridge and turn north-northwestward and begin to increase its forward speed as the cyclone starts to get caught up in the southerly flow between the ridge and an approaching mid-latitude trough. The various track model guidance is in better agreement today compared to yesterday, especially in the day 3-5 time frame. The latest NHC track forecast is little changed from the previous one, and lies near the various consensus track model solutions. FORECAST POSITIONS AND MAX WINDS INIT 26/2100Z 14.2N 50.5W 130 KT 150 MPH 12H 27/0600Z 14.7N 51.3W 130 KT 150 MPH 24H 27/1800Z 15.5N 52.5W 125 KT 145 MPH 36H 28/0600Z 16.3N 53.6W 125 KT 145 MPH 48H 28/1800Z 17.2N 54.7W 120 KT 140 MPH 60H 29/0600Z 18.1N 55.8W 120 KT 140 MPH 72H 29/1800Z 19.2N 57.4W 115 KT 130 MPH 96H 30/1800Z 22.1N 60.5W 110 KT 125 MPH 120H 01/1800Z 26.6N 62.9W 110 KT 125 MPH $$ Forecaster Latto
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