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Hurricane Sam Forecast Discussion Number 12

2021-09-25 16:41:37| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 25 2021 000 WTNT43 KNHC 251441 TCDAT3 Hurricane Sam Discussion Number 12 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Sat Sep 25 2021 Sam has continued to intensify this morning and now has a complete ring of deep convection with cloud tops colder than -70 degrees C surrounding a mostly clear, small eye. The latest subjective Dvorak intensity estimates from TAFB and SAB are both 102 kt, while the combined objective Dvorak intensity estimates from the UW-CIMSS SATCON and ADT is 110 kt. Based on this data, the initial intensity has been increased to 105 kt for this advisory, making Sam a major hurricane. The cyclone remains compact, with hurricane-force winds extending only 20 n mi from the center. The hurricane wobbled to the west over the past few hours. However, the 12-hour motion is west-northwest at 9 kt. There are no changes to the forecast track reasoning. The ridge the north of Sam responsible for its current motion is forecast to shift to the east in a couple of days as a deep-layer trough establishes itself over the western Atlantic. Sam is forecast to turn northwestward around the southwestern periphery of the ridge in about 48 h, and gradually increase its forward speed thereafter. The model guidance continues to be in very good agreement with this scenario, and the latest NHC track forecast is unchanged from the previous one. It should be noted that the westernmost guidance continues to be the ECMWF ensemble mean, whose members have had a low bias in the intensity of Sam, which is likely contributing to the westward track bias. Based on the NHC forecast, and all of the other guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5. Environmental conditions support further strengthening in the short term, and Sam is expected to become a category 4 hurricane by Sunday. In a few days, some southwesterly shear is forecast to develop as the cyclone begins to round the periphery of the subtropical ridge. Although this should cause Sam to weaken, it is still expected to remain a powerful hurricane throughout the 5-day forecast period. The latest NHC intensity forecast is near the previous one, which remains slightly above the various consensus solutions through 72 h, and then closely follows the consensus thereafter. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 12.9N 47.6W 105 KT 120 MPH 12H 26/0000Z 13.3N 48.7W 110 KT 125 MPH 24H 26/1200Z 13.7N 49.8W 115 KT 130 MPH 36H 27/0000Z 14.3N 50.8W 120 KT 140 MPH 48H 27/1200Z 15.0N 51.9W 120 KT 140 MPH 60H 28/0000Z 15.8N 53.0W 115 KT 130 MPH 72H 28/1200Z 16.6N 54.1W 115 KT 130 MPH 96H 29/1200Z 18.4N 56.5W 110 KT 125 MPH 120H 30/1200Z 20.7N 59.8W 110 KT 125 MPH $$ Forecaster Latto

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Subtropical Storm Teresa Forecast Discussion Number 3

2021-09-25 10:34:59| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Sep 25 2021 000 WTNT44 KNHC 250834 TCDAT4 Subtropical Storm Teresa Discussion Number 3 NWS National Hurricane Center Miami FL AL192021 500 AM AST Sat Sep 25 2021 Teresa is poorly organized and it likely won't be a subtropical cyclone for much longer. The cloud pattern consists of a swirl of low-level clouds and a band of showers and thunderstorms that is located more than 250 n mi northeast of the center. This patch of convection has been detaching from the low-level circulation and it appears to be more involved with an upper-level low to the east of Teresa. ASCAT-B and ASCAT-C caught the circulation several hours ago, and showed winds of 25-30 kt near it. However, that instrument did not sample the area of convection well northeast of the center, where the winds could be a little stronger. Given the degraded structure of the system, the initial intensity is lowered to a possibly generous 35 kt. Now that the upper-level low has pulled away from the subtropical storm, west-southwesterly shear is increasing across the circulation and that should prevent convective organization and any opportunity for strengthening. Due to the strong shear and dry air entrainment, Teresa is likely to become a remnant low later today or tonight and dissipate on Sunday. Teresa is moving slowly west-northwestward at 5 kt. A turn to the north is expected later today, followed by a northeast motion as the cyclone moves in the flow ahead of a deep-layer trough. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 34.5N 65.5W 35 KT 40 MPH 12H 25/1800Z 35.4N 65.3W 30 KT 35 MPH 24H 26/0600Z 37.2N 64.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 26/1800Z...DISSIPATED $$ Forecaster Cangialosi

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Hurricane Sam Forecast Discussion Number 11

2021-09-25 10:31:24| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Sep 25 2021 000 WTNT43 KNHC 250831 TCDAT3 Hurricane Sam Discussion Number 11 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sat Sep 25 2021 Sam continues to rapidly intensify. Geostationary satellite and microwave images show that the storm has a very small and distinct eye surrounded by a nearly symmetric ring of cold cloud tops. The latest Dvorak estimates range from 90 to 102 kt, and based on that data, the initial intensity is increased to 95 kt. Although Sam is nearly a major hurricane, it remains quite compact with tropical-storm-force and hurricane-force winds estimated to only extend 50 n mi and 15 n mi from the center, respectively. The hurricane is moving west-northwestward at 11 kt and is currently being steered by a mid-level ridge to its north. There has been little change to the track forecast philosophy or model guidance. Sam is expected to continue west-northwestward during the next two or three days, but it is forecast to move at a notably slower pace of 6-8 kt during most of that time period. After that time, the ridge is expected to shift eastward as a deep-layer trough becomes established over the western Atlantic. This change in the steering pattern should cause Sam to turn northwestward and speed up some by the middle of next week. The models are in fairly good agreement, and the NHC track forecast is largely an update of the previous one. Based on this forecast, and nearly all of the reliable guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5. Continued steady or rapid strengthening is expected during the next day or two as the hurricane remains in near ideal conditions of very low wind shear and warm 29 deg C waters. However, beyond that time the shear is expected to increase a little, and that will likely cause Sam to level off in strength or weaken slightly next week. The NHC intensity forecast is slightly above the models in the short term, given the impressive structure of the hurricane and favorable conditions, but falls in line with the consensus aids at the longer lead times. Regardless of how strong Sam gets, nearly all of the models indicate that it will remain a powerful hurricane during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 12.8N 46.7W 95 KT 110 MPH 12H 25/1800Z 13.1N 48.0W 110 KT 125 MPH 24H 26/0600Z 13.5N 49.3W 115 KT 130 MPH 36H 26/1800Z 14.0N 50.4W 120 KT 140 MPH 48H 27/0600Z 14.7N 51.5W 120 KT 140 MPH 60H 27/1800Z 15.4N 52.5W 115 KT 130 MPH 72H 28/0600Z 16.3N 53.7W 115 KT 130 MPH 96H 29/0600Z 18.0N 56.0W 110 KT 125 MPH 120H 30/0600Z 20.1N 59.0W 110 KT 125 MPH $$ Forecaster Cangialosi

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Subtropical Storm Teresa Forecast Discussion Number 2

2021-09-25 04:42:19| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 24 2021 000 WTNT44 KNHC 250242 TCDAT4 Subtropical Storm Teresa Discussion Number 2 NWS National Hurricane Center Miami FL AL192021 1100 PM AST Fri Sep 24 2021 Teresa's cloud pattern is not well organized, even for a subtropical cyclone. The main convective band is rather ragged in appearance, and situated 100 n mi or more to the east and northeast of the exposed low-level center. The current intensity estimate, 40 kt, is in general agreement with the latest subtropical satellite classification from TAFB, but given the disheveled appearance of the system, this may be generous. Teresa has recently been moving more westward than northward, on the northern side of a mid-level cyclonic circulation, and the current motion estimate is an uncertain 300/9 kt. During the next day or so, the cyclone is likely to turn northward and northeastward ahead of a mid-tropospheric trough that will be moving over the northeastern United States and Atlantic Canada. The official forecast is a little faster than the previous one, but along the same general trajectory. The HCCA model consensus is somewhat faster than the NHC prediction. The cyclone has been situated under an upper-level low which has kept the vertical shear fairly low thus far. However, as this low moves away from the area, Teresa will begin to experience increasing west-southwesterly shear, and no strengthening is expected. In 48 hours or so, based on the global models, the storm is forecast to become absorbed by the aforementioned trough. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 34.3N 65.4W 40 KT 45 MPH 12H 25/1200Z 35.2N 65.5W 40 KT 45 MPH 24H 26/0000Z 36.2N 64.9W 40 KT 45 MPH 36H 26/1200Z 37.8N 64.0W 40 KT 45 MPH 48H 27/0000Z...ABSORBED $$ Forecaster Pasch

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Hurricane Sam Forecast Discussion Number 10

2021-09-25 04:34:49| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 24 2021 444 WTNT43 KNHC 250234 TCDAT3 Hurricane Sam Discussion Number 10 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Fri Sep 24 2021 Sam developed a 10-n mi wide eye in infrared satellite images a few hours ago, surrounded by a ring of cloud tops as cold as about -70 degrees Celsius. The core of the hurricane remains compact. In fact, recent Synthetic Aperture Radar (SAR) data indicated that the radius of maximum winds is only about 6 n mi. Dvorak estimates were T5.0/90 kt from TAFB and T4.5/77 kt from SAB at 0000 UTC, and given the additional clearing of the eye since then, Sam's intensity is raised to 85 kt. Sam has been moving toward the west-northwest during the past 6 to 12 hours with a motion of 285/12 kt. The track forecast reasoning remains the same, and the model guidance is packed more tightly than normal through day 5. A blocking ridge lying to the north and northwest is expected to cause Sam to slow down over the weekend and maintain a motion toward the west or west-northwest. On days 3 through 5, the ridge is expected to shift eastward, with a deep-layer trough developing over the western Atlantic. This should allow Sam to turn northwestward toward the end of the forecast period. The NHC track forecast has not budged much and remains close to the TVCA consensus aid. Based on this forecast, and nearly all of the reliable guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5. Deep-layer shear is expected to be less than 10 kt for the next day or two, and Sam will be heading toward waters of higher oceanic heat content. These conditions favor continued strengthening, and the hurricane is forecast to become a major hurricane on Saturday and possibly reach a peak intensity in 2 to 3 days. For this period, the NHC forecast is close to the HCCA consensus aid and is near the high end of the guidance envelope. As is usually the case with hurricanes this small and strong, fluctuations in intensity due to eyewall replacement cycles will be possible. On days 3 through 5, some increase in shear as Sam approaches the deep-layer western Atlantic trough is possible, and modest weakening is shown at those times. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 12.4N 45.9W 85 KT 100 MPH 12H 25/1200Z 12.7N 47.4W 100 KT 115 MPH 24H 26/0000Z 13.1N 48.8W 110 KT 125 MPH 36H 26/1200Z 13.5N 50.0W 115 KT 130 MPH 48H 27/0000Z 14.1N 51.2W 120 KT 140 MPH 60H 27/1200Z 14.8N 52.4W 120 KT 140 MPH 72H 28/0000Z 15.7N 53.5W 115 KT 130 MPH 96H 29/0000Z 17.4N 55.8W 110 KT 125 MPH 120H 30/0000Z 19.5N 58.8W 110 KT 125 MPH $$ Forecaster Berg

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