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Hurricane Sam Forecast Discussion Number 9

2021-09-24 22:57:16| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 24 2021 000 WTNT43 KNHC 242057 TCDAT3 Hurricane Sam Discussion Number 9 NWS National Hurricane Center Miami FL AL182021 500 PM AST Fri Sep 24 2021 Sam's structure on satellite has improved this afternoon, with overshooting cloud tops rotating quickly around the small central dense overcast, providing the appearance that the hurricane is mixing out the dry air that affected its core structure this morning. In fact, the last few visible satellite images suggest that a small eye is clearing out within the cirrus canopy. Interestingly, a recent 1930 UTC SSMIS pass suggests concentric bands are also forming in Sam's core structure and it remains to be seen how that structure will affect the wind field evolution of the hurricane. The latest subjective Dvorak intensities estimates form TAFB and SAB were a consensus T4.5/75 kt, while the latest SATCON and ADT estimates from UW-CIMSS have also increased this afternoon. The current advisory intensity has been set to 75-kt. Given the recent trends on satellite, this may be conservative. The motion of Sam has remained on the same heading, though the storm is beginning to slow down with the estimate now at 280/10 kt. An additional slowdown in forward motion is anticipated in the short-term as the mid-level ridging shuffles to the northwest of the tropical cyclone, impeding its forward motion. However, after 72 hours, this same ridge is expected to shift back to the northeast of Sam as a deep-layer trough off the Eastern United States coastline becomes established. Track guidance spread increases towards the end of the forecast, with the ECMWF and GFS remaining at odds, with the former on the equatorward side, and the latter on the poleward side of the track envelope. The consensus aids, however, have changed little from the previous advisory and the latest NHC track forecast is quite similar to the previous one. Now that Sam appears to have mixed out the dry air seen this morning, intensification, likely rapid, is resuming. The only fly in the ointment is the current concentric banding structure on microwave imagery, which hints at the possibility of an eyewall replacement cycle. Assuming the smaller eye will not collapse in the short term, rapid intensification appears likely over the next 12-24 hours and the intensity has been raised in the short term, taking Sam to major hurricane intensity by tomorrow. The latest NHC intensity forecast now shows a slightly higher peak at 120-kt in 48 hours followed by very gradual decay, due to a subtle increase in vertical wind shear, potentially some upwelling effects due to the slow forward notion, and eyewall replacement cycles that could lead to a broadening of the wind field. The latest NHC intensity forecast starts out on the high side of the intensity guidance but ends up very close to the HCCA consensus aid by the end of the forecast period. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 12.1N 44.8W 75 KT 85 MPH 12H 25/0600Z 12.3N 46.5W 90 KT 105 MPH 24H 25/1800Z 12.7N 48.1W 105 KT 120 MPH 36H 26/0600Z 13.1N 49.3W 115 KT 130 MPH 48H 26/1800Z 13.5N 50.5W 120 KT 140 MPH 60H 27/0600Z 14.3N 51.7W 115 KT 130 MPH 72H 27/1800Z 15.1N 52.9W 115 KT 130 MPH 96H 28/1800Z 16.9N 55.1W 110 KT 125 MPH 120H 29/1800Z 18.9N 58.1W 110 KT 125 MPH $$ Forecaster Papin

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Subtropical Storm Teresa Forecast Discussion Number 1

2021-09-24 22:56:46| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Sep 24 2021 000 WTNT44 KNHC 242056 TCDAT4 Subtropical Storm Teresa Discussion Number 1 NWS National Hurricane Center Miami FL AL192021 500 PM AST Fri Sep 24 2021 The disturbance just north of Bermuda interacting with a mid- to upper-level low has developed a prominent band of deep convection within its eastern semicircle as well as a well-defined surface center. Additionally, the system is not displaying any significant baroclinicity (i.e., frontal boundaries), so it is not an extratropical cyclone. All of which indicates that the system has evolved into a subtropical cyclone. ASCAT-C scatterometer data from 1440Z suggested peak winds were around 40 kt, which is the basis for calling the system Subtropical Storm Teresa. The system is moving toward the northwest at 12 kt is it rounds the northern part of the mid- to upper-level low. By Saturday, Teresa should turn northward and then northeastward, as it begins to be caught up in the mid-latitude westerlies. The forecast is based upon the TVCN track consensus technique. Teresa will not be long-lived. A developing extratropical system forming off of New England should absorb Teresa between 36 and 48 hours. Until then, the subtropical storm has a small window to intensify slightly while traversing lukewarm water and encountering moderate vertical shear. If deep convection develops near the system's center, then Teresa could evolve into a tropical storm. However, it is more likely that Teresa will remain a subtropical storm until dissipation in around two days. It is worth noting that Teresa will likely be the 9th so-called "shortie" of the 2021 hurricane season -- systems that are short- lived and relatively weak. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 34.5N 64.5W 40 KT 45 MPH 12H 25/0600Z 35.4N 65.5W 45 KT 50 MPH 24H 25/1800Z 36.3N 65.5W 45 KT 50 MPH 36H 26/0600Z 37.2N 64.4W 40 KT 45 MPH 48H 26/1800Z...DISSIPATED $$ Forecaster Landsea

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Hurricane Sam Forecast Discussion Number 8

2021-09-24 16:55:11| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Sep 24 2021 000 WTNT43 KNHC 241455 TCDAT3 Hurricane Sam Discussion Number 8 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Fri Sep 24 2021 After rapidly intensifying from a tropical depression to a hurricane over the past 24 hours, Sam is taking a momentary pause from intensification. The structure on visible satellite imagery remains quite healthy, with tightly coiled outer bands and a small but very cold central dense overcast. However, microwave passes at 0957 UTC and 1211 UTC on the 89-91 GHz channels show an erosion of the inner-core structure on the west side, which could be related to the dry-air slots mentioned in the previous advisory entraining into the core, temporarily disrupting the structure underneath the cirrus. The latest subjective Dvorak intensity estimates from TAFB and SAB were T4.0/4.0-65 kt and T4.0/4.5-75 kt while the latest UW-CIMSS ADT and SATCON objective estimates were both at 59 kt. The current intensity is being maintained at 65 kt for this advisory, taking a blend of these subjective and objective estimates. Sam remains a very small tropical cyclone, with the latest scatterometer wind data indicating that tropical-storm and hurricane-force winds have have not expanded much from last night. Sam continues to move just north of due west, but is beginning to gradually slow down at 280/12 kt. Over the next 24-36 hours, Sam is expected to maintain a general west to west-northwest heading but will slow down gradually as the strongest mid-level ridging becomes oriented more to the northwest of the hurricane. This ridge repositioning is thanks in part to a strong deep-layer trough well northeast of Sam that is digging in south of the Azores. Towards the end of the forecast, this trough will cutoff and gradually decay, allow the mid-level ridging to build-in back east as another deep-layer trough becomes established just off the eastern United States coastline. In general, the track guidance is in fairly good agreement on this general solution. However, differences remain in the details between how strong the ridge will be immediately to the north of Sam at the end of the 5-day forecast. The deterministic GFS and Canadian models show weaker ridging that erodes sooner, allowing a bit more poleward motion, while the ECMWF has stronger ridging that places its track on the equatorward side of the guidance envelope. The consensus track aid TCVN blends the aforementioned model guidance and has changed little this cycle, and the latest NHC track forecast is largely an update of the previous advisory. While the shear remains very low over Sam as diagnosed by the GFS- and ECMWF-based SHIPS guidance, the 700-500 mb layer mean relative humidity is a bit on the low side, and could possibly explain why Sam was susceptible to the dry-air intrusion that disrupted the core structure this morning. However, the vertical wind shear is expected to remain very low for the next 48-60 hours, and the hurricane should be able to easily mix out the dry air over the inner-core. The wind field also remains very compact, so once the inner-core closes back off, rapid intensification is expected to resume. The latest NHC intensity forecast still shows Sam becoming a major hurricane by tomorrow and currently has a peak intensity as a category 4 hurricane in 48-72 hours. Thereafter, a bit more vertical wind shear, and the likelihood of eyewall replacement cycles are likely to lead to fluctuations in the intensity that are difficult to predict ahead of time. The current intensity forecast remains close to the HCCA and IVCN consensus guidance. Regardless of the details, Sam is expected to be a significant hurricane through the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 11.8N 43.7W 65 KT 75 MPH 12H 25/0000Z 12.0N 45.4W 75 KT 85 MPH 24H 25/1200Z 12.3N 47.1W 90 KT 105 MPH 36H 26/0000Z 12.7N 48.4W 105 KT 120 MPH 48H 26/1200Z 13.1N 49.7W 115 KT 130 MPH 60H 27/0000Z 13.7N 50.9W 115 KT 130 MPH 72H 27/1200Z 14.6N 52.1W 115 KT 130 MPH 96H 28/1200Z 16.4N 54.8W 110 KT 125 MPH 120H 29/1200Z 18.2N 57.7W 110 KT 125 MPH $$ Forecaster Papin

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Hurricane Sam Forecast Discussion Number 7

2021-09-24 10:32:03| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 24 2021 000 WTNT43 KNHC 240831 TCDAT3 Hurricane Sam Discussion Number 7 NWS National Hurricane Center Miami FL AL182021 500 AM AST Fri Sep 24 2021 Sam continues to rapidly intensify. Satellite images show a small, but well-developed inner core and pronounced curved bands that wrap most of the way around the center. There are some dry slots, however, between the core and bands. The latest satellite intensity estimates range from 55 to 77 kt, and based on that data and the continued improvement in the cyclone's structure, the initial wind speed is increased to 65 kt. This makes Sam a hurricane, the seventh one of the 2021 Atlantic hurricane season. It should be noted that Sam has a compact wind field, with hurricane-force and tropical-storm-force winds estimated to only extend outward up to 15 and 50 n mi from the center, respectively. Sam is moving westward at about 13 kt, and this general motion should continue for another 12 to 24 hours as it moves in the flow on the south side of a mid-level ridge. After that time, a decrease in forward speed and a turn to the west-northwest are expected, and the official forecast has Sam moving at a slow pace of only 6-8 kt during the 48-96-hour time period. By the middle of next week, the ridge is forecast to slide eastward as a trough moves over the western Atlantic. In response, the hurricane will likely turn northwestward as it approaches the northern Leeward Islands. The models have changed little this cycle with the GFS still on the northern side of the guidance envelope and the ECMWF on the southern side. The NHC track forecast is essentially an update of the previous one and lies between the GFS and ECMWF models, and near the consensus aids. The large scale environmental conditions all appear favorable for continued rapid intensification during the next day or so as the hurricane is expected to remain over warm 29 deg C waters and in very low wind shear conditions. All of the SHIPS rapid intensification indices are well above the climatological means, and the NHC intensity forecast calls for Sam to become a major hurricane by early Saturday. Beyond a couple of days, the environment is likely to become a little less ideal, and most of the models show Sam leveling off in strength, and so does the official forecast. This intensity prediction lies near a blend of the FSSE, HCCA, and IVCN consensus models. Regardless of the details, Sam is expected to be a significant hurricane during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 11.5N 42.2W 65 KT 75 MPH 12H 24/1800Z 11.7N 44.0W 80 KT 90 MPH 24H 25/0600Z 12.1N 45.9W 95 KT 110 MPH 36H 25/1800Z 12.5N 47.4W 105 KT 120 MPH 48H 26/0600Z 12.9N 48.7W 115 KT 130 MPH 60H 26/1800Z 13.4N 50.0W 115 KT 130 MPH 72H 27/0600Z 14.1N 51.1W 110 KT 125 MPH 96H 28/0600Z 15.9N 53.8W 110 KT 125 MPH 120H 29/0600Z 17.8N 56.6W 110 KT 125 MPH $$ Forecaster Cangialosi

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Tropical Storm Sam Forecast Discussion Number 6

2021-09-24 04:40:34| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 23 2021 000 WTNT43 KNHC 240240 TCDAT3 Tropical Storm Sam Discussion Number 6 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Thu Sep 23 2021 Microwave data from around the time of the previous advisory revealed that Sam has developed a well-defined inner core, including a nearly closed eyewall feature in the 91-GHz channel. Dvorak estimates from TAFB and SAB have increased to T3.5 and T4.0, respectively, and the initial intensity is therefore estimated to be 60 kt. Sam's 24-hour intensity change qualifies as rapid intensification, since the cyclone was only a 30-kt tropical depression at this time yesterday. Sam is small, however. Recent scatterometer data showed that tropical-storm-force winds only extend a maximum of 40 n mi from the center, and the radius of maximum winds is estimated to be about 15 n mi. This small size is likely to have implications on Sam's future intensity. Sam is moving a little slower toward the west (280/13 kt). A mid-tropospheric ridge is expected to build across the Caribbean Sea and extend northeastward over the central Atlantic during the next few days. This blocking high is expected to cause Sam to slow down to 6 to 7 kt by late Saturday and Sunday while it maintains a westward to west-northwestward heading. Global models suggest the ridge may weaken and shift eastward a bit by days 4 and 5, allowing Sam to turn to the northwest and speed up a little by the end of the forecast period. The ECMWF, HWRF, and HCCA models lie along the southern edge of the main pack of models, while the ECENS ensemble mean is even farther south. The GFS and the HMON are on the northern side. Partially due to an adjustment of the initial position, the new NHC track forecast has been shifted southward a bit from the previous forecast, and it lies just south of the TVCA consensus aid--but not as far to the left as the previously mentioned southern models. Environmental conditions and Sam's structure and size are ideal for continued rapid intensification (RI). The SHIPS RI index for a 30-kt increase in 24 hours is nearly 50 percent. The DTOPS methodology, which uses inputs from the statistical-dynamical and deterministic models, shows RI indices for various forecast periods exceeding 90 percent. Therefore, rapid strengthening is being forecast at least for the next 36 hours, with Sam becoming a hurricane very soon and then a major hurricane by Friday night or Saturday morning. One caveat to this forecast is that although Sam's small size more readily allows for RI to occur, it can also make the cyclone more prone to weakening if, for example, vertical shear increases. By 48 hours, the intensity models show the strengthening trend leveling off, and the NHC intensity forecast follows suit. Fluctuations in intensity from days 3 through 5 are likely, both due to normal internal dynamics within the storm and the cyclone's response to the surrounding environment in relation to its small size. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 11.2N 40.9W 60 KT 70 MPH 12H 24/1200Z 11.4N 42.9W 75 KT 85 MPH 24H 25/0000Z 11.8N 45.0W 90 KT 105 MPH 36H 25/1200Z 12.1N 46.6W 105 KT 120 MPH 48H 26/0000Z 12.5N 47.8W 110 KT 125 MPH 60H 26/1200Z 13.0N 49.1W 110 KT 125 MPH 72H 27/0000Z 13.5N 50.4W 110 KT 125 MPH 96H 28/0000Z 15.3N 53.2W 110 KT 125 MPH 120H 29/0000Z 17.3N 56.1W 110 KT 125 MPH $$ Forecaster Berg

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