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Tropical Storm Victor Forecast Discussion Number 12

2021-10-02 10:42:34| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Oct 02 2021 118 WTNT45 KNHC 020842 TCDAT5 Tropical Storm Victor Discussion Number 12 NWS National Hurricane Center Miami FL AL202021 500 AM AST Sat Oct 02 2021 Victor's satellite appearance has continued to degrade this morning with the closest convection now located more than 150 nmi northeast of the fully exposed low-level circulation center. Thus, Victor no longer technically meets the convective criterion of a tropical cyclone. However, tropical cyclone status is being maintained for this advisory out of continuity with the previous advisory and just in case a burst of deep convection redevelops later today near the still-well-defined low-level circulation center. Victor's intensity has been lowered to 45 kt based on a subjective Dvorak current intensity estimate of CI3.0/45 kt from TAFB, and the typical decay rate and spin down of a non-convective vortex over open water. The motion estimate remains west-northwestward, or 300/11 kt. Victor is forecast to move west-northwestward to northwestward around the southwestern periphery of a deep-layer subtropical ridge for the next 72-96 hours or until dissipation occurs. The new NHC forecast track is similar to the previous advisory track, and lies close to the tightly packed simple- and corrected-consensus track models. Deep-layer south-southwesterly vertical wind shear of around 20 kt and entrainment of dry mid-level air have taken their toll on Victor. These unfavorable environmental parameters are expected to worsen, resulting in continued weakening throughout the forecast period. Therefore, Victor is forecast to become a tropical depression by Sunday, degenerate into a remnant low by Monday, and dissipate by Wednesday. However, the current weakening trend will be hastened if organized convection does not return within the next 12 hours. The new official intensity forecast is a little lower than the previous advisory, and is also lower than most of the intensity guidance due to Victor's aforementioned severely degraded convective pattern. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 12.8N 36.3W 45 KT 50 MPH 12H 02/1800Z 13.8N 37.7W 35 KT 40 MPH 24H 03/0600Z 15.5N 39.6W 30 KT 35 MPH 36H 03/1800Z 17.6N 41.8W 25 KT 30 MPH 48H 04/0600Z 19.8N 43.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 04/1800Z 21.7N 45.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 05/0600Z 23.1N 47.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 06/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Victor Forecast Discussion Number 11

2021-10-02 04:46:02| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Oct 01 2021 000 WTNT45 KNHC 020245 TCDAT5 Tropical Storm Victor Discussion Number 11 NWS National Hurricane Center Miami FL AL202021 1100 PM AST Fri Oct 01 2021 The structure of Victor this evening continues to degrade with the low-level center becoming decoupled from the mid- to upper-level circulation associated with the deeper convection. While the tropical storm does continue to produce a region of deep convection with cloud tops colder than -70 C, this activity is organized in a linear band that is now more than 150 n mi northeast of the exposed low-level center as seen on Proxy-Vis satellite imagery. Both ASCAT-B/C clipped the western half of Victor's circulation, showing peak winds of 40 kt on the far edge of the pass. In addition, the latest subjective and objective satellite intensity estimates have been falling this evening. The latest intensity for this advisory has been lowered to 50 kt this advisory, assuming the scatterometer data missed higher wind values to the northeast of Victor closer to the convection. However, given Victor's current structure, this estimate may still be generous. Now that the low-level cloud swirl is readily apparent on satellite imagery, it is somewhat easier to track Victor this evening, with the estimated motion still west-northwest at 290/11 kt. Because the cyclone is also becoming more vertically shallow, the primarily steering feature will be a large low-level subtropical ridge centered to the north, which should guide Victor on a general west-northwestward to northwestward heading for the next 2-3 days at a similar forward motion. The latest track guidance has shifted westward this cycle, likely in response to Victor being a weaker cyclone less coupled to the deep convection. The latest NHC track forecast was also shifted a bit west over the forecast period, staying closer to the consensus aids HCCA and TVCA, though it is worth nothing this is still not as far west as the latest GFS or ECMWF runs. While the deep-layer 200-850 hPa vertical wind shear has not been prohibitively strong today (15-20 kt), more substantial mid-level shear underneath the outflow layer (25-30 kt) appears to be responsible for the current disheveled appearance of Victor. This shear is related to a large upper-level cutoff low upstream of the tropical storm. Even though the current shear is not expected to increase much more in the short-term, the mid-level environment over Victor is expected to continue drying as the existing shear will import very dry air upstream into the core of the cyclone. All of the guidance responds to these unfavorable conditions by gradually weakening Victor over the next few days, and the latest NHC intensity forecast has been lowered a bit more compared to the previous advisory. While pulses of deep convection are likely to continue north of Victor over the next several days, the circulation is expected to gradually lose definition, and the global and high-res regional hurricane models now open up the system into a trough between 72-96 hours. The latest NHC intensity forecast now follows suit, showing dissipation by 96 hours. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 12.3N 35.3W 50 KT 60 MPH 12H 02/1200Z 13.2N 36.9W 45 KT 50 MPH 24H 03/0000Z 14.6N 38.7W 40 KT 45 MPH 36H 03/1200Z 16.5N 40.6W 40 KT 45 MPH 48H 04/0000Z 18.5N 42.6W 35 KT 40 MPH 60H 04/1200Z 20.6N 44.7W 35 KT 40 MPH 72H 05/0000Z 22.2N 46.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 06/0000Z...DISSIPATED $$ Forecaster Papin

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Hurricane Sam Forecast Discussion Number 38

2021-10-02 04:41:43| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Oct 01 2021 000 WTNT43 KNHC 020241 TCDAT3 Hurricane Sam Discussion Number 38 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Fri Oct 01 2021 The satellite appearance of Sam has degraded slightly during the past few hours, with the eye becoming more cloud filled and the eyewall cloud tops become a little less symmetric. Various subjective and objective satellite intensity estimates are in the 115-130 kt range, and based on the slight decay the initial intensity is lowered to 125 kt. An Air Force Reserve Hurricane Hunter aircraft is currently enroute to investigate Sam. Sam is making its forecast turn toward the north-northeast with the initial motion now 020/15 kt. There is no change to the track forecast philosophy and little change to the forecast track from the previous advisory. The steering flow between a deep-layer ridge to the northeast and east of Sam and a large mid-latitude low pressure system centered over Atlantic Canada should cause Sam to turn northeastward during the next 24 h, followed by a continued northeastward motion as the system become entrained in southwesterly mid-latitude flow. The models are in fairly good agreement, although there is some spread in both the track and the speed due to model differences in the cyclone's evolution as it becomes extratropical. The new forecast track lies close to the center of the guidance envelope and the various consensus models. Slow weakening is expected to continue during the next 2-3 days as Sam moves over cooler sea surface temperatures and begins extratropical transition. This is likely to be complete by 72 h, with the global models suggesting that Sam will evolve into a warm-core seclusion extratropical low. Based on this and the guidance, it is likely that the cyclone will maintain hurricane strength until after the transition is done. By the end of the forecast period, the extratropical low is expected to be decaying over the far north Atlantic. The new intensity forecast lies at the upper edge of the intensity guidance. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands the Greater Antilles, the Bahamas, and Bermuda during the next couple of days. Swells are expected to reach the United States east coast and Atlantic Canada on Saturday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are possible on Bermuda during the next several hours, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 31.4N 61.2W 125 KT 145 MPH 12H 02/1200Z 33.5N 60.2W 120 KT 140 MPH 24H 03/0000Z 35.8N 58.3W 110 KT 125 MPH 36H 03/1200Z 37.7N 55.6W 100 KT 115 MPH 48H 04/0000Z 39.6N 52.2W 95 KT 110 MPH 60H 04/1200Z 43.0N 47.3W 90 KT 105 MPH 72H 05/0000Z 47.2N 43.2W 80 KT 90 MPH...POST-TROP/EXTRATROP 96H 06/0000Z 50.5N 38.0W 65 KT 75 MPH...POST-TROP/EXTRATROP 120H 07/0000Z 54.5N 27.5W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Beven

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Tropical Storm Victor Forecast Discussion Number 10

2021-10-01 22:43:32| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Oct 01 2021 000 WTNT45 KNHC 012043 TCDAT5 Tropical Storm Victor Discussion Number 10 NWS National Hurricane Center Miami FL AL202021 500 PM AST Fri Oct 01 2021 Victor is looking disorganized. Deep convection has decreased in organization and coverage this afternoon. No recent microwave imagery has been available to assess the structure of the storm, but visible images from MET-11 and GOES-17 suggest that the circulation of the tropical storm is still elongated southwest-to-northeast. The intensity estimate remains 55 kt for this advisory, but this is on the high end of the recent estimates and could be generous. Compared to the large changes made this morning, almost no change was made to the official track forecast this afternoon. The model consensus has shifted substantially back to the right, and now lies very near the previous NHC forecast. Victor is forecast to turn northwestward tomorrow, and then continue on that heading through early next week, steered by ridging to the northeast. The new NHC forecast is very near the model consensus through the end of the forecast. Despite the recent agreement between the NHC forecast and the consensus, confidence in the forecast will remain low until we see better run-to-run consistency in the track model guidance. Shear and dry air may already be taking a toll on Victor, and SHIPS diagnostics from the GFS and ECMWF suggest that the environment will get worse with time. All of the intensity models forecast that Victor will weaken over the next several days in response to the hostile environment, and this is reflected in the NHC forecast, which is heavily based on the IVCN multi-model consensus. By day 5, all of the dynamical models indicate that Victor will likely degenerate into a trough of low pressure, so dissipation is shown. Several models, including the HWRF, GFS, and ECMWF indicate dissipation could occur sooner than that. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 12.0N 34.3W 55 KT 65 MPH 12H 02/0600Z 12.7N 35.5W 50 KT 60 MPH 24H 02/1800Z 13.9N 37.2W 45 KT 50 MPH 36H 03/0600Z 15.6N 39.0W 45 KT 50 MPH 48H 03/1800Z 17.6N 40.9W 40 KT 45 MPH 60H 04/0600Z 19.7N 42.9W 40 KT 45 MPH 72H 04/1800Z 21.5N 44.5W 35 KT 40 MPH 96H 05/1800Z 24.5N 47.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 06/1800Z...DISSIPATED $$ Forecaster D. Zelinsky

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Hurricane Sam Forecast Discussion Number 36

2021-10-01 16:52:51| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Oct 01 2021 000 WTNT43 KNHC 011452 TCDAT3 Hurricane Sam Discussion Number 36 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Fri Oct 01 2021 Sam remains a powerful category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale over the subtropical central Atlantic. Satellite images show that the hurricane still has a circular and quite distinct eye and closed eyewall. However, an 0827 UTC SSMIS pass indicated that a dry slot was present on the eastern side of the circulation between the eyewall and rainbands. The latest Dvorak classifications from TAFB, SAB, and CIMSS at the University of Wisconsin range from 102-115 kt. However, Air Force reconnaissance data from earlier this morning showed that the winds were much higher, and in fact, the Dvorak estimates have had a low bias compared to the Hurricane Hunter data for the past 24-36 hours. Therefore, the initial intensity is held at 130 kt for now. Another Air Force Hurricane Hunter aircraft is scheduled to investigate Sam this afternoon. An ASCAT-A pass from around 12Z indicated that Sam's tropical-storm-force wind radii have expanded, and the initial wind radii were adjusted based on that data. In addition, the eye of Sam passed about 60 n mi east of NOAA buoy 41049 earlier this morning. The buoy reported a minimum pressure of 998 mb, maximum winds of around 45 kt, gusts to 62 kt, and maximum seas of about 22 ft. The major hurricane has turned to the north and is now moving faster. The latest initial motion estimate is 355/18 kt. The large-scale pattern consists of a deep-layer ridge to the northeast of Sam and a large low pressure system centered over Atlantic Canada. The steering flow between these features should cause Sam to move northeastward at a sightly slower pace this weekend. By early next week, the deep-layer low is expected to retreat northward, which should cause Sam to turn a little more to the right. The models are in fairly good agreement, but there are some notable speed differences with the UKMET model being a fast outlier. The new NHC track forecast is slightly to the right of the previous one at 60 and 72 h, but is otherwise very similar. This forecast lies near the middle of the guidance envelope. Sam is expected to slowly weaken during the next several days as it moves over progressively cooler waters and into an environment of stronger shear. The hurricane will likely begin extratropical transition in a couple of days when it crosses the 26 degree C isotherm, and this transition is expected to be completed by 96 hours. All of the model guidance shows steady weakening through the forecast period, and so does the NHC forecast. This prediction is close to the IVCN, IVDR, and HCCA models in the short term and near the GFS model during the extratropical portion of the forecast. Even though Sam is likely to weaken, it is still expected to be a significant storm over the next several days. Key Messages: 1. Swells generated by Sam will impact the northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next couple of days. Swells are expected to reach Bermuda and the Bahamas this afternoon, and then spread to the United States east coast and Atlantic Canada on Saturday. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are possible on Bermuda beginning tonight or early Saturday, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 28.4N 61.8W 130 KT 150 MPH 12H 02/0000Z 30.7N 61.5W 125 KT 145 MPH 24H 02/1200Z 33.4N 60.2W 110 KT 125 MPH 36H 03/0000Z 35.6N 58.4W 100 KT 115 MPH 48H 03/1200Z 37.2N 56.0W 95 KT 110 MPH 60H 04/0000Z 38.7N 52.9W 90 KT 105 MPH 72H 04/1200Z 41.9N 48.6W 85 KT 100 MPH 96H 05/1200Z 49.0N 42.2W 75 KT 85 MPH...POST-TROP/EXTRATROP 120H 06/1200Z 51.0N 34.3W 60 KT 70 MPH...POST-TROP/EXTRATROP $$ Forecaster Cangialosi/Nepaul

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