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Tropical Storm Victor Forecast Discussion Number 9

2021-10-01 16:52:25| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Oct 01 2021 000 WTNT45 KNHC 011452 TCDAT5 Tropical Storm Victor Discussion Number 9 NWS National Hurricane Center Miami FL AL202021 1100 AM AST Fri Oct 01 2021 Since the last advisory, Victor's deep convection has become limited to the northern portion of its circulation. Convective outflow is limited to the south, which indicates that the changes in the structure of the tropical storm are due at least in part to southerly wind shear. Microwave and ASCAT data between 0800 and 1200 UTC also indicate that the center of Victor has become elongated, and may be trying to reform to the north, closer to the convection. Even with that reformation, an 1128 ASCAT-B overpass revealed that the center of Victor is located substantially south of previous estimates, which has necessitated a significant change to the track forecast. With the new position, the initial motion estimate is now 295/13 kt. The tropical storm is still generally expected to turn northwestward over the weekend and then continue on that heading through early next week, steered by the southwest periphery of a mid-level ridge centered over the eastern Atlantic. Nearly all of the dynamical models are showing this turn occuring slightly later than earlier forecasts, which results in a track well to the west of the previous NHC forecast. Combined with the updated analysis of Victor's position, the new NHC forecast has been shifted over 100 n mi to the southwest by day 4. However, this is still on the far east side of the guidance envelope so additional changes to the track forecast might be needed this afternoon. The aforementioned ASCAT data showed peak winds near 50 kt, which supports an intensity of 55 kt, assuming a little undersampling due to the resolution of the instrument. The intensity analysis is also supported by the latest TAFB Dvorak estimate. Despite the large changes to the track forecast, no change of note was made to the intensity forecast. Increasing shear and a dry surrounding environment should cause Victor to weaken during the next few days. Some models even indicate it could dissipate before the end of the forecast period. The NHC forecast is based on the intensity model consensus. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 11.5N 33.5W 55 KT 65 MPH 12H 02/0000Z 12.0N 34.8W 55 KT 65 MPH 24H 02/1200Z 13.1N 36.4W 50 KT 60 MPH 36H 03/0000Z 14.7N 38.1W 45 KT 50 MPH 48H 03/1200Z 16.6N 40.0W 45 KT 50 MPH 60H 04/0000Z 18.7N 42.1W 40 KT 45 MPH 72H 04/1200Z 20.6N 43.9W 35 KT 40 MPH 96H 05/1200Z 23.6N 46.5W 30 KT 35 MPH 120H 06/1200Z 27.1N 46.9W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster D. Zelinsky

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Hurricane Sam Forecast Discussion Number 35

2021-10-01 10:46:48| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Oct 01 2021 000 WTNT43 KNHC 010846 TCDAT3 Hurricane Sam Discussion Number 35 NWS National Hurricane Center Miami FL AL182021 500 AM AST Fri Oct 01 2021 An Air Force Reserve Hurricane Hunter aircraft investigated Sam a few hours ago and found that the already-intense hurricane had strengthened a bit since yesterday afternoon's reconnaissance flight. The plane measured a peak 700-mb flight-level wind of 147 kt in the northeastern eyewall, and using a typical 90-percent reduction from that altitude, the intensity is estimated to be 130 kt. Dropsondes released in the eye also indicated that the central pressure had fallen to 934 mb. This drop in pressure is likely due to a combination of the strengthening and an expansion of the tropical-storm- and hurricane-force wind fields, which was confirmed by the aircraft and scatterometer data from last evening. Sam is moving a little faster toward the north-northwest (345/15 kt) as it curves around the western periphery of a mid-tropospheric ridge located over the eastern and central subtropical Atlantic. The hurricane is expected to turn northward later today and then northeastward by Saturday night--passing well to the east of Bermuda--when it begins to move between the ridge and a mid- to upper-level low located over the northeastern United States and Atlantic Canada. Interaction and a possible merger with the aforementioned low during extratropical transition is likely to cause a sudden north-northeastward acceleration to the southeast of Newfoundland on day 4, followed by a slower motion toward the east-northeast on day 5. The track models are very tightly clustered for much of the 5-day forecast period, yielding high confidence in the NHC track forecast. The new NHC forecast is relatively unchanged from the previous forecast, and closely follows the GFEX and TVCA consensus aids. Although fluctuations in intensity will still be possible, a general weakening trend is anticipated in the coming days as Sam moves over a gradually cooler ocean, reaching sub-26C waters by day 3. However, the weakening is not expected to be rapid due to continued upper-level divergence and only light-to-moderate deep-layer shear, and Sam could remain a major hurricane at least through Saturday night. Global models now indicate that Sam should complete extratropical transition by day 4, and baroclinic forcing is likely to keep the system as a potent hurricane-force post-tropical cyclone through the end of the forecast period. The NHC intensity forecast is essentially an update of the previous forecast, lying close to the HCCA and IVCN consensus aids. 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 today, and then spread to the United States east coast and Atlantic Canada this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are expected on Bermuda beginning tonight or early Saturday, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 26.6N 61.7W 130 KT 150 MPH 12H 01/1800Z 28.8N 62.0W 125 KT 145 MPH 24H 02/0600Z 31.7N 61.2W 115 KT 130 MPH 36H 02/1800Z 34.2N 59.7W 105 KT 120 MPH 48H 03/0600Z 36.0N 57.8W 95 KT 110 MPH 60H 03/1800Z 37.7N 55.0W 90 KT 105 MPH 72H 04/0600Z 40.1N 51.4W 85 KT 100 MPH 96H 05/0600Z 47.7N 44.8W 75 KT 85 MPH...POST-TROP/EXTRATROP 120H 06/0600Z 50.1N 38.1W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Berg

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Tropical Storm Victor Forecast Discussion Number 8

2021-10-01 10:42:18| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Oct 01 2021 000 WTNT45 KNHC 010842 TCDAT5 Tropical Storm Victor Discussion Number 8 NWS National Hurricane Center Miami FL AL202021 500 AM AST Fri Oct 01 2021 Infrared and microwave satellite imagery indicate that Victor has continued to become a little better organized, with a 0342Z AMSR2 overpass showing that a 75-percent-closed mid-level eye feature had formed. However, both data sources also revealed that a pronounced dry slot has formed in the southeastern quadrant. Upper-level outflow remains well established in all quadrants except to the south where it is somewhat restricted. Despite the slightly improved satellite signatures, subjective and objective intensity classifications have not changed since the previous advisory, so the intensity remains at 50 kt for this advisory. The initial motion estimate is 305/13 kt. Victor has made a slight jog toward the northwest, but this is likely a temporary motion due to redevelopment of the center farther into the convective cloud shield. The cyclone should resume a west-northwestward motion later today and maintain that motion for another day or so as Victor moves along the southwestern periphery of a strong subtropical ridge situated over the east-central Atlantic. The latest model runs continue to show a broad mid- to upper-level low developing over the central Atlantic to the west of Victor in the 24-72-h forecast period, which will act to erode the western portion of the ridge and create a deep-layer southerly flow pattern across the cyclone. As a result, Victor is forecast to turn northwestward by the weekend and then move northward by early next week. Owing to a more robust mid-/upper-level low forecast to develop than previously expected, the latest NHC track guidance has made a significant westward shift by at least 100 nmi at 72 h and nearly 200 nmi on days 4 and 5. This westward shift is due to Victor now forecast to weaken faster and become more vertically shallow, with the dominant steering flow shifting to low-level easterlies. The new NHC track forecast has also been shifted westward, but not as far west as the tightly packed consensus models, all of which lie well to the east of the westernmost GFS solution. Victor is expected to remain in a somewhat favorable environment for the next 18 h or so, which should allow for at least some slight strengthening to occur. Thereafter, however, southwesterly deep-layer vertical wind shear is forecast by the GFS and ECMWF models to increase to 20-25 kt in 24 hours and 30-35 kt by 72 h. These hostile shear conditions, in conjunction with a drier air mass, should result in gradual weakening during the 24-120-h period, with Victor becoming a remnant low on day 5. The new official intensity forecast is a little below the previous advisory, but it is above the consensus intensity models ICON, HCCA, and FSSE, due to the recent development of the aforementioned primitive eye feature. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 11.7N 32.3W 50 KT 60 MPH 12H 01/1800Z 12.4N 33.6W 55 KT 65 MPH 24H 02/0600Z 13.4N 35.3W 55 KT 65 MPH 36H 02/1800Z 14.9N 36.8W 50 KT 60 MPH 48H 03/0600Z 16.7N 38.6W 45 KT 50 MPH 60H 03/1800Z 19.0N 40.5W 40 KT 45 MPH 72H 04/0600Z 21.0N 42.0W 35 KT 40 MPH 96H 05/0600Z 24.0N 44.0W 30 KT 35 MPH 120H 06/0600Z 28.0N 45.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart

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Hurricane Sam Forecast Discussion Number 34

2021-10-01 04:53:43| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 30 2021 000 WTNT43 KNHC 010253 TCDAT3 Hurricane Sam Discussion Number 34 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Thu Sep 30 2021 Sam's satellite structure on infrared remains quite impressive this evening, with a circular eye staying quite warm (+10-20C) in comparison to the ring of cold cloud tops associated with the eyewall (-60 to -70C). The most recent microwave imagery was a SSMIS pass at 1957 UTC, which indicated the eyewall remains quite intense, if just a little asymmetric with a stronger western semicircle. This eyewall asymmetry is consistent with a touch of light southerly shear affecting the cyclone as diagnosed by ECMWF-SHIPS. However, this shear does not appear to be having a significant impact on the hurricane's intensity. While there have not been any recent aircraft data to determine Sam's intensity since the last mission departed the hurricane at around 2000 UTC, the satellite intensity estimates have not changed much this evening. Thus, the initial intensity has been held at 125-kt this advisory. Another Air Force Reserve Reconnaissance Mission will be flying into Sam overnight to get a better sense of the current intensity. Following Sam's eye tonight, the hurricane is beginning to make a gradual turn northward at a faster motion, with the latest estimate now north-northwest at 345/14 kt. The track guidance in the short-term remains in good agreement that Sam should continue to turn to the north, north-northeast, and then northeastward as it is steered around the western edge of a large subtropical ridge over the central Atlantic. This track should take the hurricane east of Bermuda. While there is a large deep-layer trough located north of Sam, this feature will not initially capture the cyclone, as this trough first de-amplifies and merges with a larger deep-layer trough located over the far eastern Provinces of Canada. In fact, Sam's forward motion to the northeast in 48-72 hours may even slow a tad as it remains just south of the stronger upper-level westerlies. However, by 96-120 hours, the deep-layer low over Canada is expected to dig southeastward into North Atlantic, and will likely capture Sam as the hurricane rapidly accelerates ahead of this amplifying trough. It is in the 72-120 hour time-frame that a lot of the track guidance diverges, mostly related to the timing and degree of Sam's interaction of with this trough ejecting out of Canada. For now, the official NHC track forecast has elected to stay close to the track consensus TVCN, which has preformed well with this hurricane so far. This latest track is just a bit northeast and faster than the previous one after 72 hours. Sam still has another 24 hours under warm 28-29 C sea-surface temperatures while the southerly vertical wind shear is expected to remain light. Thus, the hurricane is expected to maintain Category 4 intensity over the next day or so, with short-term intensity changes likely to be controlled by inner core dynamics (such as eyewall replacement cycles). Thereafter, sea surface temperatures decrease markedly as southerly shear is also expected to gradually increase. This combination should lead to steady weakening. After 72 hours, the rate of weakening has actually been slowed a tad, since the ECMWF, GFS, and Canadian models all show Sam receiving significant baroclinic support as it undergoes a dynamic extratropical transition while the hurricane phases with the large baroclinic trough emerging off Canada. Simulated satellite imagery from these models suggest that Sam's transition to an extratropical cyclone should take place between 96-120 hours, with the resulting cyclone producing a large wind footprint across the North Atlantic basin. Key Messages: 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by tomorrow, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning is in effect for the island. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 25.0N 61.2W 125 KT 145 MPH 12H 01/1200Z 27.2N 61.8W 125 KT 145 MPH 24H 02/0000Z 30.2N 61.6W 115 KT 130 MPH 36H 02/1200Z 32.9N 60.5W 105 KT 120 MPH 48H 03/0000Z 35.0N 58.9W 95 KT 110 MPH 60H 03/1200Z 36.6N 56.8W 85 KT 100 MPH 72H 04/0000Z 38.7N 53.8W 80 KT 90 MPH 96H 05/0000Z 44.6N 47.2W 75 KT 85 MPH 120H 06/0000Z 48.9N 42.5W 70 KT 80 MPH...POST-TROP/EXTRATROP $$ Forecaster Papin

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Hurricane Sam Forecast Discussion Number 33

2021-09-30 22:57:39| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 30 2021 000 WTNT43 KNHC 302057 TCDAT3 Hurricane Sam Discussion Number 33 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 30 2021 Sam remains a formidable hurricane, with a well-organized cloud pattern on satellite imagery. The 25 n mi wide eye is surrounded by a fairly symmetric pattern of deep convective cloud tops, and the upper-level outflow pattern is very well defined. An Air Force Reserve Unit Hurricane Hunter aircraft investigated the system and found peak 700-mb flight level winds of 133 kt, and a highest unflagged SFMR-observed surface wind of 125 kt. Based on these observations, the advisory intensity is kept at 125 kt. Measurements from the aircraft also showed that the central pressure remains quite low, near 938 mb. Sam will continue to traverse waters of high oceanic heat content for another 12-18 hours, and the shear should remain fairly low for the next few days. Therefore, some intensification could still occur overnight. In any event, the system is expected to maintain major hurricane status for the next 36-48 hours. Over the weekend, a gradual weakening trend should commence due to cooler SSTs. However, Sam will likely remain a powerful cyclone with hurricane- force winds until the end of the forecast period. By 120 hours, simulated satellite imagery from the ECMWF and GFS models forecast the system's cloud pattern to resemble that of an extratropical low, and this is reflected in the official forecast, which shows extratropical status at that time. The official intensity forecast remains close to the HCCA corrected consensus model solution. Center fixes continue to show a gradual increase in forward speed, and the hurricane is moving northwestward, or about 320/12 kt. During the next 24-36 hours, Sam should turn toward the north as it moves around the western periphery of a large subtropical high over the eastern and central Atlantic. Then, the cyclone is expected to move generally northeastward in the flow between the anticyclone and a trough near Atlantic Canada. There is fairly close agreement in the track models through 60 hours or so. However, there are some significant differences in the model-predicted track of Sam at higher latitudes, probably due to variations in how the system interacts with the trough in the various guidance. The NHC track forecast follows the latest model consensus, TVCN. Although the core of Sam is predicted to pass east of Bermuda early Saturday, the tropical-storm-force wind field is forecast to expand and could pass very close to the island beginning late Friday night or early Saturday. Therefore, a Tropical Storm Warning has been issued for Bermuda. Key Messages: 1. Swells generated by Sam will impact the Northern Leeward Islands and the Greater Antilles, including Puerto Rico, during the next few days. Swells are expected to reach Bermuda and the Bahamas by Friday, and then spread to the United States east coast by this weekend. These swells could cause life-threatening surf and rip current conditions. Please consult products from your local weather office. 2. Tropical storm conditions are expected on Bermuda beginning Friday night or early Saturday, and a Tropical Storm Warning has been issued for that island. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 23.6N 60.9W 125 KT 145 MPH 12H 01/0600Z 25.6N 61.5W 130 KT 150 MPH 24H 01/1800Z 28.5N 61.9W 125 KT 145 MPH 36H 02/0600Z 31.3N 61.4W 110 KT 125 MPH 48H 02/1800Z 33.8N 60.1W 100 KT 115 MPH 60H 03/0600Z 35.8N 58.3W 90 KT 105 MPH 72H 03/1800Z 37.4N 56.2W 85 KT 100 MPH 96H 04/1800Z 42.5N 50.0W 75 KT 85 MPH 120H 05/1800Z 47.5N 46.0W 65 KT 75 MPH...POST-TROP/EXTRATROP $$ Forecaster Pasch

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