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Tropical Storm Peter Forecast Discussion Number 12

2021-09-21 16:57:08| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Sep 21 2021 000 WTNT41 KNHC 211457 TCDAT1 Tropical Storm Peter Discussion Number 12 NWS National Hurricane Center Miami FL AL162021 1100 AM AST Tue Sep 21 2021 Like the last several days, this morning's visible satellite imagery indicates that Peter has become increasingly detached from the remaining fragmented convection that is firing more than 100 n mi east of the center. An Air Force Reserve Reconnaissance aircraft has been sampling the storm this morning and so far has yet to find any flight level (925 mb) or SFMR winds above tropical storm force, although they weren't able to fly the full pattern in the northeastern quadrant. The initial intensity has been lowered to 35 kt for this advisory, though the data so far suggests this estimate is generous. After taking a jog to the west overnight, Peter appears to have resumed a slower west-northwest motion at 290/8 kt. Over the next several days, Peter is expected to continue to slow down as it gradually turns to the north and then north-northeast as a deep-layer cyclone to the north is forecast to create a weakness in the current low-level ridging. The track guidance is in general agreement with this scenario, but differences remain in how sharp this rightward turn will be. The latest NHC track forecast has elected to stay close to the track consensus models TVCN and HCCA, and is a bit right of the previous track. After holding its own against a fairly hostile upper-level wind environment during the last 24-48 hours, Peter appears to finally be succumbing to the combination of 25-30 kt of deep-layer vertical wind shear helping to import sub-50 percent mid-level relative humidity over the center. This shear is not forecast to abate for the next 36-48 hours, and it now appears likely Peter will not be able to survive this unfavorable environment. The latest NHC intensity forecast now weakens Peter to a tropical depression in 12 hours, and degenerates it to a remnant low in 48 hours. However, given the current fragile state of the low-level circulation, it is distinctly possible that Peter may open up into a trough even sooner in the next 24-48 hours. Key Messages: 1. Rainfall around the southern periphery of Tropical Storm Peter could lead to areas of urban and small stream flooding through today across northern Hispaniola, Puerto Rico, portions of the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 21/1500Z 19.9N 64.7W 35 KT 40 MPH 12H 22/0000Z 20.7N 65.7W 30 KT 35 MPH 24H 22/1200Z 21.7N 67.0W 30 KT 35 MPH 36H 23/0000Z 22.7N 67.5W 25 KT 30 MPH 48H 23/1200Z 23.4N 67.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 24/0000Z 24.4N 67.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 24/1200Z 25.4N 66.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 25/1200Z 27.5N 64.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 26/1200Z 29.8N 62.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Papin

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Tropical Storm Rose Forecast Discussion Number 9

2021-09-21 10:37:16| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Sep 21 2021 000 WTNT42 KNHC 210837 TCDAT2 Tropical Storm Rose Discussion Number 9 NWS National Hurricane Center Miami FL AL172021 500 AM AST Tue Sep 21 2021 Rose has lost organization over the past several hours. Satellite images show a fully exposed low-level center with only a small area of deep convection lingering in the southeastern quadrant. The Dvorak classifications have decreased, and the initial intensity is nudged downward to 40 kt. Rose is moving northwestward at 17 kt. This general heading but with a decrease in forward speed is expected during the next couple of days as the storm is steered by the flow on the southwest side of a low- to mid-level ridge. By late this week and over the weekend, a sharp turn to the north and then the northeast is expected as Rose becomes embedded in the flow on the south side of a broad deep-layer trough. Although there is some spread in the guidance, the models all show the same general theme. The NHC track forecast lies near the middle of the guidance envelope. The environment is expected to become increasingly unfavorable for Rose during the next several days with westerly shear increasing and dry air expected to wrap into the circulation. These conditions should promote a weakening trend, and the cyclone is forecast to become a remnant low by the weekend when it should be over cooler waters and in an environment of more than 30 kt of shear. Some of the models suggest that Rose could become a remnant low or open into a trough sooner than that, and if the current trend continues that is a distinct possibility. The new forecast is largely an update of the previous one, but does show the storm becoming a remnant low a day earlier. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 21.0N 36.4W 40 KT 45 MPH 12H 21/1800Z 22.4N 37.2W 40 KT 45 MPH 24H 22/0600Z 23.8N 38.3W 35 KT 40 MPH 36H 22/1800Z 25.0N 39.5W 30 KT 35 MPH 48H 23/0600Z 26.4N 40.6W 30 KT 35 MPH 60H 23/1800Z 27.7N 41.0W 30 KT 35 MPH 72H 24/0600Z 29.0N 40.4W 30 KT 35 MPH 96H 25/0600Z 30.7N 36.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 26/0600Z 32.8N 31.4W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Cangialosi

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Tropical Storm Peter Forecast Discussion Number 10

2021-09-21 04:50:09| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 20 2021 000 WTNT41 KNHC 210250 TCDAT1 Tropical Storm Peter Discussion Number 10 NWS National Hurricane Center Miami FL AL162021 1100 PM AST Mon Sep 20 2021 Peter has proven to be a resilient tropical cyclone, despite strong upper-level wind shear that continues to displace its deep convection well east of its now exposed low-level center. Data from an Air Force Reserve Hurricane Hunter aircraft investigating Peter tonight, as well as recent scatterometer data, indicate that the cyclone has maintained its tropical storm intensity. An ASCAT-A pass shows several 40-kt wind vectors, with tropical-storm-force winds extending outward up to 150 n mi from the center in the northeastern quadrant. The aircraft has found peak 925-mb flight-level winds of 47 kt and SFMR winds of around 35 kt, although it did not sample the area where ASCAT depicted the strongest winds. The initial intensity is held at a possibly generous 45 kt for this advisory based on the scatterometer data. An upper-level trough to the northwest of Peter should maintain 20 to 30 kt of vertical wind shear over the cyclone for the next 2-3 days. Thus, intensification seems unlikely during this period, despite 29 deg C SSTs along Peter's forecast track. If the tropical cyclone can endure these hostile upper-level winds, it could survive through the entire forecast period, although the drier mid-level environment at higher latitudes will also work against Peter later this week. However, an alternative scenario that has been favored by the GFS is that Peter weakens sooner due to a lack of sustained convection and opens up into a trough late this week. The long-range forecast is further complicated by the potential development of another non-tropical low to the north of Peter later this week, which could interact with or absorb Peter. The official NHC intensity forecast is similar to the previous one and only shows gradual weakening over the next several days, which is in good agreement with the HCCA and IVDR consensus aids. Data from the aircraft indicate that the center has moved westward and slowed down a bit over the past several hours, and Peter's initial motion is estimated to be 285/10 kt. Peter is expected to continue moving generally west-northwestward through Tuesday, as it is steered around the southern extent of a low- to mid-level ridge over the central and western Atlantic. By Wednesday, a mid-level cutoff low is forecast to develop to the north of Peter over the western Atlantic, which will induce a weakness in the steering ridge. Therefore, the cyclone is forecast to slow down and turn northward and then north-northeastward through the latter part of the week and into the weekend. There are some larger along-track differences noted in the guidance at days 4-5, with the ECMWF moving or re-forming the center much farther north than the rest of the track guidance. The official NHC track forecast is shifted slightly to the right at 48 h and beyond, based on the latest TVCA and HCCA consensus aids. At longer ranges, the forecast is of much lower confidence and trends a bit slower than the consensus aids, which are heavily influenced by the outlying ECMWF solution. Key Messages: 1. Rainfall around the southern periphery of Tropical Storm Peter may lead to areas of urban and small stream flooding through Tuesday across northern Hispaniola, Puerto Rico, the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 19.8N 62.8W 45 KT 50 MPH 12H 21/1200Z 20.4N 64.2W 40 KT 45 MPH 24H 22/0000Z 21.3N 66.0W 40 KT 45 MPH 36H 22/1200Z 22.3N 67.2W 35 KT 40 MPH 48H 23/0000Z 23.2N 67.7W 35 KT 40 MPH 60H 23/1200Z 24.0N 67.7W 30 KT 35 MPH 72H 24/0000Z 25.1N 67.2W 30 KT 35 MPH 96H 25/0000Z 27.5N 66.0W 30 KT 35 MPH 120H 26/0000Z 30.5N 64.5W 30 KT 35 MPH $$ Forecaster Reinhart

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Tropical Storm Rose Forecast Discussion Number 8

2021-09-21 04:45:43| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 20 2021 000 WTNT42 KNHC 210245 TCDAT2 Tropical Storm Rose Discussion Number 8 NWS National Hurricane Center Miami FL AL172021 1100 PM AST Mon Sep 20 2021 Although Rose's convective pattern does not appear to be as organized as it was earlier today, a couple of ASCAT overpasses this evening indicated that the tropical cyclone has strengthened since this morning. Both ASCAT-A and -B instruments detected 40-45 kt winds around the southeastern portion of the circulation, and the initial intensity was raised to 45 kt around 0000 UTC with the issuance of a Tropical Cyclone Update. That intensity is maintained for this advisory, and it is also in agreement with subjective Dvorak data T-numbers of 3.0 from TAFB and SAB. The ASCAT data also indicated that the center of Rose is located to the west of the previous estimates, near the northwestern edge of the convective mass noted in satellite imagery. As a result of the center re-location, the initial motion is a somewhat uncertain 320/13 kt. Although the short-term portion of the track forecast has been adjusted westward due to the more westward initial position, there has been no overall change in forecast track reasoning for this advisory. Rose should continue northwestward around the western portion of a strong subtropical ridge during the next couple of days. Beyond that time, a mid- to upper-level trough over the north-central Atlantic should cause Rose to recurve northward, then northeastward, and finally east-northeastward by the end of the forecast period. The dynamical models are in general agreement on this scenario but there are some differences in how sharp Rose will turn, and how fast it will move northeastward late in the period. The new NHC track forecast shows a somewhat sharper turn than the previous advisory to be closer to the latest consensus aids. Rose has likely reached its peak intensity as increasing shear and the entrainment of drier mid-level air are likely to cause weakening by this time tomorrow. Additional weakening is expected after that time as a further increase in shear is anticipated when Rose nears the aforementioned trough in 2 to 3 days. The updated NHC wind speed forecast is slightly stronger at 12 and 24 hours due to the higher initial intensity, but is unchanged after that time. Given the expected hostile environmental conditions, it would not be surprising to see Rose wither more quickly and degenerate into a remnant low sooner than indicated below. That is indicated by at least some of the global model guidance. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 18.9N 35.4W 45 KT 50 MPH 12H 21/1200Z 20.5N 36.5W 45 KT 50 MPH 24H 22/0000Z 22.2N 37.6W 40 KT 45 MPH 36H 22/1200Z 23.5N 38.7W 35 KT 40 MPH 48H 23/0000Z 24.8N 39.9W 30 KT 35 MPH 60H 23/1200Z 26.1N 40.7W 30 KT 35 MPH 72H 24/0000Z 27.3N 40.8W 30 KT 35 MPH 96H 25/0000Z 29.0N 38.2W 25 KT 30 MPH 120H 26/0000Z 30.5N 33.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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Tropical Storm Rose Forecast Discussion Number 7

2021-09-20 22:34:04| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Sep 20 2021 000 WTNT42 KNHC 202033 TCDAT2 Tropical Storm Rose Discussion Number 7 NWS National Hurricane Center Miami FL AL172021 500 PM AST Mon Sep 20 2021 Satellite images show that Rose hasn't changed much during the past several hours, with the center on the northern side of the cloud shield. The last microwave pass from a few hours ago showed that the mid- and low-level centers of the cyclone remained 60-90 n mi apart, which is probably a sign that it isn't strengthening. The initial wind speed remains 35 kt based on continuity, and hopefully scatterometer data will be available for the next advisory due to the recent high bias of the conventional satellite estimates. The window for strengthening of the tropical cyclone is closing soon due to increasing shear and likely intrusions of dry mid-level air. These factors should cause the storm to start a weakening trend sometime tomorrow. The long-term future of Rose doesn't look golden either due to further increasing shear from an incoming upper- level trough. The new forecast is just a shade lower than the previous one, near the model consensus, with Rose likely sinking to a depression in a few days and degenerating to a remnant low by day 5. The initial motion remains northwestward at about 14 kt. Rose is is likely to continue moving in that general direction during the next couple of days around the southwestern and western periphery of a strong subtropical ridge. Beyond that time, guidance is coming into good agreement on a mid- to upper-level trough over the north-central Atlantic causing the cyclone to recurve to the north, northeast and even east-northeastward by the weekend. While the models don't agree on how sharp of a turn will occur, they are at least consistent that this trough will take the storm into a hostile environment and decay. The new forecast is again adjusted well to northeast at long range, and still might not be far enough to the east. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 18.4N 34.4W 35 KT 40 MPH 12H 21/0600Z 20.0N 35.6W 40 KT 45 MPH 24H 21/1800Z 21.8N 36.7W 35 KT 40 MPH 36H 22/0600Z 23.3N 37.8W 35 KT 40 MPH 48H 22/1800Z 24.5N 38.9W 30 KT 35 MPH 60H 23/0600Z 26.0N 39.9W 30 KT 35 MPH 72H 23/1800Z 27.3N 40.2W 30 KT 35 MPH 96H 24/1800Z 29.2N 38.8W 25 KT 30 MPH 120H 25/1800Z 31.5N 34.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Blake

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