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Tropical Storm Peter Forecast Discussion Number 7
2021-09-20 10:48:50| National Hurricane Center (Atlantic)
Issued at 500 AM AST Mon Sep 20 2021 000 WTNT41 KNHC 200848 CCA TCDAT1 Tropical Storm Peter Discussion Number 7...Corrected NWS National Hurricane Center Miami FL AL162021 500 AM AST Mon Sep 20 2021 Corrected name of storm in first and second paragraphs Peter is a strongly sheared tropical storm. Reconnaissance wind data, conventional satellite imagery, and passive microwave satellite data indicate that Peter's center is located just to the west of the deep convection. Data from the aircraft support holding the intensity at 45 kt for now based on 925-mb flight-level and SFMR winds of 53 kt and 41 kt, respectively, on their last leg when the aircraft came in from the northeast. The central pressure had also increased only slightly to 1006 mb. Peter has continued to move west-northwestward, or 295/12 kt. There are no significant changes to the previous track forecast or reasoning. Peter is expected to move around the southwestern and western periphery of a subtropical ridge for the next couple of days, followed by a slow northward motion on days 3-5 as the cyclone moves through a weakness in the ridge induced by the southward-moving former Tropical Storm Odette. The new NHC track forecast is very similar to the previous advisory track, and lies just to the east of the tightly packed consensus track models. Some fluctuations in intensity will be possible during the next 24-36 hours owing to 5-10 kt fluctuations in the magnitude and also the direction of the deep-layer vertical wind shear. However, by 48 hours and beyond, a slow weakening trend is expected due to the shear increasing from the southwest and west-southwest of near 30 kt. The new official intensity forecast is essentially just an update of the previous advisory, and closely follows the IVCN and HCCA intensity consensus models. Based on the latest track, intensity, and wind radii forecasts, no tropical storm watches or warnings are required for the northern Leeward Islands, Virgin Islands, or Puerto Rico at this time. However, locally heavy rain is possible on Monday and Tuesday when Peter is expected to pass to the north of these locations. Key Messages: Rainfall around the southern periphery of Tropical Storm Peter may lead to areas of urban and small stream flooding through Tuesday across Puerto Rico, the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 19.1N 59.5W 45 KT 50 MPH 12H 20/1800Z 19.7N 61.4W 40 KT 45 MPH 24H 21/0600Z 20.4N 63.6W 40 KT 45 MPH 36H 21/1800Z 21.4N 65.5W 35 KT 40 MPH 48H 22/0600Z 22.4N 67.2W 35 KT 40 MPH 60H 22/1800Z 23.2N 68.0W 30 KT 35 MPH 72H 23/0600Z 24.2N 68.4W 30 KT 35 MPH 96H 24/0600Z 26.2N 67.8W 25 KT 30 MPH 120H 25/0600Z 28.5N 66.8W 30 KT 35 MPH $$ Forecaster Stewart
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Tropical Storm Peter Forecast Discussion Number 6
2021-09-20 04:51:09| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 19 2021 000 WTNT41 KNHC 200251 TCDAT1 Tropical Storm Peter Discussion Number 6 NWS National Hurricane Center Miami FL AL162021 1100 PM AST Sun Sep 19 2021 Peter is a sheared tropical cyclone. Satellite imagery and microwave data indicate that an earlier convective burst near the center of Peter has collapsed tonight. Although the low-level center is now displaced at least 60 n mi west of the edge of the convective cloud mass, an Air Force Reserve Hurricane Hunter aircraft investigating Peter has found the cyclone is a bit stronger tonight. The aircraft measured flight-level (925 mb) winds of 58 kt and SFMR winds of 45 kt or so. These data support raising the initial intensity to 45 kt for this advisory. Peter's wind field is very asymmetric, with tropical-storm-force winds extending outward up to 120 n mi from the center only in its northeast quadrant. The moderate to strong southwesterly vertical wind shear that is plaguing the system is forecast to persist during the next several days. Thus, despite sufficient oceanic heat content along its forecast track, the official NHC intensity forecast does not show any further intensification. In fact, some gradual weakening is forecast since the system appears likely to struggle sustaining organized convection near its center, as suggested by GFS and ECMWF model simulated satellite imagery. The official NHC intensity forecast has been adjusted upward in the near-term to account for the stronger initial intensity, but otherwise closely follows the HCCA and IVCN aids and shows gradual weakening this week. While the official NHC forecast shows Peter remaining a tropical cyclone through the forecast period, the GFS suggests Peter could struggle to even maintain its closed low-level circulation in the coming days. Therefore, it is plausible that the cyclone could degenerate into an open wave and weaken somewhat quicker than forecast. Peter is moving west-northwestward, or 295/12 kt, along the southwestern periphery of a subtropical ridge over the central Atlantic. This general motion should continue for the next couple of days. Thereafter, the track forecast becomes more challenging. A low- to mid-level ridge is expected to build over the western Atlantic by midweek, which would keep the weakening cyclone on a more northwestward trajectory. But, a mid-level shortwave is forecast to drop southward and erode the southern extent of the ridge, which should eventually draw Peter more northward during the middle and latter parts of the week. There is more spread noted in the track guidance at days 3-5, as the timing of this northward turn is uncertain. The official NHC track forecast has been adjusted slightly to the left of the previous one, and it lies near the center of the guidance envelope and closer to the TVCA and HCCA aids. Based on the latest track, intensity, and wind radii forecasts, no tropical storm watches or warnings are required for the northern Leeward Islands, Virgin Islands, or Puerto Rico at this time. However, locally heavy rain is possible on Monday and Tuesday when Peter is expected to pass to the north of these locations. 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 Puerto Rico, the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 18.6N 58.5W 45 KT 50 MPH 12H 20/1200Z 19.3N 60.6W 40 KT 45 MPH 24H 21/0000Z 20.1N 62.9W 40 KT 45 MPH 36H 21/1200Z 21.0N 65.0W 35 KT 40 MPH 48H 22/0000Z 22.0N 66.7W 35 KT 40 MPH 60H 22/1200Z 23.0N 67.9W 30 KT 35 MPH 72H 23/0000Z 23.9N 68.6W 30 KT 35 MPH 96H 24/0000Z 25.7N 68.5W 25 KT 30 MPH 120H 25/0000Z 28.0N 67.5W 25 KT 30 MPH $$ Forecaster Reinhart
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Tropical Storm Rose Forecast Discussion Number 4
2021-09-20 04:50:08| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 19 2021 000 WTNT42 KNHC 200250 TCDAT2 Tropical Storm Rose Discussion Number 4 NWS National Hurricane Center Miami FL AL172021 1100 PM AST Sun Sep 19 2021 The center of Rose has been very difficult to locate in infrared satellite imagery this evening. However, an earlier ASCAT-A overpass as well as an SSMIS microwave image from around of the time of the previous advisory were both helpful in establishing the initial position, motion, and intensity of the tropical cyclone. Rose's center is located on the eastern edge of the main convective mass due to some southeasterly shear. The ASCAT ambiguity data revealed an area of 30-35 kt winds around the southeastern portion of the circulation, so the wind speed remains 35 kt for this advisory. This is also in agreement with subjective Dvorak T-numbers of T2.5 form both SAB and TAFB. The ASCAT data indicated that Rose is located slightly west of the earlier estimates, and the initial motion estimate is now northwestward or 320/14 kt. The cyclone is forecast to move generally northwestward during the next several days around the southwestern and western portions of a subtropical ridge located over the far eastern Atlantic. After day 4, a strong mid- to upper-level trough over the north-central Atlantic should cause Rose to turn northward. The dynamical models are in reasonably good agreement through about 60-72 hours, but there is growing east-to-west (cross-track) spread after that time. The NHC track forecast is not too different from the previous advisory, and lies close to the various consensus models in deference to the increasing model spread late in the period. Rose has about 24 hours over warm waters and in low vertical wind shear conditions in which to strengthen, and the new NHC intensity forecast for that time is unchanged from before. After 24 hours, increasing westerly shear is likely to result in some gradual weakening. A further increase in shear is anticipated by day 3 as Rose approaches the aforementioned trough. This is likely to result in additional weakening, and Rose is forecast to weaken to a tropical depression by day 4. Some of the global model guidance suggests that weakening could occur faster than indicated below, and it is possible that Rose will degenerate into a remnant low by the end of the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 15.3N 31.1W 35 KT 40 MPH 12H 20/1200Z 16.9N 32.6W 40 KT 45 MPH 24H 21/0000Z 18.9N 34.2W 45 KT 50 MPH 36H 21/1200Z 20.9N 35.5W 40 KT 45 MPH 48H 22/0000Z 22.6N 36.6W 40 KT 45 MPH 60H 22/1200Z 23.9N 37.5W 40 KT 45 MPH 72H 23/0000Z 25.0N 38.7W 35 KT 40 MPH 96H 24/0000Z 27.1N 40.8W 30 KT 35 MPH 120H 25/0000Z 29.0N 40.5W 30 KT 35 MPH $$ Forecaster Brown
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Tropical Storm Peter Forecast Discussion Number 5
2021-09-19 22:40:06| National Hurricane Center (Atlantic)
Issued at 500 PM AST Sun Sep 19 2021 000 WTNT41 KNHC 192039 TCDAT1 Tropical Storm Peter Discussion Number 5 NWS National Hurricane Center Miami FL AL162021 500 PM AST Sun Sep 19 2021 Over the past few hours deep convection with cloud tops as cold as -85 degrees C have developed near and to the east of the center of Peter. An Air Force Reserve Hurricane Hunter aircraft investigating the system earlier provided data during several legs of the flight that confirmed an intensity of 40 kt. A blend of the latest Dvorak T- and CI- numbers from TAFB are consistent with that data, and therefore the initial intensity remains 40 kt for this advisory. The vertical wind shear that stripped Peter of its deep convection earlier today is forecast to increase further through tonight and persist for at least a few days. Global model simulated satellite imagery suggests this latest burst of convection will also become removed from the center within several hours, with the cyclone struggling to maintain persistent deep convection throughout much of the 5-day forecast period. Therefore, despite being over very warm waters, Peter is forecast to slowly weaken over the next several days. The intensity model guidance is in decent agreement on this scenario, and the NHC forecast remains near the various intensity consensus solutions. As mentioned in the previous discussion, there are some indications, particularly by the GFS, that Peter could open back into a wave within a few days which adds some additional uncertainty to the intensity forecast. Peter's initial motion remains 290/15 kt. The storm is forecast to continue to move in this west-northwestward direction for the next couple of days as it is steered to the south of a subtropical ridge. This ridge is expected to weaken in a few days which should cause the cyclone to slow its forward motion and turn northwestward. Late in the forecast period a turn to the north and possibly northeast is expected to occur as Peter gets caught in the flow around a large trough to its north. The model guidance has shifted westward beyond day 2, in part due to a faster forward motion. While the timing of the cyclone's turn to the north remains the same, the NHC forecast was shifted to the left beyond 48 h, but still remains to the east of the consensus. Based on the track, intensity, and wind radii forecast, no tropical storm watches or warnings are required for the northern Leeward Islands, Virgin Islands or Puerto Rico. However, interests there should monitor the progress of the system as locally heavy rain is possible on Monday and Tuesday when it is expected to pass to the north of the area. Key Messages: 1. Rainfall around the southern periphery of Tropical Storm Peter may lead to areas of urban and small stream flooding from late today into Tuesday across Puerto Rico, the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 18.4N 57.8W 40 KT 45 MPH 12H 20/0600Z 19.1N 59.8W 40 KT 45 MPH 24H 20/1800Z 19.9N 62.1W 35 KT 40 MPH 36H 21/0600Z 20.8N 64.3W 35 KT 40 MPH 48H 21/1800Z 21.9N 66.0W 35 KT 40 MPH 60H 22/0600Z 22.9N 67.2W 35 KT 40 MPH 72H 22/1800Z 24.1N 68.2W 30 KT 35 MPH 96H 23/1800Z 26.1N 68.3W 30 KT 35 MPH 120H 24/1800Z 27.7N 67.5W 30 KT 35 MPH $$ Forecaster Latto
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Tropical Storm Rose Forecast Discussion Number 3
2021-09-19 22:39:36| National Hurricane Center (Atlantic)
Issued at 800 PM CVT Sun Sep 19 2021 000 WTNT42 KNHC 192039 TCDAT2 Tropical Storm Rose Discussion Number 3 NWS National Hurricane Center Miami FL AL172021 800 PM CVT Sun Sep 19 2021 Satellite images show that deep convection has increased in coverage near the low-level center during the last 6 hours, while the overall structure of the tropical cyclone has also improved. Based on the improved organization and consensus T-2.5 numbers from TAFB and SAB, the initial intensity is set to 35 kt, making Rose the seventeenth named storm of the busy 2021 Atlantic hurricane season. Only 2005 and 2020 have had the seventeenth named storm on an earlier date. The 12-hour motion is north-northwest, or 330/14. However, Rose appears to be moving more in a northwest direction during the past few hours. This is probably the start of a northwest motion that should continue through day 4 as the system moves along the southwestern and western periphery of a subtropical ridge that extends from Africa across the Cabo Verde Islands. A turn to the north is predicted in about 5 days, as Rose comes under the influence of a strong mid-level trough approaching from the northwest. No significant changes were made to the previous track forecast. Environmental conditions are conducive for strengthening during the next day or so, with low wind shear, SSTs near 27C, and sufficient mid-level atmospheric moisture. Therefore, a little more strengthening is shown compared to the previous NHC forecast. The new NHC forecast is in agreement with the various consensus aids, but slightly below the SHIPS model, in deference to the global models which show a weaker cyclone. Westerly wind shear will likely increase in about 36 hours as a strong upper-level trough approaches from the northwest. Given that the cyclone is forecast to remain rather small, it will likely be vulnerable to the higher shear so weakening is forecast in the 36-72 h time period. Beyond day 3, a further increase in wind shear is expected to cause Rose to weaken to a tropical depression. Although not explicitly forecast, it is possible that the cyclone could become a remnant low in 4-5 days if the shear is too much for the system to handle. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 14.3N 29.9W 35 KT 40 MPH 12H 20/0600Z 15.9N 31.4W 40 KT 45 MPH 24H 20/1800Z 17.9N 33.3W 45 KT 50 MPH 36H 21/0600Z 20.0N 34.8W 45 KT 50 MPH 48H 21/1800Z 22.0N 36.0W 40 KT 45 MPH 60H 22/0600Z 23.5N 37.0W 40 KT 45 MPH 72H 22/1800Z 24.7N 38.0W 35 KT 40 MPH 96H 23/1800Z 26.7N 40.6W 30 KT 35 MPH 120H 24/1800Z 28.7N 41.2W 30 KT 35 MPH $$ Forecaster Hagen/Blake
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