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Tropical Storm Rene Forecast Discussion Number 17

2020-09-11 10:42:23| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 11 2020 000 WTNT43 KNHC 110842 TCDAT3 Tropical Storm Rene Discussion Number 17 NWS National Hurricane Center Miami FL AL182020 500 AM AST Fri Sep 11 2020 An AMSR2-GM1 microwave pass revealed that Rene's surface center is farther separated from the shrinking deep convection. The 0000 UTC FV3 and ECMWF model soundings indicate that east-southeasterly 30-35 kt shear near 300 mb is temporarily undercutting the diffluent southerly flow aloft. The initial intensity is held at 40 kt and is based on the subjective Dvorak estimates from TAFB and SAB, and an ADT estimates yielding 39 kt. The shear is expected to relax during the next 36 to 48 hours which should allow for gradual intensification. By mid-period, Rene is forecast to move into an area of increasing west-northwesterly shear, which should induce a weakening trend. It's worth noting that the ECMWF and the FV3 Decay SHIPS show very little strengthening through the period and it appears to be due to the cyclone moving into an even more inhibiting thermodynamic environment, in addition to the aforementioned shear. In fact, the relative humidity in the mid portions of the atmospheric is less than 49 percent beyond day 3. The NHC forecast sides with the various intensity consensus models and is above the statistical-dynamical guidance. Rene continues to move west-northwestward, or 290/9 kt within the southwestern periphery of a mid-tropospheric ridge extending from western Africa to the eastern Atlantic. A turn toward the northwest on Saturday, followed by a north-northwestward and northward motion with a decrease in forward speed is forecast Sunday and Sunday night. Early next week, a subtropical ridge is forecast to build to the northwest of Rene, in response to Tropical Storm Paulette moving northeastward over the northern central Atlantic. This change in the steering pattern should cause the cyclone to begin a southwestward to west-southwestward drift through the end of the period. The official track forecast is close to the previous advisory through day 4, then it's nudged a little south to conform with the HCCA and TVCA multi-model aids. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 19.7N 38.5W 40 KT 45 MPH 12H 11/1800Z 20.4N 40.2W 45 KT 50 MPH 24H 12/0600Z 21.6N 42.4W 50 KT 60 MPH 36H 12/1800Z 23.2N 44.3W 60 KT 70 MPH 48H 13/0600Z 25.0N 45.6W 60 KT 70 MPH 60H 13/1800Z 26.6N 46.1W 55 KT 65 MPH 72H 14/0600Z 27.4N 46.0W 50 KT 60 MPH 96H 15/0600Z 26.9N 46.4W 40 KT 45 MPH 120H 16/0600Z 26.1N 48.8W 35 KT 40 MPH $$ Forecaster Roberts

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Tropical Storm Paulette Forecast Discussion Number 18

2020-09-11 10:40:23| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 11 2020 000 WTNT42 KNHC 110840 TCDAT2 Tropical Storm Paulette Discussion Number 18 NWS National Hurricane Center Miami FL AL172020 500 AM AST Fri Sep 11 2020 Paulette continues to experience the effects of 35-40 kt of southwesterly vertical shear, which has caused the center to occasionally become exposed to the south and southwest of the primary convective bursts. Since the various satellite intensity estimates are unchanged since the last scatterometer overpass, the initial intensity is held at 55 kt. However, this could be a little generous. The initial motion is west-northwestward or 300/9 kt. There is little change to either the forecast track philosophy or the forecast track from the last advisory. The track guidance models remain in good agreement between a generally northwest to west-northwestward motion over the next 3-4 days as the cyclone is steered by the subtropical ridge to its north and northeast. Around day 4, the cyclone is expected to recurve into the westerlies through a break in the ridge, with this turn to the northeast expected at about the time of closest approach to Bermuda. The new forecast track lies close to the various consensus models. It should be noted that there is still sufficient spread in the track guidance such that it is too soon to know what types of impacts Paulette could have on Bermuda early next week. The global models are in reasonable agreement that the shear over Paulette should subside, although it may be another 24 h before it subsides enough that significant strengthening can occur. After that, conditions should be favorable for strengthening until the system again encounters strong upper-level winds near 120 h after recurvature. The intensity guidance is in good agreement that Paulette will strengthen to a hurricane during this time and reach a peak intensity in about 96 h, and the new intensity forecast has only minor changes from the previous forecast. The new forecast is close to, but a little weaker than the various intensity consensus models. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane this weekend and make its closest approach to the island on Monday and Tuesday. While the exact details of Paulette's track and intensity near the island are not yet known, the risk of strong winds, storm surge, and heavy rainfall on Bermuda continues to increase. 2. Swells produced by Paulette are expected to affect portions of the Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, and the southeastern United States into the weekend. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 23.1N 51.7W 55 KT 65 MPH 12H 11/1800Z 23.9N 53.1W 55 KT 65 MPH 24H 12/0600Z 25.3N 54.9W 55 KT 65 MPH 36H 12/1800Z 26.8N 56.7W 60 KT 70 MPH 48H 13/0600Z 28.2N 58.8W 65 KT 75 MPH 60H 13/1800Z 29.5N 61.2W 75 KT 85 MPH 72H 14/0600Z 30.7N 63.3W 85 KT 100 MPH 96H 15/0600Z 33.5N 64.0W 90 KT 105 MPH 120H 16/0600Z 36.0N 59.5W 90 KT 105 MPH $$ Forecaster Beven

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Tropical Storm Paulette Forecast Discussion Number 17

2020-09-11 04:51:20| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 10 2020 000 WTNT42 KNHC 110251 TCDAT2 Tropical Storm Paulette Discussion Number 17 NWS National Hurricane Center Miami FL AL172020 1100 PM AST Thu Sep 10 2020 Paulette has been fighting off intense vertical wind shear, which is analyzed by UW-CIMSS to be on the order of 35-40 kt out of the southwest. Deep convection with cold cloud tops of -70 to -80 degrees C have persisted near and to the northeast of the center throughout the evening. A late arriving ASCAT overpass revealed that Paulette is stronger than previously analyzed, with believable wind vectors of 50-55 kt in the northeastern quadrant. There were some values even higher than these. However, the ASCAT ambiguities and satellite imagery suggest most of those are either rain contaminated or too close to the edge of the ASCAT pass to be trustworthy. Nonetheless, the other values indicate that the initial intensity is now at least 55 kt. The SHIPS guidance suggests that the current shear over Paulette will begin to gradually decrease tomorrow and continue to decrease into the weekend. By late Saturday the guidance indicates the shear could fall to 10 kt or less. By that time, the cyclone is forecast to be traversing over waters with SSTs near 29 C. The only negative environmental factor could be some dry air in the vicinity of the system. However that same dry air has not seemed to have that much of an effect on Paulette as of late. With shear expected to remain fairly strong the next 24 h, no strengthening is anticipated during that time. After 24 h, Paulette should gradually intensify into early next week. The NHC intensity forecast for this advisory has been increased across the board. In the near term this change is due to the adjusted initial intensity. Beyond 24 h, the latest NHC forecast is close to a blend of the multi-model consensus values, including the HFIP corrected consensus and the Florida State Superensemble. Paulette is now moving northwest, or 305/09 kt. There is no change to the forecast track reasoning from the previous advisory. The forecast models are in good agreement between a generally northwest to west-northwestward motion over the next 3-4 days as the cyclone is steered around a subtropical ridge to its north. Around day 4, about the same time the system would be making its closest approach to Bermuda, a turn to the northeast is expected as the system rounds the western periphery of the ridge. The latest NHC track forecast is little changed from the previous one, and is near the tracks of the various consensus aids. It should be noted that there is sufficient spread in the track guidance such that it is too soon to know what types of impacts Paulette could have on Bermuda early next week. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane this weekend and make its closest approach to the island on Monday and Tuesday. While the exact details of Paulette's track and intensity near the island are not yet known, the risk of strong winds, storm surge, and heavy rainfall on Bermuda continues to increase. 2. Swells produced by Paulette are expected to affect portions of the Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, and the southeastern United States into the weekend. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 22.7N 50.9W 55 KT 65 MPH 12H 11/1200Z 23.4N 52.4W 55 KT 65 MPH 24H 12/0000Z 24.6N 54.1W 55 KT 65 MPH 36H 12/1200Z 26.1N 55.8W 60 KT 70 MPH 48H 13/0000Z 27.6N 57.6W 65 KT 75 MPH 60H 13/1200Z 28.9N 59.9W 70 KT 80 MPH 72H 14/0000Z 30.0N 62.3W 80 KT 90 MPH 96H 15/0000Z 32.6N 65.0W 85 KT 100 MPH 120H 16/0000Z 35.0N 62.5W 90 KT 105 MPH $$ Forecaster Latto

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Tropical Storm Paulette Forecast Discussion Number 16

2020-09-10 22:34:21| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 10 2020 000 WTNT42 KNHC 102034 TCDAT2 Tropical Storm Paulette Discussion Number 16 NWS National Hurricane Center Miami FL AL172020 500 PM AST Thu Sep 10 2020 Continuous bursts of deep convection have been ongoing to the north and northeast of Paulette's center of circulation, with the cyclone being affected by nearly 40 kt of deep-layer southwesterly shear. The highest intensity estimates are Dvorak classifications of T3.0/45 kt from TAFB and SAB, and that value remains the initial intensity. The shear should reach its peak magnitude this evening, which is likely to cause Paulette to weaken slightly during the next 24 hours. However, gradual re-strengthening is forecast to begin in about 36 hours, and the rate of intensification is expected to increase in 2 to 3 days when the shear could fall to 10 kt or less, along with a more unstable atmosphere and warmer sea surface temperatures. Paulette is now forecast to become a hurricane by day 3 and continue to intensify through the end of the forecast period. As was advertised in the previous forecast package, the new NHC forecast intensities have been bumped up on days 3 through 5 and now lie near or just below the IVCN intensity consensus and the HCCA corrected consensus aid. Paulette's heading over the past 6-12 hours has been toward the west-northwest, or 300/8 kt. The cyclone's trajectory is expected to oscillate between northwest and west-northwest for the next 4 days, being dictated by the strength of the subtropical ridge to the north and the depth of the steering flow depending on Paulette's intensity. The updated NHC track forecast during this period has been nudged a bit to the north, mostly to account for an adjustment of the initial position. The model guidance all agree that Paulette should turn northward around the western side of a central Atlantic high pressure area by day 5, with the expected hurricane likely to make a tight recurvature near Bermuda. There remains some uncertainty on exactly where that turn will occur, but for now the NHC forecast lies between the tighter-turning ECMWF and HCCA models and the more gradually-turning GFS and HWRF models. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane this weekend and make its closest approach to the island on Monday and Tuesday. While the exact details of Paulette's track and intensity near the island are not yet known, the risk of strong winds, storm surge, and heavy rainfall on Bermuda continues to increase. 2. Swells produced by Paulette are expected to affect portions of the Leeward Islands, the Greater Antilles, the Bahamas, Bermuda, and the southeastern United States into the weekend. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 22.1N 50.1W 45 KT 50 MPH 12H 11/0600Z 22.5N 51.5W 40 KT 45 MPH 24H 11/1800Z 23.6N 53.4W 40 KT 45 MPH 36H 12/0600Z 25.0N 55.1W 45 KT 50 MPH 48H 12/1800Z 26.6N 56.8W 45 KT 50 MPH 60H 13/0600Z 28.0N 58.9W 55 KT 65 MPH 72H 13/1800Z 29.2N 61.4W 65 KT 75 MPH 96H 14/1800Z 31.5N 65.5W 75 KT 85 MPH 120H 15/1800Z 34.5N 64.0W 80 KT 90 MPH $$ Forecaster Berg

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Tropical Storm Rene Forecast Discussion Number 15

2020-09-10 22:31:47| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 10 2020 000 WTNT43 KNHC 102031 TCDAT3 Tropical Storm Rene Discussion Number 15 NWS National Hurricane Center Miami FL AL182020 500 PM AST Thu Sep 10 2020 It appeared that the storm had become better organized this morning, but that development trend seems to have been at least temporarily interrupted. Rene's central features have become rather ragged-looking, and deep convection has diminished somewhat. It appears that the system is being disrupted a bit by easterly flow that is undercutting the outflow layer. There is also an apparent dearth of low-level inflow over the southern and southeastern portions of the circulation at this time. Dvorak intensity estimates from both TAFB and SAB remain T3.0/45 kt, so the current intensity estimate is unchanged from the previous advisory. Assuming that the upper-level winds will soon become a little more conducive for strengthening, Rene is forecast to become a hurricane by the weekend. The official intensity forecast is a little below the latest model consensus. Rene continues its west-northwestward movement with a motion of near 285/10 kt. The cyclone is expected to turn northwestward and then north-northwestward for the next few days, while moving around the western periphery of a mid-level anticyclone. Later in the forecast period, A mid-level high is predicted to build to the northwest of Rene, which should slow down the cyclone's forward progress and cause it to turn to the left. At the present time, it appears that Rene will remain sufficiently separated from Paulette, which is located about 800 n mi to the west, for there not to be a significant binary interaction between the two storms. The official track forecast remains close to the simple dynamical model consensus, TVCN. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 18.9N 36.8W 45 KT 50 MPH 12H 11/0600Z 19.4N 38.1W 50 KT 60 MPH 24H 11/1800Z 20.2N 40.1W 55 KT 65 MPH 36H 12/0600Z 21.3N 42.2W 60 KT 70 MPH 48H 12/1800Z 22.7N 44.1W 65 KT 75 MPH 60H 13/0600Z 24.3N 45.7W 65 KT 75 MPH 72H 13/1800Z 25.8N 46.7W 60 KT 70 MPH 96H 14/1800Z 27.3N 47.5W 50 KT 60 MPH 120H 15/1800Z 27.5N 49.5W 45 KT 50 MPH $$ Forecaster Pasch

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