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Tropical Storm Sally Forecast Discussion Number 5

2020-09-12 22:56:54| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Sat Sep 12 2020 000 WTNT44 KNHC 122056 TCDAT4 Tropical Storm Sally Discussion Number 5 NWS National Hurricane Center Miami FL AL192020 500 PM EDT Sat Sep 12 2020 Sally continues to gradually become better organized, with developing convective banding features primarily over the southern portion of the circulation at this time. Upper-level outflow is still rather limited over the northern semicircle, possibly due to a weak upper low near the Georgia/Florida border. The latter feature is likely to soon move away from the area, however. Based on earlier scatterometer passes that showed tropical-storm-force winds over the Straits of Florida, the advisory intensity remains 35 kt. With very warm waters and a moist tropospheric environment ahead of Sally, strengthening seems likely. Some increase in westerly shear over the northern Gulf in 48-72 hours could at least slow down the intensification process around that time. The official intensity forecast is not far from the latest model consensus and a little above the previous forecast. Fixes from an Air Force Hurricane Hurricane aircraft, radar, satellite, and surface observations indicate a slow, generally westward motion of about 280/6 kt. A weakening mid-level high pressure system to the northeast of Sally should cause a generally west-northwestward to northwestward motion for the next few days. Because of the weakening steering currents, the cyclone should move rather slowly while it approaches the northern Gulf Coast. However, there is expected to be enough of a narrow north-south oriented ridge to the east of Sally in 3-4 days to steer the cyclone northward across the coast. The official track forecast has been nudged just slightly west of the previous one and lies between the simple and corrected dynamical track model consensus predictions. Users are reminded to not focus on the exact details of the track or intensity forecast as the average NHC track error at 96 h is around 150 miles and the average intensity error is around 15 mph. In addition, winds, storm surge, and rainfall hazards will extend far from the center. KEY MESSAGES: 1. Life threatening storm surge is possible along the Gulf Coast beginning on Monday, and a Storm Surge Watch is in effect for areas outside the southeast Louisiana Hurricane and Storm Damage Risk Reduction System from the Mouth of the Mississippi River to the Alabama/Florida border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are possible by early Tuesday from Grand Isle, Louisiana to the Alabama/Florida border, including Metropolitan New Orleans, with tropical storm conditions possible by Monday. 3. Sally is expected to produce flash flooding across portions of southern Florida and prolong existing minor river flooding across central Florida through Sunday. Flash, urban, and minor to moderate river flooding is likely across portions of the central Gulf Coast from Sunday through the middle of next week. 4. Tropical storm conditions are possible early next week in portions of the Florida Panhandle, where a Tropical Storm Watch is in effect. Wind gusts to tropical-storm force could occur over portions of the southern Florida Peninsula and the Florida Keys this evening. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 25.7N 81.9W 35 KT 40 MPH 12H 13/0600Z 26.4N 83.4W 40 KT 45 MPH 24H 13/1800Z 27.3N 85.3W 50 KT 60 MPH 36H 14/0600Z 28.0N 86.9W 60 KT 70 MPH 48H 14/1800Z 28.5N 88.1W 70 KT 80 MPH 60H 15/0600Z 29.0N 89.0W 75 KT 85 MPH 72H 15/1800Z 29.7N 89.3W 75 KT 85 MPH 96H 16/1800Z 31.1N 89.2W 55 KT 65 MPH...INLAND 120H 17/1800Z 33.1N 87.0W 30 KT 35 MPH...INLAND $$ Forecaster Pasch

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Tropical Storm Paulette Forecast Discussion Number 24

2020-09-12 22:46:25| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 12 2020 000 WTNT42 KNHC 122046 TCDAT2 Tropical Storm Paulette Discussion Number 24 NWS National Hurricane Center Miami FL AL172020 500 PM AST Sat Sep 12 2020 Convective cloud tops have cooled again as deep convection has redeveloped since this morning, with an eye-like feature noted in visible satellite imagery. Since the storm is still being affected by 20-25 kt of southerly shear, it is assumed that the low-level center is still displaced to the south of this feature a bit. But it won't be surprising if the low- and mid-level centers become collocated soon as the shear drops significantly during the next 6-12 hours. The initial intensity remains 60 kt, and we should get a much better handle on Paulette's maximum winds and structure once the first Air Force Hurricane Hunter aircraft reaches the storm this evening. The motion remains northwestward (305/13 kt) due to a break in the subtropical ridge to the north-northeast of Paulette. A mid-tropospheric high currently centered near the U.S. Mid-Atlantic coast is expected to slide eastward to the north of Paulette during the next 24 hours, which should cause the cyclone to turn west-northwestward tonight. Once the high becomes established over the central Atlantic, Paulette is expected to recurve around its western periphery toward a longwave trough over the northeastern United States Sunday night through Monday night. On days 3 through 5 of the forecast period, Paulette should be accelerating northeastward toward the north Atlantic, embedded in the mid-latitude westerlies. The new NHC track forecast is embedded among the very tightly clustered guidance envelope, and no significant changes were made from the morning forecast. As stated above, deep-layer shear is expected to be 10 kt or less between 12 and 48 hours. While ocean waters are plenty warm at 28-29 degrees Celsius, the main possible limiting factor for strengthening would be mid-level dry air, with the SHIPS model diagnosing ambient relative humidities of 30-40 percent for the next couple of days. But, if the shear remains low enough, that dry air is unlikely to infiltrate the circulation, and Paulette is therefore expected to intensify during the next 2-3 days. The NHC intensity forecast is close to the SHIPS, HCCA, and HWRF solutions, showing a peak intensity just under the major hurricane threshold in 2-3 days, when Paulette should be starting to move away from Bermuda. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane on Sunday and be near the island Sunday night and Monday. A prolonged period of strong winds, storm surge, and heavy rainfall is expected on Bermuda beginning Sunday evening, and a hurricane warning is in effect for the island. Preparations to protect life and property should be rushed to completion. 2. Swells produced by Paulette are affecting portions of the Leeward Islands, the Greater Antilles, the Bahamas, and Bermuda, and are expected to spread westward to the east coast of the United States during the next day or two. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 28.4N 58.5W 60 KT 70 MPH 12H 13/0600Z 29.2N 60.4W 65 KT 75 MPH 24H 13/1800Z 30.3N 62.8W 75 KT 85 MPH 36H 14/0600Z 31.8N 64.6W 85 KT 100 MPH 48H 14/1800Z 33.5N 64.9W 90 KT 105 MPH 60H 15/0600Z 35.3N 63.2W 95 KT 110 MPH 72H 15/1800Z 37.0N 59.8W 95 KT 110 MPH 96H 16/1800Z 40.0N 52.0W 85 KT 100 MPH 120H 17/1800Z 43.5N 44.0W 70 KT 80 MPH $$ Forecaster Berg

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Tropical Depression Rene Forecast Discussion Number 23

2020-09-12 22:34:57| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 12 2020 000 WTNT43 KNHC 122034 TCDAT3 Tropical Depression Rene Discussion Number 23 NWS National Hurricane Center Miami FL AL182020 500 PM AST Sat Sep 12 2020 A fragmented band of deep convection has recently developed over the northern portion of the circulation, but the associated cloud tops are already warming, suggesting that dry air is continuing to hinder development. Dvorak classifications of T2.0 (30 kt) from both TAFB and SAB, and the earlier ASCAT data are the basis for the 30-kt initial intensity. The depression is expected to remain within an area of dry mid-level air and increasing west- northwesterly shear on Sunday, and these factors are likely to contribute to gradual weakening over the next couple of days. While the shear is forecast to abate later in the period, the SHIPS guidance indicates mid-level relative humidity values of 30-40 percent, which suggest weakening is likely to continue. The global models also weaken the system with the UK and ECMWF showing dissipation within 4-5 days. The new NHC intensity forecast is again lowered from before and now calls for Rene to become a remnant low in 72 h and dissipate by day 5. Both of these events, however, could occur much sooner. Rene is moving northwestward at about 12 kt. A blocking ridge of high pressure is forecast to build to the northwest of Rene on Sunday which is expected to considerably slow the forward progress of the cyclone. As the ridge continues to shift eastward and build, Rene or its remnants are forecast to turn west-southwestward in the low-level steering flow early next week. The overall guidance envelope as changed little from this morning, and the new NHC track forecast is essentially an update of the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 24.3N 45.6W 30 KT 35 MPH 12H 13/0600Z 25.6N 46.7W 30 KT 35 MPH 24H 13/1800Z 26.9N 47.7W 30 KT 35 MPH 36H 14/0600Z 27.4N 48.3W 25 KT 30 MPH 48H 14/1800Z 27.2N 48.8W 25 KT 30 MPH 60H 15/0600Z 26.7N 49.6W 25 KT 30 MPH 72H 15/1800Z 25.8N 51.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 16/1800Z 24.7N 54.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 17/1800Z...DISSIPATED $$ Forecaster Brown

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Tropical Depression Rene Forecast Discussion Number 22

2020-09-12 16:58:46| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 12 2020 000 WTNT43 KNHC 121458 TCDAT3 Tropical Depression Rene Discussion Number 22 NWS National Hurricane Center Miami FL AL182020 1100 AM AST Sat Sep 12 2020 The deep convection associated with Rene has gradually decreased over the past several hours, with only a few sporadic patches remaining. A recent ASCAT overpass showed peak winds of 28 kt in the northeastern quadrant, while the latest Dvorak intensity estimates from TAFB and SAB suggest the system is 30 kt. Therefore the initial intensity is being lowered to 30 kt. The depression is forecast to battle dry air for the next several days, with SHIPS guidance indicating that the mid-level relative humidity will decrease to under 40 percent in a few days. Also, it appears that there will be increasing subsidence over the system starting in a couple of days due to a ridge building to the northwest of the cyclone. Neither of these conditions bode well for maintaining organized deep convection. And, with it anticipated that the depression will struggle to maintain convection it is expected to slowly spin down over the next several days. The official forecast shows Rene degenerating into a remnant low in 5 days, but that timing is low confidence, as the system may generate just enough convection over warm waters to continue to be classified as a tropical cyclone longer than that. The alternative scenario is that the system could become a remnant low or open into a trough even sooner. The NHC intensity forecast is a little lower than the previous one mainly due to the decrease in initial intensity, and is in good agreement with the various intensity consensus values. Rene continues to move northwestward, now at 11 kt. A northwestward motion around the western periphery of a mid-level ridge over the eastern Atlantic should continue into tonight. On Sunday, Rene is forecast to slow down as the ridge builds to the northwest and north of the cyclone. By Tuesday, Rene is expected to turn westward and then west-southwestward to southwestward under the influence of the building ridge. The NHC track forecast is very little changed from the previous one, and is near the middle of the track guidance. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 23.2N 44.4W 30 KT 35 MPH 12H 13/0000Z 24.5N 45.8W 30 KT 35 MPH 24H 13/1200Z 26.0N 47.0W 30 KT 35 MPH 36H 14/0000Z 27.0N 47.7W 30 KT 35 MPH 48H 14/1200Z 27.3N 48.2W 25 KT 30 MPH 60H 15/0000Z 27.0N 48.7W 25 KT 30 MPH 72H 15/1200Z 26.3N 49.8W 25 KT 30 MPH 96H 16/1200Z 25.0N 52.6W 25 KT 30 MPH 120H 17/1200Z 24.0N 56.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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Tropical Storm Paulette Forecast Discussion Number 23

2020-09-12 16:56:51| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 12 2020 000 WTNT42 KNHC 121456 TCDAT2 Tropical Storm Paulette Discussion Number 23 NWS National Hurricane Center Miami FL AL172020 1100 AM AST Sat Sep 12 2020 Paulette's structure is going through a bit of an evolution as the deep-layer shear backs from southwesterly to southeasterly. Most of the deep convection had waned earlier this morning, but new convection is beginning to develop near the center as the cyclone reconstitutes itself. For now, the initial intensity remains 60 kt. An overnight AMSR microwave pass showed that Paulette has well-defined low-level banding features, and this structure should allow the cyclone to strengthen as vertical shear falls below 10 kt during the next 12-24 hours. Strengthening is anticipated to continue through day 3, and after that time, gradual weakening is possible due to increasing southwesterly shear. The intensity guidance has been fairly stable for the past few forecast cycles, and no significant changes were required to this new NHC intensity forecast, which lies close to the HCCA model and just a bit above the intensity consensus. Paulette is likely to be strengthening while it makes its closest approach to Bermuda in about 48 hours, and the GFS-based SHIPS and HWRF models each suggest that there is a possibility of it becoming a major hurricane in 72 hours. Paulette is moving northwestward (310/13 kt), with a break in the subtropical ridge located due north of the storm. The ridge may strengthen briefly over the next day or so, causing Paulette to move west-northwestward for a short time, but it should resume the northwestward motion by late Sunday. After that time, an approaching longwave trough over the northeastern United States will erode the ridge eastward, causing Paulette to recurve sharply toward the north and then northeast between days 2 and 3. The hurricane is then expected to accelerate east-northeastward toward the north Atlantic on days 4 and 5, embedded within the mid-latitude flow. The track guidance is well clustered and has not shown any significant shifts over the past few forecast cycles. Therefore, the new NHC track forecast is similar to the previous one. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane on Sunday and be near the island Sunday night and Monday. A prolonged period of strong winds, storm surge, and heavy rainfall is expected on Bermuda beginning late Sunday evening, and a hurricane warning has been issued for the island. Preparations to protect life and property should be rushed to completion. 2. Swells produced by Paulette are affecting portions of the Leeward Islands, the Greater Antilles, the Bahamas, and Bermuda, and are expected to spread westward to the east coast of the United States during the next day or two. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 12/1500Z 27.5N 57.2W 60 KT 70 MPH 12H 13/0000Z 28.5N 59.1W 65 KT 75 MPH 24H 13/1200Z 29.5N 61.4W 75 KT 85 MPH 36H 14/0000Z 30.8N 63.6W 80 KT 90 MPH 48H 14/1200Z 32.4N 64.8W 85 KT 100 MPH 60H 15/0000Z 34.2N 64.2W 90 KT 105 MPH 72H 15/1200Z 35.8N 61.5W 95 KT 110 MPH 96H 16/1200Z 39.0N 54.5W 90 KT 105 MPH 120H 17/1200Z 42.0N 47.0W 75 KT 85 MPH $$ Forecaster Berg

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