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Hurricane Paulette Forecast Discussion Number 25

2020-09-13 04:40:13| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Sep 12 2020 000 WTNT42 KNHC 130240 TCDAT2 Hurricane Paulette Discussion Number 25 NWS National Hurricane Center Miami FL AL172020 1100 PM AST Sat Sep 12 2020 An Air Force Reserve Hurricane Hunter aircraft made multiple passes through the center of Paulette earlier this evening. The plane reported max 850-mb flight-level winds of 78 kt and believable SFMR winds of 58 kt. A dropsonde in the southeast quadrant measured adjusted mean boundary layer winds of just over 60 kt while multiple center drops revealed that the minimum pressure is now down to 981 mb. Taken together, these observations support an intensity of 65 kt, making Paulette the 6th hurricane of the 2020 Atlantic season. The plane found the Paulette has turned west-northwestward and is nearly exactly on the previous forecast track. With no major changes in the track guidance either, there is no reason to make any changes of note to the previous NHC track forecast at this time. A mid-level ridge building to the north of Paulette should keep the hurricane moving generally west-northwestward for the next day or so. The ridge and a mid-level trough to the west should then slide eastward by early Monday, causing Paulette to slow and turn northward, moving very near or over Bermuda Monday morning. Paulette is then forecast to turn northeastward and accelerate in that direction by Tuesday. The NHC forecast remains in the middle of the tightly clustered guidance suite at all forecast hours. Paulette appears to be getting more vertically aligned as the shear that has affected the cyclone for the past several days slowly decreases. Further intensification appears likely through early Monday given the warm SSTs below Paulette and favorable upper-air environment that is expected to develop around the cyclone. Dry air in the surrounding environment could be a limiting factor, but all of the intensity guidance continues to indicate that Paulette will be a dangerous hurricane when it approaches Bermuda late Sunday and early Monday. Further intensification will then be possible for another day or so after the hurricane begins to move away from Bermuda and it could be near major hurricane strength by the middle of next week. The NHC intensity forecast is near the middle of the latest intensity guidance throughout the forecast period. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane on Sunday and will 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, Bermuda, and the east coast of the United States. These swells could cause life-threatening surf and rip current conditions. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 28.9N 59.7W 65 KT 75 MPH 12H 13/1200Z 29.7N 61.6W 70 KT 80 MPH 24H 14/0000Z 31.0N 63.8W 80 KT 90 MPH 36H 14/1200Z 32.6N 65.0W 85 KT 100 MPH 48H 15/0000Z 34.5N 64.0W 90 KT 105 MPH 60H 15/1200Z 36.2N 61.0W 95 KT 110 MPH 72H 16/0000Z 37.7N 57.5W 95 KT 110 MPH 96H 17/0000Z 41.0N 49.0W 85 KT 100 MPH 120H 18/0000Z 43.5N 42.0W 70 KT 80 MPH $$ Forecaster Zelinsky

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Tropical Depression Sixteen-E Forecast Discussion Number 1

2020-09-13 04:38:29| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Sat Sep 12 2020 000 WTPZ41 KNHC 130238 TCDEP1 Tropical Depression Sixteen-E Discussion Number 1 NWS National Hurricane Center Miami FL EP162020 900 PM MDT Sat Sep 12 2020 The last few visible satellite images indicated convection growing closer to the center of a low pressure area that NHC has been tracking for the past few days. Since the low was already well-defined this afternoon, the development of organized deep convection means that a tropical depression has formed. The initial wind speed is set to 30 kt, which is a blend of the TAFB/SAB fixes and earlier ASCAT data. The depression is embedded within a moderate easterly wind-shear environment, partially related to its position in the larger ITCZ/monsoon trough structure. Global model guidance is in reasonable agreement on the large-scale circulation becoming focused on the new tropical cyclone and a slight relaxation of the shear during the next couple of days. This change should lead to gradual intensification, and the new forecast is between the model consensus and the corrected-consensus aids. Beyond 2-3 days, decreasing SSTs and increasing dry air entrainment should promote weakening, and that is reflected below. The initial motion estimate is an uncertain northwestward or 315/08 kt. A mid-level ridge is forecast to build westward, causing the cyclone to move west-northwestward for a few days after the weekend. A westward turn is forecast at long range due to the shallow cyclone feeling the low-level ridge. One big uncertainty with this forecast is how it interacts with the larger ITCZ/monsoon trough circulation, including the remnants of Invest 91E. For now, it is assumed that only a slight slow down occurs as the depression becomes the dominant circulation in the area, and the forecast is more consistent with the GFS- or UKMET-based guidance. However, a slower ECMWF-like solution is also possible, but is being given less weight at this time. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 16.7N 112.9W 30 KT 35 MPH 12H 13/1200Z 17.5N 113.8W 35 KT 40 MPH 24H 14/0000Z 18.3N 115.4W 40 KT 45 MPH 36H 14/1200Z 18.9N 116.9W 45 KT 50 MPH 48H 15/0000Z 19.5N 118.5W 50 KT 60 MPH 60H 15/1200Z 20.0N 119.9W 55 KT 65 MPH 72H 16/0000Z 20.5N 121.0W 55 KT 65 MPH 96H 17/0000Z 21.5N 123.5W 45 KT 50 MPH 120H 18/0000Z 21.5N 126.0W 30 KT 35 MPH $$ Forecaster Blake

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Tropical Storm Sally Forecast Discussion Number 6

2020-09-13 04:38:29| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Sat Sep 12 2020 000 WTNT44 KNHC 130238 TCDAT4 Tropical Storm Sally Discussion Number 6 NWS National Hurricane Center Miami FL AL192020 1100 PM EDT Sat Sep 12 2020 Sally's structure remains somewhat disorganized this evening. While there has been a recent increase in deep convection near the low- level center, there is a large CDO feature with -70 to -80C tops displaced 50 to 60 miles to the south and southeast. This structure could be due to the 10 to 15 kt of northwesterly shear analyzed by the SHIPS model and UW-CIMSS. The initial intensity remains 35 kt based on the latest Dvorak estimates from TAFB and SAB, and the central pressure of 1003 mb is based on surface observations in the Naples area as the center passed offshore before 00Z. An Air Force Reserve Hurricane Hunter aircraft will be investigating Sally overnight to provide more information on the storm's intensity and structure. Given the current structure, only gradual strengthening is expected in the short term. However, once the cyclone develops more of an inner core, it should be able to take advantage of warm SSTs and low shear over the Gulf of Mexico during the next couple of days. Steady intensification to hurricane strength is forecast through 48 hours, with a leveling off of the intensity in 60-72 hours as the shear is expected to increase. Overall, the intensity guidance is a little less bullish this cycle, with the HWRF more aggressive while the remainder of the guidance shows a slower rate of intensification. The NHC intensity forecast through landfall remains unchanged from the previous advisory, but now lies a little above HCCA and the intensity consensus. The initial motion estimate is a somewhat uncertain 285/07. Sally is currently being steered by a mid-level ridge centered over the Carolinas that will shift eastward and weaken as a longwave trough moves into the eastern United States over the next 24 to 36 hours. During this time, Sally should move generally west-northwestward or northwestward with a decrease in forward speed on Monday. By 48 hours, the trough lifts out and a narrow ridge builds in north and east of Sally, which should result in the tropical cyclone turning north-northwestward and then northward at less than 5 kt as it approaches the north-central Gulf Coast and moves inland over the Mississippi Valley. After that time, the next trough should induce a faster northeastward motion by day 5. There is a fair bit of spread in the track guidance after 24 hours, with the COAMPS-TC well to the right and the GEFS ensemble mean and UKMET to the left. However, the GFS and ECMWF are in good agreement through 60 hours near the middle of the guidance envelope. The NHC track is similar to the previous one and lies in the middle of the guidance and is near the TVCA multi-model consensus and HCCA. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 72 h is around 110 miles and the average intensity error is around 15 mph. In addition, wind, storm surge, and rainfall hazards will extend far from the center. Unfortunately, confidence is increasing that Sally's expected slow forward speed near the Gulf Coast will exacerbate the storm surge and heavy rainfall threats. 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 southeastern 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 and urban flooding and minor to moderate river flooding are 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 lower Florida Keys overnight. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 26.3N 82.9W 35 KT 40 MPH 12H 13/1200Z 27.0N 84.4W 40 KT 45 MPH 24H 14/0000Z 27.7N 86.0W 50 KT 60 MPH 36H 14/1200Z 28.3N 87.5W 60 KT 70 MPH 48H 15/0000Z 28.8N 88.6W 70 KT 80 MPH 60H 15/1200Z 29.4N 89.2W 75 KT 85 MPH 72H 16/0000Z 30.1N 89.5W 75 KT 85 MPH 96H 17/0000Z 31.7N 89.1W 50 KT 60 MPH...INLAND 120H 18/0000Z 33.5N 86.5W 25 KT 30 MPH...INLAND $$ Forecaster Brennan

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Tropical Depression Rene Forecast Discussion Number 24

2020-09-13 04:35:00| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Sep 12 2020 000 WTNT43 KNHC 130234 TCDAT3 Tropical Depression Rene Discussion Number 24 NWS National Hurricane Center Miami FL AL182020 1100 PM AST Sat Sep 12 2020 Satellite images indicate that a band of convection has recently increased on the western side of the circulation, otherwise the center remains mostly exposed. The initial wind speed is reduced to 25 kt based on scatterometer data. The small cyclone is likely to gradually spin down due to persistent moderate shear and a fairly dry environment. The hostile conditions are not likely to change too much, so Rene is forecast to lose all convection in a day or two and become a remnant low. The new NHC intensity forecast is lower than the previous one due to the weaker initial intensity and follows the latest global model consensus. The depression is moving northwestward at about 11 kt. A blocking ridge of high pressure is forecast to build to the northwest or Rene, causing the cyclone to slow down and then turn westward and west-southwestward early next week. The guidance is similar to the previous package, and no significant track changes were made to the official NHC forecast. The forecast still might hold onto the circulation too long, and the remnant low could dissipate into a trough of low pressure sooner than indicated below. FORECAST POSITIONS AND MAX WINDS INIT 13/0300Z 25.3N 46.4W 25 KT 30 MPH 12H 13/1200Z 26.4N 47.3W 25 KT 30 MPH 24H 14/0000Z 27.3N 48.1W 25 KT 30 MPH 36H 14/1200Z 27.4N 48.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 15/0000Z 27.2N 49.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 15/1200Z 26.5N 50.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 16/0000Z...DISSIPATED $$ Forecaster Blake

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Tropical Depression Twenty Forecast Discussion Number 1

2020-09-12 23:03:24| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 12 2020 000 WTNT45 KNHC 122103 TCDAT5 Tropical Depression Twenty Discussion Number 1 NWS National Hurricane Center Miami FL AL202020 500 PM AST Sat Sep 12 2020 The tropical wave and associated area of low pressure that NHC has been tracking since it emerged off of Africa a couple of days ago has become sufficiently organized to be designated as a tropical depression. A curved band of deep convection developed early this morning and persisted just to the west of a well-defined low level circulation throughout the day. An earlier ASCAT overpass showed that 25-30 kt winds over the northwestern portion of the circulation, which is the basis for the initial intensity being set at 30 kt. The depression has a rather large circulation, with the radius of maximum winds nearly 100 n mi from the center and the overall wind field appearing to extend outward over 300 n mi. The environment surrounding the cyclone over the next 36 h is characterized by moderate northeasterly vertical wind shear and plenty of warm water and atmospheric moisture. These factors are supportive of gradual strengthening, however, due to the large size of the system, it may take some time for it to consolidate. The NHC intensity forecast shows only slight strengthening through 36 h as the system consolidates, and that portion of the forecast is well below the intensity guidance. By early next week, the wind shear is expected to decrease to under 10 kt and a faster rate of intensification is indicated from 36-96 h in anticipation of the cyclone having a better structure to take advantage of the lower shear. After 96 h the intensity is held steady as northwesterly shear is forecast to increase while the system encounters some slightly drier air and moves over lower oceanic heat content. The NHC intensity forecast beyond 36 h starts well below most of the guidance, and trends close to the IVCN/ICON later on in the forecast period. The depression is moving toward the west-northwest at 8 kt, steered by a mid-level ridge to its north. This ridge is forecast to build westward over the next few days, which should result in a continued general west-northwest motion, perhaps at a slightly faster forward speed early next week. By the middle of next week, a weakness is forecast to develop in the ridge, partially due to interaction of Paulette and a mid- to- upper level trough over the northern Atlantic at that time, and the cyclone should turn to the northwest into this weakness. Overall, track guidance from the global and regional models is in decent agreement on this scenario, and the NHC forecast is between the HFIP corrected consensus HCCA and the TVCN multimodel consensus. FORECAST POSITIONS AND MAX WINDS INIT 12/2100Z 11.4N 33.5W 30 KT 35 MPH 12H 13/0600Z 11.5N 34.5W 30 KT 35 MPH 24H 13/1800Z 12.1N 36.1W 35 KT 40 MPH 36H 14/0600Z 12.8N 38.3W 40 KT 45 MPH 48H 14/1800Z 13.3N 40.7W 50 KT 60 MPH 60H 15/0600Z 14.0N 42.8W 60 KT 70 MPH 72H 15/1800Z 15.0N 44.5W 70 KT 80 MPH 96H 16/1800Z 17.7N 47.3W 80 KT 90 MPH 120H 17/1800Z 21.0N 49.7W 80 KT 90 MPH $$ Forecaster Latto

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