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Tropical Storm Sally Forecast Discussion Number 8
2020-09-13 16:57:34| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Sun Sep 13 2020 133 WTNT44 KNHC 131457 TCDAT4 Tropical Storm Sally Discussion Number 8 NWS National Hurricane Center Miami FL AL192020 1100 AM EDT Sun Sep 13 2020 Although the center of Sally remains near the northwestern edge of the deep convection, there is a large area of convection and some banding evident over the southeastern portion of the circulation. An Air Force Reserve Hurricane Hunter aircraft investigating the storm this morning reported a peak 850-mb flight-level wind of 62 kt, and believable SFMR winds of around 50 kt, and these data are the basis for the 50-kt initial wind speed. The plane reported minimum pressures in the 996-998 mb range, with the higher value being the most recent information available. Northwesterly shear continues over the cyclone, but this shear is expected to decrease later today and tonight as Sally moves beneath a narrow upper-level ridge axis. This more conducive upper-level pattern is expected to allow the tropical storm to strengthen while it moves over the north-central Gulf of Mexico tonight and Monday. Most of the intensity guidance calls for Sally to become a hurricane in about 24 hours and so does the official forecast. Additional strengthening is expected after that time and Sally could be slightly stronger at landfall than indicated below since it is forecast to reach the coast between the 36 h forecast point and 48 h when it is inland over southeast Louisiana. The NHC intensity forecast is close to the consensus aids through 24 hours and at or just above the SHIPS, LGEM and HFIP corrected consensus aids at 36 and 48 hours. Sally is moving west-northwestward at about 10 kt. The tropical storm should continue on that general heading and speed over the next 12 to 24 hours as it steered around the southern flank of a mid-level ridge. After 24 hours, Sally is expected to be near the western portion of the ridge which should cause the storm to slow down and turn northwestward. The global models have trended toward slightly more ridging over the northern Gulf during the next 24 hours, and the track guidance has edged westward. The NHC track has been adjusted slightly westward and lies near the lastest run of the GFS, but is not as far west as the ECMWF and the various consensus aids. As Sally rounds the ridge in 48 to 72 h, the steering flow is expected to be quite weak, and a slow northward motion is forecast at that time. Afterward, a north-northeastward to northeastward motion should commence as the cyclone moves in that direction ahead of a short-wave trough. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 36-48 h is 60-80 miles and the average intensity error is 10-15 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. An extremely dangerous and life-threatening storm surge is now expected, and a Storm Surge Warning is in effect for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from Port Fourchon, Louisiana to the Mississippi/Alabama border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are expected by late Monday from Grand Isle, Louisiana to Ocean Springs, Mississippi, including Metropolitan New Orleans, with tropical storm conditions likely by Monday. Preparations should be rushed to completion in those areas. 3. Sally is expected to produce flash flooding across southwest and central Florida and prolong existing minor river flooding across west-central Florida through Monday. Widespread significant flash flooding and minor to isolated major river flooding is likely across portions of the central Gulf Coast Monday through the middle of the week, with flooding impacts spreading farther into the Southeast in the middle to late parts of the week. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 27.5N 84.9W 50 KT 60 MPH 12H 14/0000Z 28.0N 86.6W 55 KT 65 MPH 24H 14/1200Z 28.5N 88.2W 65 KT 75 MPH 36H 15/0000Z 28.9N 89.4W 80 KT 90 MPH 48H 15/1200Z 29.7N 90.2W 65 KT 75 MPH...INLAND 60H 16/0000Z 30.3N 90.5W 50 KT 60 MPH...INLAND 72H 16/1200Z 31.5N 90.3W 35 KT 40 MPH...INLAND 96H 17/1200Z 33.6N 88.3W 25 KT 30 MPH...INLAND 120H 18/1200Z 34.8N 83.8W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown
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Tropical Depression Twenty Forecast Discussion Number 4
2020-09-13 16:56:07| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sun Sep 13 2020 000 WTNT45 KNHC 131455 TCDAT5 Tropical Depression Twenty Discussion Number 4 NWS National Hurricane Center Miami FL AL202020 1100 AM AST Sun Sep 13 2020 Convective banding features are generally limited to the southern semicircle of the circulation with the center exposed just to the north. A recent ASCAT pass indicated that the strongest winds--up to 30 kt--were primarily located in the southerly and southwesterly monsoonal flow trailing the depression, although some stronger winds are beginning to develop just west of the center. Light-to- moderate northerly shear is currently affecting the depression, but this shear is expected to decrease during the next 24-48 hours. In addition, global model fields indicate that the cyclone should become increasingly separated from the ITCZ/monsoon trough. Steady strengthening is anticipated for much of the forecast period, and the NHC intensity forecast closely follows the HCCA aid and the IVCN intensity consensus. This new forecast is a little higher than the previous prediction, especially on days 3 through 5. There are two main points that suggest this forecast could potentially go even higher in later forecast cycles: 1. the HWRF model shows more significant strengthening at the latter part of the period, bringing the system to major hurricane strength, and 2. the SHIPS Rapid Intensification guidance indicates that there is a 50-50 chance that the system will strengthen by at least 65 kt over the next 3 days, which is 9-10 times higher than the climatological mean. The depression is moving west-northwestward (290/9 kt) to the south of a large mid-tropospheric high centered over the central subtropical Atlantic. This feature should continue to drive the cyclone westward or west-northwestward for the next 2-3 days. After that time, the mid-level high is expected to shift northward and elongate, and the potentially intensifying hurricane is likely to acquire more poleward motion, moving northwestward and a little slower on days 3-5. Most of the track models are clustered close together, except for the HWRF which has a trajectory farther to the south and west. The new NHC track forecast has been shifted a bit westward compared to the previous prediction, close to the GFS-ECMWF consensus but not as far to the left as the latest TVCA and HCCA solutions. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 12.3N 36.4W 30 KT 35 MPH 12H 14/0000Z 12.8N 38.0W 35 KT 40 MPH 24H 14/1200Z 13.3N 40.6W 45 KT 50 MPH 36H 15/0000Z 13.7N 43.0W 55 KT 65 MPH 48H 15/1200Z 14.4N 45.1W 65 KT 75 MPH 60H 16/0000Z 15.3N 46.7W 75 KT 85 MPH 72H 16/1200Z 16.3N 48.2W 85 KT 100 MPH 96H 17/1200Z 19.0N 51.0W 95 KT 110 MPH 120H 18/1200Z 22.0N 53.5W 95 KT 110 MPH $$ Forecaster Berg
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Tropical Storm Karina Forecast Discussion Number 3
2020-09-13 16:53:37| National Hurricane Center (East Pacific)
Issued at 900 AM MDT Sun Sep 13 2020 000 WTPZ41 KNHC 131453 TCDEP1 Tropical Storm Karina Discussion Number 3 NWS National Hurricane Center Miami FL EP162020 900 AM MDT Sun Sep 13 2020 Karina's center is a little bit closer to the deep convection this morning, resulting in intensity estimates increasing slightly. Based on Dvorak estimates of T3.0/45 kt and T2.5/35 kt from TAFB and SAB, respectively, and an overnight SATCON estimate of 42 kt, Karina's initial intensity is assumed to be 40 kt. The moderate northeasterly shear currently affecting the cyclone is not expected to change appreciably during the next few days. However, Karina will remain over ocean waters warmer than 26 degrees Celsius for the next 48 hours, which should allow for a gradual intensification trend during that period. The shear is forecast to lighten up in 3-4 days, but by then Karina will be over colder waters and in a drier, more stable environment, which should induce weakening. Karina is likely to lose all of its deep convection and become a remnant low by the end of the forecast period. The NHC intensity forecast is generally close to the HCCA aid and the IVCN intensity consensus and shows weakening starting a little sooner than before. Karina is moving toward the west-northwest (300/9 kt), south of a mid-level ridge axis which extends from northwestern Mexico southwestward over the Pacific. This ridge should keep Karina on this general heading with a very gradually decreasing forward speed through day 4. Once the cyclone weakens to a remnant low, it is expected to turn westward, steered by lower-level trade winds. No significant changes in the track forecast were required compared to the previous prediction, and the official forecast lies generally close to the multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 17.6N 114.7W 40 KT 45 MPH 12H 14/0000Z 18.3N 115.8W 45 KT 50 MPH 24H 14/1200Z 19.0N 117.4W 45 KT 50 MPH 36H 15/0000Z 19.7N 119.0W 50 KT 60 MPH 48H 15/1200Z 20.2N 120.4W 55 KT 65 MPH 60H 16/0000Z 20.9N 121.6W 50 KT 60 MPH 72H 16/1200Z 21.7N 122.8W 50 KT 60 MPH 96H 17/1200Z 22.6N 125.2W 35 KT 40 MPH 120H 18/1200Z 22.8N 127.7W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg
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Hurricane Paulette Forecast Discussion Number 27
2020-09-13 16:50:35| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sun Sep 13 2020 000 WTNT42 KNHC 131450 TCDAT2 Hurricane Paulette Discussion Number 27 NWS National Hurricane Center Miami FL AL172020 1100 AM AST Sun Sep 13 2020 An Air Force Hurricane Hunter aircraft investigating Paulette this morning found peak 700-mb flight-level winds of 92 kt, which reduces to around 75 kt at the surface. However, around that same time the peak winds measured by the SFMR instrument on board the aircraft were only 58 kt. It is possible that the stronger winds suggested by the flight-level values were not reaching the surface at that location. Later on in the flight, the SFMR measured 64-kt winds in the northwestern quadrant. Since there has been no notable change to the structure of the hurricane since the aircraft was in the system a few hours ago, the initial intensity is being held at 70 kt and is a compromise of those two different peak values measured by the aircraft. Infrared satellite imagery has been showing some dry air intrusion over the eastern portion of the circulation, causing a break in the eyewall there over the past few hours. There is currently deep convection firing around the remainder of the eyewall, and therefore it is anticipated the dry air will be worked out of the system soon. Very low wind shear and warm waters will support strengthening over the next couple of days, and Paulette is expected to be a dangerous hurricane as it nears Bermuda Monday morning. The window for strengthening should continue for another 12-24 h after the cyclone passes Bermuda, and Paulette could become a major hurricane during that time. After 48 h, vertical wind shear is forecast to rapidly increase. After 72 h, Paulette is expected to cross the 26 degree C SST isotherm, and by 120 h those SSTs will be near 22 degrees C. The NHC forecast shows a weakening trend beginning after 48 h due to the negative environmental factors. By 120 h, the global models suggest that Paulette will have completed a transition to an extratropical cyclone. The only change to the NHC intensity forecast from the previous advisory was a slight upward adjustment over the first few days due to the increase in strength found by the aircraft this morning. This forecast is in between the LGEM and HFIP corrected consensus, HCCA. Paulette is now moving northwestward at 12 kt, to the southwest of a mid-level ridge. This motion should continue until just after the cyclone passes Bermuda. Later on Monday, the hurricane should turn north, then on Monday night northeastward, as it rounds the periphery of the ridge. After turning northeastward, the cyclone is expected to accelerate as it gets picked up in the mid-latitude flow ahead of an approaching mid- to upper-level trough. Later on in the forecast period, a slower eastward motion is indicated once the aforementioned trough bypasses the cyclone. The latest NHC forecast track is little changed from the previous one through 48 h and lies in the middle of the latest global and regional track model guidance. Beyond 48 h, the NHC forecast is a little faster than the previous one, and lies near the tightly clustered consensus track guidance. On the forecast track, tropical storm conditions should reach Bermuda by this evening, with hurricane force winds arriving there overnight. Key Messages: 1. Paulette is expected to approach Bermuda as a hurricane today and will be near the island tonight and Monday. A prolonged period of strong winds, storm surge, and heavy rainfall is expected on Bermuda beginning this 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 13/1500Z 29.9N 61.9W 70 KT 80 MPH 12H 14/0000Z 30.9N 63.6W 80 KT 90 MPH 24H 14/1200Z 32.8N 64.8W 90 KT 105 MPH 36H 15/0000Z 35.0N 63.5W 95 KT 110 MPH 48H 15/1200Z 37.0N 60.0W 100 KT 115 MPH 60H 16/0000Z 39.1N 55.1W 95 KT 110 MPH 72H 16/1200Z 41.4N 49.1W 85 KT 100 MPH 96H 17/1200Z 43.9N 42.0W 65 KT 75 MPH 120H 18/1200Z 44.0N 36.6W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Latto
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Tropical Storm Sally Forecast Discussion Number 7
2020-09-13 11:42:02| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sun Sep 13 2020 000 WTNT44 KNHC 130941 CCA TCDAT4 Tropical Storm Sally Discussion Number 7...Corrected NWS National Hurricane Center Miami FL AL192020 500 AM EDT Sun Sep 13 2020 Corrected for inland status at 60 and 72 hourrs A large burst of convection has formed near the center of Sally this morning, and Air Force Reserve Hurricane Hunter data indicate that the storm is strengthening. Peak flight-level winds were up to 57 kt at 925 mb, with believable SFMR winds of up to 45 kt, and pressure falling a couple of mb to 1001. The initial wind speed is set to 45 kt for this advisory. While northwesterly shear has been affecting Sally, the global models suggest that the shear will decrease later today as it moves under a narrow ridge. Models all respond to these changing conditions by showing intensification, but they disagree on the rate of change. It is an extremely tricky forecast because of how this might happen close to landfall, but guidance is generally 5-10 kt higher than the last cycle so the intensity forecast has been raised by those amounts up through landfall. It is worth noting that rapid intensification is a possibility if the system develops a more solid inner core on Monday, and stronger solutions like the HWRF model can't be dismissed. Sally is moving west-northwestward at a faster speed this morning. The biggest change to note is that almost every model has shifted westward (faster) due to narrow ridging developing across the northern Gulf Coast between the storm and an exiting mid-latitude trough. The flow gets quite weak near and after landfall, so the cyclone should slow down on Tuesday/Wednesday and turn northward. After that time, the next trough should induce a faster northeastward motion by day 5. The new track is shifted westward between 30-45 n mi near the coast of Louisiana, and is still east of the GFS and ECMWF models. Users are reminded to not focus on the exact details of the track or intensity forecasts, as the average NHC track error at 48 h is around 80-90 miles and the average intensity error is around 10-15 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. An extremely dangerous and life-threatening storm surge is now expected, and a Storm Surge Warning is in effect for areas outside the southeastern Louisiana Hurricane and Storm Damage Risk Reduction System from Port Fourchon Louisiana to the Mississippi/Alabama border. Residents in these areas should follow any advice given by local officials. 2. Hurricane conditions are expected by early Tuesday from Grand Isle, Louisiana to Ocean Springs, Mississippi, including Metropolitan New Orleans, with tropical storm conditions likely by Monday. Preparations should be rushed to completion in those areas. 3. Tropical Storm Sally is expected to produce flash flooding across southern and central Florida and prolong existing minor river flooding across Central Florida through Monday. Significant flash flooding and minor to isolated major river flooding is likely across portions of the Central Gulf Coast from Monday through the middle of the week. FORECAST POSITIONS AND MAX WINDS INIT 13/0900Z 27.0N 84.0W 45 KT 50 MPH 12H 13/1800Z 27.7N 85.4W 50 KT 60 MPH 24H 14/0600Z 28.3N 87.2W 60 KT 70 MPH 36H 14/1800Z 28.7N 88.7W 70 KT 80 MPH 48H 15/0600Z 29.2N 89.7W 85 KT 100 MPH 60H 15/1800Z 30.1N 90.0W 70 KT 80 MPH...INLAND 72H 16/0600Z 31.0N 90.0W 50 KT 60 MPH...INLAND 96H 17/0600Z 32.8N 88.6W 30 KT 35 MPH...INLAND 120H 18/0600Z 34.5N 84.5W 20 KT 25 MPH...INLAND $$ Forecaster Blake
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