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Tropical Storm Dorian Forecast Discussion Number 6

2019-08-25 23:05:28| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sun Aug 25 2019 000 WTNT45 KNHC 252105 TCDAT5 Tropical Storm Dorian Discussion Number 6 NWS National Hurricane Center Miami FL AL052019 500 PM AST Sun Aug 25 2019 While still looking a bit ragged, convection has continued to increase near the center of Dorian since the last advisory, and a just-received SSMI/S overpass shows a convective band wrapped about three-quarters of the way around the center. Various Dvorak-based objective and subjective satellite intensity estimates are in the 35-45 kt range, while recent microwave-based estimates are in the 45-60 kt range. Based on these data, the initial intensity is increased to 45 kt. The initial motion remains 280/12 kt. The subtropical ridge to the north of the cyclone should steer it generally westward to west-northwestward for the next 72 h or so, with a more northwestward motion possible thereafter as Dorian approaches an upper-level low pressure area forecast to be over the north-central Caribbean. The model guidance has not shifted much since the previous advisory. However, some of the normally reliable models are in disagreement. The ECMWF and GFS keep Dorian weaker and farther south, and they are on the left side of the guidance envelope. The HWRF and UKMET have more northerly tracks, and forecast Dorian to pass near Puerto Rico and then near or north of Hispaniola. The various consensus models are between these extremes, and the earlier forecast is now close to them. Thus, the new forecast track is little changed from the previous forecast, and it calls for Dorian to move through the Windward Islands between 36-48 h and cross Hispaniola between 96-120 h. Dorian still appears to be ingesting dry air based on the somewhat ragged convective pattern. This entrainment is expected to continue sporadically for the next 2-3 days, and this could slow intensification despite an environment of light to moderate shear. The intensity guidance is split on how much intensification could occur during this time. The statistical-dynamical SHIPS and LGEM models showing more intensification and forecast Dorian to become a hurricane. On the dynamical side, the latest HWRF run shows little development, and the GFS/ECMWF keep the system weak to the point where it degenerates into a tropical wave before reaching Hispaniola. The intensity forecast follows the general trend of the statistical-dynamical models, but shows less intensification out of respect for the dynamical models. The intensity forecast becomes even lower confidence after 72 h due to uncertainties in how much shear and land that Dorian will encounter. The new forecast calls for Dorian to weaken to a depression due to passage over Hispaniola. However, the large range of possibilities also includes both Dorian going north of Hispaniola and remaining a hurricane and the small cyclone dissipating completely over the island. Key Messages: 1. Dorian is expected to bring tropical storm conditions to portions of the Lesser Antilles, where tropical storm watches and warnings are in effect. Residents in these areas should refer to advice from local government officials and products from their local meteorological service for additional information. 2. Dorian is expected to produce 2 to 4 inches of rainfall across portions of the Lesser Antilles, with isolated amounts as high as 6 inches. 3. While it is too soon to determine the specific time or magnitude of possible direct impacts in Puerto Rico, the Virgin Islands, or Hispaniola interests in those areas should monitor the progress of Dorian. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 11.5N 54.2W 45 KT 50 MPH 12H 26/0600Z 11.8N 55.9W 50 KT 60 MPH 24H 26/1800Z 12.4N 58.2W 55 KT 65 MPH 36H 27/0600Z 13.1N 60.3W 55 KT 65 MPH 48H 27/1800Z 14.1N 62.3W 60 KT 70 MPH 72H 28/1800Z 16.4N 66.4W 65 KT 75 MPH 96H 29/1800Z 18.5N 70.0W 60 KT 70 MPH...INLAND 120H 30/1800Z 20.5N 73.0W 25 KT 30 MPH...OVER WATER $$ Forecaster Beven

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Post-Tropical Cyclone Ivo Forecast Discussion Number 19

2019-08-25 22:31:58| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Sun Aug 25 2019 563 WTPZ45 KNHC 252031 TCDEP5 Post-Tropical Cyclone Ivo Discussion Number 19 NWS National Hurricane Center Miami FL EP102019 200 PM PDT Sun Aug 25 2019 The initial motion is 340/07 kt. Ivo has slowed down due to the system weakening and becoming vertically shallow. A slow north-northwestward motion is expected today and this evening, followed by a turn toward the north late tonight or early Monday morning. The official forecast track is similar to the previous advisory track, and lies close to the middle of the tightly clustered consensus models. The initial intensity of 25 kt is based mainly on continuity with the previous forecast. The inner-core region of Ivo's circulation has been devoid of any convection for more than 12 hours. What little bit of thunderstorm activity that does exist well to the north-northwest and west of the center is not showing any signs of feeding back onto the cyclone and, therefore, Ivo has degenerated into a remnant low pressure system. Additional spin down of the shallow vortex is forecast due to the cyclone moving over sub-22 deg C sea-surface temperatures and continuing to ingest more cool and stable air, with dissipation likely by Tuesday. Although Ivo is no longer a tropical cyclone, significant swells generated by the low are still affecting portions of the west coast of the Baja California peninsula and southern California. These swells will continue into tonight and could cause dangerous rip currents before subsiding on Monday. See products from your local weather office for additional information. This is the last NHC advisory on Ivo. For additional information on the remnant low, please see High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFEPI, WMO header FZPN02 KWBC, and on the web at ocean.weather.gov/shtml/NFDHSFEPI.php . FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 26.0N 117.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 12H 26/0600Z 26.9N 118.2W 20 KT 25 MPH...POST-TROP/REMNT LOW 24H 26/1800Z 27.8N 118.2W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 27/0600Z 27.7N 117.8W 15 KT 15 MPH...POST-TROP/REMNT LOW 48H 27/1800Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Dorian Forecast Discussion Number 5

2019-08-25 16:49:27| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Aug 25 2019 000 WTNT45 KNHC 251449 TCDAT5 Tropical Storm Dorian Discussion Number 5 NWS National Hurricane Center Miami FL AL052019 1100 AM AST Sun Aug 25 2019 The convective organization of Dorian has improved a little since the last advisory, as a more persistent area of central convection has formed along with increased, but ragged, outer banding. However, the various satellite intensity estimates remain clustered around 35 kt, and that remains the initial intensity. A combination of 1-minute GOES-16 visible imagery and microwave satellite data have helped pinpoint the center, and the initial motion is 280/12. The subtropical ridge to the north of the cyclone should steer it generally westward to west-northwestward for the next 3-4 days, with a more northwestward motion possible thereafter as Dorian approaches an upper-level low pressure area forecast to be over the north-central Caribbean. The model guidance continues the previous trend of being south of and a bit faster than the previous runs, and as a result the new forecast track is again shifted a little to the south. The new forecast lies between the previous forecast and the various consensus models. However, it is south of the forecasts of the HWRF, the UKMET, and the UKMET Ensemble mean. Additional adjustments to the track may be required on the next advisory if the current model trends continue. While vertical wind shear is decreasing over Dorian, the cyclone still appears to be ingesting dry air based on the ragged convective pattern. Some dry air entrainment is expected to continue for the next few days, and based on this the new intensity forecast again calls for gradual strengthening through 72 h. This part of the intensity forecast is in best agreement with the SHIPS model, and it lies near the upper edge of the intensity guidance. The intensity forecast becomes low confidence after 72 h due to uncertainties in the amount of shear and land that Dorian will encounter. The new forecast calls for some weakening due to shear before Dorian moves over Hispaniola, followed by weakening to a depression due to passage over the island. However, the large range of possibilities includes both Dorian going north of Hispaniola and remaining a hurricane and the small cyclone dissipating completely over Hispaniola. Key Messages: 1. A Tropical Storm Warning has been issued for Barbados, and a Tropical Storm Watch is in effect for St. Lucia, St. Vincent, and the Grenadines. Additional watches and warnings for other portions of the Lesser Antilles could be required later today. 2. It is too soon to determine the specific timing or magnitude of impacts in Puerto Rico, the Virgin Islands, or Hispaniola, but interests in those areas should monitor the progress of Dorian. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 11.2N 52.9W 35 KT 40 MPH 12H 26/0000Z 11.5N 54.6W 40 KT 45 MPH 24H 26/1200Z 12.0N 56.9W 45 KT 50 MPH 36H 27/0000Z 12.7N 59.1W 50 KT 60 MPH 48H 27/1200Z 13.5N 61.2W 55 KT 65 MPH 72H 28/1200Z 15.6N 65.3W 65 KT 75 MPH 96H 29/1200Z 17.5N 69.0W 60 KT 70 MPH 120H 30/1200Z 20.0N 72.0W 25 KT 30 MPH $$ Forecaster Beven

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Tropical Storm Dorian Forecast Discussion Number 4

2019-08-25 10:49:52| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Aug 25 2019 000 WTNT45 KNHC 250849 TCDAT5 Tropical Storm Dorian Discussion Number 4 NWS National Hurricane Center Miami FL AL052019 500 AM AST Sun Aug 25 2019 The convective organization of Dorian is about the same as it was several hour ago with the storm only producing small, generally short-lived thunderstorm areas. GOES-16 1-minute satellite images have been particularly helpful in locating the center in between bands of convection to the north and south of the cyclone. The initial wind speed is kept at 35 kt based on the most recent TAFB Dvorak fix. Dorian continues moving westward or 280/11 kt. A west to west-northwestward course of the tropical cyclone is expected during the next several days as a mid-level ridge remains in place over the southwestern Atlantic Ocean. There has been a noticeable change in the guidance tonight, with many models faster and to the southwest of earlier runs. This is not uncommon in the early stages of tropical Atlantic cyclones where the guidance has historically demonstrated a slow, poleward bias. Given the lack of interaction with even features at the subtropical latitudes, it makes sense to follow the trend of the latest guidance and shift the track about 30 n mi southwest during most of the forecast. Notably, the GFS and ECMWF models are still south of the new NHC prediction, and further adjustments could be required later today. The intensity forecast appears to be a battle between an increasingly conducive oceanic and shear environment during the next couple of days versus plentiful dry air in the mid-levels of the atmosphere. These mixed conditions primarily point toward slow strengthening during that time while Dorian approaches the Windward Islands, and no change has been made to the first part of the forecast. While Dorian still has a chance to become a hurricane over the eastern Caribbean, model guidance has been trending toward a higher-shear environment caused by a large upper low in a few days. Thus the intensity forecast is reduced from the previous one at long range, although it remains above most of the guidance. It should be stressed that Dorian is likely to be a difficult cyclone to forecast due to the marginal environment it is embedded within and its small size. Key Messages: 1. A Tropical Storm Watch has been issued for Barbados, and additional watches and warnings for the Windward and Leeward Islands will likely be required later today. 2. It is too soon to determine the specific timing or magnitude of impacts in Puerto Rico or Hispaniola, but interests in those areas should monitor the progress of Dorian. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 11.0N 51.6W 35 KT 40 MPH 12H 25/1800Z 11.3N 53.3W 40 KT 45 MPH 24H 26/0600Z 11.8N 55.5W 45 KT 50 MPH 36H 26/1800Z 12.4N 57.7W 50 KT 60 MPH 48H 27/0600Z 13.2N 59.9W 55 KT 65 MPH 72H 28/0600Z 15.2N 63.9W 65 KT 75 MPH 96H 29/0600Z 17.1N 67.6W 60 KT 70 MPH 120H 30/0600Z 19.5N 71.0W 35 KT 40 MPH...INLAND $$ Forecaster Blake

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Tropical Storm Dorian Forecast Discussion Number 3

2019-08-25 04:48:49| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Aug 24 2019 000 WTNT45 KNHC 250248 TCDAT5 Tropical Storm Dorian Discussion Number 3 NWS National Hurricane Center Miami FL AL052019 1100 PM AST Sat Aug 24 2019 Dorian's structure hasn't changed substantially since the last advisory. Although southeasterly shear and surrounding dry air appear to be limiting convection, the most recent available microwave imagery showed that the small cyclone is maintaining well-defined convective banding. The initial intensity is still 35 kt, based primarily on the most recent TAFB Dvorak fix. Confidence in the intensity forecast is particularly low due to a number of factors. The small size of Dorian could make it susceptible to large short-term swings in intensity (up or down). While SHIPS diagnostics suggest that the wind shear will be below 10 kt for at least the next 72 h, UW-CIMSS shear diagnostics indicate that the current shear is higher, more like 15-20 kt. And the intensity guidance spread is also notably large, further decreasing confidence in the forecast. For now, the NHC intensity forecast has not been significantly changed, and shows Dorian near hurricane strength when it approaches the Lesser Antilles in a few days. This is a little above the intensity consensus and it should be stressed that it is still too soon to pinpoint potential wind impacts across those islands, given the large uncertainty in the forecast. The official forecast then shows Dorian maintaining hurricane strength across the northeast Caribbean, but it is certainly possible that it could be weaker, especially given the expected surrounding dry environment and a possible increase in wind shear forecast by the GFS and ECMWF models. By the end of the forecast period, the NHC intensity forecast is very close to HCCA and IVCN. An ASCAT-B overpass at 0024 UTC revealed that Dorian is moving slightly faster than previously estimated, now 280/12 kt. Unlike the intensity guidance, the track models are in quite good agreement on the track of Dorian. The tropical storm should continue moving westward to west-northwestward at a similar forward speed, steered primarily by a mid-level ridge stretching across most of the central Atlantic. The NHC forecast is nearly on top of the previous advisory and remains very near the TVCN and HCCA consensus aids. Key Messages: 1. Dorian is forecast to strengthen and could be near hurricane strength when it approaches the Lesser Antilles on Tuesday. 2. It is too soon to determine the specific timing or magnitude of impacts in the Lesser Antilles, but tropical storm or hurricane watches will likely be needed for a portion of the islands on Sunday. FORECAST POSITIONS AND MAX WINDS INIT 25/0300Z 10.9N 50.4W 35 KT 40 MPH 12H 25/1200Z 11.2N 52.0W 40 KT 45 MPH 24H 26/0000Z 11.7N 54.1W 45 KT 50 MPH 36H 26/1200Z 12.5N 56.2W 50 KT 60 MPH 48H 27/0000Z 13.2N 58.2W 55 KT 65 MPH 72H 28/0000Z 15.0N 62.3W 65 KT 75 MPH 96H 29/0000Z 17.0N 66.5W 70 KT 80 MPH 120H 30/0000Z 19.0N 70.0W 55 KT 65 MPH...INLAND $$ Forecaster Zelinsky

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