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Tropical Storm Harvey Forecast Discussion Number 8

2017-08-19 10:43:01| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Aug 19 2017 000 WTNT44 KNHC 190842 TCDAT4 Tropical Storm Harvey Discussion Number 8 NWS National Hurricane Center Miami FL AL092017 500 AM AST Sat Aug 19 2017 GOES-16 shortwave infrared imagery shows the partially exposed center of Harvey on the eastern edge of a large area of deep convection. While the center is closer to the convection than a few hours ago, cirrus clouds be clearly seen moving from northeast to southwest across the system; an indication of the persistent shear. Satellite estimates suggest the maximum winds of Harvey remain 35 kt. The storm has picked up some forward speed with a recent motion estimate of 275/18. A low- to mid-level ridge extending across the western Atlantic should keep Harvey on a fast westward course across the Caribbean Sea for the next 48 hours. Thereafter, the ridge weakens across the Gulf of Mexico due to a strong mid/upper-level low currently seen there on water vapor images. This steering flow change should cause Harvey to slow down and turn a little more to the west-northwest in the northwestern Caribbean Sea, with an even slower motion forecast for Harvey in the Bay of Campeche. Model guidance is in fairly good agreement on this scenario, and no significant changes were made to the previous forecast. Harvey should be moving into a more conducive environment for strengthening early next week since the strong northeasterly shear that has been affecting the cyclone is forecast by almost all of the guidance to weaken within about 24 hours. In a few days, most of the global models show a favorable upper-level environment for intensification, and Harvey could be near hurricane strength between the 72 hour forecast period and landfall. There has been little change to the guidance so the new intensity forecast is similar to the previous one, close to the model consensus. However, with a weak storm moving so quickly across the central Caribbean, one always has to be careful of the system opening up into a wave. This is a plausible alternative scenario still suggested by the GFS and ECMWF models. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 13.8N 65.9W 35 KT 40 MPH 12H 19/1800Z 14.0N 68.7W 35 KT 40 MPH 24H 20/0600Z 14.3N 72.7W 40 KT 45 MPH 36H 20/1800Z 14.6N 76.7W 45 KT 50 MPH 48H 21/0600Z 15.2N 80.1W 50 KT 60 MPH 72H 22/0600Z 17.0N 86.5W 55 KT 65 MPH 96H 23/0600Z 18.3N 90.5W 35 KT 40 MPH...INLAND 120H 24/0600Z 19.3N 93.0W 40 KT 45 MPH...OVER WATER $$ Forecaster Blake

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Tropical Storm Kenneth Forecast Discussion Number 4

2017-08-19 10:39:59| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sat Aug 19 2017 000 WTPZ43 KNHC 190839 TCDEP3 Tropical Storm Kenneth Discussion Number 4 NWS National Hurricane Center Miami FL EP132017 200 AM PDT Sat Aug 19 2017 Microwave data reveal that Kenneth is not well organized yet with the low- and mid-level centers well separated. The low-level center is located on the northeastern edge of the convection due to northerly shear as diagnosed by the SHIPS model. The overall outflow pattern is well established. A partial ASCAT pass around 0500 UTC and Dvorak estimates indicate that the initial intensity remains at 35 kt. The shear is forecast to decrease a little, and Kenneth will be moving over warm waters for the next two days. On this basis, the NHC forecast calls for some strengthening, and Kenneth could become a hurricane by Sunday. By the end of the forecast period, the cyclone is expected to be moving over cooler waters and weakening should then begin. The best estimate of the initial motion is toward the west-northwest or 285 degrees at 14 kt. The cyclone is embedded within the flow around the subtropical high, and this track should continue for the next day or two. After that time, Kenneth should have reached the western end of ridge, and it should then gradually turn more to the northwest or north-northwest. For the next 2 to 3 days, the track guidance is pretty tightly clustered, increasing the confidence in the forecast. Beyond 3 days, the forecast is less certain since the guidance spread is large. The NHC forecast is in the middle of the guidance envelope which is bounded by the ECMWF on the west side and by the GFS and the HWRF models on the right. The official forecast does not depart much from the previous one during the first 3 days, and is shifted a little bit to the right thereafter. FORECAST POSITIONS AND MAX WINDS INIT 19/0900Z 15.6N 120.2W 35 KT 40 MPH 12H 19/1800Z 16.0N 122.3W 45 KT 50 MPH 24H 20/0600Z 16.6N 125.0W 55 KT 65 MPH 36H 20/1800Z 17.5N 127.5W 65 KT 75 MPH 48H 21/0600Z 18.5N 129.5W 65 KT 75 MPH 72H 22/0600Z 20.5N 131.5W 65 KT 75 MPH 96H 23/0600Z 23.5N 132.5W 50 KT 60 MPH 120H 24/0600Z 26.5N 133.5W 40 KT 45 MPH $$ Forecaster Avila

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Tropical Storm Harvey Forecast Discussion Number 7

2017-08-19 04:33:57| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Aug 18 2017 000 WTNT44 KNHC 190233 TCDAT4 Tropical Storm Harvey Discussion Number 7 NWS National Hurricane Center Miami FL AL092017 1100 PM AST Fri Aug 18 2017 Microwave and conventional geostationary satellite images indicate that Harvey's center is still displaced to the east of a persistent cluster of deep convection due to about 15 kt of northeasterly shear. With no notable changes in the cyclone's structure since the last advisory, the maximum wind estimate remains 35 kt, which is in line with the latest Dvorak CI numbers. Recent microwave fixes suggest that Harvey's center may have slowed down or wobbled northward temporarily, and the 12-hour motion estimate is a slightly slower 275/16 kt. A low- to mid-level ridge extending across the western Atlantic should keep Harvey on a swift westward course across the Caribbean Sea for the next 48 hours. Once Harvey moves into the northwestern Caribbean Sea by 72 hours, it will reach the western edge of the ridge, and it will likely slow down and gain some latitude due to a cut-off low over the northwestern Gulf of Mexico. Although the track guidance envelope has shifted slightly northward on this cycle, especially around the 72-hour period, there are no significant changes from six hours ago. Northeasterly or northerly shear is forecast to continue for the next 36 hours or so, which should prevent significant strengthening while Harvey moves westward across the eastern and central Caribbean Sea. By 48 hours, the vertical shear over the cyclone drops drastically, but the system's fast motion could still limit the amount of intensification that will occur. The best opportunity for strengthening would likely be around day 3 when Harvey begins to slow down as it approaches Belize and the Yucatan Peninsula. The updated NHC intensity forecast is the same as the previous one during the first 48 hours, and it was nudged a little lower at 72 hours to fall closer to the HFIP Corrected Consensus (HCCA) model and the ICON intensity consensus. A lot of uncertainty remains, however, since models like SHIPS and HWRF bring Harvey near or to hurricane intensity before it reaches land while the GFS and ECMWF continue to weaken the circulation while it moves across the Caribbean Sea. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 13.7N 64.1W 35 KT 40 MPH 12H 19/1200Z 14.0N 66.8W 35 KT 40 MPH 24H 20/0000Z 14.3N 70.4W 40 KT 45 MPH 36H 20/1200Z 14.5N 74.3W 40 KT 45 MPH 48H 21/0000Z 14.8N 78.1W 45 KT 50 MPH 72H 22/0000Z 16.5N 84.7W 55 KT 65 MPH 96H 23/0000Z 18.0N 89.5W 45 KT 50 MPH...INLAND 120H 24/0000Z 19.0N 92.5W 40 KT 45 MPH...OVER WATER $$ Forecaster Berg

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Tropical Storm Kenneth Forecast Discussion Number 3

2017-08-19 04:31:56| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Fri Aug 18 2017 000 WTPZ43 KNHC 190231 TCDEP3 Tropical Storm Kenneth Discussion Number 3 NWS National Hurricane Center Miami FL EP132017 800 PM PDT Fri Aug 18 2017 Conventional satellite imagery and an earlier SSMI/S overpass show that the depression has been strengthening during the past several hours. Impressive outer curved banding has developed in the eastern and northern portions of the cyclone, and the last few visible images are showing early signs of a small central dense overcast, or inner core formation. Based on the overall improvement of the cloud pattern and Dvorak satellite intensity estimates of 35 kt from SAB and TAFB, the depression is upgraded to Tropical Storm Kenneth. The northeasterly shear is diminishing, the oceanic temperatures are warm, and the low to mid levels of the atmosphere are moist, all bolstering further strengthening through the 48-hour period. Afterward, decreasing sea surface temperatures and an intruding stable air mass from the north should result in gradual weakening. No significant changes were made to the previous intensity forecast, and the NHC forecast is based primarily on the IVCN multi-model consensus. The initial motion is estimated to be westward, or 280/13 kt. Kenneth is embedded in the easterly mid-level flow of a subtropical ridge extending over the eastern Pacific from northern Mexico. The cyclone should commence a gradual turn toward the west-northwest around the 24-hour period as a mid- to upper-level low located just southwest of the southern California coast amplifies and erodes the ridge to the northwest of Kenneth. Kenneth's motion is expected to be further influenced by this growing weakness in the ridge by decreasing in forward speed, turning northwestward on day 3, and then north-northwestward around the 96-hour period. The guidance suite is surprisingly tightly clustered through the entire forecast period. The NHC track forecast is similar to the previous one, and is hedged toward the HCCA corrected consensus model. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 15.0N 119.1W 35 KT 40 MPH 12H 19/1200Z 15.4N 121.2W 45 KT 50 MPH 24H 20/0000Z 16.0N 124.1W 55 KT 65 MPH 36H 20/1200Z 16.8N 126.7W 65 KT 75 MPH 48H 21/0000Z 17.7N 128.9W 75 KT 85 MPH 72H 22/0000Z 19.6N 131.8W 70 KT 80 MPH 96H 23/0000Z 22.3N 133.1W 60 KT 70 MPH 120H 24/0000Z 25.2N 133.9W 45 KT 50 MPH $$ Forecaster Roberts

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Tropical Depression Thirteen-E Forecast Discussion Number 2

2017-08-18 22:36:53| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Fri Aug 18 2017 000 WTPZ43 KNHC 182036 TCDEP3 Tropical Depression Thirteen-E Discussion Number 2 NWS National Hurricane Center Miami FL EP132017 200 PM PDT Fri Aug 18 2017 The tropical depression is slowly becoming more organized. Moderate upper-level northeasterly winds are inhibiting outflow in the northeast quadrant of the cyclone, but banding convection is present in all quadrants. A pair of ASCAT passes around 1700 and 1800 UTC confirmed that the depression has a well-defined center and showed maximum winds of just below 30 kt. The initial intensity is therefore held at 30 kt. The ASCAT data also indicated that the center of circulation is a little to the south of the previous estimated position, perhaps in part due to a reorganization of the center closer to the main mass of convection. Due to the relocation of the estimated center to the south, the official forecast has been shifted to the south and west throughout the forecast period. Otherwise, the reasoning behind the forecast has not changed, and the depression should continue to be steered generally westward to west-northwestward for the next 3 days by a mid-level ridge. A weakness in the ridge is forecast to develop around day 4 that should cause the cyclone to turn toward the northwest and slow down. The only change in the track guidance is a slight increase in forward speed through 36 h, and confidence is fairly high in the track forecast due to a tight clustering of the global models. No change has been made to the intensity forecast, which remains very close to the multi-model consensus. The light northeasterly shear currently affecting the cyclone is forecast to decrease over the next 12 to 24 h. Once that happens, the cyclone will be embedded within an environment consisting of low shear, warm SSTs, and sufficient moisture to support strengthening. After about 72 h, cooler waters and a more stable environment should cause steady weakening. By day 5, SSTs are expected to be be below 24 C, and the cyclone will likely be close to becoming a remnant low. FORECAST POSITIONS AND MAX WINDS INIT 18/2100Z 14.8N 117.9W 30 KT 35 MPH 12H 19/0600Z 15.1N 119.9W 35 KT 40 MPH 24H 19/1800Z 15.6N 122.7W 45 KT 50 MPH 36H 20/0600Z 16.4N 125.5W 55 KT 65 MPH 48H 20/1800Z 17.2N 128.0W 65 KT 75 MPH 72H 21/1800Z 18.9N 131.6W 75 KT 85 MPH 96H 22/1800Z 21.0N 133.3W 65 KT 75 MPH 120H 23/1800Z 23.9N 133.9W 45 KT 50 MPH $$ Forecaster Zelinsky

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