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Tropical Storm Hernan Forecast Discussion Number 8

2020-08-28 04:34:40| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Thu Aug 27 2020 135 WTPZ43 KNHC 280234 TCDEP3 Tropical Storm Hernan Discussion Number 8 NWS National Hurricane Center Miami FL EP132020 900 PM MDT Thu Aug 27 2020 A burst of strong convection has developed near the center of Hernan during the past several hours, but overall the storm remains poorly organized. Indeed, a westerly surface wind observed in the Isla Marias Islands to the north of the center makes it unclear if a closed circulation still exists, a question that upcoming scatterometer data will hopefully resolve. The initial intensity remains 35 kt based mainly on satellite intensity estimates from TAFB and SAB. The initial motion is now north-northwestward or 335/7 kt. The cyclone is expected to turn northwestward and then west- northwestward as it rotates around the northeastern side of the approaching Tropical Storm Iselle. The new forecast track is nudged to the north of the previous track and lies near the various consensus models. A combination of continued strong easterly shear and interaction with the larger Iselle should cause Hernan to weaken during the next 48 h or so. The new intensity forecast shows the system weakening to a depression after 12 h and degenerating to a remnant low by 36 h. After that, the cyclone is currently forecast to degenerate to a trough by 60 h as it gets absorbed into Iselle, and several of the global models suggest this could happen earlier. The new intensity forecast is an update of the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 28/0300Z 20.5N 107.0W 35 KT 40 MPH 12H 28/1200Z 21.6N 108.3W 35 KT 40 MPH 24H 29/0000Z 22.3N 110.0W 30 KT 35 MPH 36H 29/1200Z 23.0N 111.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 30/0000Z 23.4N 113.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 30/1200Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Laura Forecast Discussion Number 32

2020-08-27 22:43:19| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Thu Aug 27 2020 000 WTNT43 KNHC 272043 TCDAT3 Tropical Storm Laura Discussion Number 32 NWS National Hurricane Center Miami FL AL132020 400 PM CDT Thu Aug 27 2020 Laura has continued to weaken this afternoon. The satellite and radar presentation of the tropical cyclone has continued to degrade, and the center has now moved into southern Arkansas. The initial intensity has been reduced to 45 kt, and is based on a blend of surface observations, Doppler radar data, and typical over land tropical cyclone filling rates. Sustained tropical storm force winds have been observed in northern Louisiana, and wind gusts to nearly 50 kt were reported in southern Arkansas earlier this afternoon. Laura will continue to rapidly weaken during the next 6-12 hours, and it is expected to become a tropical depression either this evening or overnight. The extratropical remnants could strengthen over the western Atlantic early next week, and the track and intensity forecast for that time is based on guidance from the NOAA Ocean Prediction Center. The tropical storm has turned north-northeastward or 015/13 kt. The cyclone is forecast to turn northeastward, and then east- northeastward as it becomes embedded within the mid-latitude westerlies on Friday. This motion will take Laura or its remnants across the central Appalachians and to the Mid-Atlantic states on Saturday. After that time, the system should accelerate east-northeastward to northeastward over the western Atlantic. The updated NHC track forecast remains similar to the previous advisory and is close to the various consensus models. Key Messages: 1. Tropical storm force winds, especially in gusts, will continue near the center of Laura over portions of extreme northern Louisiana and Arkansas this evening. 2. Flash flooding along small streams, urban areas, and roadways will continue across portions of Louisiana, Mississippi and Arkansas. Additional rainfall will also lead to minor to moderate freshwater river flooding. The heavy rainfall threat and flash and urban flooding potential will spread northeastward into the middle-Mississippi, lower Ohio and Tennessee Valleys, and Mid-Atlantic States Friday and Saturday. 3. A few tornadoes are possible this evening across central and eastern Arkansas into Mississippi. The risk for a few tornadoes should redevelop Friday afternoon and evening across parts of the Mid-South and Tennessee Valley regions. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 33.4N 92.8W 45 KT 50 MPH...INLAND 12H 28/0600Z 35.3N 91.7W 30 KT 35 MPH...INLAND 24H 28/1800Z 36.7N 89.3W 25 KT 30 MPH...INLAND 36H 29/0600Z 37.4N 85.1W 25 KT 30 MPH...POST-TROP/INLAND 48H 29/1800Z 38.2N 78.6W 25 KT 30 MPH...POST-TROP/INLAND 60H 30/0600Z 39.6N 70.9W 35 KT 40 MPH...POST-TROP/EXTRATROP 72H 30/1800Z 43.1N 63.4W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 31/1800Z 48.8N 50.7W 45 KT 50 MPH...POST-TROP/EXTRATROP 120H 01/1800Z...DISSIPATED $$ Forecaster Brown

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Tropical Storm Hernan Forecast Discussion Number 7

2020-08-27 22:34:16| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Thu Aug 27 2020 000 WTPZ43 KNHC 272034 TCDEP3 Tropical Storm Hernan Discussion Number 7 NWS National Hurricane Center Miami FL EP132020 300 PM MDT Thu Aug 27 2020 Hernan has had a rather ragged appearance today with intermittent convection firing around, but never really over the center. There were no ASCAT overpasses during the day that directly sampled the system's circulation. The latest Dvorak intensity estimates from TAFB and SAB support 35 kt and 30 kt, respectively, while the UW-CIMSS ADT and SATCON numbers range from 30 to 40 kt. Based on a blend of these values, the initial intensity is being lowered to 35 kt. Hernan should at least maintain its current strength through tonight as it remains over very warm waters while battling moderate easterly shear. This shear is not forecast to abate during the forecast period. And by 24 h, the cyclone is expected to begin moving over progressively cooler waters. The combination of these two factors should induce a weakening trend. By 60 h, simulated satellite imagery by global models suggest that the cyclone will become devoid of deep convection. After that time, the remnant low is expected to be absorbed within the broader circulation of Iselle to its west. The latest NHC intensity forecast is in agreement with the various intensity consensus aids. Hernan is now moving northwest at about 5 kt. There is no change to the track forecast reasoning. Hernan is expected to remain caught within a large-scale, eastern North Pacific cyclonic gyre during the next few days, which will result in Hernan moving counter-clockwise around the eastern and northern periphery of the gyre. The cyclone is expected to continue to parallel the coast of southwestern Mexico through this evening and should begin to increase its forward speed tonight. A turn to the west-northwest then west is forecast Friday through Friday night. The westward motion should continue until the system becomes absorbed early next week. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 19.6N 106.5W 35 KT 40 MPH 12H 28/0600Z 20.7N 107.6W 35 KT 40 MPH 24H 28/1800Z 21.6N 109.4W 30 KT 35 MPH 36H 29/0600Z 22.3N 111.1W 30 KT 35 MPH 48H 29/1800Z 22.5N 113.0W 25 KT 30 MPH 60H 30/0600Z 22.6N 114.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 30/1800Z...ABSORBED $$ Forecaster Latto

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Tropical Storm Iselle Forecast Discussion Number 6

2020-08-27 22:32:49| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Thu Aug 27 2020 000 WTPZ44 KNHC 272032 TCDEP4 Tropical Storm Iselle Discussion Number 6 NWS National Hurricane Center Miami FL EP142020 200 PM PDT Thu Aug 27 2020 Since the previous advisory, a new burst of deep convection developed near the center of Iselle the persisted for much of the morning. It was not until recently that the easterly shear has begun to push the convection to the west of the center. The latest Dvorak estimates from TAFB and SAB as well as data from a recent ASCAT overpass all agree that the initial intensity has increased to 45 kt. Moderate-to-strong easterly to northeasterly vertical wind shear should prevent Iselle from further strengthening over the next couple of days. After that time, the shear is forecast to relax somewhat. However, by that time the cyclone will then be moving over SSTs of only 26 C and into a more stable atmospheric environment. This should cause the deep convection to gradually dissipate, and the cyclone is expected to weaken late this weekend into early next week before degenerating into a remnant low. The latest NHC intensity forecast is close to the LGEM guidance. Iselle continues moving northeastward at 4 kt, embedded in a large monsoon gyre. The steering flow is forecast to become more southeasterly this weekend inside the gyre, resulting in a gradual turn to the northwest. As the cyclone breaks north of the gyre and becomes shallow, a turn to the west is anticipated while the system becomes steered by a low- to mid- level ridge to its north. The latest NHC track forecast was nudged just a little to the right of the previous one due to a shift in the guidance, and lies in between the previous forecast and the various track consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 17.2N 115.8W 45 KT 50 MPH 12H 28/0600Z 17.8N 115.3W 45 KT 50 MPH 24H 28/1800Z 18.6N 114.7W 45 KT 50 MPH 36H 29/0600Z 19.6N 114.4W 40 KT 45 MPH 48H 29/1800Z 20.8N 114.3W 35 KT 40 MPH 60H 30/0600Z 21.9N 114.6W 30 KT 35 MPH 72H 30/1800Z 22.8N 115.4W 25 KT 30 MPH 96H 31/1800Z 23.2N 116.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 01/1800Z 23.1N 117.8W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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Tropical Storm Iselle Forecast Discussion Number 5

2020-08-27 16:50:42| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Thu Aug 27 2020 000 WTPZ44 KNHC 271450 TCDEP4 Tropical Storm Iselle Discussion Number 5 NWS National Hurricane Center Miami FL EP142020 800 AM PDT Thu Aug 27 2020 Iselle consists of a rather ragged looking area of deep convection being sheared to the southwest of a partially exposed low-level center. A blend of the latest satellite intensity estimates from TAFB and UW-CIMSS support keeping the initial intensity at 40 kt. Moderate-to-strong easterly to northeasterly vertical wind shear will likely prevent Iselle from strengthening over the next couple of days. After that time, the shear is forecast to relax somewhat. However, the cyclone will then be moving over SSTs of only 26 C and into a more stable atmospheric environment. This should cause the deep convection to gradually dissipate, and the cyclone is expected to weaken this weekend before degenerating into a remnant low early next week. The latest NHC intensity forecast is a blend of the IVCN consensus and SHIPS guidance. Iselle is moving northeastward at 4 kt, embedded in a large monsoon gyre. The steering flow is forecast to become more southerly this weekend inside the gyre, resulting in a more northward motion. As the cyclone breaks north of the gyre, a turn to the northwest then west is anticipated while the system becomes steered by a low- to mid- level ridge to its north. The latest NHC track forecast is little changed from the previous one, and lies in between the HFIP corrected consensus HCCA and the TVCX track consensus. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 17.0N 115.8W 40 KT 45 MPH 12H 28/0000Z 17.5N 115.4W 40 KT 45 MPH 24H 28/1200Z 18.2N 115.0W 40 KT 45 MPH 36H 29/0000Z 18.9N 114.6W 40 KT 45 MPH 48H 29/1200Z 19.9N 114.2W 40 KT 45 MPH 60H 30/0000Z 21.2N 114.4W 35 KT 40 MPH 72H 30/1200Z 22.2N 115.3W 30 KT 35 MPH 96H 31/1200Z 22.5N 116.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 01/1200Z 22.3N 117.3W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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