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Tropical Depression Iselle Forecast Discussion Number 17

2020-08-30 16:36:12| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Sun Aug 30 2020 000 WTPZ44 KNHC 301436 TCDEP4 Tropical Depression Iselle Discussion Number 17 NWS National Hurricane Center Miami FL EP142020 900 AM MDT Sun Aug 30 2020 Iselle is skirting the line between tropical cyclone and remnant low, producing a small amount of deep convection during the past few hours. While this isn't particularly organized thunderstorm activity, for now it is enough to keep advisories going another 6 hours. The initial intensity is set to 30 kt, near the latest TAFB T-number. Iselle is still expected to degenerate into a remnant low later today due to persistent easterly shear and a dry and stable environment. Model guidance all show the system becoming a trough by Tuesday, so the dissipation phase has been pushed ahead from the last forecast. The guidance also remains in good agreement on a northward track today, followed by a north-northwest turn overnight. The track is shifted a bit to the east because of a more eastward initial position, otherwise it is unchanged. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 23.0N 113.3W 30 KT 35 MPH 12H 31/0000Z 24.0N 113.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 31/1200Z 25.0N 113.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 01/0000Z...DISSIPATED $$ Forecaster Blake

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Tropical Depression Iselle Forecast Discussion Number 16

2020-08-30 10:36:37| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Sun Aug 30 2020 000 WTPZ44 KNHC 300836 TCDEP4 Tropical Depression Iselle Discussion Number 16 NWS National Hurricane Center Miami FL EP142020 300 AM MDT Sun Aug 30 2020 Iselle has produced very little deep convection since around 00Z. Earlier partial ASCAT data indicated that the maximum winds have decreased to 30 kt, which is not surprising given Iselle's lack of organization. Iselle is moving into a drier and more stable environment and over progressively cooler SSTs. While some slight redevelopment of convection is possible during the next few hours, it is unlikely to be very well organized and Iselle is forecast become post-tropical later today. The remnants of Iselle will then continue to gradually spin down for another day or two until it dissipates entirely in a couple of days. The cyclone has moved generally north-northeastward to northward during the pas few hours and is slowing down. The guidance is in good agreement that a northward to north-northwestward motion is likely until Iselle dissipates. No changes of note were made to the NHC track or intensity forecasts. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 22.3N 114.0W 30 KT 35 MPH 12H 30/1800Z 23.3N 114.1W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 31/0600Z 24.2N 114.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 31/1800Z 24.9N 114.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 01/0600Z 26.0N 115.0W 15 KT 15 MPH...POST-TROP/REMNT LOW 60H 01/1800Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Iselle Forecast Discussion Number 15

2020-08-30 04:32:01| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Sat Aug 29 2020 000 WTPZ44 KNHC 300231 TCDEP4 Tropical Storm Iselle Discussion Number 15 NWS National Hurricane Center Miami FL EP142020 900 PM MDT Sat Aug 29 2020 Most of the deep convection associated with Iselle has been displaced to the west of the center due to strong easterly shear caused by a large upper-level anticyclone to the northeast. Based on the decreased organization, it is assumed that the cyclone has been gradually spinning down this evening, and the current intensity estimate is set at 35 kt. Although the shear may relax a bit in a day or so, Iselle will be moving over increasingly cooler waters during that time. Therefore, continued weakening is anticipated, and the system will likely degenerate into a remnant low in 12 to 24 hours. The official intensity forecast is at the high end of the model guidance. The cyclone has been moving north-northeastward, at around 020/8 kt, on the west side of a mid-level high pressure area. The weakening and increasingly shallow system should gradually turn to the left under the influence of the lower-level flow. The official track forecast is about in the middle of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 21.8N 114.2W 35 KT 40 MPH 12H 30/1200Z 22.9N 114.3W 30 KT 35 MPH 24H 31/0000Z 23.9N 114.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 31/1200Z 24.5N 114.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 01/0000Z 25.0N 115.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 01/1200Z 25.5N 115.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 02/0000Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm Iselle Forecast Discussion Number 14

2020-08-29 22:38:58| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Sat Aug 29 2020 000 WTPZ44 KNHC 292038 TCDEP4 Tropical Storm Iselle Discussion Number 14 NWS National Hurricane Center Miami FL EP142020 300 PM MDT Sat Aug 29 2020 Iselle has become less organized during the past several hours, as the area of convection southwest of the center is now smaller and farther from the center than previously. This is likely due to the combined effects of continued easterly shear and decreasing sea surface temperatures under the storm. A recent ASCAT overpass shows that maximum sustained winds remain near 40 kt, but the tropical-storm-force winds are now occurring only over the southeastern quadrant. The initial motion is north-northeastward or 025/6 kt. Iselle should turn toward the north during the next several hours as it is steered by a mid-level ridge to the east. The northward motion should continue for another day or so, followed by a turn toward the northwest as Iselle becomes a weak and shallow system. The new forecast track again has minor adjustment from the previous track based mainly on the initial position and motion. The shear and cooler water along the forecast track should cause Iselle to steadily weaken, and the new forecast track has no changes from the previous forecast. The forecast, which calls for the cyclone to weaken to a depression by 24 h, to degenerate to a remnant low by 36 h, and to weaken to a trough by 72 h, again lies at the upper edge of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 21.0N 114.5W 40 KT 45 MPH 12H 30/0600Z 22.1N 114.5W 35 KT 40 MPH 24H 30/1800Z 23.4N 114.8W 30 KT 35 MPH 36H 31/0600Z 24.2N 115.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 31/1800Z 24.8N 115.6W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 01/0600Z 25.4N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 01/1800Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Iselle Forecast Discussion Number 13

2020-08-29 16:33:22| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Sat Aug 29 2020 000 WTPZ44 KNHC 291433 TCDEP4 Tropical Storm Iselle Discussion Number 13 NWS National Hurricane Center Miami FL EP142020 900 AM MDT Sat Aug 29 2020 Iselle has changed little in organization since the last advisory, as the storm has a classic shear pattern in satellite imagery with the low-level center near the northeastern edge of the convective mass. Satellite intensity estimates have changed little in the last 6 h, so the initial intensity remains 40 kt. The cyclone has tracked a little to the right with the initial motion now north-northeastward or 025/6 kt. This should be short lived, and Iselle is expected to resume a northward motion later today or tonight. The northward motion should continue for another day or so, followed by a turn toward the northwest as Iselle becomes a weak and shallow system. The new forecast track is tweaked a little from the previous forecast based on the initial position and motion. Iselle is moving over decreasing sea surface temperatures, and it continues to experience easterly vertical shear. This combination should cause steady weakening, and the new intensity forecast follows the previous forecast in calling for the system to weaken to a depression by 24 h, to degenerate to a remnant low by 36 h, and to weaken to a trough by 72 h. The new intensity forecast lies at the upper edge of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 29/1500Z 20.3N 114.7W 40 KT 45 MPH 12H 30/0000Z 21.3N 114.6W 35 KT 40 MPH 24H 30/1200Z 22.8N 114.7W 30 KT 35 MPH 36H 31/0000Z 23.9N 115.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 31/1200Z 24.7N 115.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 01/0000Z 25.4N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 01/1200Z...DISSIPATED $$ Forecaster Beven

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