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Tropical Storm ROSLYN Forecast Discussion Number 4
2016-09-26 10:35:49| National Hurricane Center (East Pacific)
Issued at 200 AM PDT MON SEP 26 2016 000 WTPZ43 KNHC 260835 TCDEP3 TROPICAL STORM ROSLYN DISCUSSION NUMBER 4 NWS NATIONAL HURRICANE CENTER MIAMI FL EP182016 200 AM PDT MON SEP 26 2016 The sheared cloud pattern in infrared satellite imagery has changed little since the previous advisory. However, a 0304Z SSMI/S pass indicated the low-level structure had improved markedly, with a sharp hooked band wrapping more than half around the now well-defined low-level circulation center. The initial intensity has been increased to 35 kt based on a Dvorak classification of T2.5/35 kt from TAFB and the improved low-level structure indicated in SSMI/S 37 GHZ data. This makes Roslyn the seventeenth named storm of the 2016 eastern North Pacific hurricane season. The initial motion estimate is 020/4 kt. The advisory position had to be adjusted more than 30 nmi to the east of the previous advisory track based on recent SSMI/S and AMSU microwave center fixes. Otherwise, the previous track forecast rationale remains unchanged. A large mid- to upper-level low currently located over the central Baja California peninsula is expected to continue moving westward to west-southwestward during the next day or two. The combined southwesterly flow between that low and a deep-layer ridge located to the east and southeast of Roslyn should keep the tropical cyclone moving slowly north-northeastward to northeastward for the next 48 hours. After that, Roslyn is expected to weaken into a shallow remnant low, turning northward on day 3 and then northwestward on day 4. The new NHC track forecast is to the right of the previous advisory track, mainly due to the more eastward initial position, and lies close to the various consensus models. Little if any strengthening is expected due to gradually increasing southwesterly vertical wind shear and a very dry mid-level moisture regime characterized by humidity values less than 40 percent. By 36 to 48 hours, the vertical shear is forecast to increase to more than 30 kt, which is expected to erode the deep convection and induce steady weakening. As a result, Roslyn is forecast to become a remnant low by 48 hours, but this could occur sooner. The GFS and ECMWF models show the remnant low dissipating by the end of the forecast period when Roslyn will be moving over sub-24C SSTs. The new intensity forecast is identical to the previous advisory and closely follows the IVCN intensity consensus model. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 16.8N 119.5W 35 KT 40 MPH 12H 26/1800Z 17.2N 119.2W 35 KT 40 MPH 24H 27/0600Z 18.0N 118.5W 35 KT 40 MPH 36H 27/1800Z 19.1N 117.6W 35 KT 40 MPH 48H 28/0600Z 20.4N 117.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 29/0600Z 22.5N 117.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 30/0600Z 24.6N 119.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 01/0600Z...DISSIPATED $$ Forecaster Stewart
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