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Tropical Storm Pilar Forecast Discussion Number 4
2017-09-24 16:33:53| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Sun Sep 24 2017 000 WTPZ43 KNHC 241433 TCDEP3 Tropical Storm Pilar Discussion Number 4 NWS National Hurricane Center Miami FL EP182017 1000 AM CDT Sun Sep 24 2017 Pilar has been a difficult tropical storm to analyze and forecast this morning. The overnight short-wave and clean infrared channels from GOES-16 were suggestive that the system had accelerated north-northwestward unexpectedly. However, the available microwave imagery was ambiguous and the spread between the SAB and TAFB Dvorak fixes was a degree. I've re-positioned Pilar north-northwestward significantly, but not as much as may be needed later once additional visible imagery becomes available. The initial motion, then, is a very uncertain 335 degrees at 7 kt, as Pilar is being advected around a deep-layer ridge located over central Mexico. Despite the initial speed up in movement, the guidance insists that Pilar should slow down shortly. The official forecast track is quite similar to the previous one, with a course along or just west of the coast of southwestern Mexico. This forecast is based upon a blend of the GFS, HWRF, and UKMET models. However, the guidance has a large spread (and some of the models could not explicitly track Pilar), and a plausible alternative scenario is that the center of Pilar makes landfall and dissipates shortly thereafter. This alternative is supported by the ECMWF, COAMPS, and HMON models. Hopefully, the 12Z model runs will be more consistent. The initial intensity of Pilar is unchanged at 40 kt, as the SAB and TAFB Dvorak Current Intensity classifications remained the same. While the SSTs are warm and the vertical shear is only moderate for the next 24 to 36 hours, interaction with the high topography of southwestern Mexico may prevent any additional intensification. After about 24 to 36 hours, the vertical shear should go way up as Pilar is affected by strong southerlies associated with a vigorous mid- to upper-tropospheric trough over the southwestern United States. The official intensity forecast is closest to the HWRF model, though none of the guidance now shows any significant intensification. This new forecast is slightly below that from the previous advisory and now calls for Pilar to become a remnant low in about two days. An alternative scenario, mentioned above, is for Pilar to make landfall and dissipate sooner. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 19.9N 105.9W 40 KT 45 MPH 12H 25/0000Z 20.4N 106.0W 40 KT 45 MPH 24H 25/1200Z 21.1N 106.0W 35 KT 40 MPH 36H 26/0000Z 21.8N 106.0W 30 KT 35 MPH 48H 26/1200Z 22.5N 106.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 27/1200Z...DISSIPATED $$ Forecaster Landsea
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