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Tropical Storm Selma Forecast Discussion Number 1
2017-10-27 10:54:49| National Hurricane Center (East Pacific)
Issued at 400 AM CDT Fri Oct 27 2017 000 WTPZ45 KNHC 270854 TCDEP5 Tropical Storm Selma Discussion Number 1 NWS National Hurricane Center Miami FL EP202017 400 AM CDT Fri Oct 27 2017 Satellite images indicate that deep convection has become more concentrated over the past several hours in the western semicircle of the low over the far eastern part of the eastern Pacific. Scatterometer data from overnight also indicate that the low has become well defined, with winds of about 35 kt. Thus, advisories are being initiated on Tropical Storm Selma. This is a rare location for a tropical storm to form in the eastern Pacific, and this is the only the second tropical storm to form on record east of 90W that didn't come from an Atlantic cyclone (the first was Alma 2008). A weak ridge of high pressure over southeastern Mexico is causing the storm to move slowly northwestward. However, this pattern is forecast to change quickly as the ridge breaks down due to a large mid-latitude trough dropping into the Gulf of Mexico during the next 24 hours. This pattern should steer Selma toward the north over the weekend, although the models are in rather poor agreement on exactly when that turn occurs. To complicate matters, there is also some chance of interaction with the Caribbean disturbance AL93, with the models showing the most interaction, such as the UKMET or ECMWF, having tracks on the eastern side of the guidance envelope. The GFS-based guidance shows little interaction and shows a track more toward Guatemala. The NHC forecast splits the difference between the model consensus and the ECMWF, and is just east of the latest NOAA-corrected consensus, necessitating a Tropical Storm Warning for El Salvador. This is a low confidence forecast due to the large spread in the guidance and a Tropical Storm Watch or Warning could be required for portions of the coast of Guatemala later today. Selma has a day or so of very warm waters and light-to-moderate shear in its path. These conditions should promote strengthening, and the NHC intensity forecast is consistent with the bulk of the models. It should be emphasized, however, that the most significant hazard expected with Selma is heavy rainfall. Up to 10 inches of rain are possible over El Salvador and southern Guatemala, which could cause life-threatening flash floods and mudslides. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 10.7N 89.5W 35 KT 40 MPH 12H 27/1800Z 11.3N 89.9W 40 KT 45 MPH 24H 28/0600Z 12.5N 90.0W 45 KT 50 MPH 36H 28/1800Z 13.7N 89.7W 35 KT 40 MPH...INLAND 48H 29/0600Z...DISSIPATED $$ Forecaster Blake
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