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