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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