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Hurricane Hilary Discussion Number  24
NWS National Hurricane Center Miami FL       EP092017
200 AM PDT Thu Jul 27 2017

Hilary has lost organization due to the effects of moderate
northerly vertical shear, with recent Windsat and GPM overpasses
indicating that the center is now near the northwestern edge of the
main convective area.  Subjective satellite intensity estimates are
75-90 kt, while estimates based on microwave sounding data and
the CIMSS ADT are 55-65 kt.  Based on a blend of these, the
initial intensity is reduced to a possibly generous 75 kt.

The large-scale models forecast Hilary to move under a cyclonic
shear axis between two upper-level high pressure areas during the
next 24-48 h.  This evolution may allow the current shear to
decrease, but it is not an ideal environment for a tropical
cyclone.  Thus, the intensity forecast calls for little change in
strength during this time, which is in good agreement with the
intensity guidance.  After 48 h, Hilary should move over cooler sea
surface temperatures, and it is likely to significantly interact
with Tropical Storm Irwin.  The large-scale models differ on the
details, but they agree that the combination of the interaction, dry
air, and the presence of cool water should cause Hilary to quickly
weaken as a result.  Thus, the new intensity forecast now calls for
the system to degenerate to a remnant low by the end of the forecast
period.

The initial motion is 290/10.  For the first 48 h or so, Hilary
should be steered generally west-northwestward by the subtropical
ridge.  After that, the track forecast becomes very uncertain due
to the interaction with Irwin, where the latter cyclone is likely
to move around the eastern and northern sides of Hilary before a
possible merger. The Canadian model merges the two cyclones in
about 72 h, while the GFS finishes the merger at about 120 h.  On
the other hand, the ECMWF delays the merger until after the forecast
period, while the UKMET keeps the two systems separate.  The new
track forecast will not attempt to show the details of the
interaction, but it will show more of a westward turn and changes in
speed during the interaction than the previous forecast.  Overall,
the new track is close to the consensus models.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 17.3N 115.1W   75 KT  85 MPH
 12H  27/1800Z 17.7N 116.5W   70 KT  80 MPH
 24H  28/0600Z 18.4N 118.2W   70 KT  80 MPH
 36H  28/1800Z 19.1N 119.9W   70 KT  80 MPH
 48H  29/0600Z 19.8N 121.7W   70 KT  80 MPH
 72H  30/0600Z 21.0N 126.0W   60 KT  70 MPH
 96H  31/0600Z 21.5N 130.5W   45 KT  50 MPH
120H  01/0600Z 22.0N 133.0W   30 KT  35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Beven

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