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Tropical Storm Irwin Discussion Number   6
NWS National Hurricane Center Miami FL       EP102017
200 PM PDT Sun Jul 23 2017

Deep convection has covered up Irwin's center since the last
advisory, a sign that the shear affecting the system continues to
diminish.  Dvorak intensity estimates have increased to T3.0/45 kt
from TAFB and SAB, but objective numbers are still at T2.5/35 kt.
Based on these estimates, and a 1748 UTC ASCAT-B pass, the initial
intensity is raised to 40 kt.

Irwin is moving westward, or 270/7 kt, between Tropical Storms Greg
and Hilary and to the south of a weak low- to mid-level ridge
extending west of the Baja California peninsula.  Since the ridge
is expected to remain weak, Irwin is forecast to move only slowly
westward through 48 hours.  After that time, its motion is likely
to become increasingly influenced by Hilary, with the two beginning
some degree of a binary interaction.  The track guidance now
indicates that Irwin will turn west-southwestward on days 3 and 4,
and then possibly turn back to the northeast around Hilary's
southern side on day 5.  With a shift in the overall guidance
envelope, the new NHC track forecast has been shifted southward
toward HCCA and the TVCN multi-model consensus, and it also now
shows Irwin stalling or drifting northward on day 5.

Irwin is located over warm sea surface temperatures and in a
relatively moist environment, and the northerly shear affecting the
cyclone should continue to decrease over the next day or so.
Therefore, additional gradual strengthening is anticipated, and
Irwin has the possibility of reaching hurricane strength in about
48 hours.  After that time, Hilary (possibly as a major hurricane)
will be getting closer to Irwin, and its upper-level outflow could
cause stronger shear to develop over Irwin.  The intensity guidance
has responded to this possibility by showing more pronounced
weakening after 48 hours, and the new NHC intensity forecast now
shows Irwin holding a steady intensity as a tropical storm on days 3
through 5.  This forecast remains closest to HCCA and the ICON
intensity consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  23/2100Z 14.9N 116.6W   40 KT  45 MPH
 12H  24/0600Z 14.8N 117.3W   45 KT  50 MPH
 24H  24/1800Z 14.7N 118.1W   50 KT  60 MPH
 36H  25/0600Z 14.6N 118.9W   60 KT  70 MPH
 48H  25/1800Z 14.5N 119.8W   65 KT  75 MPH
 72H  26/1800Z 14.0N 122.1W   55 KT  65 MPH
 96H  27/1800Z 13.5N 123.5W   55 KT  65 MPH
120H  28/1800Z 14.0N 123.5W   55 KT  65 MPH

$$
Forecaster Berg

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