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Hurricane Irwin Discussion Number  14
NWS National Hurricane Center Miami FL       EP102017
200 PM PDT Tue Jul 25 2017

Irwin's satellite presentation has improved since the last
advisory, with a well-defined eye showing up in infrared imagery,
and an elongated band extending around the western and northern
side of the circulation.  Using the eye pattern from the Dvorak
technique yields an estimate of T4.5/77 kt, so the initial intensity
is raised to 75 kt.  Irwin has already strengthened beyond what we
were originally forecasting, so it's a little difficult to know if
it will intensify further.  However, 10-15 kt of southerly shear is
expected to remain over the cyclone for the next 24 hours, and the
hurricane's slow motion could cause some upwelling of colder water.
Therefore, gradual weakening is forecast during the next 48 hours.
After that time, Irwin will begin interacting with Hurricane
Hilary, and since it has the smaller circulation of the two, it is
likely to suffer and be the one that loses intensity.  Water
temperatures also cool significantly as Irwin accelerates toward
the north, so more definitive weakening is expected by the end of
the forecast period.  Some of the global models show Irwin becoming
absorbed into Hilary's circulation before the end of the forecast
period, so it's possible that dissipation could occur by day 5.

Irwin continues to move generally westward, but that motion is
likely to become more west-southwestward from 12-48 hours as Hilary
approaches from the east.  The Fujiwhara interaction with Hilary
will then cause Irwin to stall around day 3, and then accelerate
around the southeast and then northeast side of Hilary on days 4
and 5.  Although the track models all agree on this general
scenario, there remain a large amount of spread regarding when and
where Irwin will turn toward the north.  To be conservative, the
updated NHC track forecast is near the middle of the guidance
envelope, close to the previous forecast but also to the right of
the TVCN multi-model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 15.7N 120.1W   75 KT  85 MPH
 12H  26/0600Z 15.3N 121.0W   70 KT  80 MPH
 24H  26/1800Z 14.8N 122.1W   65 KT  75 MPH
 36H  27/0600Z 14.4N 123.1W   60 KT  70 MPH
 48H  27/1800Z 14.3N 123.6W   55 KT  65 MPH
 72H  28/1800Z 14.7N 123.4W   55 KT  65 MPH
 96H  29/1800Z 17.5N 123.0W   55 KT  65 MPH
120H  30/1800Z 21.5N 125.5W   45 KT  50 MPH

$$
Forecaster Berg

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