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Tropical Storm Irwin Forecast Discussion


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000
WTPZ45 KNHC 242033
TCDEP5

Tropical Storm Irwin Discussion Number  10
NWS National Hurricane Center Miami FL       EP102017
200 PM PDT Mon Jul 24 2017

Irwin has been displaying a dimple in visible satellite imagery,
which appears to coincide with a mid-level eye that has been
apparent in recent microwave imagery.  However, the microwave data
also suggest that Irwin's circulation is tilted from south to north
with height, with the low-level center located south of the feature
noted in visible imagery.  Subjective Dvorak estimates have risen
to T3.5/55 kt from TAFB and T4.0/65 kt from SAB, while the
objective ADT is still down around T3.1/47 kt.  Given the cyclone's
tilted structure, I'm comfortable going down the middle and
initializing the intensity at 55 kt.

Vertical shear is currently low over Irwin and should remain so for
the next 24 hours or so.  This will likely allow Irwin to continue
strengthening, reaching hurricane status overnight.  Increased
shear, possibly enhanced by Hilary's outflow, could begin to
impinge on Irwin after 24 hours, which would likely limit the
intensification trend.  The new intensity guidance has come down a
bit after 24 hours, and only gradual weakening is anticipated
through the end of the forecast period as Irwin and Hilary
interact.  Irwin will also be approaching the 26 deg C SST isotherm
in about 5 days, which will also foster some weakening.  The
updated NHC forecast is close to the HCCA guidance, and it's just
slightly below the previous forecast from 36 hours and beyond.

Irwin's is drifting westward with an initial motion of 280/3 kt.
As Irwin and Hilary's circulations get closer together over the
next few days, Irwin will respond by first moving slowly
west-southwestward on days 2 and 3, and then turning northwestward
and accelerating behind or around the southeast side of Hilary's
circulation.  The regional HWRF and HMON models continue to show a
persistent westward track that ignores Hilary's existence, while
the global models show varying degrees of interaction between the
two cyclones.  Because of the unrealistic scenarios shown by the
regional models, the NHC track forecast continues to lie east of
the multi-model consensus on days 4 and 5, and it's very close to
HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 15.0N 117.8W   55 KT  65 MPH
 12H  25/0600Z 15.2N 118.3W   65 KT  75 MPH
 24H  25/1800Z 15.2N 119.2W   70 KT  80 MPH
 36H  26/0600Z 15.1N 120.3W   65 KT  75 MPH
 48H  26/1800Z 14.8N 121.3W   65 KT  75 MPH
 72H  27/1800Z 14.2N 122.7W   60 KT  70 MPH
 96H  28/1800Z 15.0N 123.0W   60 KT  70 MPH
120H  29/1800Z 17.5N 124.5W   55 KT  65 MPH

$$
Forecaster Berg