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Tropical Storm Irma Discussion Number   2
NWS National Hurricane Center Miami FL       AL112017
500 PM AST Wed Aug 30 2017

Irma is displaying increasingly organized symmetric and interlocking
rainbands with a well-defined outflow as seen in the visible and
infrared satellite imagery this afternoon.  Dvorak classifications
from both TAFB and SAB have gone up at 18Z, suggesting 45 and 35 kt,
respectively.  Given their low bias earlier compared with the ASCAT
scatterometer passes, the initial intensity is bumped up to 50 kt at
advisory time.

Irma is embedded within very low tropospheric vertical shear
currently and for the foreseeable future, as the tropical storm lies
underneath the upper-level anticyclone.  However, in about three
days the SSTs and the mid-level humidities that Irma should start to
encounter will become cooler and drier.  The intensity guidance
shows steady intensification for the next two to three days, then
diverges in response to the low shear/not-as-favorable
thermodynamical environment.  The official intensity forecast is
closest to the ICON consensus technique through three days, then
shows no change to day five.  This new forecast is higher than the
previous advisory, but lower than ICON at day 4 and 5 (and thus may
be conservative).

Irma is moving toward the west at about 13 kt along the south side
of the deep-layer Azores high.  A slight turn toward the west-
northwest at a slower rate of forward speed is expected during
the next three days.  As the high strengthens, Irma is expected to
turn toward the west-southwest around day 4.  There is a moderate
amount of spread among the normally reliable global and mesoscale
models in this scenario.  The official track forecast is based upon
the HFIP Corrected Consensus Approach and is nearly the same as
that from the previous advisory.

Despite the large overall envelope of the Irma's circulation, the
earlier ASCAT passes showed a very small area of tropical-storm-
force winds - only 30 nm radius.  The official size forecast is
based upon the RVCN - variable consensus technique.


FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 16.4N  31.2W   50 KT  60 MPH
 12H  31/0600Z 16.8N  32.7W   60 KT  70 MPH
 24H  31/1800Z 17.3N  34.5W   70 KT  80 MPH
 36H  01/0600Z 17.8N  36.2W   80 KT  90 MPH
 48H  01/1800Z 18.2N  38.2W   85 KT 100 MPH
 72H  02/1800Z 18.2N  43.0W   90 KT 105 MPH
 96H  03/1800Z 17.3N  47.8W   90 KT 105 MPH
120H  04/1800Z 16.5N  52.5W   90 KT 105 MPH

$$
Forecaster Landsea

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