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Hurricane Irma Discussion Number 36
NWS National Hurricane Center Miami FL AL112017
1100 PM EDT Thu Sep 07 2017

Microwave images and data from an Air Force Reserve Hurricane
Hunter aircraft indicate that Irma is developing a concentric
eyewall structure, and the plane reported an elliptical eye. The
highest flight-level and SFMR winds measured by the aircraft were
146 kt and 139 kt, respectively, so the initial intensity is lowered
slightly to 145 kt. Irma’s central pressure has fallen a bit
despite the slight reduction in winds, counterbalanced by an
expansion of the hurricane-force wind field as observed by the
plane.

If Irma has developed concentric eyewalls, then some additional
gradual weakening or fluctuations in intensity are possible over
the next day or two, despite a seemingly favorable atmospheric and
oceanic environment. The intensity models depict a very gradual
decrease in Irma’s maximum winds up until a possible landfall in
Florida, but the hurricane is nonetheless expected to still be at
or near category 4 strength at that time. After landfall, a fairly
quick decay in maximum winds is expected due to land interaction
and increased shear, although Irma’s large wind field is likely to
still produce hurricane-force winds over a large area.

Irma continues to move west-northwestward at 290/14 kt, steered by
the western extent of the subtropical ridge. Irma should maintain
this trajectory but slow down during the next 36 hours, and then
turn north-northwestward toward a break in the ridge by 72 hours.
Although there was a westward shift in the 18Z GFS, this model’s
new track puts it very close to the previous NHC forecast. As
such, no major changes to the NHC track were required on this
forecast through 72 hours, and the projected path lies between the
GFS and ECMWF, closest to the TVCN consensus and HCCA. After 72
hours, the models are showing a more pronounced turn toward the
northwest when Irma interacts with a shortwave diving southeastward
from the Central Plains, and the NHC forecast has been adjusted
accordingly at the end of the forecast period.