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Hurricane Gert Discussion Number  12
NWS National Hurricane Center Miami FL       AL082017
500 PM EDT Tue Aug 15 2017

Gert has exhibited hints of an eye in visible satellite imagery
during the day, but that feature has not appeared in infrared
images.  Still, the satellite presentation has improved since the
last advisory, and the initial intensity is set at 70 kt based on a
blend of final-T Dvorak numbers of T4.0 from TAFB and SAB and T4.7
from the UW-CIMSS ADT.  Vertical shear is expected to be relatively
low--less than 20 kt--for the next 24 hours or so, which should
allow Gert to strengthen a little more over the warm waters south
of the northern wall of the Gulf Stream.  The NHC forecast has been
increased slightly through 36 hours to account for the latest
intensity guidance, although it is still not as high as what is
shown by models such as HWRF and the HFIP Corrected Consensus.
Southwesterly shear increases substantially after 48 hours, which
should cause Gert to weaken, and the cyclone is also expected to
become extratropical by 72 hours.  There is a lot of uncertainty on
the ultimate fate of the extratropical low, but most of the global
models keep Gert as a separate entity at least through day 4.

Gert's trajectory continues to veer as it rounds the western edge
of the subtropical ridge, and the initial motion is 020/10 kt.
Gert will soon become embedded in the mid-latitude westerlies,
sandwiched between a mid-oceanic high and a large cut-off low over
eastern Canada.  This pattern will cause Gert to turn northeastward
and accelerate over the northwestern Atlantic during the next 48-72
hours.  The cyclone will then likely slow down and turn more
northward by day 4 as it interacts with the aforementioned cut-off
low.  There is very little cross-track spread among the track
models, but the latest suite of guidance has slowed down slightly
from previous runs.  As a result, the new NHC forecast is just a
little slower than the previous one, near the various multi-model
consensus aids.

Swells from Gert should spread northward along the mid-Atlantic
coast of the United States during the next few days, with the first
impacts expected to occur tonight.  These swells are likely to
produce dangerous surf and rip current conditions.  Please consult
products from your local National Weather Service office.

FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 33.7N  71.2W   70 KT  80 MPH
 12H  16/0600Z 35.3N  69.4W   75 KT  85 MPH
 24H  16/1800Z 37.4N  64.9W   80 KT  90 MPH
 36H  17/0600Z 39.7N  58.4W   80 KT  90 MPH
 48H  17/1800Z 42.9N  50.2W   70 KT  80 MPH
 72H  18/1800Z 51.5N  37.1W   50 KT  60 MPH...POST-TROP/EXTRATROP
 96H  19/1800Z 55.5N  33.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  20/1800Z...DISSIPATED

$$
Forecaster Berg

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