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Subtropical Storm Ernesto Discussion Number   4
NWS National Hurricane Center Miami FL       AL052018
1100 PM AST Wed Aug 15 2018

Although inner-core convection has increased since the previous
advisory, outer banding features have become more fragmented and
the overall cloud pattern has become elongated north-to-south.
There are also no signs of any upper-level anticyclonic outflow,
an indication that Ernesto is still a subtropical cyclone. The
initial intensity is being maintained at 35 kt based on a 2328Z
ASCAT pass that showed peak winds of 32 kt in the southeastern
quadrant, along with a radius of maximum winds of 70-80 nmi. This
intensity is consistent with a ST2.5/35 kt classification from TAFB.

The initial motion is now north-northeastward or 025/09 kt. Ernesto
has rounded the subtropical ridge axis to its south, and the
cyclone should gradually get caught up in the mid-latitude
westerlies and turn northeastward during the next 12 hours or so.
A northeastward motion along with a steadily increasing forward
speed is expected through Friday. The track guidance is tightly
packed around the previous advisory track, so the new NHC forecast
track is essentially just an extension of the previous advisory, and
lies near the track consensus models HCCA and TVCN.  On the forecast
track, Ernesto is expected to approach Ireland as an extratropical
gale area on Saturday.

Ernesto is expected to remain over sea-surface temperatures (SST)
of 25 deg C or warmer for the next 18-24 hours, along with low
vertical shear conditions of less than 10 kt. These conditions
should allow for some slight strengthening during that short time
window. By 36 hours, Ernesto is forecast to be moving over SSTs of
20 deg C and colder and into a more stable airmass, resulting in a
degeneration of the inner-core convection. As a result, Ernesto is
expected to become a post-tropical cyclone by 36 hours, and become
an extratropical gale area after merging with a frontal zone near
Ireland and the United Kingdom in about 3 days.

FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 39.7N  45.1W   35 KT  40 MPH
 12H  16/1200Z 41.3N  43.4W   40 KT  45 MPH
 24H  17/0000Z 43.7N  40.0W   40 KT  45 MPH
 36H  17/1200Z 46.3N  34.9W   40 KT  45 MPH...POST-TROPICAL
 48H  18/0000Z 49.0N  27.9W   40 KT  45 MPH...POST-TROP/EXTRATROP
 72H  19/0000Z 54.0N  12.1W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  20/0000Z...DISSIPATED

$$
Forecaster Stewart

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