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Tropical Storm HERNAN (Text)

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TROPICAL STORM HERNAN DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP082014
200 AM PDT SUN JUL 27 2014

Conventional satellite imagery and scatterometer wind data indicate
that Hernan has continued to strengthen. A small central dense
overcast has persisted over the center during the past 6 hours, and
0418 UTC and 0510 UTC ASCAT-A and ASCAT-B overpasses indicated
surface winds of 48 kt and 51 kt, respectively, were present just
east of center. Assuming that the coarse resolution ASCAT data has
under-sampled the rather small and tight inner core wind field, the
initial intensity has been increased to 55 kt. This intensity is
supported by satellite intensity estimates of 55 kt from TAFB and 56
kt from UW-CIMSS ADT.

The initial motion estimate is northwestward or 310/12 kt. Hernan is
being steered along the southwestern periphery of a strong
subtropical ridge located across the southwestern and southern
United States. This persistent steering pattern is expected to
keep Hernan moving in a general northwestward direction for the
next 2-3 days, followed by a turn toward the west when the cyclone
weakens and is steered primarily the by low-level easterly trade
wind flow.  Although the models have again shifted slightly
northward, the new NHC track forecast is just an update of the
previous advisory track and lies along the southern edge of the
guidance envelope due to the expected quicker weakening than what
the more northerly track GFS and GFDL models are forecasting.

Hernan has strengthened 20 kt in the past 12 hours, which is a
faster-than-normal rate of intensification. The cyclone has about
another 12 hours or so of favorable conditions that could allow
Hernan to achieve hurricane strength. After that, however,
increasing westerly vertical wind shear should induce at least
some weakening until around 48 hours, at which time more rapid
weakening is expected as the the system moves over sub-25C SSTs.
By Days 4 and 5, decreasing sea surface temperatures and even
stronger shear should cause Hernan to degenerate into a remnant low
pressure system.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 16.9N 110.0W   55 KT  65 MPH
 12H  27/1800Z 18.1N 111.7W   65 KT  75 MPH
 24H  28/0600Z 19.6N 114.1W   60 KT  70 MPH
 36H  28/1800Z 21.0N 116.5W   55 KT  65 MPH
 48H  29/0600Z 22.1N 118.6W   45 KT  50 MPH
 72H  30/0600Z 23.3N 122.3W   30 KT  35 MPH
 96H  31/0600Z 23.6N 125.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  01/0600Z 23.6N 128.7W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

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