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Hurricane John Discussion Number   9
NWS National Hurricane Center Miami FL       EP122018
300 PM MDT Tue Aug 07 2018

The eye of John has become more distinct in visible and infrared
satellite imagery during the past few hours, however the surrounding
convective clouds tops have warmed, and a 1642 UTC AMSU
microwave overpass indicated that the eyewall was open over the
northwestern portion of the circulation.  Although objective Dvorak
T-numbers have increased to T5.6, subjective estimates from TAFB and
SAB are unchanged so the initial intensity is kept at 90 kt.  There
is still an opportunity for John to become a major hurricane tonight
or early Wednesday while it remains over warm water and in a low
shear environment.  After that time, the hurricane will be moving
over decreasing SSTs and into a less favorable thermodynamic
environment. These conditions should result in steady to rapid
weakening and John is forecast to become a post-tropical cyclone by
day 4 when it is over SSTs of 20-21C.  There is significant spread
in the intensity guidance as to how rapid John will spin down.  The
NHC forecast is a little higher than the ICON consensus model, but
shows a faster rate of weakening than the statistical guidance.

John is moving northwestward or 320/9 kt.  The hurricane should
continue moving northwestward around the western side of a
mid-level ridge over northern Mexico during the next 2-3 days.
After that time, the system should turn westward as it weakens and
comes under the influence of the low-level flow.  The track
guidance remains in excellent agreement and the new NHC track is
essentially an update of the previous advisory.

Although the core of John is forecast to remain well offshore of
the Baja California Peninsula, an eastward shift of the forecast
track or an unexpected increase in the size of the outer wind field
of the hurricane could bring tropical-storm-force winds close to
the west coast of the peninsula.  In addition, a surge of moisture
around John's outer circulation is expected to bring some locally
heavy rainfall to portions of Baja California Sur during the next
couple of days.  Therefore, interests in the southern portion of the
Baja California Peninsula should monitor the progress of John.


FORECAST POSITIONS AND MAX WINDS

INIT  07/2100Z 18.7N 110.5W   90 KT 105 MPH
 12H  08/0600Z 19.9N 111.6W  100 KT 115 MPH
 24H  08/1800Z 21.5N 113.3W   95 KT 110 MPH
 36H  09/0600Z 23.0N 115.2W   80 KT  90 MPH
 48H  09/1800Z 24.4N 117.3W   65 KT  75 MPH
 72H  10/1800Z 26.6N 121.3W   45 KT  50 MPH
 96H  11/1800Z 27.0N 124.7W   35 KT  40 MPH...POST-TROPICAL
120H  12/1800Z 27.0N 127.3W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown

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