[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 080259
TCDAT4

HURRICANE MATTHEW DISCUSSION NUMBER  40
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL142016
1100 PM EDT FRI OCT 07 2016

Coastal Doppler radar data this evening indicate that Matthew is
gradually becoming less organized.  The eyewall has broken open
with the remaining deep convection in a band just north of the
center, and there is now little precipitation in the southeastern
quadrant.  A combination of radar winds and earlier aircraft data
suggests that the intensity has decreased slightly since the last
advisory, so the initial intensity is lowered to 90 kt.

The initial motion is now 010/10 kt.  During the next 36 hours,
Matthew should turn more northeastward as it moves along the
southern edge of a mid-latitude trough.  The forecast track, which
lies in the center of the track guidance envelope, has the center
moving near the coasts of Georgia and South Carolina during the next
12-18 hours, then near the North Carolina coast from 18-36 hours.
While this occurs, the cyclone is forecast to encounter strong
vertical wind shear and to entrain dry air associated with an
approaching frontal system.  This should result in a steady
weakening, and Matthew is now forecast to weaken to a tropical storm
by 36 hours in agreement with the SHIPS model. It is possible that
Matthew could merge with the frontal system at about 36 hours,
although none of the available guidance currently forecasts the
system to become an extratropical low.

The track and intensity forecasts become very low confidence after
36 hours due to a large diversity of model solutions.  The GFDL and
HWRF forecast Matthew to turn northeastward and become an
extratropical low near the Canadian Atlantic provinces.  The UKMET
moves Matthew eastward and eventually has it absorbed by Tropical
Storm Nicole.  The ECMWF, NAVGEM, and Canadian models show a
southwestward turn, with the cyclone or it remnants near the Bahamas
by 120 hours.  The GFS is between the UKMET and ECMWF, showing
Matthew moving far enough to the east to interact with Nicole, then
turning southward.  The new track forecast follows the previous
advisory in showing a southward/southwestward turn similar to the
ECMWF, but is east of the previous track due to an overall eastward
trend in the guidance.  Regarding the intensity, the GFS suggests
that Matthew could decay to a remnant low by 120 hours, while the
ECMWF suggests the system could still be a tropical cyclone.  Either
way, continued weakening is likely due to shear and dry air
entrainment, and the official forecast calls for Matthew to weaken
to a depression by 96 hours.

Due to the degradation of the radar signature of Matthew, the
hourly Tropical Cyclone Updates will no longer be issued.


KEY MESSAGES:

1.  We have been very fortunate that Matthew's strongest winds have
remained a short distance offshore of the Florida and Georgia coasts
thus far, but this should not be a reason to let down our guard.
Only a small deviation to the left of the forecast track could bring
these winds onshore.  The western eyewall of Matthew, which contains
hurricane-force winds, is expected to move over or very near the
coasts of Georgia and South Carolina tonight and Friday.

2.  Hurricane winds increase very rapidly with height, and occupants
of high-rise buildings along the coast are at particular risk of
strong winds.  Winds at the top of a 30-story building will average
one Saffir-Simpson category higher than the winds near the surface.

3.  The water hazards remain, even if the core of Matthew remains
offshore.  These include the danger of life-threatening inundation
from storm surge, as well as inland flooding from heavy rains from
Florida to North Carolina.

4.  The National Hurricane Center is issuing Potential Storm Surge
Flooding Maps, and Prototype Storm Surge Watch/Warning Graphics for
Matthew.  It is important to remember that the Potential Storm Surge
Flooding Map does not represent a forecast of expected inundation,
but rather depicts a reasonable worst-case scenario -- the amount of
inundation that has a 10 percent chance of being exceeded.


FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 31.2N  80.5W   90 KT 105 MPH
 12H  08/1200Z 32.5N  79.8W   80 KT  90 MPH
 24H  09/0000Z 33.6N  77.8W   70 KT  80 MPH
 36H  09/1200Z 33.9N  75.5W   60 KT  70 MPH
 48H  10/0000Z 33.0N  74.0W   55 KT  65 MPH
 72H  11/0000Z 30.0N  73.5W   40 KT  45 MPH
 96H  12/0000Z 27.5N  74.5W   30 KT  35 MPH
120H  13/0000Z 26.0N  75.5W   30 KT  35 MPH

$$
Forecaster Beven
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