Tropical Storm Humberto Forecast Discussion

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000
WTNT44 KNHC 140842
TCDAT4

Tropical Storm Humberto Discussion Number   7
NWS National Hurricane Center Miami FL       AL092019
500 AM EDT Sat Sep 14 2019

Convection associated with Humberto has increased a little since
the last advisory, as a new burst has occurred to the north of the
center.  Other convection continues in a large curved band in the
eastern semicircle.  Reports from an Air Force Reserve Hurricane
Hunter aircraft near 0500 UTC indicated that the maximum sustained
winds were near 35 kt, and that will be the initial intensity for
this advisory.  It should be noted that an upper-level trough is
located just south of Humberto, and the storm may have some
subtropical characteristics due to interaction with this trough.

While the motion is still a bit erratic, Humberto now seems to be
moving a little faster toward the northwest or 315/6 kt.  During the
next few days, a low- to mid-level ridge over the eastern United
States and the western Atlantic is expected to slide southeastward,
with a weakness in the ridge developing due to a mid- to upper-level
trough over the northeastern U.S.  This change in the steering
pattern should cause the storm to slow down and turn northward off
the east coast of Florida in 36 to 48 hours.  Subsequently, the
northeastern U.S. trough should dig southward, and this change
should turn Humberto northeastward and then eastward away from the
U.S.  The track guidance has shifted a little more to the east
during the first 72 h, and the new NHC track has been nudged
eastward as well during this time.  The new forecast track lies near
the various consensus models.

Southwesterly vertical shear and dry air entrainment caused by the
upper-level trough to the south is forecast to diminish during the
next 2-3 days while Humberto moves near or over the warm waters of
the Gulf Stream.  This should allow intensification, and Humberto
is expected to become a hurricane in about 48 h.  The intensity
forecast becomes more uncertain between 72-120 h.  Strong
upper-level winds associated with the mid-latitude trough over the
eastern United States should cause strong shear over Humberto.
However, there is a chance that baroclinic forcing from the trough
could compensate for the shear, and the GFS, ECMWF, UKMET, Canadian,
and HWRF models all show the central pressure falling during this
time. The intensity forecast will follow this guidance and show some
additional strengthening through 96 h.  Overall, the new intensity
forecast is similar to, but slightly higher than, the previous
forecast.


Key Messages:

1. Humberto will likely bring tropical-storm-force winds and heavy
rainfall to portions of the northwestern Bahamas today.  Significant
storm surge is not expected in the northwestern Bahamas from this
system. Residents there should follow any advice given by local
officials.

2. Since the forecast track has shifted farther to the east, the
chance of heavy rainfall affecting the southeastern United States
has diminished.


FORECAST POSITIONS AND MAX WINDS

INIT  14/0900Z 26.3N  76.0W   35 KT  40 MPH
 12H  14/1800Z 26.9N  76.9W   40 KT  45 MPH
 24H  15/0600Z 28.0N  77.7W   45 KT  50 MPH
 36H  15/1800Z 29.0N  77.9W   55 KT  65 MPH
 48H  16/0600Z 29.8N  77.4W   65 KT  75 MPH
 72H  17/0600Z 30.8N  74.7W   75 KT  85 MPH
 96H  18/0600Z 31.5N  71.0W   80 KT  90 MPH
120H  19/0600Z 33.0N  66.0W   75 KT  85 MPH

$$
Forecaster Beven



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