Hurricane Dorian Discussion Thread

A particularly good NHC disco tonight IMHO:

118
WTNT45 KNHC 300256
TCDAT5

Hurricane Dorian Discussion Number 23
NWS National Hurricane Center Miami FL AL052019
1100 PM AST Thu Aug 29 2019

The NOAA and Air Force Reserve Hurricane Hunter aircraft have been
tag-teaming the reconaissance of Dorian tonight, providing useful
information about what has changed during the past several hours.
They found the pressure has dropped to about 977 mb, with recent
SFMR data of about 90 kt (also supported by flight-level winds).
These data are also consistent with satellite imagery that show
Dorian with a larger, deeper central dense overcast than this
afternoon. Thus the wind speed is raised to 90 kt on this advisory.

The initial motion is about the same as before, 325/10 kt. Dorian
is expected to gradually turn to the west-northwest on Saturday,
and westward on Sunday due to a building ridge over the southwestern
Atlantic. While the model guidance generally shows this scenario in
a broad sense, there are substantial timing differences among the
guidance. The source of the uncertainty can be attributed to
challenges in forecasting the strength and orientation of the
mid-level ridge over the southeastern United States in a few days,
along with exactly where and how large Dorian is by then. As you
can imagine, with so many complex variables in play, it is no wonder
the models have been having a difficult time nailing down the path
of the hurricane. There's been a notable trend on this model cycle
toward a slower, more westward track beyond 36 hours, which can be
seen most strongly in the GFS-based guidance. The track forecast is
shifted southward beyond 36 hours, and is about 30 n mi south of the
previous one at 96 h. We will see if this southward trend in the
models continues after the dropsonde data collected by the G-IV gets
incorporated into the 00Z models. Users are reminded to not focus
on the exact forecast track, as typical forecast errors at days 4
and 5 are around 155 and 205 miles, respectively.

There is still substantial middle- and upper-level dry air on the
south side of the hurricane, as shown by tonight's G-IV mission,
which has been allowing only slow strengthening during the day.
As Dorian turns west-northwestward, however, shear should drop
somewhat due to it moving on the northeast side of an upper-level
low near the Florida Straits, and the winds aloft will no longer be
pointed toward the core, which will help decrease dry air
entrainment. All these changes should promote intensification
while Dorian moves over the 29C waters east of Florida, so the
intensity forecast is raised from the previous one, consistent with
the corrected-consensus guidance. Unfortunately, I don't see any
large-scale factors that would prevent Dorian from becoming an
extremely dangerous category 4 hurricane during the next few days.

Key Messages:

1. The risk of life-threatening storm surge and hurricane-force
winds this weekend continues to increase in the northwestern
Bahamas, and hurricane watches could be issued on Friday. Residents
should have their hurricane plan in place and listen to advice given
by local emergency officials.

2. There is an increasing likelihood of life-threatening storm surge
along portions of the Florida east coast late this weekend or early
next week, although it is too soon to determine where the highest
storm surge will occur. Residents should have their hurricane plan
in place, know if they are in a hurricane evacuation zone, and
listen to advice given by local emergency officials.

3. The risk of devastating hurricane-force winds along the Florida
east coast and peninsula late this weekend and early next week
continues to increase, although it is too soon to determine where
the strongest winds will occur.

4. Regardless of the exact track of Dorian, heavy rains are expected
to occur over portions of the Bahamas, Florida, and elsewhere in the
southeastern United States this weekend and into the middle of next
week.


FORECAST POSITIONS AND MAX WINDS

INIT 30/0300Z 23.3N 68.4W 90 KT 105 MPH
12H 30/1200Z 24.3N 69.4W 100 KT 115 MPH
24H 31/0000Z 25.4N 71.1W 105 KT 120 MPH
36H 31/1200Z 26.0N 73.0W 110 KT 125 MPH
48H 01/0000Z 26.3N 74.7W 115 KT 130 MPH
72H 02/0000Z 26.8N 77.9W 120 KT 140 MPH
96H 03/0000Z 27.0N 79.8W 120 KT 140 MPH
120H 04/0000Z 27.5N 81.0W 65 KT 75 MPH...INLAND

$$
Forecaster Blake