Aug 17 Thursday 2017

Tropical Report

Further Update: Now Tropical Storm Harvey

UPDATED: Potential Tropical Cyclone Nine Discussion Number 1
NWS National Hurricane Center Miami FL AL092017
1100 AM AST Thu Aug 17 2017

Satellite imagery, including experimental 1-minute data from
GOES-16, indicates that the circulation of the low pressure area
east of the Lesser Antilles is becoming better defined, and that a
cluster of strong convection has formed just west of the center.
Based on this and the potential for the system to become a tropical
storm before reaching the Lesser Antilles, advisories are being
initiated as a potential tropical cyclone. An Air Force Reserve
Hurricane Hunter aircraft is scheduled to investigate the system
this afternoon, and it is likely the aircraft will find a tropical
cyclone has formed.

The initial motion is 270/15. A deep-layer ridge to the north of
the system should steer it generally just north of due west through
the forecast period, with this motion expected to bring the system
through the Lesser Antilles and into the eastern Caribbean Sea in
24-36 h, into the central Caribbean by 72 h, and to the western
Caribbean by 96-120 h. The forecast track lies near the center of
the tightly clustered guidance envelope and lies near the various
consensus models.

The system is in an area of moderate easterly vertical shear, and
the large-scale models suggest that light/moderate shear should
continue through the forecast period. This seems favorable for
strengthening, and the SHIPS and LGEM models show slow, but steady,
intensification. However, the GFS and ECMWF models forecast the
system to degenerate to an easterly wave over the central Caribbean
Sea, possibly due to dry air entrainment. The intensity forecast
follows the trend of the SHIPS/LGEM models, but it respects the
GFS/ECMWF forecasts by being on the low side of the intensity
guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT 17/1500Z 13.1N 54.1W 30 KT 35 MPH…POTENTIAL TROP CYCLONE
12H 18/0000Z 13.1N 56.4W 30 KT 35 MPH…TROPICAL DEPRESSION
24H 18/1200Z 13.2N 59.8W 35 KT 40 MPH
36H 19/0000Z 13.5N 63.2W 40 KT 45 MPH
48H 19/1200Z 13.9N 66.7W 45 KT 50 MPH
72H 20/1200Z 14.5N 74.5W 55 KT 65 MPH
96H 21/1200Z 15.5N 82.0W 65 KT 75 MPH
120H 22/1200Z 17.0N 87.5W 65 KT 75 MPH

$$
Forecaster Beven

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Thu Aug 17 2017

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Hurricane
Gert, located several hundred miles south of Cape Race,
Newfoundland.

1. Morning satellite imagery shows that shower activity associated
with a low pressure system centered about 550 miles east of the
Lesser Antilles has become better organized overnight, and buoy
data indicates that the circulation is becoming better defined.
Environmental conditions appear generally conducive for development,
and a tropical depression could form later today or tonight as the
system moves westward at about 15 mph. Regardless of development,
locally heavy rainfall and gusty winds are expected to spread across
portions of the Lesser Antilles and the eastern Caribbean Sea
tonight and Friday, and interests in these areas should monitor the
progress of this system. An Air Force Reserve Hurricane Hunter
aircraft is scheduled to investigate the disturbance this afternoon.
* Formation chance through 48 hours…high…80 percent.
* Formation chance through 5 days…high…90 percent.

2. Shower activity associated with a second area of low pressure
located about midway between the coast of Africa and the Lesser
Antilles has also become a little better organized during the past
24 hours. Continued gradual development of this system is possible
during the next couple of days while it moves west-northwestward at
15 to 20 mph, but upper-level winds are expected to become less
conducive for tropical cyclone formation when the disturbance moves
north of the Leeward Islands this weekend.
* Formation chance through 48 hours…medium…50 percent.
* Formation chance through 5 days…medium…60 percent.

3. A tropical wave located over the far eastern Atlantic Ocean near
the Cabo Verde Islands is producing disorganized showers and
thunderstorms. Gradual development of this system is possible
during the next several days while it moves westward to
west-northwestward at about 15 mph.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…medium…40 percent.

Forecaster Beven

GERT BEGINS TO WEAKEN AS IT RACES TOWARD COLDER WATERS

Hurricane Gert Discussion Number 18
NWS National Hurricane Center Miami FL AL082017
500 AM AST Thu Aug 17 2017

The expected weakening trend of Gert appears to have started. The
eye of the hurricane is no longer evident in satellite images, and
the convective pattern has become less symmetric during the past
several hours. The initial wind speed is lowered a little to 85 kt,
in agreement with a blend of the Dvorak final T- and CI-numbers
from TAFB and SAB. The hurricane is crossing the north wall of the
Gulf Stream current, and it will soon be moving over substantially
colder water. These colder waters combined with a significant
increase in wind shear and drier air should cause rapid weakening
during the next day or so. Gert is now forecast to lose its
tropical characteristics in about 24 hours, when it will be over
very cool sea surface temperatures of about 16 deg C. The NHC
intensity forecast is an update of the previous one, and it
generally follows the GFS model.

Gert is embedded in fast mid-latitude flow and it is racing
east-northeastward over the central Atlantic at 34 kt. A decrease
in forward speed and a slight turn to the northeast is expected
during the next couple of days while Gert interacts with a large
extratropical low to the east of Atlantic Canada. The global
models show this extratropical system absorbing, or merging with,
Gert in about 3 days. The NHC track forecast is adjusted a little
to the right of the previous one to come into better agreement with
the latest consensus models.

The track, intensity, and wind radii forecasts have incorporated
guidance from NOAA’s Ocean Prediction Center.

Swells from Gert should continue to affect the coast of the
northeastern U.S. and Atlantic Canada through tonight. These swells
are likely to produce dangerous surf and rip current conditions.
Please consult products from your local weather forecast office for
more information.

FORECAST POSITIONS AND MAX WINDS

INIT 17/0900Z 41.7N 54.0W 85 KT 100 MPH
12H 17/1800Z 44.2N 47.4W 70 KT 80 MPH
24H 18/0600Z 48.0N 40.2W 55 KT 65 MPH…POST-TROP/EXTRATROP
36H 18/1800Z 50.7N 36.0W 50 KT 60 MPH…POST-TROP/EXTRATROP
48H 19/0600Z 52.0N 33.0W 40 KT 45 MPH…POST-TROP/EXTRATROP
72H 20/0600Z…DISSIPATED

$$
Forecaster Cangialosi

 Tropical Weather Outlook
NWS National Hurricane Center Miami FL
500 AM PDT Thu Aug 17 2017

For the eastern North Pacific…east of 140 degrees west longitude:

1. An area of low pressure is located about 600 miles south-southwest
of Manzanillo, Mexico. This system has a broad circulation, but
the associated shower and thunderstorm activity remains
disorganized. Environmental conditions are expected to be generally
conducive for gradual development, and a tropical depression may
form within the next few days while it moves west-northwestward
at 10 to 15 mph.
* Formation chance through 48 hours…medium…40 percent.
* Formation chance through 5 days…medium…60 percent.

Forecaster Zelinsky