Hurricane Gert Discussion Number 17
NWS National Hurricane Center Miami FL AL082017
1100 PM AST Wed Aug 16 2017
Gert has maintained an eye with very cold cloud tops mainly over
the northern and western portions of the circulation, which is
rather remarkable for a system at 40N latitude. The current
intensity is set a little higher, to 90 kt, in agreement with the
latest Dvorak estimates from TAFB and SAB. The cyclone has
continued to traverse warm waters, but very soon will encounter a
much cooler ocean to the north of the Gulf Stream. Thus, rapid
weakening is forecast, similar to the latest model consensus. In 36
hours the global model guidance depicts the system becoming embedded
in a distinct baroclinic zone, so Gert should have become an
extratropical storm by that time. The extratropical cyclone is
forecast to merge with another large low over the north Atlantic
after 72 hours.
Gert has continued to accelerate and is now moving
east-northeastward near 32 kt. A slightly faster motion is
expected on Thursday in the strong flow to the southeast of a
mid-tropospheric cyclone nearing Newfoundland. Later in the
forecast period, the post-tropical system is likely to turn
northeastward and slow down significantly while it interacts with
the other large low.
The track, intensity, and wind radii forecasts have incorporated
guidance from NOAA's Ocean Prediction Center.
Swells from Gert should continue to affect the northeast U.S. coast
and Atlantic Canada through Thursday. These swells are likely to
produce dangerous surf and rip current conditions. Please consult
products from your local weather forecast office for more
information.
Hurricane Gert Discussion Number 9
NWS National Hurricane Center Miami FL AL082017
1100 PM EDT Mon Aug 14 2017
Satellite images show that Gert has developed an eye, although the
convection is weak in the western eyewall. Still, microwave images
show a healthy inner-core structure, with plenty of deep convection
in the eastern eyewall. Satellite intensity estimates unanimously
support making Gert a hurricane on this advisory, and the initial
wind speed is set to 65 kt, close to the TAFB figure.
Some light or moderate northerly shear is forecast to continue to
affect Gert for the next day or so, which might limit the
intensification rate. On Wednesday, however, the shear is forecast
to drop off as Gert accelerates northeastward over warm waters with
a large increase in upper-level divergence also noted. This is
typically a favorable environment for intensification, and the
official forecast is raised from the previous one, lying between
the model consensus IVCN and the higher corrected consensus aids.
Gert is expected to complete extratropical transition within 72
hours, then gradually weaken over the North Atlantic before it
merges with a larger extratropical cyclone in 4 to 5 days.
Gert continues to move northward at about 7 kt. The storm is
expected to turn north-northeastward and begin to accelerate
Tuesday ahead of a shortwave trough approaching the northeastern
United States. Like the last cycle, the track guidance continues to
be in excellent agreement, but is generally slower than before.
The new forecast is on the fast side of the guidance, assuming that
Gert is a strong hurricane that stays vertically coherent in the
faster mid/upper-level flow.
Swells from Gert are expected to spread northward along the
mid-Atlantic coast of the United States during the next few days.
These swells are likely to produce dangerous surf and rip current
conditions. Please consult products from your local National
Weather Service office.
Tropical Storm Gert Discussion Number 4
NWS National Hurricane Center Miami FL AL082017
500 PM EDT Sun Aug 13 2017
Satellite images indicate that the tropical cyclone is gradually
strengthening. The banding features have become better established
during the last several hours, and the system has become less
vertically tilted. The Dvorak classification from TAFB and ADT
values from CIMSS at the University of Wisconsin support increasing
the initial wind speed to 35 kt, making the system Tropical Storm
Gert.
Gert is moving north-northwestward at 9 kt on the southwestern
periphery of a subtropical ridge. A northward motion is expected
tonight and Monday while the system rounds the western side of the
ridge, and Gert is expected to be about midway between Bermuda and
North Carolina on Monday and Tuesday. After that time, a
progressively faster motion to the northeast and east-northeast is
forecast as the system moves on the north side of the ridge and
becomes embedded in the mid-latitude westerlies until it dissipates
in about 5 days. The track guidance remains in very good agreement,
and only small changes were made to the previous NHC prediction.
This forecast lies near the middle of the tightly clustered guidance
envelope.
The system is over warm 29 deg C SSTs, and it will remain over
these warm waters during the next few days. In addition, the global
models indicate that the upper-level pattern should become conducive
for strengthening during the next 36 hours or so. The combination
of these conditions should allow Gert to strengthen during the next
couple of days. Thereafter, a sharp increase in west-southwesterly
shear, drier air, and decreasing sea surface temperatures should end
the strengthening trend and lead to extratropical transition in
a little more than 3 days. The NHC intensity forecast is slightly
above the previous one, but remains on the lower side of the
guidance.
The center of Gert recently passed very near NOAA buoy 41047, and
that data has been helpful in estimating the minimum pressure of the
system.
Tropical Depression Eight Discussion Number 1
NWS National Hurricane Center Miami FL AL082017
1100 PM EDT Sat Aug 12 2017
Late afternoon GOES-16 visible satellite images showed that the
circulation associated with the low pressure located northeast of
the Bahamas had become better defined. Since that time, the
night-time microphysics RGB product and other infrared satellite
pictures have continued to show an improvement in the circulation,
and recent ASCAT data show that the system has acquired a well-
defined center. Deep convection is organized in a large curved band,
however the center is located near the western edge of the band.
NOAA buoy 41046 located southeast of the center reported 25 to 30 kt
winds earlier this evening, and the scatterometer data support an
initial wind speed of 30 kt.
The depression is moving north-northwestward or 330/11 kt.
The depression should move north-northwestward to northward
around the western side of a deep-layer ridge over the central
Atlantic through Sunday night. It is expected to reach the
mid-latitude westerlies by late Monday and begin to accelerate
northeastward ahead of a short-wave trough that will be approaching
the northeastern United States. The steering currents are well
established over the western Atlantic and the track guidance is in
good agreement on the forecast scenario. The NHC track forecast lies
near the various consensus aids.
The intensity forecast is more problematic. The system is expected
to traverse warm water and remain in a low-shear environment during
the next day or so, but intrusions of dry mid-level air are likely
to limit strengthening. The statistical guidance and the HWRF favor
more significant intensification than the global models, which
generally do not show much deepening. In deference to the global
models, the NHC forecast is below the consensus, especially later in
the period. The system is likely to become extratropical in 3 to 4
days, and the extratropical portion of the forecast is based on
guidance provided by the NOAA Ocean Prediction Center.
1. An elongated area of low pressure located about midway between the
Cabo Verde Islands and the Lesser Antilles is producing only limited
shower and thunderstorm activity. Development, if any, is expected
to be slow to occur while the system moves generally
west-northwestward across the tropical Atlantic at about 15 mph.
* Formation chance through 48 hours...low...10 percent.
* Formation chance through 5 days...low...30 percent.
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