ATL: GORDON - Advisories

[cycloneye]

Tropical Depression Gordon Discussion Number 21
NWS National Hurricane Center Miami FL AL072024
1100 AM AST Mon Sep 16 2024

Deep convection has persisted, since last night, mainly over the
eastern portion of the circulation of Gordon. This activity is
limited and not particularly well organized however, with the
coldest cloud tops near -70 deg C. Based on a subjective Dvorak
classification from TAFB and objective intensity estimates from
UW-CIMSS, the advisory intensity is set at 30 kt.

The cyclone has been moving generally westward over the past day or
so while embedded in the flow on the south and southeast side of a
mid-level ridge. During the next couple of days, the ridge is
forecast to shift westward and weaken while a trough digs to the
north and northeast of Gordon. This flow evolution, along with the
interaction with a developing frontal cyclone about 10 degrees to
the north of the tropical cyclone, should result in a turn toward
the north in 36 hours or so. Some of the global models are showing
a partial merger of Gordon with the frontal wave, but it appears
likely that the system will remain distinct as a tropical cyclone
throughout the forecast period. The official track forecast has
been shifted somewhat to the east of the previous NHC prediction,
and is near or west of the dynamical model consensus tracks.

Westerly vertical wind shear over Gordon has abated, but the
environment is still a bit dry. The dynamical guidance indicates a
further decrease in shear with some increase in low- to mid-level
humidities in a few days while the system remains over warm waters.
Therefore, restrengthening is forecast to begin around 60 hours,
in general agreement with the latest intensity model consensus.

FORECAST POSITIONS AND MAX WINDS

INIT 16/1500Z 19.1N 48.1W 30 KT 35 MPH
12H 17/0000Z 19.2N 48.7W 30 KT 35 MPH
24H 17/1200Z 19.5N 49.2W 30 KT 35 MPH
36H 18/0000Z 19.9N 49.4W 30 KT 35 MPH
48H 18/1200Z 20.5N 49.1W 30 KT 35 MPH
60H 19/0000Z 21.6N 48.5W 35 KT 40 MPH
72H 19/1200Z 23.1N 47.6W 35 KT 40 MPH
96H 20/1200Z 26.0N 45.8W 40 KT 45 MPH
120H 21/1200Z 28.0N 45.0W 45 KT 50 MPH

$$
Forecaster Pasch

[cycloneye]

Tropical Depression Gordon Discussion Number 22
NWS National Hurricane Center Miami FL AL072024
500 PM AST Mon Sep 16 2024

Gordon is producing only a few sporadic bursts of deep convection,
mainly over the southern and eastern portion of its circulation.
The vertical wind shear does not appear to be very strong at this
time, as evidenced by the relatively slow motions of the cirrus
clouds over the system. Since the cyclone is situated over a
warm ocean, the suppression of convection is probably due to a
stable air mass and/or dry air. The advisory intensity is held at
30 kt in accordance with an objective estimate of 32 kt from
UW-CIMSS and a subjective Dvorak Current Intensity number of 30 kt
from TAFB.

Gordon continues to move slowly westward with a current motion
estimate of 270/5 kt. A high pressure area that was north of the
system over the past couple of days is shifting westward, and being
replaced by a mid-level trough within the next day or two. Also,
there is a frontal cyclone located about 10 degrees to the north of
Gordon. This feature, along with the trough, should cause the
tropical cyclone to turn northward and north-northeastward during
the next couple of days. Although Gordon is expected to at least
briefly interact with the frontal wave, the global model guidance
indicates that it will retain its identity and move farther
north-northeastward over the Atlantic during the forecast period.
The official forecast is shifted a little more to the east of the
previous NHC prediction and is close to the corrected model
consensus, HCCA.

Output from the statistical-dynamical SHIPS models shows some
increase in the low- to mid-level relative humidity and also
suggests some increase in instability during the forecast period.
The SHIPS output also shows generally low to moderate vertical
shear for the next few days. The official forecast shows some
restrengthening after 48 hours, but not as much of an intensity
increase as called for by the SHIPS/LGEM predictions.


FORECAST POSITIONS AND MAX WINDS

INIT 16/2100Z 19.0N 48.5W 30 KT 35 MPH
12H 17/0600Z 19.2N 49.0W 30 KT 35 MPH
24H 17/1800Z 19.7N 49.1W 30 KT 35 MPH
36H 18/0600Z 20.3N 48.8W 30 KT 35 MPH
48H 18/1800Z 21.3N 48.3W 30 KT 35 MPH
60H 19/0600Z 22.8N 47.6W 35 KT 40 MPH
72H 19/1800Z 24.6N 46.5W 40 KT 45 MPH
96H 20/1800Z 27.0N 44.5W 45 KT 50 MPH
120H 21/1800Z 29.0N 43.5W 50 KT 60 MPH

$$
Forecaster Pasch

[cycloneye]

Tropical Depression Gordon Discussion Number 23
NWS National Hurricane Center Miami FL AL072024
1100 PM AST Mon Sep 16 2024

Gordon has become a little better organized this evening, with
convection intensifying around the center-- a marked improvement
from 24 hours ago. A pair of scatterometer passes from 00-01 UTC
showed believable maximum winds of 25-30 kt, and 30 kt is chosen
out of continuity from the last advisory.

The depression has been creeping westward recently, estimated at
about 3 kt. As mid-level high pressure builds to the northeast of
the cyclone, the tropical cyclone should turn northward on Tuesday
and move faster to the north-northeast for the next few days. A
frontal wave currently seen about 10 degrees north of Gordon should
also cause the cyclone to accelerate midweek as Gordon moves around
that feature. The biggest change to the guidance this evening is
that most are a bit faster, so the latest NHC prediction is trended
in that way.

Modest increases in low- to mid-level moisture around Gordon are
shown in the model fields during the next couple of days. In
combination with generally low shear during that time, these
factors suggest that Gordon will be primed for a comeback, and the
intensity guidance generally calls for it to become a tropical storm
again around midweek. The low-shear conditions should last
through late week, so the NHC forecast is nudged upward from 2-4
days, a touch below the model consensus. The shear could get
rather prohibitive by day 5 so the intensity forecast is leveled
off then, but considerable uncertainty exists at that time frame,
with model guidance ranging from a depression to a category 2
hurricane.


FORECAST POSITIONS AND MAX WINDS

INIT 17/0300Z 19.0N 48.8W 30 KT 35 MPH
12H 17/1200Z 19.3N 49.1W 30 KT 35 MPH
24H 18/0000Z 19.9N 49.0W 30 KT 35 MPH
36H 18/1200Z 20.8N 48.6W 30 KT 35 MPH
48H 19/0000Z 22.3N 47.9W 35 KT 40 MPH
60H 19/1200Z 24.2N 47.0W 40 KT 45 MPH
72H 20/0000Z 25.8N 45.8W 45 KT 50 MPH
96H 21/0000Z 27.5N 44.3W 50 KT 60 MPH
120H 22/0000Z 29.5N 43.5W 50 KT 60 MPH

$$
Forecaster Blake

[cycloneye]

Tropical Depression Gordon Discussion Number 24
NWS National Hurricane Center Miami FL AL072024
500 AM AST Tue Sep 17 2024

After becoming a bit better organized Monday evening with persistent
deep convection over the low-level center, Gordon's convective
organization has been steady during the overnight hours. Subjective
Dvorak intensity estimates range from 25-35 kt, while recent
objective estimates from UW-CIMSS are in the 31-36 kt range. Based
on these intensity estimates as well as taking into account earlier
ASCAT data, Gordon's intensity is held at 30 kt for this advisory.

The ASCAT passes Monday evening helped locate the center, but the
center has been difficult to find since that time. Recent satellite
images suggest that the center may be near the northern edge of the
central convective area. The motion is still estimated to be
westward, or 265/3 kt, but models indicate that the northward turn
should begin within the next few hours. A frontal low currently
located several hundred miles north of Gordon has created a weakness
in the subtropical ridge, which will induce a northward and then a
north-northeastward motion at an increasing forward speed during the
next couple of days. Gordon should pass east of this feature
Wednesday night or Thursday as it is steered by mid-level high
pressure to the east of the cyclone. Some of the global models show
this ridge building to the northeast of Gordon, which could cause
Gordon to bend more toward the north in 4 to 5 days. There are
significant speed differences as well as cross-track spread at days
4 and 5. The new NHC forecast is similar to the previous one
through 48 h, but is nudged to the west of the previous official
forecast beyond 48 h, closer to the latest consensus aids. During
the 3 to 5 day time period, the NHC forecast is slower than the
latest GFS model and well to the east of the latest ECMWF global
model.

Gordon's intensity forecast is challenging. Environmental
conditions appear moist enough for Gordon to at least maintain its
intensity during the next couple of days, given the weak vertical
wind shear and warm sea-surface temperatures. However, global
models do not show Gordon intensifying much during the next couple
of days as the cyclone interacts with a weakening non-tropical low,
currently located north of Gordon. Some strengthening is likely
once Gordon moves past this feature, but there is quite a bit of
uncertainty in what the upper-level winds will look like over the
cyclone in the day 3 to 5 period. The GFS model shows moderate
southwesterly shear during that time, whereas the ECMWF and other
global models suggest stronger shear from the west or northwest,
which would be a less favorable direction and would likely prevent
further strengthening. As a result, there is a large spread in the
intensity guidance, leading to below average confidence in the NHC
intensity forecast. The NHC intensity forecast is similar to the
previous one, and remains near the low end of the intensity guidance
through the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT 17/0900Z 19.0N 49.0W 30 KT 35 MPH
12H 17/1800Z 19.5N 49.1W 30 KT 35 MPH
24H 18/0600Z 20.4N 48.7W 30 KT 35 MPH
36H 18/1800Z 21.7N 48.2W 30 KT 35 MPH
48H 19/0600Z 23.3N 47.5W 35 KT 40 MPH
60H 19/1800Z 25.1N 46.7W 45 KT 50 MPH
72H 20/0600Z 26.3N 45.8W 50 KT 60 MPH
96H 21/0600Z 27.6N 44.6W 50 KT 60 MPH
120H 22/0600Z 30.0N 44.3W 55 KT 65 MPH

$$
Forecaster Hagen

[cycloneye]

Remnants Of Gordon Discussion Number 25
NWS National Hurricane Center Miami FL AL072024
1100 AM AST Tue Sep 17 2024

Gordon is no longer a tropical cyclone. The convective structure has
degraded since the overnight hours, with only small bursts of
convection occurring to the south and east of the estimated center
position. More importantly, recent scatterometer data indicate the
system does not possess a well-defined, trackable center, with an
elongated structure more indicative of a trough of low pressure.
Therefore, this will be the final NHC advisory on the remnants of
Gordon. The initial intensity is set at 25 kt based on the
scatterometer data, although this could be generous.

The remnants are expected to move northward to north-northeastward
over open waters during the next few days while rotating around a
non-tropical low currently positioned to its north. While the
structure is unlikely to improve in the short term, there are
indications in the global models that the remnants of Gordon could
redevelop later this week once the system moves into a more moist
environment and gains some distance from the nearby frontal low. NHC
will continue to monitor the remnants of Gordon for signs of
organization and the possibility of redevelopment later this week.
Information on the potential for regeneration will be contained in
the Tropical Weather Outlook.

Additional information on this system can be found in High Seas
Forecasts issued by the National Weather Service, under AWIPS header
NFDHSFAT1, WMO header FZNT01 KWBC, and online at
ocean.weather.gov/shtml/NFDHSFAT1.php


FORECAST POSITIONS AND MAX WINDS

INIT 17/1500Z 19.5N 49.1W 25 KT 30 MPH...REMNANTS OF GORDON
12H 18/0000Z...DISSIPATED

$$
Forecaster Reinhart