ATL: KATIA - Advisories

[cycloneye]

BULLETIN
Tropical Depression Twelve Advisory Number 1
NWS National Hurricane Center Miami FL AL122023
200 PM CVT Fri Sep 01 2023

...TROPICAL DEPRESSION FORMS OVER THE EASTERN ATLANTIC...


SUMMARY OF 200 PM CVT...1500 UTC...INFORMATION
----------------------------------------------
LOCATION...19.6N 28.3W
ABOUT 375 MI...605 KM NW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...NNW OR 340 DEGREES AT 8 MPH...13 KM/H
MINIMUM CENTRAL PRESSURE...1007 MB...29.74 INCHES






Tropical Depression Twelve Discussion Number 1
NWS National Hurricane Center Miami FL AL122023
200 PM CVT Fri Sep 01 2023

The area of low pressure over the far eastern Atlantic to the
northwest of the Cabo Verde Islands has developed a well-defined
surface circulation with sufficiently organized deep convection to
be classified as a tropical cyclone. Dvorak classifications from
TAFB and SAB are a consensus T2.5, but since the convection is
loosely organized around the center, the initial intensity is
started at 30 kt.

Sea surface temperatures of 27-28 degrees Celsius and low vertical
shear during the next day or so should allow the depression to
strengthen to a tropical storm later today or on Saturday. However,
the cyclone will be reaching marginally warm waters in 24-48 hours,
and southerly shear is forecast to increase at about that same time.
The NHC forecast therefore shows weakening beginning in 48 hours,
and the system is likely to degenerate into a remnant low by 60
hours due to a loss of organized deep convection. The remnant low
is shown dissipating by day 4, following the majority of the global
model fields.

The depression is moving toward the north-northwest (340 degrees) at
7 kt between a mid-level ridge over western Africa and a mid- to
upper-level low located over the eastern Atlantic. The steering
environment changes little during the next couple of days, and the
depression is therefore expected to move toward the north-northwest
or northwest, at a slightly faster speed, through 48 hours. After
the cyclone becomes a shallower remnant low, lower-level flow is
expected to turn the system toward the west-northwest until it
dissipates. The track guidance is tightly clustered for the next 4
days, and the NHC track forecast is very close to the HCCA consensus
aid.


FORECAST POSITIONS AND MAX WINDS

INIT 01/1500Z 19.6N 28.3W 30 KT 35 MPH
12H 02/0000Z 21.0N 29.0W 35 KT 40 MPH
24H 02/1200Z 22.8N 30.2W 35 KT 40 MPH
36H 03/0000Z 24.3N 31.4W 35 KT 40 MPH
48H 03/1200Z 25.5N 32.8W 30 KT 35 MPH
60H 04/0000Z 26.0N 33.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
72H 04/1200Z 26.4N 34.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 05/1200Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Tropical Depression Twelve Discussion Number 2
NWS National Hurricane Center Miami FL AL122023
800 PM CVT Fri Sep 01 2023

A 1458 UTC AMSR-2 microwave pass showed that the depression has a
well-defined circulation, however the associated deep convection
was rather disjointed. Dvorak CI numbers classifications are still
T2.5 from TAFB, but the final-T number dropped to 2.0 from SAB, so
the system is still being classified as a 30-kt depression at this
time. Some more defined convective bands are now developing within
the circulation, so the system may be on its way to becoming a
tropical storm. ASCAT data should provide a better handle on the
maximum winds of the system this evening.

The initial motion is toward the north-northwest (345 degrees) at 10
kt, with the depression situated between a mid-level ridge over
western Africa and a mid- to upper-level low over the
eastern/central Atlantic. Another ridge near the Azores is expected
to force the depression to turn northwest and then west-northwest
around the mid-level low during the next few days. The NHC track
forecast is a little east of the previous forecast near the
multi-model consensus aids, but this is more a function of an
adjustment of the initial position. Once the system becomes a
remnant low, the low-level flow should turn it toward the northwest
and north on days 3 and 4.

The depression has a small window of relatively low shear, warm
waters, and a sufficiently moist atmosphere to strengthen to a
tropical storm tonight or on Saturday, which is shown in the
official forecast. However, the system will be moving into a much
drier atmosphere with strengthening southerly shear, which should
cause weakening to begin in about 36 hours. Organized deep
convection could also dissipate in a couple of days, and the
cyclone is forecast to become a remnant low by 48 hours. Global
model fields suggest the remnant low could last a little bit longer
than shown in the previous forecast, and dissipation is now shown a
day later on day 5.


FORECAST POSITIONS AND MAX WINDS

INIT 01/2100Z 20.8N 28.7W 30 KT 35 MPH
12H 02/0600Z 22.2N 29.5W 35 KT 40 MPH
24H 02/1800Z 24.0N 30.8W 35 KT 40 MPH
36H 03/0600Z 25.4N 32.3W 30 KT 35 MPH
48H 03/1800Z 26.3N 33.5W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 04/0600Z 26.9N 34.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 04/1800Z 27.7N 35.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 05/1800Z 29.4N 35.4W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 06/1800Z...DISSIPATED

$$
Forecaster Berg

[cycloneye]

Tropical Depression Twelve Discussion Number 3
NWS National Hurricane Center Miami FL AL122023
200 AM CVT Sat Sep 02 2023

The depression continues to produce deep bursts of convection east
of its estimated center. Scatterometer data measured the western
portion of the circulation just before 0000 UTC, showing no
tropical-storm-force winds. While there is a possibility for
stronger winds on the eastern, convective side of the circulation,
the initial intensity is held at 30 kt for this advisory.

The initial motion is estimated to be just west of north at 11 kt.
The depression is moving between a ridge over western Africa and a
mid- to upper-level low over the central Atlantic and expected to
turn to the northwest shortly. By day 3, the cyclone is expected to
slow in forward speed as the vortex becomes more shallow and turn
northward in the low-level flow. The latest NHC track forecast is
very similar to the previous advisory prediction.

The window for potential intensification is quickly closing.
Vertical wind shear is expected to increase as the cyclone moves
into an increasingly drier environment within a day or so. Most of
the intensity guidance does not show any strengthening beyond now.
However, global models are not capturing the most recent flare of
convection and may be underestimating the short-term intensity
forecast. The official prediction shows that the depression could
still become a tropical storm within 12 h. Beyond 12 h, the cyclone
is expected to gradually weaken. Simulated satellite imagery from
the global models suggests the depression should be devoid of
convection and become a remnant low in a couple of days or so.


FORECAST POSITIONS AND MAX WINDS

INIT 02/0300Z 21.6N 28.7W 30 KT 35 MPH
12H 02/1200Z 22.8N 29.7W 35 KT 40 MPH
24H 03/0000Z 24.5N 31.2W 30 KT 35 MPH
36H 03/1200Z 25.5N 32.6W 30 KT 35 MPH
48H 04/0000Z 26.2N 33.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 04/1200Z 27.1N 34.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 05/0000Z 27.8N 35.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 06/0000Z 29.2N 35.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 07/0000Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Tropical Storm Katia Discussion Number 4
NWS National Hurricane Center Miami FL AL122023
800 AM CVT Sat Sep 02 2023

The depression has become better organized overnight. Cloud tops
temperatures have decreased to -76 Celsius over the surface center
and curved banding has improved, mainly over the northern
semi-circle. UW-CIMSS AiDT and DPRINT objective satellite
intensity estimates, as well as the Dvorak estimated from TAFB
support upgrading the system to a tropical storm.

Katia is expected to be a short-lived storm as deep-layer
southerly shear should increase soon while Katia moves into an
inhibiting, dry and stable surrounding environment. As a result, a
weakening trend is forecast through the period and is in agreement
with the deterministic models and the statistical-dynamic SHIPS
intensity guidance.

Katia is estimated to be moving north-northwestward, or 330/11 kt,
and is situated midway between a subtropical ridge extending toward
the west from western Africa, and a large cut-off low centered over
the central subtropical Atlantic. A turn toward the northwest is
expected today followed by a a gradual turn toward the north at a
slower forward pace around mid-period while Katia degenerates to a
vertically shallow low-pressure system.


FORECAST POSITIONS AND MAX WINDS

INIT 02/0900Z 22.7N 29.4W 35 KT 40 MPH
12H 02/1800Z 24.1N 30.5W 35 KT 40 MPH
24H 03/0600Z 25.4N 31.8W 30 KT 35 MPH
36H 03/1800Z 26.2N 33.1W 30 KT 35 MPH...POST-TROP/REMNT LOW
48H 04/0600Z 26.8N 34.3W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 04/1800Z 27.5N 35.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 05/0600Z 28.2N 35.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 06/0600Z 29.6N 35.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 07/0600Z...DISSIPATED

$$
Forecaster Roberts

[cycloneye]

Tropical Storm Katia Tropical Cyclone Update
NWS National Hurricane Center Miami FL AL122023
1140 AM CVT Sat Sep 02 2023

...KATIA STRENGTHENING...

Satellite wind data indicate that the maximum sustained winds of
Katia have increased to 50 mph (85 km/h). A new intensity forecast
to reflect the current intensity will be issued by 2 PM CVT
(1500 UTC).


SUMMARY OF 1140 AM CVT...1240 UTC...INFORMATION
----------------------------------------------
LOCATION...23.5N 29.4W
ABOUT 630 MI...1015 KM NW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H
PRESENT MOVEMENT...NNW OR 330 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...1000 MB...29.53 INCHES

$$
Forecaster Blake

[cycloneye]

Tropical Storm Katia Discussion Number 5
NWS National Hurricane Center Miami FL AL122023
200 PM CVT Sat Sep 02 2023

Satellite images indicate the center of Katia is on the southern
side of a large central dense overcast. Scatterometer data from
this morning indicated maximum winds were at least 40-45 kt, and
the latest TAFB Dvorak estimate was 45 kt. The initial wind speed
is raised to 45 kt from the last full advisory, similar to the
morning update statement.

Katia continues moving north-northwestward, or 330/10 kt. The
motion should gradually bend toward the northwest today due to the
storm moving between a large middle- to upper-level low to the west
and a narrow mid-latitude ridge over the eastern Atlantic. The
northwestward motion is likely to continue until about Monday, when
the system becomes weaker and Katia gets stuck in the low-level
subtropical ridge. The only notable change to the track forecast is
that it has been adjusted to the east in the short-term due to the
initial position and is very close to the previous one at the end.

The storm is probably near its peak intensity with all models
showing an increase in dry mid-level air near the center later
today. Combined with moderate southerly shear, this should be
enough to choke off deep convection in Katia's core and cause
gradual weakening. In about 2 days, the environment becomes quite
dry, which should finish off any remaining thunderstorm activity,
and Katia is forecast to degenerate into a remnant low at that time.
The NHC intensity forecast is raised early on to reflect the higher
initial wind speed and is blended back with the guidance by the end
of the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT 02/1500Z 23.9N 29.7W 45 KT 50 MPH
12H 03/0000Z 25.1N 30.8W 45 KT 50 MPH
24H 03/1200Z 26.2N 32.1W 40 KT 45 MPH
36H 04/0000Z 27.0N 33.5W 35 KT 40 MPH
48H 04/1200Z 27.6N 34.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 05/0000Z 28.5N 35.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 05/1200Z 29.1N 36.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 06/1200Z...DISSIPATED

$$
Forecaster Blake

[cycloneye]

BULLETIN
Tropical Storm Katia Advisory Number 6
NWS National Hurricane Center Miami FL AL122023
500 PM AST Sat Sep 02 2023

...KATIA OVERACHIEVING IN THE EASTERN ATLANTIC BUT EXPECTED TO
WEAKEN TOMORROW...


SUMMARY OF 500 PM AST...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...25.0N 30.5W
ABOUT 750 MI...1205 KM NNW OF THE CABO VERDE ISLANDS
MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H
PRESENT MOVEMENT...NW OR 320 DEGREES AT 13 MPH...20 KM/H
MINIMUM CENTRAL PRESSURE...998 MB...29.47 INCHES




Tropical Storm Katia Discussion Number 6
NWS National Hurricane Center Miami FL AL122023
500 PM AST Sat Sep 02 2023

Deep convection continues near and north of the center of Katia,
with microwave data showing a tilted vortex due to the southerly
shear. Overall, the system is showing more internal structure than
this morning, which is confirmed by increasing satellite estimates
from many sources which mostly lie between 45-55 kt. The initial
wind speed is raised to 50 kt as a blend of this data.

Katia has turned northwestward at about 11 kt. The storm should
move between a large middle- to upper-level low to the west
and a narrow mid-latitude ridge over the eastern Atlantic for the
next couple of days. The biggest changes from this morning are
that uncertainty is increasing in a few days when Katia, or the
remnants, reaches a col point between a mid-latitude ridge building
over the central Atlantic and deep trough over the northeastern
Atlantic. The guidance suite literally has motions in all cardinal
directions after that point, indicating a very low confidence
situation, and the GEFS ensemble lows show the proverbial squashed
spider pattern. Thus, the new forecast stalls the system before
dissipating, but it will require some time to sort out the
long-range details.

The earlier microwave data was showing a growing distance between
the mid-level and low-level centers of the cyclone, suggesting that
southerly shear could be winning the battle soon (also indicated
by the latest infrared satellite data). A continuation of that
shear plus a slow increase in dry air is likely to cause Katia to
slowly weaken for the next few days. The global models show no
significant deep convection after 36 h, and remnant low status is
indicated for 48 h. Little change was made to the previous
forecast, similar to the NOAA corrected consensus guidance.


FORECAST POSITIONS AND MAX WINDS

INIT 02/2100Z 25.0N 30.5W 50 KT 60 MPH
12H 03/0600Z 25.9N 31.5W 45 KT 50 MPH
24H 03/1800Z 26.8N 32.8W 40 KT 45 MPH
36H 04/0600Z 27.5N 33.9W 35 KT 40 MPH
48H 04/1800Z 28.2N 35.0W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 05/0600Z 28.5N 35.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 05/1800Z 29.0N 35.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 06/1800Z...DISSIPATED

$$
Forecaster Blake

[cycloneye]

Tropical Storm Katia Discussion Number 7
NWS National Hurricane Center Miami FL AL122023
300 AM GMT Sun Sep 03 2023

Katia has noticeably weakened since the previous advisory. Remnants
of convection appear to be separating from the low-level circulation
and no new bursts have formed in the past few hours. Geostationary
satellite imagery suggests dry air likely has made its way into the
core of the storm. Final T-numbers from TAFB and SAB were T3.0/45
and T1.5/25 . The initial intensity has therefore been lowered to
40 kt. Scatterometer data missed the center of the storm, but
measured the winds on the eastern half, revealing the radii of
tropical-storm-force winds had decreased.

Katia is once again moving north-northwestward at about 14 kt. The
storm should turn back to the northwest shortly as it moves between
an upper-level low to the west and a narrow mid-latitude ridge over
the eastern Atlantic. Model guidance suggests it should maintain
this heading at a decreased forward speed during the next couple of
days. However, the cyclone has been moving to the east of the track
guidance envelope for the past several advisory cycles. Beyond day
2, there is still considerable uncertainty in the motion, and
therefore location, of Katia or its remnants. The latest NHC track
forecast has been shifted northward, largely due to the initial
storm position.

Model guidance still suggests Katia may have another burst of
convection later today or Monday. Environmental conditions are
expected to become increasingly unfavorable, with strong-to-moderate
vertical wind shear and dry mid-level humidities. The official
intensity forecast still shows gradual weakening through the
forecast period and shows Katia becoming a remnant low in a couple
of days, though this may occur sooner.


FORECAST POSITIONS AND MAX WINDS

INIT 03/0300Z 26.5N 31.0W 40 KT 45 MPH
12H 03/1200Z 27.5N 32.0W 35 KT 40 MPH
24H 04/0000Z 28.3N 33.1W 30 KT 35 MPH
36H 04/1200Z 28.9N 34.1W 30 KT 35 MPH
48H 05/0000Z 29.4N 34.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 05/1200Z 29.8N 35.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 06/0000Z 30.2N 35.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 07/0000Z...DISSIPATED

$$
Forecaster Bucci

[cycloneye]

Tropical Storm Katia Discussion Number 8
NWS National Hurricane Center Miami FL AL122023
900 AM GMT Sun Sep 03 2023

MET-10 proxy-vis and a series of AMSR2 microwave overpasses indicate
that the surface center is farther south than earlier thought and is
displaced about 170 mi to the south of a small patch of deep
convection. Subsequently, Katia has been devoid of convection
since 0100 UTC, and unless organized deep convection redevelops,
it will likely be classified as a post-tropical cyclone within the
next 24 hours. The initial intensity is lowered to 35 kt and is
based on a blend of the subjective satellite intensity estimates
from TAFB and SAB.

The intensity forecast philosophy remains unchanged. A gradual
spin-down of the cyclone should continue as deep-layer southerly
shear is expected to increase while Katia moves into an inhibiting,
dry and stable surrounding environment. As a result, The NHC
intensity forecast is similar to the previous one except that it
now shows Katia degenerating to a remnant low a little sooner, and
within the next 24 hours.

Katia's initial motion is estimated to be northwestward, or 310/8
kt. The cyclone is forecast to move between a large middle- to
upper-tropospheric cut-off low to the west over the subtropical
Atlantic and a northwest-to-southeast oriented mid-level ridge to
the east during the next 36 hours or so. The picture is more
explicit through the remaining portion of the forecast than in
previous global model runs. As mentioned above, the cut-off feature
situated west of Katia fills and retrogrades northwestward, while a
deep-layer mid-latitude trough digs southeastward over the northeast
Atlantic. This change in the synoptic steering pattern causes Katia
to slow to a crawl and gradually turn northward, or meander, on
Tuesday. The official track forecast is shifted toward the left of
the previous advisory to agree more with the tightly clustered
consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 03/0900Z 26.0N 31.9W 35 KT 40 MPH
12H 03/1800Z 26.6N 32.8W 30 KT 35 MPH
24H 04/0600Z 27.1N 33.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 04/1800Z 27.7N 34.3W 30 KT 35 MPH...POST-TROP/REMNT LOW
48H 05/0600Z 28.1N 34.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 05/1800Z 28.5N 34.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 06/0600Z 28.5N 34.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 07/0600Z...DISSIPATED

$$
Forecaster Roberts

[cycloneye]

Tropical Storm Katia Discussion Number 9
NWS National Hurricane Center Miami FL AL122023
300 PM GMT Sun Sep 03 2023

Katia has a large, well-defined circulation on visible satellite
imagery, but any associated deep convection is well north of the
center. Intensity estimates are mostly at tropical-storm strength,
so the initial wind speed is kept at 35 kt. Weakening should
resume later today as a combination of southerly shear and very dry
air in the mid-levels wipes out the rest of the deep convection.
The global models continue to show no organized thunderstorm
activity by tomorrow, and thus remnant low status is anticipated at
that time.

The storm is still moving northwest, but a bit slower at 6 kt.
Katia is forecast to be steered between a large middle- to
upper-tropospheric cut-off low to the west over the subtropical
Atlantic and a northwest-to-southeast oriented mid-level ridge to
the east during the next 36 hours or so. As the tropical cyclone
weakens, it should get trapped within the low-level subtropical
ridge, causing little motion in a couple of days, with the forecast
remnants of Katia drifting southeastward at at long range. Other
than a small northward adjustment based on the initial position, no
notable changes were made to the NHC predicted track.


FORECAST POSITIONS AND MAX WINDS

INIT 03/1500Z 26.7N 32.3W 35 KT 40 MPH
12H 04/0000Z 27.3N 33.1W 30 KT 35 MPH
24H 04/1200Z 27.8N 33.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 05/0000Z 28.3N 34.4W 30 KT 35 MPH...POST-TROP/REMNT LOW
48H 05/1200Z 28.5N 34.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 06/0000Z 28.3N 34.0W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 06/1200Z 27.8N 33.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 07/1200Z...DISSIPATED

$$
Forecaster Blake

[cycloneye]

Tropical Storm Katia Discussion Number 10
NWS National Hurricane Center Miami FL AL122023
900 PM GMT Sun Sep 03 2023

The center of Katia remains exposed on the latest satellite
imagery, with a large convective burst in the northern semicircle
noted this evening. The intensity estimates are a split decision,
with about half of data points suggesting tropical depression, and
the other half still a tropical storm. With the burst in
convection, the initial wind speed is held at 35 kt, and there
could be further clarity from partial scatterometer data this
evening. A combination of southerly shear and very dry mid-level air
should extinguish the thunderstorm activity early Monday and cause
gradual weakening. The global models continue to show no organized
thunderstorm activity by tomorrow, and Katia is likely to become a
remnant low in 12-24 h.

The storm is holding steady on a northwest course at 6 kt. Katia
is expected to be steered between a large middle- to upper-
tropospheric cut-off low to the west over the central subtropical
Atlantic and a mid-level ridge to the east during the next 36 hours
or so. As Katia weakens, it should get trapped within the
low-level subtropical ridge, causing little motion by Tuesday, and
a southward drift by Wednesday. No significant changes were made to
the NHC track or intensity forecasts.


FORECAST POSITIONS AND MAX WINDS

INIT 03/2100Z 27.1N 32.9W 35 KT 40 MPH
12H 04/0600Z 27.5N 33.6W 30 KT 35 MPH
24H 04/1800Z 28.0N 34.2W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 05/0600Z 28.4N 34.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 05/1800Z 28.4N 34.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 06/0600Z 28.0N 33.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 06/1800Z 27.5N 33.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 07/1800Z...DISSIPATED

$$
Forecaster Blake

[cycloneye]

Tropical Depression Katia Discussion Number 11
NWS National Hurricane Center Miami FL AL122023
300 AM GMT Mon Sep 04 2023

Deep convection has been on the decline and only a small area of
thunderstorms remain well north of the center. A recent ASCAT pass
showed peak winds in the 25-30 kt range, and based on that data,
Katia has been downgraded to a 30-kt tropical depression. This value
is also in line with the latest satellite intensity estimates.

The depression is moving slowly west-northwestward. A slow
west-northwestward to northwestward motion is expected for about
another day before the system reverses course and drifts
southeastward in the flow between a low- to mid-level ridge to
its west and a deep-layer low to the northeast. Only small
changes were made to the previous NHC track forecast, and this one
lies near the various consensus models.

West-southwesterly vertical wind shear and dry air should continue
to weaken Katia, and this system will likely become a remnant low
in 12 to 24 hours. Dissipation is expected in 3 to 4 days.


FORECAST POSITIONS AND MAX WINDS

INIT 04/0300Z 27.1N 33.4W 30 KT 35 MPH
12H 04/1200Z 27.5N 33.9W 30 KT 35 MPH
24H 05/0000Z 28.1N 34.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 05/1200Z 28.4N 34.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 06/0000Z 28.3N 34.1W 25 KT 30 MPH...POST-TROP/REMNT LOW
60H 06/1200Z 27.9N 33.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 07/0000Z 27.4N 33.0W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 08/0000Z...DISSIPATED

$$
Forecaster Cangialosi

[cycloneye]

Tropical Depression Katia Discussion Number 12
NWS National Hurricane Center Miami FL AL122023
900 AM GMT Mon Sep 04 2023

Katia barely qualifies as a tropical cyclone at this time. The
system is producing a few puffs of deep convection over the
northern portion of its circulation. Earlier scatterometer data
indicated that the cyclone had weakened to a tropical depression
and the current intensity estimate is kept at 30 kt. This is in
good agreement with various objective estimates from UW-CIMSS.

The depression is moving northwestward, or at about 310/6 kt.
Katia's northwestward progress will soon be blocked by a high
pressure area ahead of it. Then, the flow on the southwestern side
of a large cyclone over the eastern Atlantic is likely to cause the
system to make a U-Turn and move slowly southeastward to
south-southeastward in 24 to 72 hours. The official forecast is a
blend of the GFS and ECMWF solutions and is similar to the previous
NHC track.

Katia should remain in an environment of very dry low- to mid-level
air with moderate vertical shear. Therefore, the cyclone is likely
to degenerate into a remnant low by tonight, and dissipate in a few
days. This is also consistent with various global model
predictions.


FORECAST POSITIONS AND MAX WINDS

INIT 04/0900Z 27.7N 33.7W 30 KT 35 MPH
12H 04/1800Z 28.0N 34.3W 30 KT 35 MPH
24H 05/0600Z 28.4N 34.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 05/1800Z 28.5N 34.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 06/0600Z 28.2N 33.9W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 06/1800Z 27.7N 33.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 07/0600Z 26.7N 33.1W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 08/0600Z...DISSIPATED

$$
Forecaster Pasch

[cycloneye]

Tropical Depression Katia Discussion Number 13
NWS National Hurricane Center Miami FL AL122023
300 PM GMT Mon Sep 04 2023

Katia continues to produce small areas of convection well north of
the center, but they aren't showing much organization. If the
convective pattern does not improve, Katia will likely be declared
a remnant low this evening. A partial scatterometer pass indicated
maximum winds of 30 kt, so this will stay as the initial wind
speed.

The cyclone should gradually weaken in a moderate shear, low
relative humidity environment, and this is indicated in the NHC
forecast and all of the model guidance. The dry environment will
likely cause Katia to become a remnant low soon, similar to the
model simulated satellite forecasts. Katia is moving slower to the
northwest, and should executive a slow clockwise loop over the next
couple of days as its poleward motion is blocked by a low-level
ridge. A steadier southward course should begin late Wednesday due
to the ridge building to Katia's west. No notable changes were
made to the intensity or track forecasts.


FORECAST POSITIONS AND MAX WINDS

INIT 04/1500Z 28.0N 34.0W 30 KT 35 MPH
12H 05/0000Z 28.4N 34.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 05/1200Z 28.6N 34.6W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 06/0000Z 28.6N 34.3W 20 KT 25 MPH...POST-TROP/REMNT LOW
48H 06/1200Z 28.1N 33.8W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 07/0000Z 27.2N 33.4W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 07/1200Z 26.0N 33.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 08/1200Z...DISSIPATED

$$
Forecaster Blake

[cycloneye]

Post-Tropical Cyclone Katia Discussion Number 14
NWS National Hurricane Center Miami FL AL122023
900 PM GMT Mon Sep 04 2023

The convection clock has run out on Katia with no organized
thunderstorm activity for most of the day. No additional organized
convection is expected either due to very dry air aloft and
continued shear. Thus, the system is declared post-tropical, and
this is the last NHC advisory. The winds are set to 30 kt in
accordance with an earlier scatterometer pass.

Katia isn't moving much now, and it is forecast to drift in a small
clockwise loop over the next day or two within the subtropical
ridge. The remnants of the system should move a bit faster
southward on Wednesday, gradually weakening due to the unfavorable
environment, and open up into a trough within 3 days.

Additional information on this system can be found in High Seas
Forecasts issued by Meteo France under WMO header FQNT50 LFPW and
available on the web at
www.meteofrance.com/previsions-meteo-ma ... randlarge/
metarea2.


FORECAST POSITIONS AND MAX WINDS

INIT 04/2100Z 28.0N 34.4W 30 KT 35 MPH...POST-TROP/REMNT LOW
12H 05/0600Z 28.2N 34.7W 25 KT 30 MPH...POST-TROP/REMNT LOW
24H 05/1800Z 28.4N 34.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
36H 06/0600Z 28.1N 34.2W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 06/1800Z 27.5N 33.7W 20 KT 25 MPH...POST-TROP/REMNT LOW
60H 07/0600Z 26.5N 33.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 07/1800Z...DISSIPATED

$$
Forecaster Blake