ATL: DON - Post-Tropical - Discussion

[Teban54]

Don is a strange system. Satellite images show that convection has increased in aerial coverage in the southeastern quadrant of the cyclone, although it is still not well defined in banding features or other organizational metrics. The convection is also closer to the center, and the upper-level low near Don has weakened. In some ways it has actually gained some tropical characteristics since yesterday despite remaining over cool waters, but the convective organization is still shy of a tropical cyclone. Regardless of the scientific curiosity, there are no signs that the winds have strengthened, and Don is best classified as a 30-kt subtropical depression on this advisory.

[Sciencerocks]

[cycloneye]

Will transition to fully tropical on Tuesday.

Both the
regional-hurricane models and global models show Don maintaining or
increasing convective activity near its center as it moves
southward. For these reasons, the latest intensity forecast now
shows the system transitioning to a tropical cyclone a bit earlier,
and also shows some gradual intensification beginning at 48 hours as
the system moves over these warmer waters. A bit more
intensification is shown in the latter part of the forecast, but
still remains under the majority of the guidance in the extended
range given the larger-than-normal uncertainty in the location and
structure of the system at the end of the forecast period.

[ChrisH-UK]

Dom has some convection on the east side.

[cycloneye]

AL, 05, 2023071712, , BEST, 0, 387N, 438W, 30, 1011, TD

[cycloneye]

Transitioned to fully tropical.

Satellite images indicate that Don continues to produce a cluster of
deep convection, displaced east of the center due to shear. Despite
the cool waters, this deep convection has persisted overnight, and
the cloud pattern most resembles a sheared tropical cyclone,
although the circulation remains elongated. This structure is
confirmed by the latest TAFB Dvorak estimate, which gave a tropical
classification for the first time at 12 UTC. Thus, Don is changed to
a tropical cyclone on this advisory, although it is still a 30-kt
depression on the basis of a recent scatterometer pass at 1229 UTC
that showed a small area of winds around 30 kt.

[Sciencerocks]

[ThomasW]

ASCAT looks to indicate a TS now:

[wxman57]

ASCAT looks to indicate a TS now:
https://cdn.discordapp.com/attachments/716215692617187328/1130547476210921492/20230717_131156.jpg

Yeah, NHC will likely upgrade Don to a TS this afternoon.

[Sciencerocks]

[Hurricane2022]

The environment around Don is forecast to become more conducive for gradual intensification in a couple of days when the cyclone moves over warmer waters with light-to-moderate shear, tempered by plenty of mid-level dry air. Surprisingly, many of the recent regional hurricane models show a hurricane forming over 24-25C waters in several days time, which is hard to believe given the seemingly marginal large-scale conditions. Additionally, Don will have to cross its own cool wake in 4-5 days, which isn't well accounted for in the models yet. The new forecast is bumped up at day 5 but remains below the model consensus.

[cainjamin]

Don is looking fully tropical now. You can see the upper level anticyclone forming with the fanning cirrus clouds.

[Subtrop]

AL, 05, 2023071800, , BEST, 0, 378N, 408W, 35, 1007, TS,

[Sciencerocks]

 https://twitter.com/VortixWx/status/1681108368927125504

[Sciencerocks]

[cycloneye]

Looks like Don may get more ACE if it moves west instead of north quickly.

From the discussion:

The convective structure of Don this evening has improved somewhat,
with a decent area of -55 to -65C cold cloud tops centered a bit to
the northeast of the low-level center. The center is also perhaps a
bit better defined than earlier, with the ongoing convective
activity possibly helping to tighten the center up. While subjective
Dvorak classifications have not increased much this evening, an
ASCAT-C pass at 2302 UTC caught the eastern portion of the
circulation, with several believable wind vectors of up to 35 kt.
Thus, Don is being upgraded to a tropical storm this advisory with
sustained winds of 35 kt.

The latter part of the forecast is likely
dependent on the Don's ultimate track. A further left track, like
the GFS and HAFS-A/B, may take the cyclone over warmer SSTs, and
result in a stronger storm. However, a more rightward track like
the ECMWF would take Don over its cold wake and likely would limit
additional intensification.

[Sciencerocks]

[LarryWx]

Don has a small chance to eventually become the season's first hurricane, perhaps Thu or Fri. What do others here think about that possibility?

[wxman57]

NHC should downgrade Don in the next advisory. Zero convection, disorganized circulation. More of a remnant low than a depression. May get up to minimal TS strength over the next day or two, but no hurricane.

[Subtrop]

Tropical Storm Don Discussion Number 18
NWS National Hurricane Center Miami FL AL052023
300 PM GMT Tue Jul 18 2023

Don has struggled to produce deep convection today, despite the
storm moving over warmer waters, with only a few moderate bands on
the eastern side of the circulation. A scatterometer pass near
1200 UTC showed maximum winds just above 30 kt, and it seems like
the system is vacillating between 30 and 35 kt as an initial wind
speed, based primarily on convective trends. I don't really want
to be chasing every burst of convection, so the conservative thing
is to wait a little longer before downgrading it to a depression.

The system should not change much in intensity over the next day or
so while it encounters a slightly more conducive (but still
marginal) environment. It also wouldn't be a surprise if Don became
a depression again in the short-term. Modest intensification is
forecast later this week while the system encounters warmer SSTs,
higher mid-level moisture, and the shear remains light or moderate.
The new forecast is similar to the previous one, below the model
consensus given the overall mediocre environment for strengthening.

Don has turned south-southeastward at about 8 kt. The storm is
about halfway through making a large anticyclonic loop over the
central Atlantic, bending around a blocking ridge in the
mid-latitudes. Don should move more slowly southward overnight and
Wednesday as it sits beneath the ridge, then gradually accelerate
northwestward and northward as it moves around the ridge. The track
guidance is coming into better agreement on this course,
conveniently converging somewhat on the last NHC track forecast, so
no significant changes were required.



FORECAST POSITIONS AND MAX WINDS

INIT 18/1500Z 35.6N 39.3W 35 KT 40 MPH
12H 19/0000Z 34.6N 39.0W 35 KT 40 MPH
24H 19/1200Z 33.9N 39.4W 35 KT 40 MPH
36H 20/0000Z 33.7N 40.5W 40 KT 45 MPH
48H 20/1200Z 34.0N 41.7W 40 KT 45 MPH
60H 21/0000Z 34.7N 43.3W 45 KT 50 MPH
72H 21/1200Z 35.7N 45.1W 45 KT 50 MPH
96H 22/1200Z 39.0N 49.3W 45 KT 50 MPH
120H 23/1200Z 44.0N 49.5W 45 KT 50 MPH

$$
Forecaster Blake