ATL: SAM - Remnants - Discussion

[aspen]

The Euro ensemble forecasts of stronger=north make no sense to me. Further south = over warmer waters and further from the possible TUTT.

Recent runs have been showing a deeper TUTT axis extending farther south. Incidentally, while this trend induces more of a southerly shortwave component to Sam's trajectory, it also results in stronger shear over the storm. So in this case a weaker Sam would be more likely to track farther west. Given the pattern, I still fail to see how Sam could follow the bullish solutions intensity-wise and impact the Caribbean. The situation seems to be either/or so far. Stronger = OTS, weaker = Caribbean.

This is no Peter. The environment that Sam will be in for the next 72-120 hours — 28-29C SSTs, a favorable anticyclone with outflow in all quadrants, low shear, and ample atmospheric moisture — will mean it’ll likely be a 110-130 kt major by the time it reaches that possible TUTT. It could cause weakening or just halt intensification, but I don’t anticipate a huge impact unless the TUTT is very strong.

Aren’t there cases of strong outflow eroding upper-level features like ULLs?

[ElectricStorm]

New convection firing near the center, looks like it's coming along nicely

[Iceresistance]

If Sam does not get above 15°N by 55°W, then the Leeward Islands & Lesser Antilles are in serious danger for Hurricane Impacts . . .

[Teban54]

I smell a repeat of the "is this an eye or a dry slot" debate during Larry.

[Zonacane]

I smell a repeat of the "is this an eye or a dry slot" debate during Larry.
https://i.imgur.com/jfxK6dQ.png
https://i.imgur.com/aNTJdx0.png

Sam is vertically stacked, so that's an eyewall.

[MGC]

Sam has "the look" so I doubt there is little to hold him back. A bit of SAL to his north but I don't think it will be much of a factor....MGC

[Buck]

If/When Sam becomes a hurricane, 2021 will officially meet the NOAA's latest forecast in all metrics (15–21 storms, 7–10 hurricanes, 3–5 major hurricanes). And there's still a week left in September. I imagine Sam will boost our ACE a bit too.

[tolakram]

floater: https://weather.cod.edu/satrad/?parms=meso-meso1-02-12-1-100-1&checked=map&colorbar=undefined

Not sure it's vertically stacked yet, but it may be the angle we're looking at through the eye like feature.

[AutoPenalti]

I smell a repeat of the "is this an eye or a dry slot" debate during Larry.
https://i.imgur.com/jfxK6dQ.png
https://i.imgur.com/aNTJdx0.png

Sam is vertically stacked, so that's an eyewall.

It's close.

[aspen]

Levi will probably start video updates on Sam in the next day or so.
https://mobile.twitter.com/TropicalTidb ... gr%5Etweet

[Zonacane]

I smell a repeat of the "is this an eye or a dry slot" debate during Larry.
https://i.imgur.com/jfxK6dQ.png
https://i.imgur.com/aNTJdx0.png

Sam is vertically stacked, so that's an eyewall.

It's close.

Microwaves showed the curved band wrapping up earlier. I would say that's an eyewall

[Iceresistance]

Sam is vertically stacked, so that's an eyewall.

It's close.

Microwaves showed the curved band wrapping up earlier. I would say that's an eyewall

Evidence of this?

[Zonacane]

It's close.

Microwaves showed the curved band wrapping up earlier. I would say that's an eyewall

Evidence of this?

Tropical Storm Sam Discussion Number 4
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Thu Sep 23 2021

Overnight and this morning, the satellite structure of the tropical
cyclone has been improving, with prominent curved bands, and obvious
low-level cloud motions that indicate the circulation is becoming
established within the convection. In particular, an SSMIS microwave
pass at 0653 UTC indicated a substantial improvement in the
convective structure, with a well-defined curved band wrapping
three-quarters of the way around the center in both the 91- and
37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated
a tight, well-defined circulation had formed, with peak winds of
44-kt on the north side of the vortex. Subjective Dvorak satellite
intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from
TAFB. In addition, the latest objective intensity estimates from
UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given
the recent scatterometer data, the intensity has been set to 45-kt
for this advisory. Thus, Tropical Depression 18 has been upgraded to
Tropical Storm Sam. It is noteworthy that this is the 2nd earliest
formation of the 18th named storm in the Atlantic basin, moving
ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion
at 280/14 kt. A prominent mid-level ridge remains entrenched to the
north of the cyclone, and this feature should continue to steer Sam
to the west-northwest over the next 2-3 days. Over this period, the
ridge orientation actually shifts more westward ahead of the cyclone
as a deep-layer trough digs in well northeast of Sam. This gradual
change in the synoptic pattern should also result in a slowdown in
Sam's forward motion over the next 72 hours. While the track
guidance remains in good agreement over this period, larger spread
begins to take shape beyond the day 3 forecast. Similar to
yesterday, the ECMWF model and its ensemble mean flanks the
southwest side of the guidance envelop, while the GFS is on the
northeast side by day 5. These differences appear to be partially
related to the mid-level ridge intensity on the western end, which
could begin to be eroded by a mid-latitude trough off the eastern
United States in 120-h. In addition, differences in both the size
and intensity of Sam may also play a role in its ultimate track
evolution. For this advisory, the latest NHC track forecast remains
close to the consensus aids TCVN and TCVE, which represents a slight
shift westward and a bit faster motion compared to the previous
forecast. Because Sam is forecast to slow down, it remains too early
to determine what impacts, direct or indirect, could be felt by the
Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that
the low-level center has become better aligned with the mid-level
vortex, perhaps a bit earlier than anticipated given the
scatterometer data from last night. This improvement in structure,
combined with favorable low vertical wind shear under 10 kt and warm
28-29 C sea-surface temperatures, argues for significant
intensification. In fact, the GFS-SHIPS rapid intensification index
now give Sam a 39 percent chance of a 65-kt increase over the next
72 hours, which is more than 7 times the climatological value. Thus,
the NHC intensity forecast has been raised quite a bit from the
prior one, especially in the short-term. The latest forecast now
makes Sam a hurricane in 36 hours, and a major hurricane in 72
hours. This intensity forecast is just a shade under the latest HCCA
consensus aid. Thereafter, a more gradual intensification rate is
forecast. By this period it is possible that Sam could undergo hard
to predict inner-core changes such as eyewall replacement cycles. In
addition, the ECMWF-SHIPS suggests that the shear may also increase
a little in days 4-5 which also argues for a bit slower rate of
intensification for this period.


FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 10.9N 38.1W 45 KT 50 MPH
12H 24/0000Z 11.3N 40.3W 50 KT 60 MPH
24H 24/1200Z 11.9N 42.9W 60 KT 70 MPH
36H 25/0000Z 12.3N 45.0W 70 KT 80 MPH
48H 25/1200Z 12.7N 46.7W 80 KT 90 MPH
60H 26/0000Z 13.2N 48.2W 90 KT 105 MPH
72H 26/1200Z 13.7N 49.4W 100 KT 115 MPH
96H 27/1200Z 15.3N 52.1W 105 KT 120 MPH
120H 28/1200Z 17.4N 55.2W 110 KT 125 MPH

$$
Forecaster Papin

[aspen]

It's close.

Microwaves showed the curved band wrapping up earlier. I would say that's an eyewall

Evidence of this?

[InfernoFlameCat]

Best looking 50 mph storm yet.

[Iceresistance]

Microwaves showed the curved band wrapping up earlier. I would say that's an eyewall

Evidence of this?

https://rammb-data.cira.colostate.edu/tc_realtime/products/storms/2021al18/amsusr89/2021al18_amsusr89_202109231214.gif

The Northern Band probably needs to die away if Sam were to become a powerful storm, I hope that the band stays longer to limit Sam somewhat . . .

[Zonacane]

Best looking 50 mph storm yet.

Pretty clear-cut hurricane based on the convective structure.

[grapealcoholic]

I smell a repeat of the "is this an eye or a dry slot" debate during Larry.
https://i.imgur.com/jfxK6dQ.png
https://i.imgur.com/aNTJdx0.png

I think the LLC is a little bit south of the warm spot

[InfernoFlameCat]

That is an eye. There is deep convection wrapping around it.

[Teban54]

Slightly off-topic, but Papin is really getting the looks of a young Stewart or something. He may have been a bit aggressive, as seen from the 100 kt initial peak forecast yesterday and the straight-to-45-kts TS classification today. However, his solo advisories are also incredibly detailed and informative. In fact, this is the longest discussion I have seen in a while:

347
WTNT43 KNHC 231500
TCDAT3

Tropical Storm Sam Discussion Number 4
NWS National Hurricane Center Miami FL AL182021
1100 AM AST Thu Sep 23 2021

Overnight and this morning, the satellite structure of the tropical
cyclone has been improving, with prominent curved bands, and obvious
low-level cloud motions that indicate the circulation is becoming
established within the convection. In particular, an SSMIS microwave
pass at 0653 UTC indicated a substantial improvement in the
convective structure, with a well-defined curved band wrapping
three-quarters of the way around the center in both the 91- and
37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated
a tight, well-defined circulation had formed, with peak winds of
44-kt on the north side of the vortex. Subjective Dvorak satellite
intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from
TAFB. In addition, the latest objective intensity estimates from
UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given
the recent scatterometer data, the intensity has been set to 45-kt
for this advisory. Thus, Tropical Depression 18 has been upgraded to
Tropical Storm Sam. It is noteworthy that this is the 2nd earliest
formation of the 18th named storm in the Atlantic basin, moving
ahead of the 2005 hurricane season, and only trailing last year.

Sam continues to move to the west-northwest with an estimated motion
at 280/14 kt. A prominent mid-level ridge remains entrenched to the
north of the cyclone, and this feature should continue to steer Sam
to the west-northwest over the next 2-3 days. Over this period, the
ridge orientation actually shifts more westward ahead of the cyclone
as a deep-layer trough digs in well northeast of Sam. This gradual
change in the synoptic pattern should also result in a slowdown in
Sam's forward motion over the next 72 hours. While the track
guidance remains in good agreement over this period, larger spread
begins to take shape beyond the day 3 forecast. Similar to
yesterday, the ECMWF model and its ensemble mean flanks the
southwest side of the guidance envelop, while the GFS is on the
northeast side by day 5. These differences appear to be partially
related to the mid-level ridge intensity on the western end, which
could begin to be eroded by a mid-latitude trough off the eastern
United States in 120-h. In addition, differences in both the size
and intensity of Sam may also play a role in its ultimate track
evolution. For this advisory, the latest NHC track forecast remains
close to the consensus aids TCVN and TCVE, which represents a slight
shift westward and a bit faster motion compared to the previous
forecast. Because Sam is forecast to slow down, it remains too early
to determine what impacts, direct or indirect, could be felt by the
Lesser Antilles by this cyclone.

The previously mentioned SSMIS microwave imagery also indicated that
the low-level center has become better aligned with the mid-level
vortex, perhaps a bit earlier than anticipated given the
scatterometer data from last night. This improvement in structure,
combined with favorable low vertical wind shear under 10 kt and warm
28-29 C sea-surface temperatures, argues for significant
intensification. In fact, the GFS-SHIPS rapid intensification index
now give Sam a 39 percent chance of a 65-kt increase over the next
72 hours, which is more than 7 times the climatological value. Thus,
the NHC intensity forecast has been raised quite a bit from the
prior one, especially in the short-term. The latest forecast now
makes Sam a hurricane in 36 hours, and a major hurricane in 72
hours. This intensity forecast is just a shade under the latest HCCA
consensus aid. Thereafter, a more gradual intensification rate is
forecast. By this period it is possible that Sam could undergo hard
to predict inner-core changes such as eyewall replacement cycles. In
addition, the ECMWF-SHIPS suggests that the shear may also increase
a little in days 4-5 which also argues for a bit slower rate of
intensification for this period.


FORECAST POSITIONS AND MAX WINDS

INIT 23/1500Z 10.9N 38.1W 45 KT 50 MPH
12H 24/0000Z 11.3N 40.3W 50 KT 60 MPH
24H 24/1200Z 11.9N 42.9W 60 KT 70 MPH
36H 25/0000Z 12.3N 45.0W 70 KT 80 MPH
48H 25/1200Z 12.7N 46.7W 80 KT 90 MPH
60H 26/0000Z 13.2N 48.2W 90 KT 105 MPH
72H 26/1200Z 13.7N 49.4W 100 KT 115 MPH
96H 27/1200Z 15.3N 52.1W 105 KT 120 MPH
120H 28/1200Z 17.4N 55.2W 110 KT 125 MPH

$$
Forecaster Papin