ATL: SAM - Remnants - Discussion

[SconnieCane]

Eye is almost gone on satellite, convection has eroded. I think Sam is about to kiss goodbye to major hurricane intensity.

This was unexpected, 5 PM advisory called for no less than 110kt through 5 days.

[cycloneye]

A NOAA Hurricane Hunter aircraft investigating Sam this afternoon
and evening found that the major hurricane likely peaked in
intensity at around 135 kt with a central pressure of about 929 mb
between 1900-2200 UTC when the eye contracted down to about 7 nmi in
diameter.

155 mph 929mb peak, much better. How right now im wondering if Sam can even recover from what ever is causing this rapid weakening.

Three factors:

1-ERC

2-Shear.

3-Upwelling as it is moving slow.

[zhukm29]

Quite an unfortunate turn of events today, but this just shows how much luck still plays a role in the tropics, from missed ASCAT passes to poorly-timed recon. For example, Kate was able to be designated because an ASCAT pass just so happened to catch it while it had 45 mph winds (to everyone's surprise at the time), but had that scatterometer been in a different place at that time, Kate would have likely remained as TD10, and Sam would have instead been Rose. However, for that scatterometer to be in a position to catch Kate at peak strength, it likely missed previous systems in earlier years that could have potentially been designated.

In hindsight, things could have been a lot different had recon left just a few hours earlier, but no one could have known that Sam would collapse in remarkable fashion. It's really a game of hours - one hour earlier, and we could've known Sam's true peak, but now we'll have to do guesswork in reanalysis, and the debate over cat 4 or cat 5 will likely be in the conversation for years (similar to Jose 2017).

[us89]

I'm interested in their reasoning for the 929 mb peak. The first eye dropsonde when recon first got in measured a 932 mb splash pressure with 30kt wind, which correlates to a 929mb central pressure. And that was even after the IR appearance started degrading a bit. Based on that alone I'd have to think the actual pressure minimum was at least a few mb lower a few hours before.

[zhukm29]

Definitely - Sam had already begun degrading about an hour before recon arrived. Especially considering how small the storm is (and how fast it weakened during the first few passes), I would not be surprised if it was stronger than 929 several hours earlier when it looked the best.

I'm interested in their reasoning for the 929 mb peak. The first eye dropsonde when recon first got in measured a 932 mb splash pressure with 30kt wind, which correlates to a 929mb central pressure. And that was even after the IR appearance started degrading a bit. Based on that alone I'd have to think the actual pressure minimum was at least a few mb lower a few hours before.

[Teban54]

A NOAA Hurricane Hunter aircraft investigating Sam this afternoon
and evening found that the major hurricane likely peaked in
intensity at around 135 kt with a central pressure of about 929 mb
between 1900-2200 UTC when the eye contracted down to about 7 nmi in
diameter.

Given that Sam had clearly started weakening when the 929 mb pressure was recorded (as seen from the excellent plot by TheAustinMan), I think there's a pretty good chance its actual peak was stronger than 135/929, based on the assumption that pressure was rising at the time of the first recon pass. At this point it's basically a coin flip whether it did reach 140 kt.

In the past, they extrapolated lowest pressure between recons for Patricia, but not for Eta (IIRC).

Update: Here's what I found on Eta: (from TCR)

The estimated maximum intensity of Eta, 130 kt from 0000 to 0600 UTC 3 November, is
based on AFRES observations, using a blend of the highest 700 mb flight-level winds of 137 kt,
which adjusts to an intensity of 123 kt, and the peak SFMR-observed surface winds of 135 kt.
The 130-kt intensity is also supported by a maximum eyewall dropsonde wind speed of 129 kt
averaged over the lowest 150 m of the sounding at 0306 UTC. Eta’s estimated minimum central
pressure of 922 mb at 0600 UTC 3 November is based on an AFRES eye dropsonde
measurement of 925 mb with splash winds of 14 kt at 0358 UTC 3 November, and the possibility
that the pressure was still falling at that time.

The advisory right after the recon mission had the pressure at 923 mb, so even though they did discuss the possibility of pressure falling further, the TCR only lowered the pressure by 1 mb despite a 5mb/hr pressure fall recorded during recon.

[Teban54]

Assuming the pressure stayed at 929 mbar, the question of 155 mph vs 160 mph becomes a bit foggy; however, perhaps we could look at the structure of Emily 2005 as her peak was 160/929 but was initially a 155/929 peak for reference to what Sam *could* have been?

Emily's case was completely different as its peak was derived directly from flight-level winds:

Emily’s maximum sustained winds are estimated to be 140 kt, based on 700-mb flight-level winds of 153 kt observed at 0324Z 17 July (Fig. 2). Application of the standard 90% adjustment gives a surface estimate of 138 kt - just above the category 5 threshold. Two surface-adjusted flight-level observations of 136 kt during the preceding several hours also met the category 5 threshold. The lowest central pressure reported by reconnaissance aircraft was 929 mb, at 2341 UTC 16 July.

[ElectricStorm]

Doesn't look like that shear lasted very long, so hopefully it can get going again and have another peak. Probably a low end Cat 3 right now after taking a huge hit, but with new towers firing and wrapping around the center, hopefully it can recover.

[ScottNAtlanta]

It already looks like it is recovering. The deep convection has wrapped around the center. Just goes to show that during EWRCs these storms are more vulnerable if something is off. Especially small ones.

[Craters]

While the setting sun has some influence on temperatures, the low heat capacity of the ocean tells us that most of these temperature changes will be driven by the storm's actual structure and by extension intensity.

Great post, AustinMan. One thing -- did you mean maybe "high thermal inertia" or "high heat capacity" of the ocean or something similar where the italics are in that sentence above? As I know you're well aware, ocean water has a huge heat capacity...

[Teban54]

It already looks like it is recovering. The deep convection has wrapped around the center. Just goes to show that during EWRCs these storms are more vulnerable if something is off. Especially small ones.

That didn't last long as CDO to the west has diminished again. However, you can clearly see both the outer and inner eyewall naked on IR now, which is something you don't see very often.

My guess (without any formal reasoning) is that the EWRC went poorly because of external factors (possibly upwelling and/or shear) and is still not completed yet, but now it's trying to get back on track. Think the inner eyewall should disappear shortly and convective bursts should reach the western half of the outer eyewall again.

(Edit: Image removed to prevent spamming. See the start of the GIF below.)

[Teban54]

Very rarely do you see an EWRC with this kind of clarity:

[Hammy]

It knows its coming into more frequent recon range so the inner core decides to fall apart entirely.

[Teban54]

It knows its coming into more frequent recon range so the inner core decides to fall apart entirely.

Looks like it's somehow clearing out an eye again even though the MLC is all messed up. It did fall apart completely I guess, but it's clearly on its way to recovery now.

[kevin]

I think we've seen Sam at his worst now and it's been slowly improving its structure now. Very strong convection just fired near the center and is closing off the eye again.

[Teban54]

If I were to make a guess, I'll probably be greeted with something that resembles a Cat 3/4 again when I wake up.

[aspen]

Sam really got knocked down a few pegs overnight. I’m glad to see the NHC mentioned what its peak probably was in the 11pm advisory; no special advisory was needed, it seems.

A microwave pass from 6z shows that Sam is trying to re-develop its shattered eyewall. Hopefully it won’t be a mess when recon arrives later today.

[ChrisH-UK]

Sam this morning.

Source https://col.st/BlIjP

[kevin]

Sam is improving quite quickly now, eyewall seems to be restored/restoring and the eye is clearing out again on IR. Pretty impressive considering how much of a mess it was after its EWRC.

[aspen]

Is the next low-level recon expected for 15:30z or 17:30z?