2020 TCRs

[ColdMiser123]

Why not send an email to the NHC and/or Ronnie Berg inquiring as to why the NEXRAD data were not applied to Sally’s case?

In my view, Sally should be upgraded to 100 kt, Jose to 145–50 kt, and Joaquin to 140 kt. Matthew should be listed as 130 kt.

This was a good suggestion! I reached out to Robbie and he responded promptly to me.

The most interesting tidbit from his email to me was that the typical 80% reduction factor at 1200 meters, where those 121-122 kt velocity bins were found, accounts for undersampling by aircraft measurements (which is up to 8% as he wrote in the TCR). With radial velocity measurements though, there is little to no undersampling, so the factor should actually be even lower compared to the standard 80% conversion for a height of ~1200 meters. He also mentioned the importance of averaging multiple bins (i.e. a 4 bin average) in order to filter out noise among the velocity bins, which can impact the analysis as well. He concluded by saying the balance of evidence (including the radar derived wind data) led to an estimate of 95 kt overall.

So they did analyze the WSR-88D velocity bins, but they just didn't include it into the report. As a result, the Sally TCR is a more complete analysis than I initially thought. This was a learning experience for me, since there are two different conversion factors you should use for radar derived winds versus winds measured by aircraft. And it also highlights that there is a lot of extra hard work being done behind the scenes, even if it isn't being included in the report.

Thank you for seeking to clarify the matter. In this case I now concur with the NHC’s estimate of 95 kt, given the reasoning being presented.

No problem! Getting to the truth of the matter is most important no matter the situation.

And I agree, 95 kt is the best estimate at this point. Definitely a borderline case, but an intensity estimate of 100 kt would be more tenuous relative to 95 kt.

[ncforecaster89]

This was a good suggestion! I reached out to Robbie and he responded promptly to me.

The most interesting tidbit from his email to me was that the typical 80% reduction factor at 1200 meters, where those 121-122 kt velocity bins were found, accounts for undersampling by aircraft measurements (which is up to 8% as he wrote in the TCR). With radial velocity measurements though, there is little to no undersampling, so the factor should actually be even lower compared to the standard 80% conversion for a height of ~1200 meters. He also mentioned the importance of averaging multiple bins (i.e. a 4 bin average) in order to filter out noise among the velocity bins, which can impact the analysis as well. He concluded by saying the balance of evidence (including the radar derived wind data) led to an estimate of 95 kt overall.

So they did analyze the WSR-88D velocity bins, but they just didn't include it into the report. As a result, the Sally TCR is a more complete analysis than I initially thought. This was a learning experience for me, since there are two different conversion factors you should use for radar derived winds versus winds measured by aircraft. And it also highlights that there is a lot of extra hard work being done behind the scenes, even if it isn't being included in the report.

Thank you for seeking to clarify the matter. In this case I now concur with the NHC’s estimate of 95 kt, given the reasoning being presented.

No problem! Getting to the truth of the matter is most important no matter the situation.

And I agree, 95 kt is the best estimate at this point. Definitely a borderline case, but an intensity estimate of 100 kt would be more tenuous relative to 95 kt.

To be completely honest, I’d lean towards 100 kt based on the 98 kt sustained wind measured 3 nm inland of Orange Beach at the height of 18 m. Although that reading corresponds to roughly 95 kt at 10m, it’s highly unlikely that it was positioned in the perfect location to measure the absolute peak wind anywhere within the eyewall. It supports the 700 mb FLW to surface estimate of 100 kt...showing the winds were most certainly mixing down to the surface effectively.

As we’ve discussed, it’s about as close to a borderline case as there could be, and as a result, relies on a bit of subjectivity. In these types of cases, I’m inclined to err on the high-end than assume the lower end, when there’s sufficient data supportive of that intensity.

[TorSkk]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

[ElectricStorm]

That makes zero sense to me they're the experts.

Anyway that guarantees Eta and Laura aren't getting an upgrade. I didn't expect them to anyway tho.

[TorSkk]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

Peak Intensity
Iota’s estimated peak intensity of 135 kt at 1200 UTC 16 November is based on a blend
of a 1218 UTC reconnaissance aircraft 700-mb flight-level wind speed of 147 kt, which results in
an equivalent surface wind speed estimate of 132 kt, and an SFMR surface wind speed estimate
of 140 kt. Although there was a subsequent SFMR wind speed estimate of 143 kt observed at
1324 UTC, that value was associated with a 700-mb flight-level wind speed of only 129 kt (116-
kt equivalent surface wind speed). Furthermore, that SFMR wind estimate was obtained from the
southern semicircle (i.e., left side), which is typically the weakest portion of a westward-moving
hurricane such as Iota. Due to the small RMW of approximately 8 n mi, significant undersampling
of Iota’s peak winds was not considered to be a factor.

Operationally, Iota was assessed as a 140-kt category 5 hurricane. However, ongoing
research suggests there is a high bias of SFMR wind speeds in these high wind regimes, and the
final best track peak intensity was adjusted downward by 5 kt, closer to the standard reduction of
the peak flight level winds. While a 5-kt change is typical for post-analysis best track intensity
changes, this decrease in Iota’s peak intensity crosses the threshold from Category 5 to Category
4. This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Also, the sampling of SFMR winds near coastlines or sharp ocean current gradients where
significant breaking waves occur can result in a high bias of SFMR wind speed estimates (Uhlhorn
and Black 2003, p. 102). The interaction of Iota’s wind field with the islands of Providencia and
Santa Catalina likely caused complex wave interactions to the lee or west side of the archipelago,
producing constructive interference, wave steepening, and subsequent breaking waves. These
breaking waves could have resulted in some high bias in the SFMR surface winds in that area.

[AnnularCane]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

What??? Well...I guess they have their reasons.

But to me she'll always be a 5.

[TorSkk]

This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Maybe we are going to see a reanalysis of recent high-end storms sometime in the future? If they "recalibrate" the SFMR readings it would certainly change some peaks

[cainjamin]

This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Maybe we are going to see a reanalysis of recent high-end storms sometime in the future? If they "recalibrate" the SFMR readings it would certainly change some peaks

Matthew is definitely the most borderline case out of the other storms mentioned, and I don't think it would have been operationally upgraded to a 5 if the NHC was aware of SFMR's high bias in 2015. Irma was knocked back by 5kt as well in its TCR (and maybe Dorian too? I can't remember). Either way, it does seem clear that the NHC is going to be taking those high SFMR measurements with a grain of salt for the foreseeable future.

Hopefully we get some powerful fish storms this season that the NHC can do some high-end SFMR research in. The more data they can get their hands on the more reliable intensity estimates will be.

[NotoSans]

Perhaps fun to note that Iota’s report was written by Stewart, who bumped Matthew’s intensity up to 145kt based solely on SFMR data back in 2016.

Indicates a significant shift in attitude towards SFMR winds by the NHC.

[CrazyC83]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

Peak Intensity
Iota’s estimated peak intensity of 135 kt at 1200 UTC 16 November is based on a blend
of a 1218 UTC reconnaissance aircraft 700-mb flight-level wind speed of 147 kt, which results in
an equivalent surface wind speed estimate of 132 kt, and an SFMR surface wind speed estimate
of 140 kt. Although there was a subsequent SFMR wind speed estimate of 143 kt observed at
1324 UTC, that value was associated with a 700-mb flight-level wind speed of only 129 kt (116-
kt equivalent surface wind speed). Furthermore, that SFMR wind estimate was obtained from the
southern semicircle (i.e., left side), which is typically the weakest portion of a westward-moving
hurricane such as Iota. Due to the small RMW of approximately 8 n mi, significant undersampling
of Iota’s peak winds was not considered to be a factor.

Operationally, Iota was assessed as a 140-kt category 5 hurricane. However, ongoing
research suggests there is a high bias of SFMR wind speeds in these high wind regimes, and the
final best track peak intensity was adjusted downward by 5 kt, closer to the standard reduction of
the peak flight level winds. While a 5-kt change is typical for post-analysis best track intensity
changes, this decrease in Iota’s peak intensity crosses the threshold from Category 5 to Category
4. This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Also, the sampling of SFMR winds near coastlines or sharp ocean current gradients where
significant breaking waves occur can result in a high bias of SFMR wind speed estimates (Uhlhorn
and Black 2003, p. 102). The interaction of Iota’s wind field with the islands of Providencia and
Santa Catalina likely caused complex wave interactions to the lee or west side of the archipelago,
producing constructive interference, wave steepening, and subsequent breaking waves. These
breaking waves could have resulted in some high bias in the SFMR surface winds in that area.

That does make some sense when looking through the data. That 143 definitely should be discarded, but the 140 is worth investigating further. I'd have - for now - kept the operational 140 kt with a high degree of uncertainty, but made a note as well about likely changes, mainly because Dvorak and the P-W relationship also supported 140.

Regardless, I think 125 kt might be a bit generous for the landfall intensity of Iota. There was clear weakening at that point and the data at 00Z probably supported 125 kt (142 FL/116 SFMR). The satellite signature in the last 4 hours did deteriorate some more. I'd have assessed it at 115 kt with a pressure of 928 mb.

[CrazyC83]

Perhaps fun to note that Iota’s report was written by Stewart, who bumped Matthew’s intensity up to 145kt based solely on SFMR data back in 2016.

Indicates a significant shift in attitude towards SFMR winds by the NHC.

In 2016, the views of the SFMR running way high were not yet seen as an issue. They updated the system I believe in 2014, and the only storms that such could have been an issue in 2015 (Patricia and Joaquin) had SFMR largely as expected with the flight-level winds in intense storms.

[aspen]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

This is a massive bruh moment. Like, it makes ZERO sense in the context of Matthew (upped to 145 kt entirely on SFMR) and Zeta (upped to 100 kt in the same season). Yet another addition to 2020’s multiple controversial post-season decisions, from not retiring storms like Isaias and Sally to keeping Sally as a Cat 2, although that makes more sense.

2020 really shouldn’t have given up after Iota if it wanted to continue the Cat 5 streak, and with Iota getting the boot, Matthew might too.

[aspen]

This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Maybe we are going to see a reanalysis of recent high-end storms sometime in the future? If they "recalibrate" the SFMR readings it would certainly change some peaks

Matthew is definitely the most borderline case out of the other storms mentioned, and I don't think it would have been operationally upgraded to a 5 if the NHC was aware of SFMR's high bias in 2015. Irma was knocked back by 5kt as well in its TCR (and maybe Dorian too? I can't remember). Either way, it does seem clear that the NHC is going to be taking those high SFMR measurements with a grain of salt for the foreseeable future.

Hopefully we get some powerful fish storms this season that the NHC can do some high-end SFMR research in. The more data they can get their hands on the more reliable intensity estimates will be.

No, Dorian was kept at 160 kt. Even non-contaminated SFMR readings were getting to 165-170 kt at times, and dropsondes showed winds were mixing down pretty well.

[aspen]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

Peak Intensity
Iota’s estimated peak intensity of 135 kt at 1200 UTC 16 November is based on a blend
of a 1218 UTC reconnaissance aircraft 700-mb flight-level wind speed of 147 kt, which results in
an equivalent surface wind speed estimate of 132 kt, and an SFMR surface wind speed estimate
of 140 kt. Although there was a subsequent SFMR wind speed estimate of 143 kt observed at
1324 UTC, that value was associated with a 700-mb flight-level wind speed of only 129 kt (116-
kt equivalent surface wind speed). Furthermore, that SFMR wind estimate was obtained from the
southern semicircle (i.e., left side), which is typically the weakest portion of a westward-moving
hurricane such as Iota. Due to the small RMW of approximately 8 n mi, significant undersampling
of Iota’s peak winds was not considered to be a factor.

Operationally, Iota was assessed as a 140-kt category 5 hurricane. However, ongoing
research suggests there is a high bias of SFMR wind speeds in these high wind regimes, and the
final best track peak intensity was adjusted downward by 5 kt, closer to the standard reduction of
the peak flight level winds. While a 5-kt change is typical for post-analysis best track intensity
changes, this decrease in Iota’s peak intensity crosses the threshold from Category 5 to Category
4. This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Also, the sampling of SFMR winds near coastlines or sharp ocean current gradients where
significant breaking waves occur can result in a high bias of SFMR wind speed estimates (Uhlhorn
and Black 2003, p. 102). The interaction of Iota’s wind field with the islands of Providencia and
Santa Catalina likely caused complex wave interactions to the lee or west side of the archipelago,
producing constructive interference, wave steepening, and subsequent breaking waves. These
breaking waves could have resulted in some high bias in the SFMR surface winds in that area.

The wording suggests Iota, as well as Matthew, Irma, Jose, Maria, and Dorian, could all get re-re-analyses after more SFMR research is done. I think they said something similar about perhaps needing to revise Dorian’s peak in its TCR.

[Shell Mound]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

Peak Intensity
Iota’s estimated peak intensity of 135 kt at 1200 UTC 16 November is based on a blend
of a 1218 UTC reconnaissance aircraft 700-mb flight-level wind speed of 147 kt, which results in
an equivalent surface wind speed estimate of 132 kt, and an SFMR surface wind speed estimate
of 140 kt. Although there was a subsequent SFMR wind speed estimate of 143 kt observed at
1324 UTC, that value was associated with a 700-mb flight-level wind speed of only 129 kt (116-
kt equivalent surface wind speed). Furthermore, that SFMR wind estimate was obtained from the
southern semicircle (i.e., left side), which is typically the weakest portion of a westward-moving
hurricane such as Iota. Due to the small RMW of approximately 8 n mi, significant undersampling
of Iota’s peak winds was not considered to be a factor.

Operationally, Iota was assessed as a 140-kt category 5 hurricane. However, ongoing
research suggests there is a high bias of SFMR wind speeds in these high wind regimes, and the
final best track peak intensity was adjusted downward by 5 kt, closer to the standard reduction of
the peak flight level winds. While a 5-kt change is typical for post-analysis best track intensity
changes, this decrease in Iota’s peak intensity crosses the threshold from Category 5 to Category
4. This change is well within the typical range of uncertainty in NHC’s post-storm intensity
analyses, but the discrepancy between dropsonde-based reductions of peak flight-level winds
and SFMR-derived surface winds leads to greater-than-normal uncertainty in the peak intensity
of Iota, as has been noted in previous intense hurricanes such as Matthew in 2016, Irma, Jose,
and Maria in 2017, and Dorian in 2019. Future adjustments may be needed to Iota’s estimated
peak intensity once SFMR data at high wind speeds are recalibrated.

Also, the sampling of SFMR winds near coastlines or sharp ocean current gradients where
significant breaking waves occur can result in a high bias of SFMR wind speed estimates (Uhlhorn
and Black 2003, p. 102). The interaction of Iota’s wind field with the islands of Providencia and
Santa Catalina likely caused complex wave interactions to the lee or west side of the archipelago,
producing constructive interference, wave steepening, and subsequent breaking waves. These
breaking waves could have resulted in some high bias in the SFMR surface winds in that area.

That does make some sense when looking through the data. That 143 definitely should be discarded, but the 140 is worth investigating further. I'd have - for now - kept the operational 140 kt with a high degree of uncertainty, but made a note as well about likely changes, mainly because Dvorak and the P-W relationship also supported 140.

Regardless, I think 125 kt might be a bit generous for the landfall intensity of Iota. There was clear weakening at that point and the data at 00Z probably supported 125 kt (142 FL/116 SFMR). The satellite signature in the last 4 hours did deteriorate some more. I'd have assessed it at 115 kt with a pressure of 928 mb.

On the other hand, the images from Haulover, Nicaragua (cf. pp. 43–4 in the TCR), do indicate extremely severe wind damage to coconut palms and structures. The eye made landfall just south of Haulover, so that location clearly experienced the strongest winds. Note that numerous, mature palms were not only shredded, but also physically snapped, while deciduous trees and other vegetation were stripped of their leaves and shorn of their branches. The damage is consistent with that observed in some high-end tropical cyclones like Haiyan and Dorian, though not quite at their level of intensity. Matthew produced very similar damage during its 130-kt landfall in Haiti, however. I think the 125-kt intensity at landfall is reasonable, taking into account contextual indicators as well as uncertainty and extrapolation.

On a related note:

 https://twitter.com/philklotzbach/status/1394748236443832320

[ElectricStorm]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

This is a massive bruh moment. Like, it makes ZERO sense in the context of Matthew (upped to 145 kt entirely on SFMR) and Zeta (upped to 100 kt in the same season). Yet another addition to 2020’s multiple controversial post-season decisions, from not retiring storms like Isaias and Sally to keeping Sally as a Cat 2, although that makes more sense.

2020 really shouldn’t have given up after Iota if it wanted to continue the Cat 5 streak, and with Iota getting the boot, Matthew might too.

Matthew should be downgraded, like right now. There's no way it was stronger than Iota. Which means the Cat 5 streak in reality is likely 3 years, which while still impressive, it's much less so than a 5 year streak

[aspen]

Iota is out and downgraded to cat 4, 135 kt

https://www.nhc.noaa.gov/data/tcr/AL312020_Iota.pdf

This is a massive bruh moment. Like, it makes ZERO sense in the context of Matthew (upped to 145 kt entirely on SFMR) and Zeta (upped to 100 kt in the same season). Yet another addition to 2020’s multiple controversial post-season decisions, from not retiring storms like Isaias and Sally to keeping Sally as a Cat 2, although that makes more sense.

2020 really shouldn’t have given up after Iota if it wanted to continue the Cat 5 streak, and with Iota getting the boot, Matthew might too.

Matthew should be downgraded, like right now. There's no way it was stronger than Iota. Which means the Cat 5 streak in reality is likely 3 years, which while still impressive, it's much less so than a 5 year streak

If the NHC is now so conservative about SFMR in post-season analysis, could they potentially drop Laura and/or Eta to 125 kt? Could Michael also be at risk for a downgrade?

[CrazyC83]

This is a massive bruh moment. Like, it makes ZERO sense in the context of Matthew (upped to 145 kt entirely on SFMR) and Zeta (upped to 100 kt in the same season). Yet another addition to 2020’s multiple controversial post-season decisions, from not retiring storms like Isaias and Sally to keeping Sally as a Cat 2, although that makes more sense.

2020 really shouldn’t have given up after Iota if it wanted to continue the Cat 5 streak, and with Iota getting the boot, Matthew might too.

Matthew should be downgraded, like right now. There's no way it was stronger than Iota. Which means the Cat 5 streak in reality is likely 3 years, which while still impressive, it's much less so than a 5 year streak

https://i.imgur.com/8WU7074.png
If the NHC is now so conservative about SFMR in post-season analysis, could they potentially drop Laura and/or Eta to 125 kt? Could Michael also be at risk for a downgrade?

Michael, if anything, could get upgraded to 145 kt as that is what radar data supports.

Laura I think stays at 130 or goes to 135 as there was also a flight-level reading of 148 kt (supports 133 at the surface). Eta will be a tough one with very limited data.

[Shell Mound]

Matthew should be downgraded, like right now. There's no way it was stronger than Iota. Which means the Cat 5 streak in reality is likely 3 years, which while still impressive, it's much less so than a 5 year streak

https://i.imgur.com/8WU7074.png
If the NHC is now so conservative about SFMR in post-season analysis, could they potentially drop Laura and/or Eta to 125 kt? Could Michael also be at risk for a downgrade?

Michael, if anything, could get upgraded to 145 kt as that is what radar data supports.

Laura I think stays at 130 or goes to 135 as there was also a flight-level reading of 148 kt (supports 133 at the surface). Eta will be a tough one with very limited data.

How reliable are radar data? For example, what were Doppler-derived velocities over Golden Meadow, LA, at the time of its 82-kt reading during Zeta?

[Meteophile]

Well everyone should stop shouting about categories.

That said, this 5kts downgrade is... Surprising. I thought i would learn about this storm and the measurement techniques reading the official nhc paper, but i'd say it seems pretty poor. I'm not a specialist so i may sh*t up, but if "high bias" means "more uncertainty" and not "overestimate" (sorry i don't even know this my english level is not that good), then why taking the low estimate and not the average estimate ? I mean: reliable numbers and measurements = 153-157kts so 155kts / unreliable = 135-155kts so 145kts. What i have understood from the nhc is that 153-157 - > 153 and 135-155 - > 135. (correct me if you have understood what i'm talking about and if i'm wrong)

Btw, my personal intensity estimate would be closer to 145kts for this storm (datas, wind damage even after the storm's weakening, etc...) but again i'm not a meteorologist.