ATL: NANA - Remnants - Discussion

[CrazyC83]

How comfortable would the NHC be with a post-storm downgrade? Looking at the evidence:

* The 62 kt SFMR was likely valid. However, that would be just below hurricane force.

* The 72 kt winds at 700 were much higher than the winds at 850 at the same location. The SFMR - would be reliable as the water is very deep there - was only in the low 50s at the same time. I'd suggest that reading is probably too high.

* The surface observation at Carrie Bow Cay of 53 kt, if 10-min sustained, would translate to 59 kt. That would validate the SFMR reading.

* The pressure was likely steady state in the hours leading up to landfall. My estimate at landfall is 993 mb, based on the reading (adjusted) at Carrie Bow.

Personally, I would set the intensity - both peak and landfall - at 60 kt. However, I have a hard time imagining the NHC downgrading given the standard level of uncertainty. A post-storm downgrade is very rare.

According to the NHC, the observation at Carrie Bow Cay was a one-minute sustained wind at 10 m. If I recall correctly, until recently the NHC would have classified a storm on the basis of contextual information, e.g., whether flight-level (700-mb) winds correlated closely with winds observed just below that level and/or nearby, taking into account SFMR biases, sondes, radar data (whether the winds were contaminated by rainfall, were in rain-free areas, matched ASCAT and surface data in situ, including ship, land, and buoy, taking into account density of coverage...), et al. as well. Now the NHC seems to be basing estimates of surface winds on the averaged peak 10-s observations from reconnaissance-based FL and SFMR data, without really taking into account other factors such as pressure trends, convective and/or structural organisation, surface vs. aircraft observations, discrepancies between FL and lower-level and/or SFMR-derived winds, etc. Five or ten years ago the NHC likely would have looked at the data you mentioned, taken into account context, and went with estimated peak winds of 55 (maybe 60) knots instead of 65 knots. Furthermore, reports from the area of landfall in Belize showed only minor roof damage to a relative handful of structures in Dangriga, Silk Grass, and Hopkins. A strong but compact tropical storm could easily cause similar damage, given the comparatively poor standards of construction in rural Belize vs., say, Bermuda or the Bahamas. Personally, I think this was likely 55 knots/993–5 mb around the time of landfall in Belize, since Carrie Bow Cay likely experienced the RMW and experienced the strongest convection on the NNW side of a WSW-moving TC. I normally don’t disagree with the NHC, but I am puzzled as to why this was considered a Cat-1.

That all tends to support a 55 or 60 kt intensity then, since Carrie Bow Cay had good exposure and was in the RMW, although it's possible it was not the *absolute* maximum winds. I do think Nana was not a hurricane since none of the other data conclusively supported such. If I wrote the TCR, I'd give Nana a post-season downgrade, but will the NHC?

[Shell Mound]

How comfortable would the NHC be with a post-storm downgrade? Looking at the evidence:

* The 62 kt SFMR was likely valid. However, that would be just below hurricane force.

* The 72 kt winds at 700 were much higher than the winds at 850 at the same location. The SFMR - would be reliable as the water is very deep there - was only in the low 50s at the same time. I'd suggest that reading is probably too high.

* The surface observation at Carrie Bow Cay of 53 kt, if 10-min sustained, would translate to 59 kt. That would validate the SFMR reading.

* The pressure was likely steady state in the hours leading up to landfall. My estimate at landfall is 993 mb, based on the reading (adjusted) at Carrie Bow.

Personally, I would set the intensity - both peak and landfall - at 60 kt. However, I have a hard time imagining the NHC downgrading given the standard level of uncertainty. A post-storm downgrade is very rare.

According to the NHC, the observation at Carrie Bow Cay was a one-minute sustained wind at 10 m. If I recall correctly, until recently the NHC would have classified a storm on the basis of contextual information, e.g., whether flight-level (700-mb) winds correlated closely with winds observed just below that level and/or nearby, taking into account SFMR biases, sondes, radar data (whether the winds were contaminated by rainfall, were in rain-free areas, matched ASCAT and surface data in situ, including ship, land, and buoy, taking into account density of coverage...), et al. as well. Now the NHC seems to be basing estimates of surface winds on the averaged peak 10-s observations from reconnaissance-based FL and SFMR data, without really taking into account other factors such as pressure trends, convective and/or structural organisation, surface vs. aircraft observations, discrepancies between FL and lower-level and/or SFMR-derived winds, etc. Five or ten years ago the NHC likely would have looked at the data you mentioned, taken into account context, and went with estimated peak winds of 55 (maybe 60) knots instead of 65 knots. Furthermore, reports from the area of landfall in Belize showed only minor roof damage to a relative handful of structures in Dangriga, Silk Grass, and Hopkins. A strong but compact tropical storm could easily cause similar damage, given the comparatively poor standards of construction in rural Belize vs., say, Bermuda or the Bahamas. Personally, I think this was likely 55 knots/993–5 mb around the time of landfall in Belize, since Carrie Bow Cay likely experienced the RMW and experienced the strongest convection on the NNW side of a WSW-moving TC. I normally don’t disagree with the NHC, but I am puzzled as to why this was considered a Cat-1.

That all tends to support a 55 or 60 kt intensity then, since Carrie Bow Cay had good exposure and was in the RMW, although it's possible it was not the *absolute* maximum winds. I do think Nana was not a hurricane since none of the other data conclusively supported such. If I wrote the TCR, I'd give Nana a post-season downgrade, but will the NHC?

Have you considered emailing the forecasters who handled Nana and presenting some of these issues with the data (respectfully, of course)? Normally I find that the NHC does fantastic work, but for once I feel rather strongly about this, and, as you stated, Nana is as strong a candidate for a downgrade as any hurricane yet observed. I think careful analysis will find that, on balance, the evidence supporting a peak of 55 to 60 knots is well substantiated—being far more substantial than the evidence supporting hurricane status.

[Shell Mound]

According to the NHC, the observation at Carrie Bow Cay was a one-minute sustained wind at 10 m. If I recall correctly, until recently the NHC would have classified a storm on the basis of contextual information, e.g., whether flight-level (700-mb) winds correlated closely with winds observed just below that level and/or nearby, taking into account SFMR biases, sondes, radar data (whether the winds were contaminated by rainfall, were in rain-free areas, matched ASCAT and surface data in situ, including ship, land, and buoy, taking into account density of coverage...), et al. as well. Now the NHC seems to be basing estimates of surface winds on the averaged peak 10-s observations from reconnaissance-based FL and SFMR data, without really taking into account other factors such as pressure trends, convective and/or structural organisation, surface vs. aircraft observations, discrepancies between FL and lower-level and/or SFMR-derived winds, etc. Five or ten years ago the NHC likely would have looked at the data you mentioned, taken into account context, and went with estimated peak winds of 55 (maybe 60) knots instead of 65 knots. Furthermore, reports from the area of landfall in Belize showed only minor roof damage to a relative handful of structures in Dangriga, Silk Grass, and Hopkins. A strong but compact tropical storm could easily cause similar damage, given the comparatively poor standards of construction in rural Belize vs., say, Bermuda or the Bahamas. Personally, I think this was likely 55 knots/993–5 mb around the time of landfall in Belize, since Carrie Bow Cay likely experienced the RMW and experienced the strongest convection on the NNW side of a WSW-moving TC. I normally don’t disagree with the NHC, but I am puzzled as to why this was considered a Cat-1.

That all tends to support a 55 or 60 kt intensity then, since Carrie Bow Cay had good exposure and was in the RMW, although it's possible it was not the *absolute* maximum winds. I do think Nana was not a hurricane since none of the other data conclusively supported such. If I wrote the TCR, I'd give Nana a post-season downgrade, but will the NHC?

Have you considered emailing the forecasters who handled Nana and presenting some of these issues with the data (respectfully, of course)? Normally I find that the NHC does fantastic work, but for once I feel rather strongly about this, and, as you stated, Nana is as strong a candidate for a downgrade as any hurricane yet observed. I think careful analysis will find that, on balance, the evidence supporting a peak of 55 to 60 knots is well substantiated—being far more substantial than the evidence supporting hurricane status.

Bump...