"Tropical Storm" Isabel Assessment - VA Climatolo

[wxman57]

Good reading from the Virginia State Climatology office on "Tropical Storm" Isabel. They used the term Tropical Storm Isabel because there was no evidence of hurricane-force sustained wind in Virginia:

http://climate.virginia.edu/advisory/2003/ad03-05.htm

As a side note, I talked with one of the foremost experts on wind damage in the country (with Haag engineering) a few days ago about their assessment of Isabel's winds on the NC coast. I told him that a number of people online were convinced they went through a near Cat 3 hurricane on the Outer Banks. His reaction was a thumbs-down. I asked him what he meant and he said that he only saw a few indications of 75 mph sustained winds based on post-storm damage analysis. Most of the severe damage was caused more by poor building practices than by higher winds. He said that Isabel appeared to barely produce hurricane-force winds, and that most of the damage in NC was from sub-75 mph wind.

[ncweatherwizard]

I tend to agree with that. For instance, at my home about 30 miles northwest of Elizabeth City, I recorded a maximum sustained wind of only 41 mph, with a highest gust of 65mph. Of course, that is a different avenue of damage; trees down because of very wet earth, as compared to damage along the coast. Really Isabel was not that strong; there was just so "play it up" going on because it was at one time a Cat. 5, and that mindset continues well after the hurricane passed.

I think I said that right.

[Stormsfury]

I agree and disagree at the same time.

Here's my reasoning.

Yes, sustained winds of 75 mph or higher were far and few between to find. I'm sure more analyses and reanalyses will be forthcoming.

However, IMO, this didn't reduce the potential for strong, quicker, and sharper damaging wind gusts much higher than the usual sustained/gust couplet you'd normally see in a storm with a well-established ring of convection around its core. With the dry air entrainment and this kind of setup resembled a lot like microbursting.

Isabel fought like hell basically to sustain convection around it core with a mixture of dry air entrainment and semi-hostile conditions. The fly in the ointment was the fact of how well the wind field ALOFT was established. Under normal circumstances, with sustainable deep convection, Isabel's flight level winds supported a CAT 3 storm. However, one must reduce that wind flow down to 70% with a storm with a less than solid core of deep convection. Even more so, without sustainable deep convection and frictional forces of land reduced the sustained winds even more.

Another point was the fact of the very strong high to its NE which kept Isabel from moving out to sea and provided a strong pressure gradient on its Eastern and Northeastern Quads well away from the center.

Isabel was bad ... it could have been a lot worse.

SF

[wxman57]

I agree and disagree at the same time.

Here's my reasoning.

Yes, sustained winds of 75 mph or higher were far and few between to find. I'm sure more analyses and reanalyses will be forthcoming.

However, IMO, this didn't reduce the potential for strong, quicker, and sharper damaging wind gusts much higher than the usual sustained/gust couplet you'd normally see in a storm with a well-established ring of convection around its core. With the dry air entrainment and this kind of setup resembled a lot like microbursting.

Isabel fought like hell basically to sustain convection around it core with a mixture of dry air entrainment and semi-hostile conditions. The fly in the ointment was the fact of how well the wind field ALOFT was established. Under normal circumstances, with sustainable deep convection, Isabel's flight level winds supported a CAT 3 storm. However, one must reduce that wind flow down to 70% with a storm with a less than solid core of deep convection. Even more so, without sustainable deep convection and frictional forces of land reduced the sustained winds even more.

Another point was the fact of the very strong high to its NE which kept Isabel from moving out to sea and provided a strong pressure gradient on its Eastern and Northeastern Quads well away from the center.

Isabel was bad ... it could have been a lot worse.

SF

Good points about the spot downbursts of higher winds. Isabel's winds aloft didn't transport down to the surface as well as with most storms, but there was the potential for higher gusts than with a typical hurricane with those sustained winds. With Lili last year, the vast majority of wind instruments along its path measured about 40-50 mph winds, but there were gusts nearly double that or even higher.

It's amazing that even a relatively weak hurricane can have so much power. Imagine if Isabel hit the same area as it had been a few days earlier - with 120-135 kt winds ALL AROUND the center and gusts 30-40 kts above that. That would have easily produced 4-6 times the wind force, leaving not much standing.

[Anonymous]

It's amazing that even a relatively weak hurricane can have so much power. Imagine if Isabel hit the same area as it had been a few days earlier - with 120-135 kt winds ALL AROUND the center and gusts 30-40 kts above that. That would have easily produced 4-6 times the wind force, leaving not much standing.

I hope the Mid-Atlantic never sees one like that. It would have been devastating. But I think it is a mistake to call Isabel (at landfall) "relatively weak" though. There are plenty of Cat 1 and Cat 2 hurricanes that have been much smaller in size with weaker winds aloft than Isabel and smaller less-widespread storm surges. If one of these types of storms had come ashore instead of Isabel the damage would have been much less and much less widespread. If Isabel had come in as a tiny-sized Cat 3 storm would it have caused more damage or less? If the answer is less then perhaps the current scale of measurement is not adequate. The current scale does not account for the size of the storm, the wind speeds aloft or the pressure -- so calling Isabel weak simply because it did not have 100 mph sustained winds at the surface seems wrong to me. I guess it depends on what your a defining as weak. Was Isabel a weak storm? I don't think so. Were the winds of Isabel at the surface weak for a Hurricane? Yes, I could buy that argument.

[george_r_1961]

Well I live in Newport News, and while winds may not have been sustained at hurricane force there were certainly enough frequent giusts above 74 mph. In fact a certified instrument in Hampton, a neighboring city, recorded a peak wind gust of 93 mph. Im guessstimating sustained winds here were around 50-60 for about 2 hours.

[wxman57]

It's amazing that even a relatively weak hurricane can have so much power. Imagine if Isabel hit the same area as it had been a few days earlier - with 120-135 kt winds ALL AROUND the center and gusts 30-40 kts above that. That would have easily produced 4-6 times the wind force, leaving not much standing.

I hope the Mid-Atlantic never sees one like that. It would have been devastating. But I think it is a mistake to call Isabel (at landfall) "relatively weak" though. There are plenty of Cat 1 and Cat 2 hurricanes that have been much smaller in size with weaker winds aloft than Isabel and smaller less-widespread storm surges. If one of these types of storms had come ashore instead of Isabel the damage would have been much less and much less widespread. If Isabel had come in as a tiny-sized Cat 3 storm would it have caused more damage or less? If the answer is less then perhaps the current scale of measurement is not adequate. The current scale does not account for the size of the storm, the wind speeds aloft or the pressure -- so calling Isabel weak simply because it did not have 100 mph sustained winds at the surface seems wrong to me. I guess it depends on what your a defining as weak. Was Isabel a weak storm? I don't think so. Were the winds of Isabel at the surface weak for a Hurricane? Yes, I could buy that argument.

The key word was <b>relatively</b>. When Isabel's at landfall were closer to 8 times weaker (in terms of force/area) than when it was at its peak, and about 4 times weaker than a solid Cat 3. But Isabel was by no means weak, it was probably the strongest storm to hit that area in over 100 years, as most stronger storms only grazed NC/VA with their weaker sides.