Wilma top sustained winds below 85 mph in Dade, Broward

[zlaxier]

Posted on Sat, Oct. 29, 2005

85 mph winds shredded region

Experts debated whether brief microbursts or shoddy construction caused widespread damage during Wilma.

BY JOHN DORSCHNER

jdorschner@herald.com

While Wilma inflicted heavy damage on South Florida, its power wasn't that astounding -- blowing through Miami-Dade and Broward with Category 1 sustained winds generally below 85 mph, according to data released Friday.

That's the preliminary assessment of hurricane scientists, who are just beginning to study a storm that darkened more than 80 percent of South Florida homes, closed schools and many businesses for at least a week and caused brutal devastation in some areas, knocking down power poles, ripping sides off buildings and shattering countless storm-resistant windows.

Even gusts measured by the National Weather Bureau didn't reach higher than 112 mph in Miami-Dade and 108 mph in Broward, according to the preliminary numbers from the National Weather Service.

This contrast between minimal hurricane winds and severe pockets of extreme damage might mean that brief, devastating winds called microbursts touched down in some places.

''It could be microbursts or it could be questionable construction,'' said Sharanya Majumdar, a hurricane researcher at the University of Miami who believes Wilma will eventually be classified a Category 1 as it passed through South Florida. ``Or it may have been a little of both.''

Some experts thought the problem was construction. ''Something failed,'' said Eugenio Santiago, a veteran structural engineer. ``Either it was the designs or the standards. We shouldn't have had this destruction. It looks like Andrew [a Category 5] came through.''

The debate about wind bursts vs. shoddy construction is likely to last months, if not years, as it did after Andrew blasted through in 1992.

A key discussion point: the power poles of Florida Power & Light. Thousands of thick concrete and wooden poles, supposedly designed to withstand winds of 119 miles, collapsed during Wilma.

The utility reported the major collapses of poles occurred in random, isolated areas, such as a stretch along Tamiami Trail near the Miccosukee Reservation, where 48 poles were cracked, and along U.S. 441 near Belle Glade, where 75 poles cracked.

FPL Vice President Geisha Williams said the poles were built ''to the highest standard in the utility industry'' and she believes an unusual force felled the poles. ''I'm very curious,'' she said earlier this week, ``whether there were some tornadoes or microbursts moving through the area.''

She said FPL engineers were starting to study the question, but it could be weeks, if not months, before their report comes in.

Meanwhile, Santiago, a building official for the village of Key Biscayne, said he can't believe that the poles were truly built to withstand 119 mph winds. ''How can you have concrete snapping like that?'' he asked.

Santiago noted state statutes exempt FPL's poles from inspection by local building officials. He wonders, among other things, if the poles were planted deep enough in the ground.

''Now they're trying to play the tornado card?'' Santiago said. ``Did they have tornadoes all over Dade County and nobody saw them?''

Others also suspect tornadoes or other strong, brief winds were at fault. Roberto Smith, the property manager at Broward Financial Centre, a downtown Fort Lauderdale office building, told The Herald he thought a tornado could be responsible for knocking out up to 200 windows at that high rise.

But there is no proof of tornadoes spinning out of Wilma. After the hurricane passed, The Herald asked the Weather Bureau whether it had spotted any tornadoes on its radar. The answer was no.

Still, hurricane scientists are just starting their research, and the lack of tornado sightings doesn't preclude microbursts, winds that can be 175 mph and may only last a second or two.

The concept of microbursts was discovered and developed by Ted Fujita, a University of Chicago wind expert who spent almost a year studying Andrew's effect on South Dade. For 1,500 hours, he pored over 2,000 aerial photos, going block by block, and he discovered random patterns of utter devastation caused by these microbursts.

''Damage is caused by peak winds,'' Fujita said. ''It doesn't matter how long they last.'' Fujita, an internationally renowned wind expert dubbed ''The Tornado Man'' by his peers, died in 1998.

Mark Powell, a hurricane researcher with the National Oceanic and Atmospheric Administration, said the problem with microbursts is that ''they're almost impossible to observe'' because they are so brief. ''There are a lot of other possible things'' that could explain extreme damage.

Among Powell's suspects: high-speed sustained winds pounding at a building, and the difference in wind speed between a lull and a sudden gust. ``The lulls are as important as the gusts, with that going back and forth. Duration is another important factor, and how much wind direction changed.''

Another factor could have been Wilma coming in from the southwest. Most hurricanes move over the flat surface of the ocean and arrive on shore with a relatively steady wind, but Wilma, moving in over Florida's southwest coast and then the Everglades, was hitting structures and dips, creating uneven swirls of wind and perhaps picking up more debris along the way than an Atlantic storm.

In fact, FPL President Armando Olivera said the 241 substations knocked out by the storm suffered primarily because of debris blown on them. ''Why did so much debris fall on them?'' he asked at a Wednesday news conference.

Hurricane scientists are just beginning their studies of Wilma. Powell and others at the Hurricane Research Division are starting to assemble data, and it's possible estimates of wind speed will change because some reporting stations could have been knocked out during the most intense winds.

In the meantime, they are making some preliminary observations:

• By the time it reached South Florida, Wilma was uncommonly dry for a hurricane: It dumped only three-fourths of an inch at Miami International Airport and three inches at the Fort Lauderdale airport.

• The tops of high-rises are more vulnerable than their bases. A Category 2 hurricane at ground level could be a Category 3 by the 25th floor, said Majumdar, the UM scientist. Because the destruction impact is squared, that means a 100 mph wind at ground level could be 44 percent more destructive at the upper level of a high-rise building.

Of course, said Santiago, the structural engineer, high-rises are supposed to be built with that consideration in mind.

Herald staff writers Debbie Cenziper, Patrick Danner, Tim Henderson and Martin Merzer contributed to this report.

[Scorpion]

I agree, although there definitely had to be some parts that got Cat 2 winds. I mean, Wilma was 110 kts at landfall... how could it weaken to a 1 moving so fast over Florida?

[wxman57]

I agree, although there definitely had to be some parts that got Cat 2 winds. I mean, Wilma was 110 kts at landfall... how could it weaken to a 1 moving so fast over Florida?

It's not a question of weakening that fast, it's a matter of marine winds vs. winds over land. The maximum sustained winds in a hurricane are measured over water. They're marine winds, over a generally frictionless environment - basically laminar flow. As soon as the wind field moves over a rough terrain caused by buildings/trees, the wind speed at the surface is reduced by frictional effects. That's why a Cat 1 or Cat 2 hurricane over water may not produce hurricane-rofce sustained wind over land beyond the beaches. It may take a Category 3 hurricane (over water) at landfall to produce Category 1 winds over land for any significant distance inland.

So it's not necessarily weakening of the storm, it's frictional wind reduction over land. Now just because those surface winds are reduced by friction doesn't mean the strong winds just above the surface don't occasionally dip down and produce some very high wind gusts (brief bursts of wind lasting less than 60 seconds). We saw some extreme examples of gustiness with Lili in 2002. While surface sustained winds dropped quickly to only 40-50 mph across south-central LA, gusts reached 100 mph or more - more than twice the sustained wind.

[sponger]

Absolute BS! Western Broward definately saw 90-100 mph. Microbursts wouldnt cause such wide spread damage. A strong hurricane would. I ask again, when was the last time a Cat 1 knocked out 95% of the power?

I know it make for good press, but its a little early for that. The difference between Miami and Northern Broward and Southern Palm Baeach is tremendous.

[Scorpion]

Oh wow, I never really thought of that. Regardless, I was surprised at the ferocity of Cat 1 winds. They are quite powerful and damaging, especially when mixed in with gusts/microbursts of 100+. I can't imagine Cat 3 winds.

[Derek Ortt]

Sponger,

what can I say? The wind readings are not good enough for you I take it because they don't agree with you; thus, they have to be BS

[wxman57]

Absolute BS! Western Broward definately saw 90-100 mph. Microbursts wouldnt cause such wide spread damage. A strong hurricane would. I ask again, when was the last time a Cat 1 knocked out 95% of the power?

I know it make for good press, but its a little early for that. The difference between Miami and Northern Broward and Southern Palm Baeach is tremendous.

You don't seem to comprehend the difference between a sustained wind and a gust. Hurricanes are measured by sustained wind - the average wind speed over a 1 minute time period. A sustained wind of 70-85 mph does not preclude the possibility of gusts over 100 mph. Wind GUSTS of 100-115 mph were observed across south Florida, but SUSTAINED winds were not that high.

[rainydaze]

I have a question....isn't it true that over Florida Wilma and the cold front crashed into each other....couldn't that have had an effect on the winds?In a normal situation when cold air meets warm air isn't there turbulence?

[Aquawind]

Simlar Damage from some Derecho Events..

http://www.spc.noaa.gov/misc/AbtDerecho ... 98page.htm

http://earthbulletin.amnh.org/D/2/

Paul

[Derek Ortt]

the Herald did a wonderful job of misquoting Sharan and equated Miami-Dade as all of south Florida

[brunota2003]

Absolute BS! Western Broward definately saw 90-100 mph. Microbursts wouldnt cause such wide spread damage. A strong hurricane would. I ask again, when was the last time a Cat 1 knocked out 95% of the power?

I know it make for good press, but its a little early for that. The difference between Miami and Northern Broward and Southern Palm Baeach is tremendous.

Olphelia was ONLY a Cat 1, and she knocked out to most of everyone, my county ONLY got TS force winds, and a good 90% of the people lost their power...

[Brent]

60 mph winds can cause widespread power outages.

Maybe now I'll stop hearing people saying "Oh it's only a Cat 1 or 2, I'm not going to do anything". Most people who go through a hurricane don't even experience hurricane force winds.

[thunderchief]

...its the gusts that do alot of the damage, anyway.

[zlaxier]

So do we ever see the maximum sustained winds hit land or are they always at least 1 category lower because of frictional effects of land?

Was Andrew really a Cat 5 over land in sustained winds or were the sustained winds in the 140mph range in Homestead and Cutler Ridge and the real damage were caused by the 175mph-200mph 2-3 second gusts and microbursts? I think I read that the NOAA report on the upgrade said Cat 5 sustained winds were concentrated on a small part of the coastline directly fronting Biscayne Bay.

[Brent]

So do we ever see the maximum sustained winds hit land or are they always at least 1 category lower because of frictional effects of land?

Was Andrew really a Cat 5 over land in sustained winds or were the sustained winds in the 140mph range in Homestead and Cutler Ridge and the real damage were caused by the 175mph-200mph 2-3 second gusts and microbursts? I think I read that the NOAA report on the upgrade said Cat 5 sustained winds were concentrated on a small part of the coastline directly fronting Biscayne Bay.

Correct. Nope the maximum sustained winds will never hit land(unless it's the beachfront with nothing to block the wind).

[bvigal]

Well I must say, Mr. DORSCHNER has redeemed himself from his previous article of misinformation! Hmmm, wonder if he got some feedback on that one??

[curtadams]

So do we ever see the maximum sustained winds hit land or are they always at least 1 category lower because of frictional effects of land?

Was Andrew really a Cat 5 over land in sustained winds or were the sustained winds in the 140mph range in Homestead and Cutler Ridge and the real damage were caused by the 175mph-200mph 2-3 second gusts and microbursts? I think I read that the NOAA report on the upgrade said Cat 5 sustained winds were concentrated on a small part of the coastline directly fronting Biscayne Bay.

The quote from Fujita in the header article says that it's the peak winds, not the sustained, that cause the damage. My limited experience looking at damage photos as part of these windspeed arguments very much agrees. It's astonishing how inconsistent the damage is. Storms tend to get gustier onshore, too, since the friction slows groundlevel winds substantially but has much less effect on higher winds.

[quandary]

On the topic of Mr. Fujita... wasn't his primary area of study tornadoes? With respect to that, wouldn't his statement of maximum winds causing damage be more applicable to tornado damage. Katrina seemed to suggest that sustained winds of hurricane force over the course of hours and hours causes equal devastation to winds of higher categories over a short period of time. Also, a study of Wilma over the Yucatan (possibly the best case of extended periods of sustained winds) would help this.

[quandary]

Derek, you're with the HRD right? Are they going to do another book of sorts about the 2004 and/or 2005 hurricane seasons? I read the book about the 1999 season + Mitch. It was amazing. 2004/2005 have been only more amazing than that period and certainly deserve the depth and accessibility that book provided.

[Recurve]

The Keys got right-front-quad winds coming mostly over water. Damage from surge was the major impact here; bayside surge was like nothing seen in most people's lifetime in the Keys.
Gusts seem to have caused tree damage that was much worse than could be expected from TS sustained winds, but that's what gusts do of course.
Most building damage is to awnings and screen porches, plus damage from trees falling on roofs. Many shingles were blown off roofs in exposed areas. Parts of roofs were blown off, including roof decking on taller houses.
Concrete power poles and their transmission stood up against the winds. Few power poles were broken or blown over in the Upper Keys (don't know about Key West).