4PM Tuesday 9/12/2023 – A Libyan flood in Derna (also spelled Darnah) killed thousands and authorities say more than 10,000 are missing.
At this hour, exact numbers are hard to come by as large areas are still cut off from rescue workers and reporters. But to put those estimates in perspective, Harvey killed 36 in Harris County.
The Cause: Months of Rain Fall in Day
Torrential rains from Mediterranean storm Daniel burst two dams, wiped out four bridges and flooded large areas of Derna. The flash floods swept homes, bodies and vehicles into the sea while knocking out communications in large parts of the area.
Libya is normally one of the driest countries in the world. The Sahara dominates most of the country’s climate. But the Mediterranean also influences rainfall along the northern coast.
In Al Bayda, about 50 miles west, Libya’s National Meteorological Centre said that 16 inches of rain fell between 10 Sep 8am and 11 Sep 8am, a new rainfall record.
Weather Spark shows that average precipitation in Derna during September amounts to less than an inch. And even in the wettest years, Derna receives at most 4 inches per month.
Weather Spark also indicates that Derna receives rainfall on less than one day in an average September.
Terrain Funneled Floodwater Toward City
Derna lies along the coast of the Mediterranean Sea, at the end of a long, narrow, natural valley, called a wadi, which is dry for much of the year.
As the city was inundated by Daniel, which made landfall in Libya on Sunday night, the wadi funneled rushing water into the center of the city. Riverbanks swelled, bridges were washed out and two dams farther up the wadi burst, adding their waters to the deluge.
Tropical-like storms rarely happen in the Mediterranean, but they do happen occasionally. And when they do, meteorologists call them “Medicanes,” short for Mediterranean hurricanes.
World and Social Media Focus on Flood
YouTube is awash in videos from the Derna flood.
NBC reported that a quarter of the city of 125,000 people was destroyed.
This CBS story shows pictures of the incredible destruction. This flood didn’t invade homes slowly, but leave them standing. It pummeled homes and whole neighborhoods.
Google “flood Derna Libya” to find hundreds of links and the latest news. Stories are being updated almost hourly.
Parallels With Harvey
Perusing many of those stories revealed several parallels with Hurricane Harvey which struck the Houston area in 2017.
Both areas encountered record rainfalls. Harvey dumped far more rain than Daniel. But then Daniel’s rainfall fell over a much drier area where infrastructure was designed to handle smaller amounts. Still, both were records, and both overwhelmed local infrastructure with unexpectedly high volumes.
Both Houston and Derna lay downstream from dams. During Harvey, the SJRA cited fear of failure of the Lake Conroe Dam as the reason for its massive release. In Derna, the dams just failed.
Lack of Warning
During Harvey, downstream residents received no warning that Lake Conroe would open its massive floodgates. Many stepped out of bed in the middle of the night to find themselves in knee deep water. CNN reported that there was no warning that the Derna dams could fail and no order to evacuate. Many who tried to evacuate found themselves cut off by collapsed bridges, washed out roads, and rising water, just as in Houston.
Lack of Maintenance
Aljazeera indicated that lack of maintenance may have played a role in the Derna disaster. The dams that failed hadn’t been maintained for years, even though severe erosion had been reported in scientific journals for more than a decade. Likewise, deferred maintenance played a role in flooding the Lake Houston Area during Harvey. During the drought in 2011, the water level in Lake Houston fell so low that the City of Houston could have removed sediment with dump trucks instead of dredges. But Mayor Anise Parker refused to do so. The accumulated sediment later wound up blocking the San Jacinto West Fork and contributing to the flooding of thousands of homes in the Humble/Kingwood area.
Enforcement of Building Codes
In Libya, political turmoil since 2011 reportedly distracted government from the mundane task of enforcing building codes. As a consequence, many hastily constructed buildings went up without adequate safeguards or supervision, and were swept away during the flood. Sound familiar?
Learning from Disasters
We should learn from the past and others’ mistakes. I wonder how many other communities have been surprised by:
- Record rainfalls
- Dam failures or unexpectedly high dam releases
- Warning systems that failed and left people with no time or way to evacuate
- Impacts of deferred maintenance
- Lax enforcement of building codes
I hope the world learns the lessons of the Derna disaster so that we can compare them to other floods and get smarter about mitigation measures that make a difference.
Posted by Bob Rehak on 9/12/2023
2205 Days since Hurricane Harvey
We have two tropical systems in the Atlantic this morning – Hurricane Lee and Tropical Storm Margo which is projected to intensify into a hurricane by tomorrow.
I love looking at the satellite imagery of earth on the National Hurricane Center website. It lets you verify all the complex descriptions of weather developments. And this morning when I went to look at the satellite imagery of the full Atlantic, I was startled by what appeared to be a pair of eyeballs staring back at me, perfectly centered in the sunrise.
And the cloud formations below them loosely resembled a nose and a frown.
Frankly, it looked a bit spooky to me. A cosmic coincidence? But one that triggered thoughts of Halloween a little more than a month away.
Please share with your kids and grandkids. This could be an interesting way to teach them to keep their eyes on the weather.
Posted by Bob Rehak on 9/11/2023
2204 Days since Hurricane Harvey
Today, September 10 is the statistical peak of hurricane season.
NOAA calculates the peak by looking at the number of storms per day in the last hundred years. And September 10th takes the prize with approximately 95 named storms. That’s almost one per year on this date. You would have about a 5% chance of NOT having a named storm in the Atlantic Basin on September 10th, according to this data.
And true to form, today, the Atlantic has two named storms, Lee and Margot, as I write this. However, neither is anywhere near the Caribbean or Gulf of Mexico.
NOAA hasn’t really been collecting hurricane data back 100 years. The chart above is based on 77 years of data, but “normalized” to 100 years to make the frame of reference more intuitive.
The 77-year period extends from 1944 to 2020, starting at the beginning of the aircraft reconnaissance era.
So Far This Year Vs. Average Season
Usually, by September 9, we’ve had eight named storms in the Atlantic Basin. That would be the H storm. However, this year, we’re already on the M storm. That puts us five storms ahead of the average year.
But wait! This year we also had a bonus storm in January. Only after re-analysis did the National Hurricane Center realize that it should have named the storm. But it didn’t. So, we’re really six storms ahead of the average season so far. And that’s far above the average.
The chart above is updated monthly and does not include the two named storms now in the Atlantic.
Forecast vs. Actual to Date
So, at the mid-point, how does this season compare to predictions? If the second half of the season is anything like the first, we could easily have more named storms and more major hurricanes than predicted by NOAA in its August 10 seasonal update.
Earlier this year, forecasters increased their predictions from a normal season to an above normal season. They predicted 14-21 named storms (winds of 39 mph or greater). We’ve already had 14.
Of those, NOAA said 6-11 could become hurricanes (winds of 74 mph or greater). We’ve had 4 so far (Don, Franklin, Idalia and Lee).
And of those, forecasters said 2-5 could become major hurricanes (winds of 111 mph or greater). We’ve had 3 of those so far (Franklin, Idalia and Lee). Franklin and Idalia reached Category 4 strength. Lee briefly exploded into a Cat 5.
Forecasters have linked storm intensity to sea surface temperatures. When average storms hit very warm waters, they can intensify quickly. And that is exactly what has happened this year.
Here’s a look at sea surface temperature anomalies (departure from normal) around the world.
Virtually everything between Africa and Houston is 1-3 degrees Celsius above normal (1.8 to 5.4 degrees Fahrenheit). Sea surface temperature data is current as of Sept. 9, 2023…just one day before the peak of hurricane season.
Posted by Bob Rehak on September 10, 2023 based on information from NHC.
2203 Days since Hurricane Harvey