Zeta Hurricane rapid ascent to near Category 3 strength at landfall in Louisiana was caused in part by the very weather system that was supposed to tear it apart, a senior forecaster at the National Hurricane Center said Thursday.
The strong trough of low pressure produced ice storms in Oklahoma and Texas on Tuesday and Wednesday while en route to the Gulf of Mexico. There, its high winds in the upper atmosphere were expected to shear Zeta’s clouds and keep it at tropical storm or Category 1 status.
Instead, those winds helped draw warm air from the surface through Zeta’s eye to its top, further reducing the storm’s central air pressure and helping it strengthen, senior hurricane specialist Mike Brennan said.
Zeta took advantage of typical late October environmental conditions in the atmosphere and the waters of the Caribbean Sea and Gulf of Mexico to become a behemoth at landfall Wednesday afternoon near Cocodrie.
After being disrupted by crossing the Yucatan Peninsula on Tuesday morning, Zeta entered warm Gulf waters that afternoon and began reconstituting itself into a hurricane. That night, however, Doppler radar aboard a NOAA G-4 aircraft investigating the storm recorded three-dimensional images that provided the first warning it might go through a process called “rapid intensification,” where its top winds could become significantly stronger, said Frank Marks, Hurricane Research Division director at the National Oceanic and Atmospheric Administration.
“A hurricane can be like a drunken sailor walking down the street, something New Orleans might be familiar with,” Marks said. The tipping of the top of the storm during its zigs and zags can reduce the flow of warm air from the surface to its top, and slow development.
But the radar images showed “alignment,” Marks said: The thunderstorms creating the eye were straight up and down, leaving an empty center with room for the storm to breathe – and intensify.
With plenty of warm water over the central Gulf of Mexico and very little upper level wind shear to disrupt its thunderstorm core, Zeta’s wind speeds increased by 45 mph in the 24 hours before landfall, a classic case of rapid intensification, Brennan said.
And as it neared Louisiana, Zeta’s forward speed increased, to almost 25 mph. That nixed another potential weakener: the cooler water along the state’s coastline. Had the storm been advancing at only 5 mph, it might have mixed cooler water from the Gulf’s depths to the surface, reducing the energy fuel that was keeping it strong, Brennan said.
The forward speed likely also affected the approaching low pressure trough, Brennan said. The trough’s wind shear did catch up with Zeta at landfall, which helped reduce the amount of rainfall on the storm’s west side.
So even though the National Weather Service declared a daily record rainfall at Louis Armstrong International Airport in Kenner, with 2.93 inches on Wednesday, New Orleans’ drainage system was able to keep up with the storm’s quickly moving rain bands despite idling two of the system’s electric turbines.
With sustained winds of 100 mph at landfall, Zeta had almost reached Category 3 strength on the Saffir-Simpson wind scale. A preliminary list of highest wind speeds collected by the Slidell office of the National Weather Service includes:
Orleans Parish, private weather station at lakefront – Sustained winds of 62 mph, a gust of 77 mph
Plaquemines Parish, Belle Chasse Naval Air Station – Sustained winds of 53 mph, a gust of 88 mph
St. Bernard Parish, Bayou Bienvenue lift gate – Sustained winds of 88 mph, a gust of 112 mph.
The highest storm surge levels were measured at Waveland, Mississippi, 8.16 feet above the mean higher high water mark, which is an average of high tides at that location. In Louisiana, the highest surge levels were at:
New Canal in Lake Pontchartrain – 4.84 feet
Bonnet Carré Spillway – 4.72 feet
Port Fourchon – 3.03 feet.
In general, those heights are about two feet below the heights above sea level.
Meteorologist Ben Schott, who oversees the Slidell office of the National Weather Service, said the Zeta forecasts were very accurate for its track, and while the storm’s rapid increase in intensity was a challenge, each of the forecasts issued by his office and the National Hurricane Center warned there was a good chance that Zeta’s winds would be higher by the time it made landfall.
Schott said it’s also too soon to come to conclusions about the amount of damage Zeta caused, pointing to incorrect public early estimates of minimal damage after Category 4 Laura went ashore in August in Cameron Parish.
“Yesterday, we were trying to get the message out that the window for preparing for this storm was short, that it was a definite threat and that it was strengthening not weakening,” he said. He blames the “hurricane fatigue” of residents in south Louisiana for underestimates of Zeta’s strength before landfall.
“Right now, from a single storm, we have over 2 million people with power outages from southeast Louisiana to Birmingham, Alabama, and I would imagine that before the day is over, those power outages will be extending into Georgia,” he said. “Folks who don’t see the worst at their homes always tell you it wasn’t that bad, that it was no big deal. But the family two blocks over [who] lost their roof likely sees it as the worst ever.”
There’s a reason for Louisianan’s hurricane fatigue this year, Brennan said. Some part of the Louisiana coast has been under a tropical storm, hurricane or storm surge watch or warning for a total of 474 hours, almost 20 days.
And Schott warned that the hurricane season doesn’t end until Nov. 30. With another patch of thunderstorm activity in the Caribbean Sea given a 60 percent chance of becoming at least a tropical depression over the next five days, he said, now is not the time to let down your guard.
“This is a La Niña year,” he said, referring to the lower eastern Pacific Ocean sea surface temperatures that are associated with lower upper level wind shear in the Atlantic Ocean and the Caribbean Sea. “And as long as we have warm water, that sets up conditions in the Caribbean and the Atlantic to be conducive of storm formation through December,” he said.
Marks has another concern about this year’s active season: It’s already burning through his division’s budget for next year, since the federal fiscal year began Oct. 1. His division already has flown 51 aerial investigative missions, a record, with many occurring in October.
“Our budget doesn’t automatically increase along with the number of storms, so we’re using up resources that are going to have to protect us through next year,” he said.
Originally published at Nola