EDITOR'S NOTE: Story updated at 8 p.m. Tuesday:
Hurricane Isaac slammed into into Louisiana at 6:45 p.m. Tuesday, more or less right at the mouth of the Mississippi River, driving a wall of water nearly 9 feet high ashore in some areas as it moved slowly but relentlessly in the direction of New Orleans.
The Weather Service’s Storm Prediction center issued a tornado watch through 8 a.m. on Wednesday, stretching from Southeastern Louisiana to the western Florida panhandle, while winds of 75 mph could be felt up to 40 miles from the eye of the storm. Heavy rains have begun pelting a wide stretch of coast, and massive flooding is expected from a combination of rain and storm surge.
Because Isaac is a slow-moving storm, it stands to lash southern Louisiana and parts of Mississippi and Alabama for as long as the next 12-24 hours before moving north, ultimately passing into Arkansas sometime Thursday.
By that time, Isaac's still powerful rains may be seen, by some, as a blessing rather than a curse: as Andrew Revkin reported in his Dot Earth blog, parts of Arkansas were still in a state of extreme drought as of just a week ago, still gripped by the worst dry spell since the 1950's. If the storm stays on it's currently projected course, it should bring heavy rain to other parched states, including Missouri, Illinois, Iowa, and Indiana, by the weekend.
Shortly after noon on Tuesday, the Weather Channel reported that had been upgraded from a tropical storm to a Category 1 hurricane, with sustained winds above 74 mph, hours before its expected landfall in southeastern Louisiana Tuesday night.
Gulf Coast residents have already been bracing for a big, powerful and destructive storm that would bring torrential rains, punishing winds, dangerous coastal and inland floods, and possibly a flurry of tornadoes as well, to Louisiana, Alabama, and Mississippi.
While the official hurricane designation is an important milestone, it doesn’t change the fact that trouble is already in the cards. In anticipation of Isaac’s likely strengthening, hurricane warnings were already in effect from just east of Morgan City, La., to the Alabama-Mississippi border.
The threat is especially great because Isaac is moving at a relatively sluggish 10 mph Rick Knabb, director of the National Hurricane Center, said on Monday that Isaac was “life-threatening” and that the most serious danger from it were storm surges from the huge amounts of water it will bring, and not the strong winds. “A slow-moving, large system poses a lot of problems,” Knabb said.
The longer Isaac lingers over a given area, the longer hurricane-force winds, will batter homes, buildings and other structures. A slow storm also has more time to unload its considerable payload of moisture — 12 inches or more of rain are now forecast for New Orleans and the surrounding area, with up to 20 in some places.
“Low-lying areas inland will be especially dangerous due to flooding rains that can't drain at their normal rate,” said Brian Norcross in his Weather Underground blog. “Do NOT drive through flooded roadways. The water will be moving faster than normal, and it only takes 6 inches of water to move a car.”
Slow motion also means an extended storm surge—the wall of water a storm pushes ahead of it. Forecasters are calling for up to a 9-foot surge in parts of Louisiana, where heavy rain has already started to fall, and up to 6 feet in neighboring Mississippi.
Although it remains in directly in Isaac’s cross-hairs, New Orleans itself hasn’t been ordered to evacuate; the levees that were breached by Katrina have been rebuilt, and they’re expected to hold back a storm surge that could reach 12 feet. (The surge of nearly 28 feet that came along with Katrina was largely responsible for the more than 1,500 deaths the storm caused). But outside the levees, a storm surge of even 6 feet could prove destructive and deadly.
Once Isaac comes ashore, it will quickly weaken into a tropical storm again. That shouldn’t necessarily reassure those along its projected path, however, which currently puts it in northern Indiana by Sunday: last year, Tropical Storm Lee never even achieved hurricane status — but it still cut a memorable swath of destruction from Louisiana all the way up to Pennsylvania and beyond.