Weather Nerd

Isaac: Uncertainty Still Reigns

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Just a quick additional note, as an addendum to my post below. There still remains a remarkable amount of uncertainty regarding the details of Isaac’s track, given that we’re only 24-48 hours from landfall. Check out this “4-panel plot” showing the 48-hour positions of the Euro (top left), GFS (top right), HWRF (bottom left) and GFDL (bottom right) models, courtesy of Dr. Ryan Maue and Weather Bell Models:

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This particular forecast is, as Maue says, a nightmare. There’s just been much more uncertainty than with most hurricanes, and that remains true, even at this late date.

As an aside, it looks like Isaac may be upgraded to a hurricane — or else very, very nearly one — at 5:00 PM Eastern. Stay tuned.

P.S. Dr. Jeff Masters has some excellent analysis on the potential storm surge from Isaac:

Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac’s large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1′ this morning. Conversely, water levels have fallen by 2′ this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac’s counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA’s Hurricane Research Division put the destructive potential of Isaac’s winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs’s storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac’s storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac’s predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5′ to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5′ high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extended pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

I’m a bit wary of this sort of forecast — likely Category 1 hurricane to produce uncommonly devastating surge due to its size! — because similar predictions regarding Irene in 2011 and Ike in 2008 were not fully borne out by the reality of what occurred. However, the analogy to Gustav seems apt in this case. We shall see. Certainly, if you live in a storm surge zone, take no chances. Get the Hell out; go to higher ground.

Dr. Masters also has a good overview of the surge’s likely impact on New Orleans, including a handy map of the newly upgraded levees. Read the whole thing, as they say.