Tuesday, August 28, 2012

Outer Bands of Hurricane Isaac Reaching New Orleans...

Some of the outer spiral bands of Hurricane Isaac are spreading into the New Orleans area at this time, as you can see by the above radar image.  This trend will continue, and increase, throughout the day.

"Wait a minute, Rob - Isaac is a Tropical Storm, not a hurricane..." you say?  Wrong.  Isaac is a hurricane, it just hasn't been classified as such by the National Hurricane Center (NHC).  

With satellite presentations like this:

...and 80 mph winds being measured in a recent aircraft pass, surely and undoubtedly the NHC will finally decide to call Isaac what he has been since yesterday on the upcoming 10am advisory, but  I have given up trying to figure out what they are going to do.

The "official" classification is really irrelevant for purposes of our discussion, because if it looks like a pig and smells like a pig... you know the rest.  Where it is not irrelevant, however, is in the realm of public safety and preparedness.  You can have a "Hurricane Warning" up for as many areas as you want for as long as you want, but until the system in question is actually called a "Hurricane" by the government, some people, and in some cases, some county and municipal government agencies, won't act, or won't act in the same way that they would if the system were called a hurricane.  In a situation like this, with a storm surge of potentially disastrous levels likely to take place in the effected areas, I think the classification situation has been handled poorly so far.  

Ok, I'll get off of my soap box now and get back to the particulars on Hurricane Isaac:

The white arrow in the radar image back up at the top of the post shows the rough position of the center of Isaac as of 9am.  This is about 145 nautical miles Southeast of New Orleans.  Isaac has continued to move toward the Northwest at 7 mph for the last several hours, and this general motion is likely to continue today.  If anything, the system may actually slow its forward speed even further later today, which will make an already bad situation (with respect to storm surge, wind duration and flooding) even worse by tonight and early Wednesday.

The one thing that I do agree with the NHC on at this time is the track of the center, which is forecast to pass over or very near New Orleans late tonight or during the pre-dawn hours of Wednesday morning:

With a system as large as Isaac, it is important not to focus solely on the location or path of the center.  At this time, tropical storm force winds extend outward up to 205 miles from the center, and by the time landfall of the center takes place tonight, hurricane force winds will likely extend up to 100 miles or more from the center.

Due to the slow movement of Isaac, tropical storm and hurricane force winds will be felt across the Hurricane Warning area for an extended period of time, possibly 12-18 hours in some locations.  This, coupled with torrential rains and a long duration storm surge will create extremely dangerous, life threatening conditions from the coast of southeast Louisiana, across the New Orleans area, into southern Mississippi and portions of southern Alabama.

Tropical storm force winds will be felt along the coast of the above mentioned areas as early as this afternoon, with hurricane force winds likely later this evening and into tonight.

Similar to my concerns about the "tropical storm vs. hurricane" classification, I am also afraid that too many folks are focusing solely on the wind speed forecast with Isaac.  While a 90-100 mph hurricane will cause plenty of wind damage (and some models are still suggesting that winds will be even higher by the time of landfall), my main concern here for several days has been the storm surge and flooding potential, due to the way that Isaac will come in (with respect to the orientation of the center vs. land, etc.).  Here is the latest forecast for the storm surge impacts across the region:

As you can see, nothing has really changed.  Extreme storm surge and flooding impacts are expected within a large area of southeast Louisiana (including the New Orleans area), then Eastward along the Mississippi and Alabama coasts.  Not only will water levels be high as Isaac pushes waters from the Gulf of Mexico into the region, but this will last for a very long time, much longer than we saw with Katrina (due to the slower movement of Isaac).  I sincerely hope that the modifications that were made to the levee system over the last few years come through and do their job.

In addition to the storm surge from the sea, widespread fresh water flooding will also take place across the region due to very heavy rainfall.  Rainfall amounts in excess of 10 inches can be expected not only across southeast Louisiana, but also much of southern Mississippi and extreme southwestern Alabama:

As is typically the case with a landfalling tropical system, isolated to scattered tornadoes will also be possible, particularly to the East and Northeast of where the center makes landfall.

I hope that folks across this region are prepared for what is coming later today through Wednesday.  If you are still in the area, particularly the New Orleans area, southeast Louisiana and southern Mississippi, and have been debating whether or not to move inland, I strongly suggest that you do so immediately.  Most of the major computer forecast models suggest that rapid intensification is likely prior to landfall tonight as the center still has plenty of warm, previously undisturbed waters to travel across.  As I mentioned above, the winds are only one factor here, as the storm surge and flooding will be widespread and of a very long duration.  Conditions are likely to become very bad or worse before you have time to react, so please take the "better safe than sorry" approach.

For the latest imagery and updates on Isaac, please refer to this dedicated webpage at our sister site, WeatherGuidance.com.  In addition to more frequent posts here on the blog today and Wednesday, I will also post various shorter updates with additional images, etc., on both facebook and twitter.  If you would like to receive those updates, please be sure to follow me there if you aren't already:

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