Issac Forecast Update

Primary threat: Freshwater flooding, heavy rainfall, down tree limbs and power lines, high storm surge east of the Mississippi Delta based on topography

Secondary threat: high wind damage

Lets cut to the chase.

Soon to be Hurricane Issac is taking a very dangerous path towards Louisiana. At the current track it will make landfall right at the Mississippi River Delta. This is farther east than Hurricane Gustav made landfall, and Issac will likely make landfall at a slightly weaker intensity (and slower forward speed) than Gustav. However, the landfall location at this time has made for near optimal storm surge heights for eastern Louisiana and Mississippi due to the storm making landfall almost perpendicular to the coast. The next few figures below show the maximum surge potential for the optimal state, at various strengths. Pay additional attention to those that are specially marked in the captions.

Surge potential for a high end cat 1 (winds 90mph) in between low tide and high tide, with the storm moving onshore at a forward speed of 10mph. *MOST LIKELY SCENARIO***

Surge potential for a high end cat 1 (winds 90mph) at high tide, with the storm moving onshore at a forward speed of 10mph. ***MOST LIKELY SCENARIO***

If the land is covered by something, then it means that either a surge or rain water has predicted these areas will be flooded. I have set the color bar to be the same for all three maps. Even the cat 1 scenario is ugly but this storm looks strikingly familiar to what Gustav did in 2008. Issac makes landfall slightly eastward and puts a 10ft storm surge that is not usually seen for a cat 1 storm, due to the topography. I would look for a blend between the first map and the second map above for potential surge but there is still great uncertainty between now and the landfall. This is because Issac has picked an unfortunately bad storm track, where it will be making landfall almost perpendicular to the eastern Louisiana shoreline. I lowered the forward speed to 10mph at landfall, but Issac will have a threat that Gustav did not have: extremely heavy rainfall along the coast. Lets compare Gustav (2008) and the model output for Issac (2012).

 

Gustav rainfall

Issac's rainfall projections

Issac is interacting with a pre-existing stationary front boundary, which is why high rainfall spreads into Georgia and the Carolinas despite the storm not being anywhere near those locations. This unfortunately slows the storm down to a crawl after landfall. At this time I think rainfall will actually be between 75% and 80% of what is shown above.

And all of today (Monday) where Issac failed to increase its maximum winds but still had a pressure drop produced a wider wind field.

The semicircle near the center of the picture signals that an eye is starting to form. Dry air along the northern quadrant is still interfering with a complete eye from forming

The image above is not a surprise: Issac has struggled to form a complete eye for the past 24 hours due to entrainment of mid level dry air. Issac may still have another 12 hours before it walls off the dry air. Thank goodness the dry air is there or otherwise Issac would already be a cat 2 hurricane. In the very last image of the animation we see an eye emerging.

Model runs for storm path of Issac.

Please be safe if you are along the Gulf Coast.