Tropical Storm Isaac remains nearly stationary across Southeast Louisiana this afternoon. Maximum sustained winds are near 70 MPH, but the real story right now is the flooding. Rainfall totals have averaged 14 to 22 inches across Southeast Louisiana and the rain is not even close to being done. Some locations may break 30 inches of rainfall by the time this event is done. That kind of rainfall can cause major flooding even with the best flood control systems. We’ve already had catastrophic flooding in areas that are outside of levee control districts along with a few areas that have sustained levee over-topping. We’ll leave the damage reports to the news media and start talking about the future impacts of Isaac and if it will bring Texas the same bad weather.
Tropical Storm Isaac continues to pound the eastern half of Louisiana and much of Southern Mississippi. Here in Texas, we’re not even receiving any rainfall as of yet. While we do have east and northeast winds due to Isaac‘s circulation, we have yet to see a real cloudiness or rainfall. We can thank that to the slow movement of Isaac and no other weather systems in the United States to help move Isaac. As such, Isaac is going to move very slowly over the next two days. From the two images above, I want you to notice the wet side and dry side of this system. Most tropical systems will typically have most of their precipitation and most severe weather along and east of the center of circulation. To the west of the center, rainfall is still occurring but is much less intense because the system is pulling dry air in from the north. I’ll forego going into a detailed explanation of a hurricane’s circulation, but that would make for a nice educational post down the road.
Here is the latest forecast track from the National Hurricane Center for Isaac. Even though it will continue to weaken and no longer pose a strong wind threat, it will retain much of its tropical moisture and continue to produce heavy, flooding rains. While Louisiana doesn’t need any more rain, places like Arkansas, Missouri, and Illinois definitely do need rain as they are in a drought. Isaac will continue to move slowly to the northwest through Friday before turning northeast and begin to accelerate. For Texas, our primary impacts would occur while the storm is moving across western Louisiana and Southwest Arkansas. Based on this track, our impacts would be limited. Since the worst weather occurs along and east of the center of circulation and Texas is west of the circulation, our impacts would be minimized. I do not expect a damaging wind event nor a tornado event. If this track is correct, rainfall impacts would be limited to within 50 miles of the borders or Arkansas and Louisiana in East Texas. The gradient of impacts on the west side of Isaac are tightly packed, meaning that Isaac will have to pass close or over Texas to bring us any substantial impacts. Now that I’ve detailed potential impacts a bit, lets take a look at possible rain totals.
This graphic shows the estimated additional rainfall totals that Isaac will produce if it follows the track as shown above. The first thing I want to note is that areas around New Orleans are projected to receive an additional foot of rainfall on top of the 18 inches they have already received. It doesn’t take a genius to realize that could produce catastrophic effects in some regions. You can also see the tight gradient of rainfall on the west side of the system. If this forecast is right, most of Texas will not receive any rainfall except those right on the Arkansas/Louisiana borders. There is still some model uncertainty so I’m not totally buying that just yet as any shift in track will change this rainfall forecast substantially.
We’ll be monitoring things tonight and tomorrow and will post changes as needed.
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