NEW GFS Brings 2x More Rain to New Orleans Than Katrina

If the new GFS were to verify, Hurricane Isaac could bring 2 TIMES MORE rain to New Orleans than Katrina did. Looking at the graphic above, we see that there were a few instances of 7.0 to 9.9 inches of rainfall where Katrina made landfall on the left graphic. There are only a couple reports of 10.0 to 16.3 inches immediately along the Gulf Coast where Katrina made landfall.

On the right graphic is a total precipitation map until Hour 93 of the new GFS. It shows over 12 inches hitting much of southeast Louisiana, with a maximum amount of just over 20 inches shown on the extreme bottom right corner of the image.

If the levees keeled over in 2005 with Katrina due to 10 inches of rain (among other factors), imagine what 20 inches of rain in a large Category 2-3 hurricane could do.

I am not saying this will happen, this is only meant as a comparison and should be treated as such.

Andrew

**ISAAC OFFICIAL FORECAST**

This is The Weather Centre's official forecast for Isaac. The Weather Centre is not related to, or affiliated with the National Hurricane Center. In the event of an evacuation order, always heed NHC warnings over our advice.

This is the official forecast for Tropical Storm Isaac. We will start out with what is making the forecasts so hard to get together.

This is a 500mb chart for 96 hours out. I highlighted two things- a trough disturbance in the Northeast, as well as Tropical cyclone Isaac. We are watching the trough to see if it influences Isaac. If Isaac happens to be in a situation where the trough does influence the system, Isaac would likely make a more easterly landfall.
However, if the trough does not take control of Isaac and goes on its merry way, Isaac would be free to go west, like many new models and ensembles have been saying in the past 24 hours. This is definitely more concerning, as we are now seeing cities like Houston, Texas and New Orleans, LA under the gun. Let's take a look at these ensembles and models.

We'll start with the NCEP ensemble group. This is from 6z, as opposed to other ensemble groups that I will be posting soon that were made at 0z- six hours earlier. Anyhow, these ensemble members are showing a varied landfall anywhere from Houston, Texas to the end of the Florida Panhandle. There does seem to be some middle ground in the Louisiana/Texas border, but the members are just too spread out to confirm that.
I think that this is a perfect example of the trough not influencing Isaac. The ensembles seem to see that the influence will not occur, thus a more westward movement will begin. I do not want to speculate on the extreme member scenarios, such as the lone member showing a Houston landfall.

This is a chart of the 0z CMC Ensembles. You can partially see a consensus of the members, bringing Isaac to New Orleans. However, note the few members also hitting Houston. I don't know if you can see that through all of the disorganized and messy lines on this image, but there are a few more members hitting Houston than there are in the NCEP, and this is concerning. It is also worrying for New Orleans, with such a tight consensus compared to the NCEP.

Lastly (and possibly the scariest) is the FNMOC Ensembles. The FNMOC Ensembles are derived of the NOGAPS model, and these two are made by the Navy. The FNMOC Ensembles are taking a tighter consensus than either of the above groups to hitting the Texas/Louisiana border, and threatening Houston ten-fold over the NCEP. Given, the NOGAPS is not a good model, therefore the FNMOC Ensembles may not be either. However, given that the models have been shifting west, one can only believe that this is a logical solution (which it very well could be).

So, the ensembles are marking out targets between the extreme west Florida Panhandle down to Houston, with a favored track hitting Louisiana and in the New Orleans area. Let's take a look at the 0z model suite.

The 0z model suite is split down the middle, with some models projecting a Florida/Alabama landfall, and others hitting Louisiana head-on. I'll just cut to the chase here. Given that Isaac is still a tropical storm and has not significantly strengthened, I am siding with the western models, as the Coriolis effect that moves storms north when they are strong appears to be working against Isaac, who remains fairly weak. Thus, a more westward movement would be anticipated.

It should be noted that the ECMWF, which originally anticipated a Louisiana track, has shifted east to join the eastern camp in their solution.

Here is a quick sketch of my track and intensity forecast. I believe Isaac will stay weak for a bit before strengthening into a Category 1 hurricane. The waters are not as warm now as they can be during the tropical season, so Category 2 strengthening should occur immediately prior to landfall. I am going with a western solution due to a very good ensemble consensus, with some models also supporting this.

Andrew


Credits: NCEP/EMC for Ensembles, UW-Milwaukee for Late and Early Models

Isaac Ensembles Shifting West; Dangerous Situation Incoming

The global model ensembles are taking a shift west, posing a danger to cities like New Orleans, that were previously not in the path of Isaac. Let's take a look.

This is the NCEP Ensembles, comprised of the GFS Ensembles. In the previous 0z suite, we saw a good consensus on Isaac hitting the Florida Panhandle. However, notice the ensemble members now more spred out. This spread appears to be favoring the west more than the east, and the 12z model suite reflected a westward shift in thinking. I'm not saying that the one member showing a Houston, Texas landfall is correct, but a westward shift is certainly something to take note of, especially on such a large scale of models and ensembles.

This is the CMC Ensemble group. These members are depicting a situation similar to that of the NCEP Ensembles, but more biased towards a MS/AL landfall. This is going along with new model thinking, and is not such a bad idea. However, there remains a considerable amount of spread among the ensembles, which can be mediocre at times, to say the best. However, this time around, the CMC ENS. may have something going for them.

Next, we have the FNMOC Ensembles. The FNMOC Ensembles are based off of the NOGAPS Model, which is from the Navy. The FNMOC ENS. are showing a much more varied, yet arguably well-composed spread, with members stretched from the Florida Panhandle to southern Texas. What I find great about this run of the FNMOC Ensembles is that it is accounting for nearly every reasonable scenario out there. Given, Isaac will likely not hit Mexico/South Texas. However, the FNMOC members are covering ground from Houston to Tallahassee, which is encompassing recent model and ensemble trends. This will be one to watch.

Lastly, the ECMWF Ensembles. Because the institute that runs the ECMWF is so strict about releasing data to the public for free, we are only able to see a mean of the ensembles, this image courtesy of Americanwx. The mean ensembles show a system making landfall in western Florida. This is a nudge to the east of ensemble thinking above, but should very well be counted in, as it was great at indicating that Isaac would be a Gulf threat.

All in all, the latest ensemble runs have been very interesting. Strangely, I am hesitant to count out any of them, for each has its own distinct trait that I find crucial in this forecast. For example, the NCEP has an ensemble group that has shifted with the models, the GEM Ensembles keep the MS/AL landfall as a consensus, and the FNMOC Ensembles account for all reasonable possibilities. This is not a common thing to see, when each group has good things going for them like this.

The question is, will a disturbance moving off the East Coast pick up Isaac?

I have marked the trough moving offshore and Isaac making landfall. The models seem to be having trouble assuming whether the trough influences Isaac and pulls it north, or does not influence Isaac and the hurricane goes west, more towards Louisiana, like the ensembles were beginning to say.

I will make a better calculation on this in the morning, when the ensembles should be a bit more settled than now.

Andrew

Isaac Threatening Florida; Hurricane Warnings Hoisted

Watches and warnings line the northern Caribbean and eastern Gulf Coast this morning, as Isaac leaves Haiti to continue towards the Gulf of Mexico.

Hurricane warnings have been hoisted in the face of Isaac in Florida, as well as tropical storm warnings blanketing Cuba. Isaac has just moved off of Haiti and appears to be heading northwest, towards the Florida Keys and the Gulf of Mexico.
If Isaac makes landfall on Cuba, it appears to be brief, and only a slight change in storm strength would likely be observed. However, if Isaac plows into Cuba, which is unlikely, the tropical storm could have issues with keeping itself together.

The latest Advanced Dvorak Technique (ADT) placement of the center puts Isaac at about 19 N longitude and 74 W latitude. This indicates Isaac has been moving northwest in the past while, but the first NHC graphic appears to show a west-northwest tilt as of the current center.
From this viewpoint, it seems a brief landfall on Cuba is likely, but nothing major as far as going through much of the country.

The forecast from the National Hurricane Center takes Isaac on a northwest path into the Florida Keys before curving north and making landfall on the Florida Panhandle. This does appear to be a fair track, but any wobble in any direction could drastically change the outcome of the track of Isaac. I am looking for hurricane warnings to be spread out across western Florida, the Panhandle, and Mississippi's into Alabama's portions of the Gulf Coast when Isaac nears.

The spread of models appears to be wanting to take Isaac a bit further west, possibly into Alabama or the extreme western Florida Panhandle. Considering the Southern US has been launching special radiosondes at 6z and 18z to aid the models in tracking Isaac, I would expect this to be a good consensus. However, it looks like a few models remain unsure, even 24 hours out, on where Isaac will be.

Intensity forecasts are also very scattered, but seem to have an idea of Category 1 hurricane strength in the next 3-4 days, before making landfall and weakening and possibly degenerating into an extratropical low. After that, some models project Isaac will shoot out to sea.

Storm surge is a big concern. The NHC is projecting that there is anywhere from a 0-30% chance of storm surge levels reaching 5 feet in Florida. The values appear to be so low because of no clear model consensus, but this remains a threat either way. Such a large storm has the potential to produce a very large storm surge area, possibly well inland of western Florida.

Andrew

Isaac Strengthening, Moving Northwest

Tropical Storm Isaac is now strengthening and appears to be in the process of beginning a northwest movement. Let's look at the past track of Isaac.

University of Wisconsin-Milwaukee

The observed track of Isaac shows a recent southern dip, followed by some regulation to the 16N longitude line. Recently, however, a sharp northwest turn was observed, bringing Isaac to about 16.5N. A northward turn is expected as Isaac strengthens. Strengthening of tropical systems commonly leads to the Coriolis effect taking over and turning storms to the north. The Coriolis effect will act as a tugboat, and veer storms to the right (north) in the northern hemisphere, and this could very well be happening now with Isaac.

SSEC CIMSS ADT

The Advanced Dvorak Technique, which helps find the center of tropical systems, has pinpointed Isaac's location at about 16.5 N and 71.35 W in the Caribbean. Looking at the satellite image superimposed with the center of Isaac, we see that the center is dangerously exposed on the west side. When we have a center of circulation (COC) with such an exposed area, it can be very easy for the atmosphere to quickly weaken Isaac. If that should happen, I could see a southward wobble happen, and more model mayhem ensue.

SSEC CIMSS ADT

The ADT also shows the strength of the system, based on satellite estimates and their own Cyclone Intensity scale, which ranges from 1 to 8 on this chart. Looking at recent observations, we see Isaac has endured some weakening in the past 24 hours. However, recent strengthening appears to be rebounding Isaac to a more stable point. This cannot be confirmed, however, until the center of circulation is re-covered.

Now... Model Mayhem Resumes!

Today's 12z European ECMWF model took a 967 millibar hurricane to the central Gulf Coast, keeping the consistency we look for and value so much in model forecasts. While the storm remains a while away, I am pondering this scenario. Isaac has already been shifting west and south and been strengthening and weakening so that a major move north has not been favorable. However, now, with the models recognizing the Gulf Coast threat, and a northwest movement seen above, this could be Isaac's time to shine, as far as moving towards the Gulf.

The GFS is much weaker, taking a 994 millibar tropical system to the Florida Panhandle. I do find this possible, but it is hard to go up against the consistency of the ECMWF. While this solution was shown in the 0z model suite last night, in which data from the Hurricane Hunters in Isaac was used, the ECMWF budged only a small amount east. Should Isaac take any wobble west, or even south, the ECMWF's solution would only become more and more likely.

The SREF is split, but many are projecting a tropical system to hit western Florida and the Panhandle, similar to the GFS. However, notice the major spread along the members. Just because you can make a big spread into a mean forecast does not mean it could be right. The wide variety of forecasts is very concerning to me, and I would like to not count in the SREF until its members can get into a good, solid solution.

Isaac is playing his tricks on us, and all you can do now is prepare. I advise (under my personal discretion, NOT the NHC's) that the Gulf Coast carefully monitor Isaac. The Florida Panhandle area, west to far eastern Louisiana should keep a close eye on Isaac, and consider making preparations for possible impact. We remain a time away, but the models seem to be coming together.

Andrew

All advice and opinions shared on here are those of mine and are not those of the National Hurricane Center. In any case, always disregard my forecasts and advice in favor of the National Hurricane Center's. This is meant as an informative article and should not be used to make major decisions, such as evacuation.