Thursday, September 8, 2011

Just When You Thought We Were Safe...

Just when you thought we were safe, the latest run of computer models has done quite a flip, with several pointing toward Nate making landfall along the northern Gulf Coast.

But before we go getting all worked up, let's consider some important points:

  • All but one of those tracks that you see pointing toward the northern Gulf Coast is either a variant of the same computer model or gets its starting conditions from that computer model -- the GFS (Global Forecast System). For instance, AVNO represents the operational or real world run of the GFS model. AEMN represents a consensus of possible tracks presented by that same GFS model.  AEMI is pretty similar to AEMN. And while the GFDL is somewhat separate, it gets its starting conditions (location of Nate, condition of the surrounding atmosphere) from the GFS.
  • This is a dramatic change for the GFS model, which for most of the last couple of days had Nate heading into Mexico. One of the things we look for in forecasting tropical systems is model consistency. A dramatic change such as this gives us serious reason to question its accuracy.
  • The real key to see if we have something to worry about will be to see if this model trend continues. The GFS will have another update out within the next couple of hours. Just as important, we'll look to see if the European (ECMWF) model follows its lead. If one or both points toward the northern Gulf Coast, then we'll become a bit more concerned.
  • The National Hurricane Center has not yet bought into this scenario. Check out the official forecast track from this morning.

The bottom line here is that I want you to realize that essentially all of those lines pointing in our general direction are really the result of a single computer model run overnight. It could easily have been a fluke. However, it's typically a fairly reliable model, so we'll wait to see if others follow suit.

If you caught my 5 p.m. or 9 p.m. weathercasts on Wednesday, I showed how 2 high pressure areas would have a big impact on Nate's track. One is located over the northern Caribbean, while the other is over northern Mexico. I showed how there was essentially an opening for Nate to move north in the next day or two, but that opening was expected to close over the weekend as the 2 highs essentially merged. If those highs for some reason don't 'bridge the gap', a northward motion would certainly be possible.

As for Nate right now, 'he' looks a tad better this morning when compared to yesterday evening. Satellite imagery shows t-storm activity trying to become a bit more concentrated around the center, but it's still mostly focused southwest of the center of circulation. This seems to be indicative of at least some modest northeasterly shear.

While the official forecast does call for Nate to become a hurricane, there is considerable uncertainty about the potential impact of some extremely dry air over the northern and western Gulf of Mexico. We've seen dry air time and time again take a toll on tropical systems this season. The water vapor image below shows that dry air in the orange shading.

So, folks, while this latest run of computer models this morning gives us reason to pause, don't go getting too worked up just yet. Forthcoming runs of the models will give us a better idea of whether there is something to be concerned about or whether the overnight run was just a fluke.

We've already gotten several questions about potential impacts from Nate on LSU and Southern home football games this weekend.  Even if Nate did decide to head this way, it looks as though the motion would be slow enough to prevent any serious problems on Saturday. We'll certainly keep you updated on that in the coming days.

1 comment:

  1. I have been so impressed with your thorough tropical discussions on this blog. Thanks!!!!