A comprehensive update on the potential Gulf hurricane

By now most readers have heard about the tropical low pressure system in the Caribbean Sea, which is approaching Puerto Rico. It is not a tropical storm yet, as a reconnaissance aircraft on Wednesday failed to find a well-defined circulation. It nonetheless appears to be developing stronger thunderstorms, and is producing tropical storm-force winds. It likely will become a depression and then a storm later this week as it passes near the Bahamas and then tracks toward Florida.

The purpose of this post is to discuss what may happen after that, but before doing so, I want to discuss the fallibility of forecast models at this time, and why we don’t have that much confidence in where the system, Invest 99L for now, will go in five to seven days, if it does head toward the Gulf of Mexico.

Model inaccuracy—it’s considerable

Let’s focus on track models. They tell us where a storm will go. Although models can do a fairly good job simulating the large scale steering currents that guide storm motion, such as upper-level pressure systems, these forecasts remain sketchy when a tropical system is poorly defined. And as I noted earlier, Invest 99L has yet to develop a center of circulation.

With that said let’s look at track errors for National Hurricane Center forecasts for three classes of storms: hurricanes (which have well defined circulations), strong tropical storms (likewise) and depressions and weak tropical storms (not so much). Actually, the category we’re interested in isn’t even on this chart—”stuff that ain’t formed yet.” Thus at five days we can expect the track error to be at a very minimum of 350 nautical miles for undeveloped tropical systems, and likely much, much more.

Moreover, the chart doesn’t even go to day six or seven (because the hurricane center doesn’t produce forecasts at such a distance). However if it did, the trend line is pretty clear, days six and seven would have greater error still. In short, the track error on any model forecast you may see for where Invest 99L ultimately goes next week is likely to be hundreds, if not thousands, of miles off. The track forecasts will get better after the system gets a well defined center of circulation, and when we get closer to an ultimate landfall. But we’re probably a couple of days away from that.

Track forecast

With that said, here are a bunch of track models!

 

As you can see there is generally pretty good agreement about the system tracking toward Florida by this weekend, but after that there’s a split in whether the system will curve back into the Atlantic Ocean, or follow a more westward track into the eastern Gulf of Mexico. The critical forecast question concerns the position and strength of a ridge of high pressure over the Southeastern-Eastern United States. A weaker, more northern ridge would allow the storm to make a northerly turn before coming all the way into the Gulf of Mexico.

I’d still say areas from Texas to the US East Coast should be cognizant of this system, but I continue to believe the chances of this system getting all the way into the western Gulf of Mexico are fairly low.

Intensity of 99L?

The other key question is how strong 99L might get. The forecast models aren’t overly bullish on it strengthening, but lets face it, intensity forecast models are not particularly great. As 99L gets closer to the Bahamas I think wind shear will relax a bit, and there is ample warm water and oceanic heat content for this system to intensify fairly quickly. I’m not saying it becomes a hurricane before reaching Florida, but I wouldn’t bet against that possibility.

Wrap-up

If all you’re concerned about is whether this storm affects Texas, I’d say that remains less likely to happen than other scenarios, especially in light of some of this afternoon’s forecast models. Additionally, historically, hurricanes that strike the Houston region tend to pass through the Caribbean Sea more to the south of where Invest 99L is (see this map of strikes put together by Matt).

 

With that said we’ll need to watch the development of this system over the next couple of days, and you can be sure we’ll do so for you.

Posted at 2:15pm CT on Wednesday by Eric