Steve’s Weather Blog – 08/24/2016

Wednesday, August 24, 2016

INVEST 99-L

Good morning!

Invest 99-L is the tropical wave near the northern Leeward Islands this morning. This morning, the system doesn’t appear very well-organized on visible satellite:

Perhaps most noticeable about this loop is the huge swell of deep convection on the southern half of the low that begins the loop, then diminishes slightly before the last five frames when the convection again is on the upswing on the southwestern part of the low. The warmer clouds (cyan and more transparent blue colors) are low-level features that seem to be showing an inflow sort of pattern. There also appears to be outflow banding taking place in different sectors of the system.

The National Hurricane Center’s 8:00 AM Tropical Weather Update included this text about 99-L:

“Although environmental conditions are currently only marginally conducive for additional development, this system could become a tropical depression at any time during the next few days while it moves west-northwestward at about 15 mph across the northern Leeward Islands, near or over Puerto Rico, Hispaniola, and the Bahamas.“

MODELS

As I’ve expressed before, I again assert that no model will accurately predict a tropical system more than three days out. The success rate is WAY too low to consider it reliable.

That being said, there’s a very good likelihood the models will continue to have difficulty picking up on the influence of mid-level high pressure that is forecast over the southeastern United States. Here’s one graphic of plots:

This is compiled output of several weather models. As you can easily see, there’s not a lot of agreement beyond 72 hours. Several models (in fact, the majority of models) push the low into the Atlantic never letting it make it to Florida.

One of the more reliable models for tropical prediction is the European Model – the ECMWF. Since the Hurricane Hunters fly the 700 millibar contour for their alpha pattern, which is about 10,000 feet up but can get much lower in the middle of a tropical low. Let’s see the European model’s output for 700 millibar wind speed next Tuesday evening at 1900 CDT:

Well that’s certainly something to be concerned about.

What you’re looking at is maximum winds of 70 knots … or just over 80 miles per hour (Category 1 on the Saffir-Simpson Scale) centered about 200 miles south of Vermilion Bay, Louisiana. We’ll have to wait until tomorrow to see next Wednesday evening’s plot.

Let’s talk about intensity.

Here’s a plot of several models and the intensity of the low over time:

When you sort through all of the graph points, you find that nine of the models don’t even evolve the system into a tropical depression, eight take it to tropical storm strength, and only three evolve it into a minimal hurricane.

My Thoughts

Who knows?

And I could stop right there. But I can’t …

It’s still too early to have any certainty about the evolution of Invest 99-L. Heck, it may not even make it out of ‘tropical wave’ status. Then again, it could evolve into the CAT 3 storm that some have been waiting for, whether good or bad.

So, conservatively, I can offer this opinion as of this morning:

The low will continue on a west-northwest track for the next couple of days, tending more to the northwest tomorrow, before it begins to interact with mid-level high pressure progged to be in place over the southeastern US. This high pressure, depending upon its location and influence (strength), will be a predominant factor in which direction the low moves.

If the high doesn’t move very much from its current location over the lower Mississippi Valley, the storm could either continue turning to the northwest, then northward, affecting Florida’s west coast and the other coastal southeastern states, or the storm could cross the southern part of Florida and come into the Gulf of Mexico.

If the high does move, it would likely move to the east and north towards the Tennessee Valley or into northern Alabama or Georgia. If that happens, the low would cross Florida, come into the Gulf of Mexico, and then make a turn northwestward towards the Florida panhandle or Mobile Bay.

As of this writing, I have ZERO confidence in any forecast beyond three days from now. I just don’t. There are two huge and fluid variables that will affect my forecasting reasoning over the next few days:

1 – the evolution of the low. 2 – the movement of the mid-level high pressure over the Southeast.

We’ll chat again tomorrow about this and watch each model run with great anticipation.

Regardless of what happens, please have your Hurricane Plan ready and your supplies.

– Meteorologist Steve Taylor