Friday, August 26, 2016

Invest 99L down but not quite out

The saga that is Invest '99L' continues as we move into the weekend. The system that appeared to be on life support on Thursday has shown some signs of coming back to life today. But wind shear and dry air continue to make it an uphill battle for '99L'.

We've got a pulse

We were almost ready to announce a time of death for '99L' on Thursday, but 18 years in this business and even more tracking systems like these have taught me things can change in a hurry in the tropics. We didn't see '99L' blow up into a tropical depression or storm today, but after a day devoid of any nearby convection (t-storms) on Thursday, blossoming t-storms on the eastern side of the system let us know it still has a pulse and a chance.

Satellite image from 2:30 PM Friday afternoon shows developing t-storms associated with 99L, primarily on its eastern flank (east of the 'L' on the map).
 The developing t-storms today are likely a sign that wind shear is lessening a bit in the vicinity of '99L'. Remember, when talking about wind shear, we're referring to strong winds in the mid and upper levels of the atmosphere that tend to rip apart tropical systems. While wind shear is lower today, satellite animations and other analyses indicate it is still very much a factor and hasn't gone away. That, combined with continued dry air west of '99L' mean that conditions are still far from ideal for strengthening in the short term.

Model guidance: not so fast, 99L

If you've followed this saga through the week, you know there's been quite the battle among our computer models, particularly the GFS and the European. While the European outperforms the GFS as a whole, the GFS appears to have scored a victory over the last couple of days as it showed little becoming of '99L' even as it neared Florida.

The last few runs of the European model have now come much more in line with the GFS and essentially keep '99L' as a tropical wave moving into the eastern Gulf and then have it drawn northward without much fanfare.

Morning model runs from Friday, Aug. 26 valid for 7 p.m. Tuesday, Aug. 30. Top panel (GFS) and bottom panel (European). Credit: WeatherBell

Forecast uncertainty

I've struggled with two aspects of this forecast all along through the week. First, I couldn't understand why the GFS and then the Euro didn't have much becoming of '99L' even once it entered the Gulf of Mexico where conditions appeared to be more favorable for development. While it may not be the lone factor, additional analysis I've done today leads me to believe dry air on the western side of '99L' may continue to be a factor.

The other thing that bothered me was this forecasted turn to the north once it entered the eastern Gulf of Mexico. Most guidance was showing a fairly potent ridge of high pressure centered over the Southeast U.S. which would tend to keep the system trekking farther westward into the Gulf. However, I'm starting to believe the models are seeing a developing upper-low near Bermuda as a feature that will drift westward, weaken the ridge, and essentially help '99L' turn northward. This is a complex interaction for sure, so I still wouldn't take it as a given.

Bottom line

The latest outlook from the National Hurricane Center as of 1 p.m. Friday still gives '99L' a 60% chance of development within the next 5 days. Do not let your guard down. The good news is that most of our reliable guidance keeps '99L' to our east even if it does develop.

Please remember that forecast errors at 4-5 days and beyond can still be quite high, so this is not an all clear. It's just to say we're liking the trend for now but will continue to monitor its progress through the weekend.

A final note

For the last couple of days, I've seen many of my colleagues in the weather business calling any mention of '99L' hype or referring to it as the most hyped tropical disturbance in history. Yes, there are a few who have gotten out of hand, but for the most part, the amount of attention devoted to this underachieving tropical wave has been warranted.

Remember, we had very reliable guidance indicating the potential for a hurricane in the Gulf of Mexico, with some model runs indicating something close to a major (Cat. 3 or stronger) hurricane. I think it would be negligent on our part not to discuss the possible scenarios, particularly given the state that our region is in right now. I can promise you I will always discuss events like this in detail, with special attention paid to not over-hyping or unnecessarily scaring anyone.

The important thing is that we put everything in a proper context. Not only can this provide some useful information to you at home, but discussions like these can help spark good dialogues within the weather community on systems like '99L' that prove elusive to forecast.

Stay safe and here's hoping that the GFS and Euro are both right with not much becoming of '99L'.

No comments:

Post a Comment