Invest 96L likely to become TS Debby in the Gulf

Well, we have already had our first hurricane of the year with a surprise in Chris. The last advisory has been issued on Chris as it is no longer a tropical cyclone. All eyes are now focused on the Gulf of Mexico and Invest 96L, which is located just north of Mexico’s Yucatan Peninsula.

Invest 96L:

Invest 96L is a very broad low pressure system that has been slowly developing over the past few days over the NW Caribbean Sea and SE Gulf of Mexico. As of 8am EDT, the NHC was giving Invest 96L a high (70%) chance of becoming a tropical cyclone over the next 48 hours. There was an aircraft reconnaissance flight scheduled for today, but due to the disorganized state of the system that mission was cancelled. Satellite Imagery shows how large of a system Invest 96L is and a broad cyclonic turning is very clear in the low levels of the atmosphere:

Visible Satellite Image of Invest 96L

Most of the convection is displaced to the east of the broad center. This is happening for two reasons: strong upper-level winds from the west (wind shear) and better divergence in the upper layers of the atmosphere. To describe the second mechanism further, think of the atmosphere as a closed container of air. If the air at the top of the container spreads out to the sides of the container (diverges) then air in the rest of the container wants to naturally fill this void due to a pressure gradient. Since air cannot come from above (we are at the top of the container pressing against the lid) then it must come from below. The air that comes from below is warmer and as it rises, it cools and water vapor condenses to form clouds through convection.

In order for development to occur we need to see convection build over the broad center of the storm, which in turn will lower surface pressures and help tighten up the center to allow the development process to continue. Looking at the latest visible satellite loops it does appear that low-level clouds are starting to thicken closer to the center, which is usually the first sign that convection is trying to fire. This will be a gradual process since the system is so broad. Convection will most likely pulsate over the next 24 hours just as it has been doing throughout 96L’s lifetime. Last night we saw a large flare up and this morning the cloud tops are starting to warm again. Until convection can build over the center, consistency will be hard to achieve.

The current conditions are not the most favorable for intensification. Wind shear is at moderate levels around 20 knots. Water vapor satellite imagery shows dry air may be hampering the formation of convection over the broad center:

Water Vapor Satellite Image of 96L

I am skeptical of this dry air. Conventional water vapor satellite imagery only shows the mid to upper layers of the atmosphere. In addition to conventional satellite imagery we also can use microwave satellite imagery, which is capable of seeing the entire atmosphere. This microwave imagery is showing that overall, the Gulf is very moist. I do not see dry air being much of a problem for the development of 96L into Debby. In my eyes, the main issue is the broad nature of the system, which can be seen in vorticity analysis from CIMSS:

850mb Vorticity Analysis

This map shows the vorticity (a measure of spin in the atmosphere) at the 850mb pressure level, which is considered to be in the lower level of the atmosphere. As you can see, there is a great deal of vorticity in the Gulf of Mexico, which would normally be favorable for development of a system, however the vorticity is not concentrated in one area. Instead it is strung out and 96L is unable to circulate its winds around one tight area of low pressure. If we start to see the 850mb vorticity become concentrated in one area in a more circular appearance, then that is a good sign that a LLC (low-level center) is forming.

Forecast for Invest 96L:

Computer models have been having a hard time handling Invest 96L due to its large size, a tricky upper-level pattern, and the lack of a tight low-level circulation center. The main question will be whether or not a trough of low pressure that is forecast to deepen over the Eastern US in about three days will be strong enough to swing 96L/Debby to the NE and over FL. If this does not occur, then 96L will get trapped to the south of a developing ridge of high pressure, which will try and bend 96L back to the west, towards Texas.

There has been little consistency in the computer models’ forecasts. For example, yesterday the usually reliable ECMWF had 96L/Debby slowly developing and moving NE into FL. It then proceeded to do a 180 and brought future Debby, as what looked to be a hurricane, into Texas. The also usually reliable GFS has consistently deepened the trough enough to bring 96L to the NE towards FL in 4-5 days, but as a tropical storm. I have noticed the GFS is awkwardly handling the interaction of the vorticity of 96L and the vorticity associated with the trough that would turn it to the NE so I am unsure of the accuracy of its runs. The UKMET has a forecast similar to the ECMWF towards Texas. The latest run of the NOGAPS keeps 96L as a weak cyclone and also sends it west. The 06Z GFDL dropped the system entirely, and the other dynamic hurricane model, the HWRF, keeps 96L as a weak tropical storm and meanders it around the NE Gulf of Mexico.

To illustrate the large uncertainty in the forecast, here is the latest spaghetti plot of the computer models:

Model plot for Invest 96L

The ECMWF is not shown on the plot above.

The intensity forecast is just as tricky. Although conditions are not incredibly favorable, as described earlier, 96L will still most likely develop into Tropical Storm Debby sometime this weekend. Once a LLC establishes itself, 96L/Debby will have to battle wind shear its whole life time. Here is the 72 hour 250 mb forecast of today’s 12Z GFS:

12Z GFS 250mb forecast for 72 hrs

The GFS is telling us that there will be an upper-level low (ULL) pressure system over western Gulf of Mexico in 72 hours that will help to impose 20 knots of wind out of the SW in the upper levels of the atmosphere. The ULL is represented by the black circle just SE of Brownsville, Texas. This wind shear will most likely keep whatever develops in the Gulf from quickly intensifying. Although SSTs are plenty warm enough to support a possible hurricane in the Gulf, in my view this chance looks low at this time. ULLs are notorious for ripping apart systems. That isn’t to say 96L/Debby will be ripped apart, but if there is a ULL over the western Gulf, I would have a hard time seeing Debby grow stronger than a 60 mph tropical storm.

There are so many question marks with this system that nothing about the forecast can be said with much confidence. There is still even a chance nothing develops at all.

Nonetheless, Invest 96L needs to be closely watched by anyone living along the Gulf Coast. Chances are we will be dealing with Tropical Storm Debby in a few days, and Debby does even have a small potential to reach hurricane strength. Regardless of development, Invest 96L will continue to dump heavy rains over South Florida, Cuba, and Mexico over the next two days.

I will have an update again as soon as anything changes.

-Mike

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