Home Tropics

Tropical Update: Invest 91L Getting More Organized

Figure 1. Latest 5-day graphical tropical weather outlook from the National Hurricane Center.


Data from CIMSS shows that the multiple areas of vorticity are beginning to congeal, meaning 91L is gradually getting more organized. However, shear is still hampering development (as expected), blowing convection to the east and preventing the “center” from closing off.

Model guidance: 

All of the reliable global model guidance (the ECMWF, GFS, and UKMET) continues to predict that 91L will develop into a tropical cyclone (TC) sometime tomorrow or Monday.

Taking a look at each individual model solution: 

  • The ECMWF (euro) predicts TC genesis (development) will occur near the Yucatan Peninsula Monday evening. It then forecasts 91L to steadily intensify while tracking to the north/ north-northeast. The euro has landfall near Pensacola, FL early Thursday morning as a low-end category 1 storm.


  • The GFS predicts development to take place near the Yucatan tomorrow morning. It then forecasts 91L to become a high-end category 1 or low-end category 2 while tracking to the north/north-northeast. The GFS has landfall near Apalachicola, FL Wednesday morning.


  • The UKMET is similar to the GFS in regard to the timing of genesis. It is predicting 91L to become a TC tomorrow morning. From there the UKMET has 91L taking a north to north-northeast track. It has landfall late Thursday/early Friday just north of Cedar Key, FL as a category 2 or 3.

The consensus is a landfall between New Orleans, LA and Cedar Key between Wednesday and Friday —  potentially as a hurricane.

Our thoughts:

We believe that 91L will become a tropical depression or storm in the NW Caribbean or southern Gulf tomorrow/Monday. The National Hurricane Center put the odds of development at 90% over the next 5 days (see figure 1).

Track: A ridge of high pressure anchored over Eastern Seaboard will cause 91L to track NW into the southern Gulf on Monday. Tuesday/Wednesday, an upper trough should cause the ridge to weaken and turn 91L toward north/north-northeast. Landfall is possible anywhere from southeast Louisiana to Cedar Key, FL in the Wednesday – Friday time frame.

The ultimate track/landfall location will depend on 3 variables: how fast 91l, speed of the trough, and position/strength of the ridge.

Intensity: There is still a large amount of uncertainty regarding how strong 91L will get.

Figure 2. 10/6/18 1200 UTC intensity guidance for 91L | Credit: NCAR

The latest SHIPS intensity forecast keeps shear in the 15 – 20 knots (kts) range while 91L tracks across the Gulf. This, combined with high ocean heat content over the central Gulf, should allow for some strengthening.

The most likely scenario is that 91L peaks as a moderate to strong tropical storm, as shown by the IVCN consensus multi-model consensus (see figure 2). But it is very possible that 91L becomes a hurricane.

Interests from New Orleans to Florida should continue to closely monitor the progress of 91L.

For future updates, follow us on social media: 




*Like Hurricane Central on Facebook*

You can also have our latest website posts sent to you via email by subscribing here