Invest 91-L Located Over The Eastern Tropical Atlantic: A tropical wave, which has now been designated Invest 91-L by the National Hurricane Center, has become a little more organized today. Latest satellite imagery indicates that the thunderstorm activity around Invest 91-L has become a little more organized and concentrated as compared to what it looked like yesterday. Even though Invest 91-L is a little more organized today, I do think that we are still a couple of days away from it becoming a tropical depression. We’ll get there, but it’s going to be a slow process.
Current analysis of environmental conditions reveals that Invest 91-L is still stuck in a narrow ribbon of low wind shear that is surrounded by strong wind shear to the north and to the south of this system. Because of these marginally favorable conditions, slow development is still expected for the next few days.
A look to the west of Invest 91-L shows that the environmental conditions become much more favorable for development west of about 45-50 West Longitude. This means that I think that we’ll really see the development occur by the time the wave reaches the area about 45-50 West Longitude late this weekend or early next week.
The latest model guidance are definitely trending towards a threat for the Leeward Islands, Virgin Islands and Puerto Rico late next week. All models seem to be pointing towards this threat & it is something that needs to be watched very, very closely if you are in the Leeward Islands, Virgin Islands and Puerto Rico.
That being said, the model guidance are still struggling with their forecasts as this disturbance is still in its formative stages. It might be a few more days before we see more consistency in the guidance, especially once this system develops into a depression and a tropical storm. Until then, unfortunately, we probably will continue to see fluctuating forecasts from the model guidance, so buyer beware when looking at them.
The GFS model has been wildly inconsistent with its forecasts of Invest 91-L for the last several days. For the last few runs, however, the GFS model has trended further south with its forecast of this system & now seems to be inline with the European model and the Google Deepmind AI models. The most recent 12Z forecast run of the GFS model now shows Invest 91-L impacting most of the Leeward Islands and the northern Windward Islands as a hurricane late next week & then directly impacting the Virgin Islands and Puerto Rico as a hurricane next weekend. From there, the GFS model has trended towards a higher threat now to the Bahamas, Florida & the Southeastern United States between about September 16 and September 18.
The Canadian model, on the other hand, is showing a curve to the northwest before it reaches the Lesser Antilles leading to no impacts for the eastern Caribbean.
The European model from last night’s 00Z run showed a pretty far south path bringing Invest 91-L right across the Leeward Islands as a hurricane around next Thursday followed by a track right across Puerto Rico and the Virgin Islands as a hurricane next weekend. Beyond this, the European model forecasts Invest 91-L to impact the Bahamas around September 16 to September 18.
Now, the most recent run of the European model from 12Z today shows a much weaker system (barely a depression) into the southern Leeward Islands and the Windward Islands around Wednesday of next week. This is followed by further weakening and dissipation in the eastern Caribbean. As I said, the models have been wildly inconsistent & just cannot be believed right now, until at least we have a fully formed system.
A look at the Google Deepmind AI models, they seem to show the same southerly type track that takes Invest 91-L right across the Lesser Antilles around Wednesday of next week. Interestingly though, the Deepmind model shows dissipation shortly thereafter with no other impacts forecast by this model.
As for the ensemble guidance – The GFS ensemble model guidance do show a different outcome from its operational deterministic counterpart. Most of the members show a further north track either near or north of the northern Leeward Islands, but they have shifted further south than previous model runs.
The European ensemble models are also different than the operational European model as a majority of the members show a track near the northern Leeward Islands & then a pass east of Puerto Rico and the Virgin Islands. There are a few members, however, that do show a track right across the central and southern Leeward Islands right into Puerto Rico and the Virgin Islands around Wednesday and Thursday of next week.
Here Are My Thoughts: As I mentioned near the start of this discussion, development into a depression or a tropical storm is still a couple of days away as my own analysis shows the environmental conditions being marginally favorable for development. Quicker development then looks possible by early next week when Invest 91-L moves past the 50 West Longitude line.
The southward trend in the guidance is certainly concerning, but it’s way too early to ring any alarm bells for the Lesser Antilles, the Virgin Islands and Puerto Rico. The reason why I say this is because the overall setup ahead of Invest 91-L is very complex with dry air issues possible, which could significantly impact this system. Also, the overall small size of Invest 91-L is giving the models fits because it is a lot harder for a model to resolve a small system than a larger system. There are times that the models completely miss resolving a tropical system because of its small size. This all leads to a very complicated and uncertain forecast with Invest 91-L.
My recommendation to those of you in the Leeward Islands, the Virgin Islands and Puerto Rico is to continue to monitor Invest 91-L very closely. Fortunately, there is a lot of time to watch this system as it’s still at least 6-7 days away from affecting the islands of the eastern Caribbean. Be aware though that there is at least the possibility of this system bringing impacts in terms of wind, rain and rough seas to the Leeward Islands, the Virgin Islands and Puerto Rico during the middle and later parts of next week.
Looking beyond this, it is way, way, way too soon to tell whether Invest 91-L will end up becoming a threat to the Bahamas, Florida or the East Coast of the United States. One thing that I will say with total confidence is that you should be ignoring those forecasts from the GFS model or earlier runs of the European model showing the hurricane strike on the Bahamas and the Southeast coast of the United States. These forecasts are some two weeks away and are oftentimes extreme outliers of what ends up happening in the end. Ignore the scary social media posts that are showing these model depictions.
It’s going to take about two weeks for Invest 91-L to make it all the way to the Bahamas and the US Southeast Coast & the weather pattern, to me, just doesn’t scream East Coast threat with this system. This weather pattern looks to consist of some sort of an upper level trough of low pressure near the East Coast of the US, which should turn this system to the northwest and north. Where this actually occurs in relation to the US coastline is an unknown right now because this is something that will not happen for another 8-10 days from now.
My recommendation for those of you in the Bahamas, Bermuda or along the US coastline from Florida to the Carolinas is to just keep an eye on this system. For now though, it’s nothing to get overly worried or stressed about.

The next tropical weather discussion will be issued on Friday.










































