Science Tropical Weather

Invest 96L / Pouch P17L

The tropical disturbance that has been slowly moving westward and heading toward the Caribbean has been designated by the NHCinfoNHCNational Hurricane Center as Invest 96L as of 08_19/1800Z. This tropical disturbance was one of the waves that came of the edge of Africa but due to the very very air and some vertical shear, 96L never had a chance to try to develop into something tropical. The dry air and shear are abating and with an ULL to the NW of 96L, increased upper divergence is now allowing 96L to ventilate and possibly have an outflow for further development. I want to make it extremely clear that although 96L is looking much better, it is still way to early to determine exactly what the track and intensity of 96L will be. There has also been some discussion as to whether or not Invest 96L(pouch P17L) and Pouch 18L which is just east of Invest 96L and will the two be two separate entities or merge into one system. Again it is still too early as both disturbances are still somewhat disorganized right at the moment albeit 96L is slowly getting organized.

Satellite imagery of 96L

Credit: NOAA Satellite Services Division

Pouch Map
Pouch Map


Invest 96L is presently heading westward at around 15 MPH, and is being steered in the region of southern periphery of the subtropical ridge. If 96L continues the same speed, within 72-84 hours 96L should begin the feel a weakness in the ridge and head slightly WSW. This weakness in the ridge should be near or around the edge of the Bahamas or possibly a little later. Since 96L is still a tropical disturbance and has not even become a tropical depression, there are still too many variables so I want to exercise caution and not get ahead of ourselves and not make assumptions in what the forecast holds in the during the next 48-72 hours. If 96L continues to organize during the next day or so, an upper level anticyclone may park itself over 96L which should allow further intensification.

It would be unwise and irresposible for me to post any of the model forecasts, both track and intensity of 96L, at least for the time being. The different models are nothing but a tool for the hurricane specialists, nothing more, nothing less. I understand many want to see the different models, but the models change, sometimes with major shifts. People need to understand that the models have to taken with a grain of salt. Unless you have the skills in meteorology, be very careful as this inaccurate information has a snowball effect and causes a major headache for the NHC. When it looks as if the models begin to have a consensus, then I may post them. For now, will only post satellite imagery and any other graphics that might be useful.

Although care has been taken in preparing the information supplied through the Weather or Knot blog, Weather or Knot does not and cannot guarantee the accuracy of it. If you require an official forecast please contact your local National Weather Service Office or the National Hurricane Center.

Tropical Weather

Invest 95L

A wave that rolled off Africa a few day ago has been designated as Invest 95L. This is a system that has a very good chance to be a tropical depression within 48 hours. Invest 95L has very good cyclonic turning and it has a very moisture envelope.

(ADDENDUM – as of 5:00pm 8/21 Invest 95L developed enough to be classified and designated as Tropical Depression Six)

Satellite image of Invest 95L
LSU satellite image of Invest 95L
Water Vapor satellite image of 95L

In the water vapor image above, you can see there is some dry air to the north of Invest 95L. The forward motion of 95L is W-WNW and this should allow the system to not be hindered by the dry air. Below is an image of Wind Shear analysis. The wind shear at the moment is not to bad for the system, but it is not allowing any development. The environment for 95L is going to be changing and the wind shear will be very low. Forecast is for better conditions overall.

Wind Shear

The forecast for 95L is that it will develop into a tropical depression by sometime Monday. All models do want the system to turn NW then N and eventually curve out to the sea and away from any land masses. The problem is exactly when it will do so.


The intensity in the models seem to keep 95L in the yet to tropical storm range (at least for 48 -72 hours) for when it does develop. An outlier does want to have 95L into a hurricane but that is will have to be shown later on. Until the system fully develops, I would take the model intensity with a grain of salt.

Early intensity forecast
Tropical Weather

Invest 91L

This morning an area of some very intense thunderstorms has become better organized and has now been designated as Invest 91L. The NHC has this system at a 60% chance of becoming a tropical depression by Tuesday morning. Satellite loop imagery is indicating that the intense thunderstorm activity and intensity has been increasing. The latest satellite images seem to indicate that 91L has very good cyclonic turning and may be beginning to form the surface circulation even though 91L is still within the ITCZ and still to close to the equator. This may be the beginning for 91L to break away from the ITCZ and if so then 91L may be classified as either a tropical depression or tropical storm later today or tonight.

RGB Satellite image
AVN Satellite image

With the SST’s in the area at around 29° centigrade and the SAL to the north of 91L along with the wind shear at 10-20 knots, further development is a distinct possibility. The only negatives for any development for 91L is the MJO, which favors downward motion over the tropical Atlantic and as stated earlier, being to close to the Equator.

Later in the week, as 91L approaches the Lesser Antilles, a strong upper-level low near Puerto Rico is forecast to bring high levels of wind shear. This would hamper any quick intensification. As usual, any long range forecasts will have changes so to state where and how strong any system will be is just speculation.

Steering layer forecasts to keep 91L on a WNW track at least for the next 1-2 days, after that a WNW – NW motion is forecast. All those in the Leeward Islands should keep a eye on 91L.

Wind Shear
8am models