Invest 99L forms. Will it become Ernesto?

After a month of tranquility across the Atlantic, the Cape Verde season has kicked into gear across the Mean Development Region. On one end, a tropical wave is bringing showers and storms to the Lesser Antilles and to Puerto Rico. On the other, convection persists along the monsoonal trough, which essentially is an elongated area of low pressure that circles the globe between 5°S and 15°N. The monsoonal trough is currently located around 10°N in the Atlantic and will continue to envelop the Cape Verde season during the next 2-3 months.  In the middle is an area of low pressure embedded in a tropical wave in the central Atlantic.

99L currently has winds of 25 mph and a pressure 1009 millibars. 99L is rather disorganized with its ‘center’ embedded in the deepest thunderstorms, but the future is bright. In the short term, 99L will move just north of due west around the southern periphery of the Bermuda High. A quick look at shear analysis, water vapor forecasts and upper level flow shows quite favorable conditions at least in the short term, and very warm waters will supplement a favorable forecast for slow growth. Below is the current satellite image of 99L (11:20p ET) with the current NHC position overlayed (00z/8:00p ET). The deep oranges and reds are tall/strong thunderstorms and the cooler colors to the north represent clear skies and the Saharan Air Layer, which should remain to the north with no inhibitory effects.

Most models continue to be favorable with 99L with slow nightly development. I expect the next couple of days will be down days and up nights that will heavily lean toward diurnal thunderstorm development and organization. I am forecasting tropical depression development on either late Wednesday or on Thursday with the possibility of tropical storm strength by Friday if favorable conditions persist. My only hesitation by days 5 and 6 are an uptick in upper level shear and any land impact. 99L will be moving into the Caribbean on Friday or Saturday, likely in the central Lesser Antilles due to its current southern position. The system will continue to be guided westward by the Bermuda High for the next 4-6 days before turning WNW ahead of a trough that will be in place across the eastern United States.

As far as intensity goes, it is too early to forecast, but with favorable conditions in play slow development every day is likely. Upon reaching the lesser Antilles, I believe that a low-balled forecast of 40-45 mph is reasonable, but for those living in the islands be prepared for a higher forecast. Heavy rain and high seas can be expected whether or not this gets upgraded to a tropical cyclone.

Also of note for Floridian weather, that tropical wave I mentioned on the western side of the Atlantic…expect that to be seen by Friday in the form of enhanced thunderstorm chances and slightly lower high temperatures.


Posted on Monday, July 30th, 2012, in Florida Weather, Tropical Weather. Bookmark the permalink. 10 Comments.

  1. I am planing a trip with my 3 small children on the 6 th of Aug –
    the 11 of Aug. to St.. Johns USVI. Do you feel this storm will be a concern to cancel our vacation plans. I want to make sure my family is safe and don’t want to put them in harms way. I look forward to your response.
    Thank you


    • This tropical system should be on its way out of the USVI on the 5th with only remaining effects on the 6th, but 6-10 days out and things can change. Stay out of the water when you first arrive as surf will likely be up. Otherwise I’d say stick to your plans and assume that there will be no effects on your trip. I’ll keep you updated. Have fun!


    • Jonathan
      We are excited to leave for our trip to St Johns tomorrow. With no concerns of Ernesto. But a concern of Florence. Please up date me on your thoughts and if we are in the path of this storm, what to expect and if there is a concern not to travel. I look forward to. Your response

      Thank you


  2. Thank you. Please keep me up dated should something change for the worse.


  3. Hi there! Just checking in with you for an up date. From what I am hearing St Thomas / St Johns is not looking so good.
    When do you think the storm will hit the usvi islands? How long will the effects of the storm stick around ? Will the weather be crapy the whole time I am there meaning will I see the sun.
    Most important question of all do you still feel it will be safe for me to travel to St Johns the 6–11.


    • TD 5/Ernesto will be making it’s pass of the USVI on Saturday morning, and the only effects there will be fringe effects…at least on the islands. I would recommend anyone in the USVI stay out of the water until at least next Monday. St. Thomas only has a 17% chance of receiving tropical storm force winds, and on the current track will only see breezy conditions for about 24 hours. The system should be gone by the time you get there.


  4. What is the % for St John receiving Ernesto? Sorry I am a lay person to all this what are fringe effects, please explain. Also what kind of weather should we expect while we are on our trip?
    Windy, Rain, Cloudy, Cool , Humid, Hot , or Sunny ?……
    I not sure what u mean by staying out of the water, do you mean swimming ? Boating? Reason why I am asking is because when you arrive you fligh into St Thomas and then take a hour ferry ride to St. johns. Hope my questions don’t make me sound too stupid……..
    Thank you for your time


    • The chance of St. John seeing a direct hit from what will likely be Ernesto is very low. I would expect some wind (less than 20 mph) and scattered showers during your entire trip, which is pretty normal this time of year in the Caribbean. It will be hot and humid with rain a few of the days.

      By staying out of the water I mean boating, swimming, snorkeling, and anything else you can do in the water. As far as the ferry ride, it might be a bit bumpy, but the ferry drivers should be able to handle it.

      Have fun on the trip!


    • Thank you


  5. Just a concerned mom of 3 babies sorry for all the over loaded questions


What do you think?

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: