Watching Central Atlantic Disturbance for Tropical Development

By: Levi32 , 2:27 PM GMT on July 30, 2012

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July is closing out as a very quiet month in the Atlantic, which is to be expected in an El Nino year, but we are now heading into August where activity starts to pick up as the peak of the season nears. Evidence of this has been emerging over the last week or so as stronger tropical waves have moved off of Africa and started moving across the Atlantic. One wave is moving across the Antilles right now, bringing showers and thunderstorms, but will be quickly torn apart by the Caribbean trade winds and will not be a significant threat for development. Behind this wave, however, is an area of low pressure embedded within the ITCZ that has more potential for development over the next few days. This low has been designated Invest 99L. The CMC for a couple of runs now has shown development, and the GFS and UKMET at one point or another have both supported development of a tropical storm prior to 99L reaching the islands. The ECMWF has been relatively unimpressed. 99L is currently broad, and needs to gain latitude away from the ITCZ if it is to develop a tighter circulation. If it is going to try to develop, it will likely be about 48 hours before it tries in earnest.

There is still a lot of sinking air in the tropical Atlantic right now as the MJO upward motion pulse is over in the Pacific, away from our area of the world, so dry air and suppression of convection will be struggles that 99L will have to deal with. The system will be crossing the Antilles Islands in about 5 days, though how much latitude it will have gained by then is uncertain, and largely depends on how much it strengthens up until that point. A stronger system will tend to gain more latitude. This will become important because if 99L spends any significant amount of time in the Caribbean, it will likely die off, suffering the same fate as the wave ahead of it due to the strong trade winds there. If, however, 99L can quickly strengthen and lift north of the Caribbean island chain, it may have a better chance at becoming or remaining a tropical cyclone. The HWRF and GFDL models are supporting the idea that an on approach to the Antilles 99L will already be weakening due to the increasing trade winds in which it will be embedded, which is significant given that these two models often strengthen storms even in the face of unfavorable conditions in the deep tropics once they get going.

If 99L does move north of the Caribbean, it will likely recurve somewhere from the U.S. eastern seaboard to Bermuda due to a mean trough that is forecasted to be over the eastern U.S. during the next 10 days or so, a pattern that prevails often in summers like this one. It is too early to speculate on any potential effects that 99L could have on land masses west of the Antilles Islands.

Overall, 99L is a threat for potential development during the next 4-5 days, and may affect the Antilles Islands with tropical storm-like conditions even if it is not a tropical storm by this weekend. Explosive development is unlikely given some of the unfavorable conditions the system will be dealing with, but this opportunity does herald the start of the peak of the hurricane season, where more of these disturbances and tropical waves in the central Atlantic will try to develop, especially once they get north of 20N.

We shall see what happens!

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14. troy1993
5:23 PM GMT on July 30, 2012
Hey Levi32 do you think that we will once again go another year without having a major hurricane making landfall in the U.S and are we in a cycle where major hurricane landfalls in the U.S are becoming less common?
Member Since: July 29, 2009 Posts: 0 Comments: 208
13. floridaboy14
3:17 PM GMT on July 30, 2012
Quoting Levi32:


Mostly because of big ridging over SE Canada. You can investigate the "why" of many things by using this site to plot past data.

thanks so much Levi. Ive learned a lot from you and I appreciate your website and the tidbits you make
Member Since: July 25, 2011 Posts: 0 Comments: 1102
12. Levi32
3:11 PM GMT on July 30, 2012
Quoting floridaboy14:
interesting thanks. just curious but why did 2004 have so many landfalls in the US? Ivan Jeanne Charley Frances. They were so many. sorry for asking so many questions. im just trying to understand why.


Mostly because of big ridging over SE Canada. You can investigate the "why" of many things by using this site to plot past data.

Member Since: November 24, 2005 Posts: 635 Comments: 26543
11. floridaboy14
3:09 PM GMT on July 30, 2012
Quoting Levi32:


It sometimes can, and sometimes cannot. In the summer the NAO does not always define the weather pattern very well. Specifically, the combination of a negative PDO and negative NAO often favors eastern troughing.
interesting thanks. just curious but why did 2004 have so many landfalls in the US? Ivan Jeanne Charley Frances. They were so many. sorry for asking so many questions. im just trying to understand why.
Member Since: July 25, 2011 Posts: 0 Comments: 1102
10. Levi32
3:00 PM GMT on July 30, 2012
Quoting floridaboy14:
ok thanks and i know you mentioned something about a negative NAO which favors troughiness over the East coast so does that mean positive NAO favors ridging?


It sometimes can, and sometimes cannot. In the summer the NAO does not always define the weather pattern very well. Specifically, the combination of a negative PDO and negative NAO often favors eastern troughing.
Member Since: November 24, 2005 Posts: 635 Comments: 26543
9. floridaboy14
2:59 PM GMT on July 30, 2012
Quoting Levi32:


I didn't give weight to any particular scenario. The east coast could potentially receive long-term impacts, but it is too soon to tell. We have to get the thing to develop a bit first.



The trough is not as strong on some GFS ensemble members.
ok thanks and i know you mentioned something about a negative NAO which favors troughiness over the East coast so does that mean positive NAO favors ridging?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
8. AtHomeInTX
2:53 PM GMT on July 30, 2012
Thanks Levi. Glad you're feeling better. :)
Member Since: August 24, 2010 Posts: 0 Comments: 671
7. hydrus
2:52 PM GMT on July 30, 2012
Thank you.
Member Since: September 27, 2007 Posts: 1 Comments: 20345
6. Levi32
2:50 PM GMT on July 30, 2012
Quoting GetReal:
Impressive analysis Levi!!! I am not as confident as you that a more northward track towards the southern Bahamas would necessarily curve north missing the East coast. I am not sure at this time as to how strong that trough will be and where it will be located.


I didn't give weight to any particular scenario. The east coast could potentially receive long-term impacts, but it is too soon to tell. We have to get the thing to develop a bit first.

Quoting floridaboy14:
thanks levi for the update. good to see you feeling better. why do some of the GFS ensembles want to take 99L into florida? is it because it misses the trough?


The trough is not as strong on some GFS ensemble members.
Member Since: November 24, 2005 Posts: 635 Comments: 26543
5. floridaboy14
2:46 PM GMT on July 30, 2012
thanks levi for the update. good to see you feeling better. why do some of the GFS ensembles want to take 99L into florida? is it because it misses the trough?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
4. GetReal
2:42 PM GMT on July 30, 2012
Impressive analysis Levi!!! I am not as confident as you that a more northward track towards the southern Bahamas would necessarily curve north missing the East coast. I am not sure at this time as to how strong that trough will be and where it will be located.
Member Since: July 4, 2005 Posts: 204 Comments: 8806
3. Levi32
2:42 PM GMT on July 30, 2012
Quoting GeorgiaStormz:
you dont sound like you were sick at all


I'm over the hill now. The voice mostly came back today so I could do the video.
Member Since: November 24, 2005 Posts: 635 Comments: 26543
2. Hoff511
2:40 PM GMT on July 30, 2012
Thanks Levi!
Member Since: September 2, 2009 Posts: 0 Comments: 265
1. GeorgiaStormz
2:40 PM GMT on July 30, 2012
you dont sound like you were sick at all :)
Member Since: February 11, 2012 Posts: 0 Comments: 9718

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Levi Cowan has been tracking tropical systems since 2002, and is currently working on his bachelor's degree in physics at UAF.

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