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July 2012

Watching Central Atlantic Disturbance for Tropical Development

   Posted by Levi at 2:22pm 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!


Atlantic Still Quiet, though Tropical Wave May Bring Wetness to Bahamas and SE U.S.

   Posted by Levi at 7:44am on July 11, 2012

I apologize for the lack of posts since Debby’s dissipation. Summer has been busy so far for me.

The Atlantic remains quiet through the first 10 days of July, as was expected. It will continue to be this way through at least the end of the month the way things currently look. El Nino years favor a quick-firing start to the season in June, but a quick ramp-down to start the heart of summer. Our next real shot at tropical development will likely not come until August when the MJO comes back to the Atlantic and drags some upward motion out of the eastern Pacific, where they have a whole train of storms going, and moves it into the Atlantic as the African wave train strengthens.

Speaking of waves, the only somewhat interesting feature to watch right now is a very large, dry tropical wave along 45W that has no thunderstorms associated with it, but can be seen with a vigorous signature in the mid-level flow.

This wave will be moving WNW into the the region between the Bahamas, Bermuda, and the SE U.S. during the next 4-7 days, being steered towards this region by a strong Bermuda High to the north. While none of the computer models currently do anything with this wave, moisture will be becoming more abundant in the SW Atlantic as the wave moves in, and upper-level temperatures are and will continue to be colder than normal due to significant upper-level troughing over the Bahamas, which overlaying 28-29C waters can cause some instability. As a result, some convection can be expected off the SE U.S. coast that may bring some wetness to the Bahamas and the coastal SE U.S. over the weekend. Given the large size of the wave, tropical or subtropical development chances seem low for now, especially with no excitement from the computer models, but expect the atmosphere to become rather soupy in the region with at least some thunderstorm activity, perhaps a little more than currently forecasted by some of the models.

CMC 96-hour forecast:

If you couldn’t tell, I’m showing off some of the new graphics I have been developing in my spare time. The first image was a GDAS analysis of 650mb meridional wind/heights, and the 2nd was the well-known Canadian model forecast. These and more products can be found on the Analysis Tools page. I will be adding more as time goes on.

Overall, the Atlantic in general will likely remain void of significant tropical activity until August. Remember, a fast start doesn’t always mean an above-normal hurricane season. We should end up with a near normal storm count by the end of it.

We shall see what happens!