A tropical disturbance to watch this week
Posted August 17, 2015
Now that we're passing mid-August, we're heading into the 6-8 weeks or so that tend to be the peak period for Atlantic hurricane activity. As has been well-publicized already, due to modest sea surface temperatures and also a heightened potential for disruptive wind shear that goes along with the El Nino pattern that's building in the Pacific, this is expected to be a below-normal season for activity in the basin.
Just the same, disturbances will be generated from time to time, and it appears we'll see a series of those in the next week or so coming off west Africa. The tricky part will be the degree to which they can organize and move westward without encountering too much shear, and too little heating from the ocean, to grow into named storms and possibly hurricanes.
The first in the series of waves has moved a bit into the eastern Atlantic now and is being tracked by the National Hurricane Center, which is giving it about a 70 percent chance of becoming a tropical cyclone within the next five days. You can see from the model tracks in the WRAL Interactive Hurricane Tracker a fairly typical westerly motion through that time frame. I also pulled a couple of individual model frames from the Penn State "e-wall" Tropical section showing how the U.S. Global Forecast System (GFS) model and the European model compare to each other by the time we get to Thursday evening at 8 p.m.
In both cases, the models show rather weak circulations (the orange or orange-to-red contours in the lower middle right), with the GFS indicating a slower moving, but more compact and organized system, and the European about 450 miles or so farther west by then, but with a very much weakened system that might be hard-pressed to maintain itself as either a tropical depression or named storm.
So, while there does appear to be reason to think the circulation will have a window in the next few days to move through a region with water temperatures just warm enough, and vertical shear low enough, for some intensification, it's quite possible that later in the week it could encounter more hostile shear that causes it to weaken or dissipate. I also took a peek at an "ensemble" of GFS runs, and noted that out of 20 runs from slightly varied starting conditions, only six of those maintain the circulation once we reach Friday or Saturday.
We'll continue to track the system for any changes in its current or projected status this week and watch for how any additional disturbances exiting Africa may behave (the European model currently forecasts a much better organized storm from a new wave early next week, but that's a long time out - stay tuned!)