The same amount of caution must be used when discussing the possibility of a non-hurricane hit as when we talk about one that could hit a particular location, no matter how general. This is one of those cases and my aim here is to point out the reasons why I believe that 96L or what ever it becomes will turn away from the United States. Of course, this is how things look now, obviously the weather is subject to change but sometimes you get clues that are pretty large and cannot be ignored.
First, the here and now.
96L is a large, sprawling low pressure area without a well defined center of circulation. Such large envelopes of energy in the tropics often take time to consolidate and close off a low level center.
Dry air continues to impede development to some extent but the overall lack of upward motion or, to put it another way, sinking air, seems to be a big issue, perhaps more so than dry air. As you can see in the graphic I have posted, 96L is located in an area that is only marginal for rising motion. Sure, there are worse conditions elsewhere around the globe but it is not located in a prime area for divergence aloft which would allow for more efficient upward motion and thus convection.
The motion is currently WNW and this should continue for the next day or so. At this track and heading, showers and squally weather will spread over portions of the Lesser Antilles today and tomorrow. Eventually, Puerto Rico and parts of Hispaniola could feel the effects with heavy rain and gusty winds possible. For now, it’s the eastern most islands of the Caribbean Sea that are having to deal with this slowly developing system.
So what about the future? Some of it is complicated but overall, I think the pattern is showing us what should ultimately happen.
Right now, 96L is being driven west by a ridge of high pressure to its north. It acts like a ceiling keeping a helium balloon from escaping in to the sky while a fan blows it laterally along the plane of the ceiling. If you were to open a hole in that ceiling, up, up and away the balloon would go. The same is generally true for tropical cyclones. High pressure at the steering layers keeps them moving more or less westward. As the pressure eases off, the cyclones literally gain latitude or move poleward (north). If the Atlantic were dominated by one large area of high pressure, like we saw in 2008 with Ike, then tropical waves that develop could travel from Africa to Texas without any chance of turning north and out to sea. This does happen but has not been the case for quite some time. The reason? Troughs of low pressure that erode the Atlantic ridge down and create the hole needed for a hurricane to escape through.
Check out the second graphic I have posted. It is the 500 millibar forecast from the most recent GFS model run (valid 5 days out). Notice where 96L (or what ever it may be at the time). Also note the two well defined areas of high pressure on the map. One is over the eastern portions of the USA. The other is located over the central Atlantic. These massive domes of air will not allow a tropical cyclone to penetrate them. Instead, they have to go around like a blob of jello navigating around more dense blobs of jello. They each react to one another in subtle ways but the end result is that the smaller blobs (hurricanes) cannot go through the larger, more dense blobs (high pressure areas).
Now notice the trough over the Atlantic between the two ridges – this is the opening between the continental ridge and the Atlantic ridge. If the two were connected, then we’d be talking about a system cruising through the Caribbean Sea right now. Instead, a through of low pressure has carved out enough of an opening in in the Atlantic ridge to allow a possible escape for what ever 96L evolves in to – be it a tropical storm or a hurricane. It will “feel” that weakness or hole and likely turn away from the Southeast U.S.
The GFS global model and the ECMWF global model both show this scenario right now. Can it change? Yes it can but the overall pattern is not likely to change enough to affect the outcome and that is to send 96L packing and out to sea.
There is one glaring issue that needs to be considered. We are talking about computer model data that is days away from becoming reality. These are computer projections of the weather almost a week out in time. I am wanting to explain the situation as it appears today. This also strongly emphasizes the issue of posting graphics that show a hurricane knocking on the door of some coastal location in seven to 10 days. It’s nonsense. Especially without any explanation beyond “this is just one model run and it could be wrong”. Anyone can say that – I wanted to point out the actual players, so to speak, and explain the process in a way that I hope you can understand.
For today, we know that there is a slowly organizing tropical wave and low pressure area which is affecting parts of the eastern Caribbean Sea. Recon is currently investigating the system now and we’ll know more about its structure, etc. soon enough. From there, we will see what happens as each model cycle gives us more clues as to the eventual outcome.
In the eastern Pacific, busy is not strong enough of a word to describe the activity there. We have TS Karina along with hurricane Lowell. In addition, another large low pressure area is about to become the next depression well off the coast of Mexico. It too should strengthen in to a hurricane but is forecast to track parallel to the coast and not bring any significant impacts to the region.
I’ll have another update here late tonight.
M. Sudduth 2:25 PM ET Aug 21