Since I have had a couple of inquiries about Investigation 99L, the system currently passing through the northern Antilles, I guess this is as good a time as any to initiate coverage. I will apologize right now for a long post, but this is a complex system. Here is what we know as of noon Wednesday:
- Hurricane Hunter aircraft found a large area of 50 knot winds and heavy thunderstorms, but no closed circulation.
- The US GFS model is having a great deal of trouble initializing this system. But what’s new there. It will catch on soon.
- The Euro Model has a good initialization and is solid on its future track, along with NAVGEM and a few others.
- Everyone from Miami to Corpus Christi needs to be keeping an eye on this system!
I know what many of you are saying. “Why should we care about a system which can’t get its act together and is almost 2,000 miles away. Plus the models show the storm is going to recurve out into the Atlantic?” The answer is 99L is a paradox. It has been one of the more impressive systems coming off the African coast over the past few weeks, yet while Fiona and Gaston formed, it never could get its act together. The problem is a pesky area Saharan dry air and dust which swept off of Africa with the the thunderstorms associated with 99L. Dry air is one of the most intense nemesis of tropical systems. As 99L has crossed the tropical Atlantic, the dry air has moved in lockstep inhibiting development. Yet the system has continued to look quite strong throughout? Now normally, that Saharan dry air layer is our friend. It inhibits development and much of the time kills systems before they ever get going.
Looking at the water vapor imagery from this morning we can see what I have been talking about. Invest 99L is in the circle, while the two arrows point to areas which show up as a reddish brown on the water vapor image. That color represents extremely dry air. The kind we normally see over the plains when humidity is an abysmal 4-6 %, not what we expect over the tropical Caribbean and Atlantic. Can you see the problem though? That’s right, 99L is moving up and away from the dry air, as it moves to the west-northwest.
But with the dry air now well away from the system, should it not have gotten its act together? Yes it should. If the dry air was the only negative influence in play. Fortunately, and unfortunately, it is not. If dry air is the number one nemesis of tropical systems, then wind shear is a close second. Tropical systems need moist low pressure at the surface and high pressure where the jets fly to create the chimney effect which allows it to spin up into the monsters we know so well. Here is a look at the wind shear map from this morning.
It is a bit tough to understand at first, but I have some lines to help you. Basically, the yellow streamlines are positive for tropical development and the red streamlines are negative. Note just to the north of the white circle representing 99L is an area of intense negative wind shear. It is pointed out by the arrow and the leading edge outlined in white. Normally this too would be a good thing. But just like the dry air, 99L will soon move away form both of these negative influences and into an area much more favorable for development and moderate intensification.
Unfortunately for us on the Gulf Coast, the fact that Saharan dry air and shear have disrupted the development of 99L actually negatively impact us as well. You can kind of thing of tropical systems as domes of associated weather moving over the surface of the ocean. When they are disorganized like 99L is right now, they are very shallow domes but quite wide. So while they may cover a few hundred miles across, they are only, on average maybe 10,000 to 15,000 feet tall. As they become organized and spin up, they pull in the outer fringes and become taller and narrower. Finally as full fledged hurricanes, they become very tall and often quite wide. It is the height of the dome we are concerned with here. Remaining disorganized and thus relatively short in height, 99L is under the influence of the low level winds, which are pushing to the west and west-northwest. That points it at the Florida coast and later Gulf. Had it grown into an even moderate tropical storm, it would begin to be influenced by those steering current meteorologist talk about all the time with hurricanes. These are the strong upper level winds. Right now, there exists a weakness in the atmosphere. It is an area of low pressure between a strong upper level high in the Gulf and the massive Bermuda-Azores High which is a major player this time of year in the Atlantic. You can see what I mean here. If 99L were a well developed system, it would almost assuredly recurve into the weakness I have outlined.
But that is not to be! Over the next few days, 99L will almost certainly spin up into a strong tropical system. But the atmosphere is constantly moving and bynthe time it gets its act together somewhere near the Bahamas, it will be too late to recurve. The models sow the weakness will pinch off and a ridge of high pressure will replace it as the Gulf high shifts eastward. Thus the lower and upper-level steering will become west-northwest and then more than likely back to due west. That aims it at Florida and eventually the Gulf. Now exactly where it will be and where it will go is anyone’s guess. All we can do is look at what the models have in mind. But first, take at look at the sea surface temperatures this system is about to encounter, without the negatives of dry air or adverse shear.
Those waters are 80-85 degrees Fahrenheit and conducive to supporting a major hurricane. So it is not unusual that the models, excluding the GFS which does not have a clue at the moment, are now hinting at a growing system moving to the west-northwest, then curving back to the west and aiming at the Florida east coast as a Category I to Category III hurricane. Obviously, intensity is much more difficult to predict until we have an actual system to model. As I finish this post about 1:30 PM Wednesday, the hurricane hunters and visible satellite imagery is hinting at a nice cyclonic spin just northeast of Puerto Rico, somewhat ahead of the mass of thunderstorms. This would be consistent with the shear noted above and it is a reasonable assumption that once the shear is left behind the storms will begin wrapping around the center and then 99L will be off and running as either Hermine or Ian. I have to hedge my bets there as there is another system coming off the Cape Verde Islands which could just spin up before 99L. Thankfully, it is not a threat for a long time if ever. I hate to post the models now as the GFS, which so many US based models are based upon, just does not have a handle on the system yet. I almost does not even know it is there. But the other major models like the ECMWF, CMC, and others are locked on.
The image to left is the latest set of tracks. The ones which ahve the system just kind of wandering in the Bahamas are based on the GFS and should be discounted for now. The ones with that west-northwest to west, then west-northwest track again are more in line with the thinking of most. Do not put ANY weight in these lines. Error rates at the end of these runs are over 300 miles north and south of the center line. But you can see the worrisome trend of a system heading towards South Florida and then the Gulf. Only time will tell us for sure.
Here we have the intensity models. Once again, all those lines in the lowest white section and lower portion of the green are generated by the GFS model initialization and should be discounted until the GFS has a firm understanding there is a system out there. One of the intensity models which has done well over the years is the SHIPS model. If anything, it tends to be a bit aggressive in intensifying systems. Most of our best intensity specific models will give us a much better picture once the system gets is act together and the GFS does the same. For now it is just something to watch. But watch it we must. I know it is the last thing in the world many in our state want to even hear, but I would rather they be watching than ignoring the potential the system has. Only time will tell for sure. I will be following it here until the threat is either somewhere else or past.
