A few interesting things happened today with Invest 97L as 96L fell apart. As I mentioned last night, we would have to see the ULL pull away from 97L in order for any development to occur, and that in fact…sort of happened. I put it in that context because we saw the ULL split into two areas, one that is falling apart north of Puerto Rico and another more robust area near Hispaniola. The two areas have allowed upwelling to occur on the northern side of 97L, which is a good thing.
Generally when someone talks about upwelling, it is often in an oceanic context where something atmospheric feeds off of the surface layer of the ocean and drains it of heat. In the atmospheric context, as used here, two areas of low pressure ventilate the area of low pressure in the lower levels of the atmosphere. Here, we have a couple upper level lows whisking away mass and heat from the “center” of 97L at the mid and upper levels. This is a case where shear is favorable over a tropical system.
This evening, that effect seemed to wear out as convection near the center of 97L has waned since early evening. The “center” of 97L is north of Barbados, and continues to move generally toward the west. Earlier today, the island of Barbados had a semi-reliable westerly wind, which to me signifies that 97L took a run at trying to become a tropical depression earlier today before thunderstorm activity fell apart. 97L has winds of 30-35 mph and a pressure of 1008 mb. Most thunderstorm activity tonight is well to the east of the center of 97L.
The track of this system in the long term will nearly entirely depend on the intensity of the system. Currently, the system is being guided toward the west at the surface by the subtropical ridge, which is aligned east to west from the Bahamas eastward. There is a trough in the picture, but I believe this will be a case where the train will miss the station for the most part. The models are in two camps right now with a splitting tropical wave or a more centralized spin in the Caribbean.
Scenario #1: GFS/Euro solution: As the tropical wave reaches Puerto Rico, it produces two areas of spin (or vorticity) north and south of that island. What I think will actually happen with the second piece of energy is that it comes from a wave following 97L. This will likely take a weakness that may or may not be present north the NW. This solution takes 97L toward the west as a struggling mess, and eventually makes landfall in central America. I am much more inclined to take scenario #2:
Scenario #2: UKMET: This scenario keeps a stronger subtropical ridge and a more centralized vorticity toward the WNW over the next few days. This more centralized vorticity will form in about 24-48 hours in the central Caribbean. I believe that some of the hurricane a barotropic models are taking this scenario a little too quickly. For instance, the GFDL spins up this circulation, which currently is stretched out (Below), within 24 hours and quickly intensifies it before moving into Hispaniola.
Even though it is now September, I am not buying the rapid intensification story that numerous models are trying to sell. The truth is that this storm is elongated, and it will take a night or two for this system to centralize and develop. Only once it does that, can it tap the better than all season conditions. Note that I did not say favorable conditions. In 4-5 days we will see more favorable conditions, but in the short term there will be 15-25 kt shear around this system and a bit of dry air to work out. If this system is to get a name, it will be in 3-5 day or beyond.
This system struggles over the next 24-48 hours before showing signs of organization in the central Caribbean. 97L will keep a WNW heading until roughly 75W when it should be in the rough proximity of Jamaica as a developing system.
Scattered moderate thunderstorms to isolated strong thunderstorms will move through the central and lower lesser antilles overnight into tomorrow night. Gusty winds are probable. Seas will be choppy at times, especially in the passes between islands as the system passes through. Any localized torrential rainfall may lead to mudslides in mountainous areas of the Caribbean over the next 5 days. Stay prepared!
As we start September, we inch closer to the peak of the Atlantic Hurricane Season. During the first 10 days of the season, most storms form in the Gulf of Mexico or the Mean Development Region, which stretches from the Lesser Antilles to the coast of Africa. Hurricanes form this time of year more than any other time of year because water temperatures are the warmest and winds in the layer of the atmosphere where clouds reach their peak heights are the weakest.
The Atlantic is heating up as we reach the peak month for activity in the basin. There are currently two invests in the Mean Development Region (MDR) with a low chance of development in the short term. After Fernand made landfall last week, the basin went pretty quiet with the exception of the African wave train. A steep increase in instability, a drop in dry air, and an increasingly favorable MJO should resuscitate the Atlantic over the next week.
The first of two invests (96L) is fresh off the coast of Africa and seems to be undergoing poofogenesis before even making it to the Cape Verde Islands. The current spaghetti models show a general trend toward the northwest with a decent gain in latitude over the next few days. With environmental conditions (dry air, shear) becoming prohibitive for development, the time for this system is quickly running out.
Impact: Expect heavy rainfall in the Cape Verde Islands over the next day or two with higher than normal wave activity.
The second of two invests in the Atlantic, Invest 97L, has much more of my interest at the moment. The Hurricane Hunters showed a broad circulation 350 miles east of the lesser antilles last afternoon, and convection has been on the increase since their departure from the system. With dropsonde data in the models, the current thinking is that 97L will trend toward the west through the lesser antilles as a weak disturbance with gaining organization. An Upper Level Low is exhibiting shear in the area of 15-25 knots on the northern side of 97L, which combined with slightly less than favorable mid level moisture should keep 97L weak for the time being. The forecast challenge beyond 48 hours become whether or not the ULL disengages with 97L or not. Currently, I liken the situation to two people running down a pair of lanes toward a finish line of some sort with a tree in the middle which will obstruct both paths. That tree, is of course the island of Hispaniola. In order for any organization to take place, the ULL must either weaken or gain latitude at a faster rate than 97L. After 2 days, 97L will be out of the eastern Caribbean death zone (a climatological anomaly in the basin that prohibits growth), into warmer SST’s and ocean heat content, in slightly more humid atmospheric columns, and should be temporarily far enough from land that chances of development are decent.
Thus far, this has been one of the most climatologically “normal” systems we have tracked. The others being Fernand and Andrea. Climatology would suggest slow development over the next few days, and some of the models show enhanced development after 3 days.
Impact: Expect heavy rainfall in the central lesser antilles over the next 36 hours and enhanced rainfall chances in the Caribbean over the next 5 days. Waves should be in the 6-9 foot range in close proximity to the tropical wave as it passes, with higher waves in the open Atlantic north of the islands. Gusty conditions are possible. This may become a threat to the Greater Antilles and the Bahamas in the days to come…be prepared for tropical weather as we head into the peak of the season. More than half of the season is still ahead of us activity wise.
Special Note: This was the first post created in part using a mobile device. I should able to initiate posts and respond to twitter (@JonathanBelles) with more ease with this new capability. Expect more frequent posts over both medium.
Just a short update tonight since much of my thinking is the same as last night. Both systems in the Atlantic have developed, and the east Atlantic system has become Tropical Depression #5. The western Caribbean system seems to be organizing, but it is currently in a downward trend for the time being.
Invest 92L /the Caribbean low pressure system should continue to organize and develop over the next few days. Most of the models are very similar to what they were yesterday. There remain two distinct camps in the track models with one scenario taking 92L to the west as a weak system and the other scenario taking a stronger system to the NW then NNE toward the central Gulf. The associated tropical wave will be the main engine running this system for the majority of the next 24 hours. If 92L can get it’s self going and if it can form into a tropical cyclone by the time it hits the Yucatan Peninsula, the chances are better that the second scenario will play out. If shear remains higher and land interaction becomes a bigger problem, then the first scenario will likely play out. In either case, a lot of rainfall will be strewn from the Yucatan toward the northern Gulf Coast. From New Orleans to Atlanta to Savannah, residents can expect 4-6 inches of rain with locally higher totals. We’ll have to nail down any wind impacts when 92L gets into the gulf since it seems to early to nail down any direct path right now. I do think that 92L will become a tropical cyclone either tomorrow or early on Friday. I believe that TD (discussed below) will become Erin, but the 2nd name down the list is Fernand.
Below are the forecast tracks and the rainfall forecast from the Weather Prediction Center:
Tropical Depression Five has formed this evening in the far east Atlantic. In the very near term, TD5 will produce heavy rains and locally high winds in the southern Cape Verde Islands. TD 5 is under conditions that should allow some slow growth over the next 3 days before TD 5 hits slightly cooler waters, drier air, and a bit more shear. This may be Dorian part two, but the leeward islands may want to watch the path of this system toward the middle to end of next week. This will likely take the next name on the list, which is “Erin” .