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Watching the newest tropical depression in the Atlantic

Since our local weather across the Tennessee Valley looks quiet (without even any rain chances) for at least the next seven days, we are going to take the time in today's blog discussion to talk about newly formed Tropical Depression #7 in the Atlantic, and how it may play out as we head through the next several days. This system will gain a lot of interest in the social media world in the coming days. We are going to go ahead and get the jump on things and state what we know as of now, and also what is still unknown at this time...


Tropical Depression 7 formed out in the central Atlantic earlier this morning, a little over 800 miles east of the Leeward Islands. As of the 10:00 AM Central Time Advisory, maximum sustained winds were at 35 mph, and the system was moving westward at around 14 mph. This would put it crossing the Leeward Islands early in the day on Friday. The system is officially forecast to become Tropical Storm Fiona as early as later this afternoon or this evening. The official track then carries it through the far northern Caribbean, very near Puerto Rico and Hispaniola, through Monday morning. However, the track fan extends as far south as south of Jamaica and as far north as missing the big Caribbean islands to the north and reaching the southeastern Bahamas through five days.


Enhanced infrared satellite early this afternoon shows improving outflow aloft in association with the system, especially on the eastern semi-circle. However, there is still likely shear in the western quadrants and near the center of circulation. Convection is strengthening near the center and on the eastern side of the center of circulation, but visible satellite data suggests that the convection isn't too far from partially exposing the circulation center, likely due to shear in the environment at this time.


As you would generally expect in the middle of September, sea surface temperatures ahead of the system are very favorable for tropical development, in the mid 80s Fahrenheit. A few locations in the Caribbean and Gulf are closer to the upper 80s. There is also increasing oceanic heat content ahead of the system as it tracks westward toward the Caribbean, and that high oceanic heat content extends up into the Bahamas and off the Southeast Coast, as well as over into the Gulf. This means that that warm water is also deep, making it harder for upwelling over cooler water along the system's path to turn the environment less favorable. When all other ingredients are favorable, tropical systems are more likely to intensify over deep areas of favorably warm water.


Saharan dust also isn't a big inhibiting factor for this system. That is mainly east of the tropical depression, and the system is moving away from it. Water vapor imagery does show some mid-level dry air ahead of the system, but the depression will pass away from that by Friday, and convection ahead of the depression is also trying to work over some of that dry air with time. It's not an especially favorable factor, but it may not be a major inhibiting one either.


The main thing that may help to keep the system from strengthening quickly in the next few days is the presence of wind shear. Strong wind shear tilts thunderstorm updrafts and causes storms to be removed from the center of circulation. A strengthening tropical system needs those thunderstorms to be co-located near the center of circulation and the thunderstorm updrafts to stand upright so that the internal mechanics can work favorably for continued intensification. The depression is in an area of fairly strong wind shear right now, but that shear weakens a bit as we head closer to Friday and the weekend, but it will still be present along the track through the eastern Caribbean.


The Baron Hurricane Index shows conditions a bit marginal for tropical system development where the system is currently, but conditions gradually improve some and become more favorable as the system enters the Caribbean heading into Friday and the weekend. That won't stop the system from becoming a tropical storm as early as today, but it will likely keep the system's intensity in check for the time being, in agreement with the official forecast from the National Hurricane Center.


One big thing that makes both the track and intensity forecast fairly uncertain as we head out in time is the potential for the system to interact with Puerto Rico, Hispaniola, and possibly even Cuba. Interaction with mountainous terrain is detrimental to tropical systems because the friction slows down the surface winds and interrupts the system's inner core circulation. HOWEVER, a system that is weaker can often "survive" a direct interaction with these islands easier than a stronger system, simply because a weaker system has less of an inner core organization to disrupt. You can't disrupt what doesn't exist. Some of the big hurricanes in Gulf history have taken tracks across these mountainous islands, but did so as weak circulations, and then went on to become major hurricanes down the road. However, other systems of varying intensities have had their low-level circulations separated from their mid-level systems when interacting with these islands, and that has caused many of them to significantly weaken. We will simply have no idea of really knowing how these islands may impact the system until it actually interacts with them. If the system misses the islands to either the north or the south, there would be less land interaction and a better chance of significant development.


Many extended range model solutions, including the GFS and Euro, have a weakness in the subtropical ridge over the Bahamas and up the East Coast early next week as the system approaches the longitude of Hispaniola. This should allow a door to open for the system to turn northward. While doing so, there is the potential for the system to have impacts on the Bahamas. From there, the width of the weakness in the ridge and the potential for any incoming cold fronts from the west will dictate whether the system is more likely to turn safely northward out to sea, or if that northward turn across the Bahamas opens the door for an East Coast landfall risk. It is simply too early to know at this time. While less likely and not supported nearly as much by model data right now, there is also the potential for the system to either miss the islands in the Caribbean to the south or track westward along Cuba. IF that happens, it would increase the risk of the system getting into the Gulf at some point next week. However, that is not as likely at this time based on current data.


The overall takeaway is that this new tropical depression is likely to become Tropical Storm Fiona later today, and it does appear to be tracking westward toward the Caribbean. However, potential interaction with the islands down there and some inhibiting factors make it way too early to try to determine the ultimate strength of this system and if it may be a United States threat (outside of Puerto Rico). It DOES have a better shot than Danielle or Earl, but it is still very much uncertain whether it will track far enough westward in the coming days to end up as a United States mainland threat.


We will be watching carefully in the coming days, and we will be providing more information as the system tracks along and things start to become more certain.

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