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Tropical Depression 9 has formed in the Caribbean this morning. Eastern Gulf & Florida on alert.

Meteorologist Kelli Rosson will have a blog discussion with a full breakdown of local weather in the Tennessee Valley coming later today. This morning blog discussion is going to focus solely on newly developed Tropical Depression 9 in the Caribbean and its likely track and impacts to the eastern Gulf Coast. While it's looking less and less likely that this system will locally impact us here in the Tennessee Valley, we know that fall break is next week for many people, and a lot of folks head to places like the Gulf Coast beaches, Disney, etc., for that mini vacation. That makes it imperative that we give this storm system special coverage, despite it being increasingly unlikely that we see local impacts in our immediate viewing area.

As of 4:00 AM Central Time, Tropical Depression 9 has officially formed in the Caribbean. The morning advisory from the National Hurricane Center has maximum sustained winds at 35 mph with pressure down to an estimated 1006 millibars. Hurricane hunter aircraft will be in there this morning to investigate, and we will start getting official measured readings at that point. Movement is west-northwest at 13 mph. The center of circulation is currently located a little north of the island of Aruba within the ABC Island chain, and about 615 miles ESE of Kingston, Jamaica within the Caribbean. The official forecast from the National Hurricane Center here has this becoming Tropical Storm Hermine later today and becoming a hurricane by Sunday overnight into early Monday morning as it crosses near the Cayman Islands before crossing western Cuba and approaching southwest Florida as a strengthening upper-end Category 2 hurricane. We will talk in much more detail about the forecast for Tropical Depression 9 a bit later here in the blog discussion.


A closed low-level circulation did develop by yesterday afternoon in association with the tropical wave, but thunderstorms have developed and significantly organized with that center of circulation overnight, allowing the system to develop into a tropical depression. However, strong upper-level northerly wind shear from the outer upper-level outflow of Hurricane Fiona to the north is still shearing the system and pushing these storms to the south and west sides of where the low-level circulation is located. Until this wind shear lessens over the next few days, TD 9 will still be able to intensify, but it will be at a relatively slow pace.


Water vapor satellite imagery across Caribbean this morning has shown the northerly upper-level winds impacting TD9, but it also shows the system is in an overall environment that doesn't have much dry air to content with. This has been one of the limiting factors with many tropical waves so far this season, but that looks to not be a limiting factor now. There is dry air in place over the Gulf of Mexico right now in association with the upper-level system that has provided our cold front. However, that is likely to shift northward over the weekend ahead of the next approaching cold front in our area, and farther removing itself from what will likely be Tropical Storm or Hurricane Hermine as it approaches.


Water temperatures from the central and northwestern Caribbean up into the eastern Gulf of Mexico are very warm and very favorable for tropical development. For tropical systems to develop, you typically need sea surface temperatures of 79F or warmer, and they are currently running in the mid 80s through this entire region, with a few isolated locations in the upper 80s. In addition, this warm water over the area is very deep. That is shown by the high values of oceanic heat content over the central and western Caribbean and up into the southeastern Gulf and the Florida Straits. This means that upwelling winds from a tropical system isn't as likely to bring as cool of water to the surface as the system moves overhead, and this provides more heat content for the storm system to use for intensification purposes. Those heat content values in the northwest Caribbean and southeastern Gulf is the most favorable out of the entire Atlantic or eastern Pacific basins right now.


As mentioned earlier, strong northerly upper-level winds from the outflow of Fiona are currently impacting TD9, and this is keeping the system lopsided and will likely keep any strengthening on the slower side from the next day or so. However, as we head deeper into the weekend, Fiona pulls away and TD9 heads westward into an environment with much lighter upper-level winds and rapidly weakening shear, and by Sunday, the only strong upper-level winds shown anywhere near now TD9 are with the actual low pressure circulation itself and NOT with the outside environment. This shows that the environment will continue to become more and more favorable for TD9 to strengthen as we head deeper into the weekend, and when combining that weakening wind shear with the very warm water and lack of dry air ahead of the system, the environment from Sunday into Monday over the northwestern Caribbean may be favorable for the system to not only strengthen into a hurricane, but possibly rapidly intensify. There is the potential that this may become a major hurricane before it reaches Cuba.


So, where do we go from here? Now that we have a closed low-level center to identify and track and we are starting to get a better handle on the upper-level pattern over the United States from the weekend into early next week, models are starting to get more into agreement with the track. This is still early on in the game, and there is still uncertainty and time for changes, but models are converging on agreement on a track northwestward in the northwest Caribbean toward western Cuba and then into the southeastern Gulf by early next week. On this track, it is becoming more and more UNLIKELY that the Alabama coastline or our local area will see direct impacts from this system. We don't want you to turn your back on it and stop paying attention, but the trend for our local area and our immediate stretch of the coastline is looking better. There is still some uncertain threat to the Florida panhandle, but the trend is lessening that risk with time. The latest trends, however, are increasing the threat to the Florida peninsula. Having said that, we are still 5 or so days out from a possible landfall there, and the angle of the coastline on the west coast of Florida, combined with the northeastward angle of approach of the storm, means that very small errors in track direction (even as little as 10 degrees) can result in a shift in the point of landfall of a few counties. This makes it impossible right now to nail down specific impacts and timing for specific areas, but we urge folks down in especially the central and southern portions of Florida to go ahead and start getting into hurricane preparation mode now.


The GFS and the Euro are the two main global models that we use when doing these forecasts, and then a lot of the hurricane specific track and intensity models (the spaghetti models and such) are then derived from these. Earlier in the week, these two models had been in wild disagreement. At one point, the GFS had the storm heading for Louisiana while the Euro was showing it recurving northeast early enough to head toward the Bahamas and almost completely miss the Gulf! However, as we have gotten a low-level center to develop over the last 24 hours and we have started to get close enough in time to get a good idea of the evolution of the trough that brings our Sunday night cold front and opens a weakness for this storm to turn into early next week, these two main models are getting into much better agreement. The GFS is still a little left of the Euro, but they are converging on an idea similar to the official track that takes the system up toward Cuba and then into the southeast Gulf and into central or southwestern Florida.


The upper-level pattern shows ridging over Puerto Rico and Hispaniola building in behind Fiona. This is taking TD9 on the west-northwest track right now. As we head deeper into the weekend, a big upper-level trough in association with our Sunday night cold front moves into the eastern United States. This opens up a weakness in the ridge and allows the storm system to turn northward in the northwest Caribbean and come up into the southern Gulf of Mexico. The location of this trough and weakness in the ridge is also what is allowing this to turn northward earlier and hook northeast toward Florida instead of coming more westward toward our region. From there, the upper-level flow becomes more southwesterly to northeasterly, and that will help shift the storm northeast in the general direction of central/southern Florida and then into the west Atlantic into the mid and latter part of the week. It is too far out right now to know whether it will be a more northeasterly track out into the open waters, or if it turns more northerly toward the Carolinas. We will know much more about that as we get toward the end of the weekend and the start of next week.


As mentioned before, the environment by late weekend in the northwest Caribbean will be very favorable for strengthening, possibly rapid strengthening, and it's very possible that the storm becomes a major hurricane prior to reaching western Cuba. The GFS has the system as a major hurricane off the west coast of Florida early next week. The Euro takes more of a rightward track across the central portions of Cuba and up into the Florida Strait and the Keys before making landfall in the Everglades. This takes the storm over higher terrain in Cuba compared to the far western areas, and then it gives it less time to recover once moving out over the water. This is a main reason why, despite still showing the system landfalling as a hurricane in south Florida, it is a good deal weaker than the GFS. A track over far western Cuba means a very short time over land and over very flat and marshy, watery land. That would provide very little chance, if any, for weakening. Then, such a track would mean more time over the waters of the eastern Gulf of Mexico and more time for strengthening before making landfall in Florida. There is also the possibility that the Euro just simply isn't strong enough with the storm, even before it approaches Cuba. Despite no land interaction and a highly favorable environment, the latest run of the Euro only shows minimal hurricane force winds on its approach to Florida. What we know about the environment the system will be in suggests that the storm may be much stronger than that as it approaches Cuba. If that is the case, it may also be much stronger than shown as it approaches south Florida despite higher terrain to interact with, simply because it will have a higher intensity to come down from. This uncertainty is another reason why it is too soon to try to pinpoint specific impacts to specific areas down there.


Hurricane intensity forecasting is also the thing meteorologists struggle with the most because of internal mechanisms, such as eyewall replacement cycles, that can rapidly change the intensity of a tropical system. These systems are NOT well modeled by our computer models at all. We can come to a general idea about intensity based on the environment, track, etc., but exact details are impossible this far out. Because of these internal and sometimes unpredictable changes, we always ask folks to prepare for a category higher than what the forecast calls for, just to be on the safe side.


The bottom line right now is that we have increasing confidence that the storm system is more of a threat to the far eastern Gulf of Mexico and Florida, and a threat to our local area is becoming increasingly UNLIKELY. There is still time for change, and for that reason, we ask that you keep an eye on the forecast and don't just start ignoring this system. However, the trends for our local area and our part of the northern Gulf Coast is heading in the right direction. The system is likely to become a hurricane as it moves across western Cuba and into the southeastern Gulf. If it isn't impacted by too much terrain over Cuba (it won't be if it heads over the western tip area), there is a very real chance the storm may be a major hurricane (Category 3 or higher) as it approaches Florida. It might even attain that intensity prior to approaching Cuba. It's too soon for exact details on timing and impacts to specific areas in the threat zone, but we are getting into the range where we can start to have confidence in these general forecast ideas and work from there.


Again, the latest trends in the data suggest that the threat of a direct impact to our local area is becoming increasingly UNLIKELY. However, we know that folks get nervous anytime there is a tropical system in the Gulf, we know folks that live in our local area have friends and family in the threat area, and we know that many folks may have vacation and trip plans next week that may include the threat area. For those reasons, we will be providing special coverage for this storm over the next few days that we might not otherwise need to provide for a storm system that won't directly impact our local viewing area. Beginning this afternoon, we will have daily specific video tropical updates concerning the storm until its threat to the Gulf Coast area has ended. This is in addition to information that we will provide in our regularly scheduled forecast video weathercasts and blog discussions.

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