Synopsis: The National Hurricane Center has designated an area of interest north of Cuba as Invest 90L. On satellite 90L is still disorganized but there is a surface circulation evident. It looks like this disturbance will move out in to the Atlantic and develop into a subtropical or tropical storm, it will be named Arthur if it does develop. It may come close to North Carolina but no direct landfall is currently expected.
The potential for tropical development in this area is something that has been anticipated for several days now. This disturbance is developing from energy left behind from a front that moved into the Gulf of Mexico earlier this week. This is a common mode of development in the Gulf and near the east coast, especially this time of year.
Whether this is a subtropical or tropical storm doesn't matter too much, conditions near the storm will still be the same. Some models are hinting at 90L making a run at hurricane strength but I think that's a bit premature. So far all of our information has come from global models and our knowledge of how these storms develop. Global models have pretty good success at predicting a storm's track but don't always perform well predicting its strength. The reason is that they don't have a high enough resolution to accurately model conditions around the core of the storm. Now that this has been designated an invest we will start to get data from the models that are used to model tropical cyclones and this will help nail down the intensity forecast better than the global models can.
As it is sea surface temperatures off the east coast aren't really warm enough yet to support a storm of hurricane strength. You'll notice on the map below that there is a color reserved for 26.5C. That is considered the minimum temperature needed to sustain a storm of hurricane strength. A hurricane isn't necessarily impossible below that, but unlikely. And even though there are a few pockets exceeding that temp it doesn't go very deep at all. You'd like to see at least 50 meters of +26.5C water. You can see that on the 2nd map, which shows how deep 26C water goes below the surface. You can read more here about what is needed for a storm to form. Another way of looking at the ocean is to estimate their heat content, or the energy that is available to a developing storm. You can see this in the 3rd map and that there is very little energy available. The lack of warm water is why I think this will be a subtropical storm, because any developing system it is not going to be completely fed from warm ocean water as with a tropical storm.
Wind shear, which is changing wind speed and direction with height, isn't very conducive for development either. For tropical cyclones you want upper level outflow (think of it as exhaust) to stay located over the low level circulation and convection (thunderstorms). This allows warm moist air flowing into the center of the storm at the surface an exhaust path out the top once it has delivered its energy. When wind shear is too high this can't happen, the upper and lower levels decouple or the upper level exhaust never forms in the first place. There may be a little pocket of lower wind shear near the storm as it moves to the northeast but it looks to be marginal at best.
That's all I have for today. The main reason I wanted to send an update is to dispel any concerns there were about this storm and to refresh you all a little on how the forecast process works for tropical weather.
I'll let you know if the forecast changes. Have a great weekend.
Chris
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