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Florence hanging tough, new TD 7 hot in persuit

Well, let's deal with Florence first. There actually isn't a whole lot to say about Florence. She is still maintain strength with winds of 90 mph and headed toward Bermuda. Florence's eye and still ragged and at first glance does not appear to be too impressive of a hurricane.

While the models are calling for a direct strike on Bermuda, they rapidly diverge after about 24 hours. A lot of this scatter is likely due to differences in anticipation of Florence's transition to an extra-tropical system.

The wind speed forecasts are even less in agreement than the track forecasts. Part of the disagreement is due to the resolution of the model and the remainder involves differences handling the storm itself. The global models, such as the AVN, Eta, NGM, CMC, etc tend to underestimate the strength of a hurricane due to the maximum wind being calculated as an average over a much larger area than hurricane specific models like the GFDL and DSHIPS produce. Even with this accounted for, a weakening trend is forecast to begin very soon.

With all of this said, the cone of doom is locked squarely onto Bermuda where a hurricane warning is in effect. Tropical Storm force winds have already spread over the island, but they seem prepared. Florence seems to have reached its westernmost point and will start curving eastward at any moment.

Now what about the new Tropical Depression Seven? While it seems to be lacking a well-defined surface circulation, the convection is very healthy and looks be easilly capable of further development into a tropical storm in the short term.

TD Seven has a very small circulation that is actually too small to be well captured by most of the forecast models. At this point the BAM models are probably providing the best track forecasts and intensity forecasts are a crap shoot. The BAM only shows what would happen if the depression were to float along in the general flow and not influence that flow at all. Accordingly, forecasts are very low confidence.

Model charts for Tropical Depression Seven are available at:

TD Seven is forecast to become a tropical storm tomorrow and quickly recurve out to sea. It is worth noting that Bermuda is one again inside NHC's forecast cone for TD Seven. However, as previously mentioned this is a low confidence forecast from NHC. TD Seven may never recurve and could head directly at the United States.

But how are these two systems related? TD Seven is following right behind Florence. Looking at the GOES-12 water vapor shot shows the two systems in close proximity with a cloud streamer seemingly connecting the two storms. I suspect that this could be something called a "vorticity streamer" which is a ribbon of air containing constant "potential vorticity."

Posted by Bryan Woods | Permalink