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Tropical Depression Four Steady

Good to be back! I took an extended vacation after receiving my M. S., completing my summer course, and submitting my first journal article. Whew! Anyway, enough about me and onto the tropics. TD-4 has maintainted 30 kt (35 mph) winds in the latest advisory from NHC. Although the intensity models are forecasting TD-4 to become TS Debby in the next 12-24 hours, I think the models are probably a little off. TD-4 does have some things going for it: warm sea-surface temperatures and a low shear environment. However, as noticed by the lack of convection (Figure 1), something is still inhibiting the intensity of this storm. I think the answer lies in the dry mid-levels. A large plume of Saharan air has accompanied TD-4 off the coast of Africa. In Figure 2, if you look southwest of the center of circulation, you can see some of this Saharan air being sucked into the system. In my opinion, this is going to limit intensification. While TD-4 may eventually become a tropical storm, I think NHC's intensity forecast is high at the moment. I don't expect TD-4 to traverse the Atlantic like similar systems from 2004 and 2005 did. By the way, these Saharan air outbreaks have been especially severe this season and are likely the reason we have been having "only" a normal season thus far, since the other factors for tropical cyclone development have been quite favorable. Stay tuned to the StormTrack for more.

Figure 1. Infrared satellite image of TD-4. Notice there are no red shadings anywhere, indicating that thunderstorm activity is limited.
Figure 2. Saharan Air Layer analysis from UW-CIMSS. The hotter colors indicate where the dry, dusty air is located. Notice how the dry air is being entrained into TD-4.

Posted by Adam Moyer | Permalink