Potentially potent subtropical trouble to develop and move north early next week

   Posted by Levi at 10:14am on October 5, 2011

High pressure is now building over the central United States and starting to edge eastward, north of a trailing front over the Bahamas and NW Caribbean. This setup has been talked about for a while now as something that could easily spark development in this pattern. Now that the downward MJO pulse is finally leaving, this idea has some merit, and the models are now latching onto the potential for subtropical development in the Florida area beginning this weekend and on into early next week.

The track of such a development would likely be ultimately northward towards the SE U.S. mainland, around the western side of a big ridge that will be slowly progressing eastward during the next 7 days. As mentioned yesterday, this ridge looks like it may get pushed east fast enough to trap whatever development may occur south of it. The models are now moving towards that idea, and if a system is trapped, it would eventually find a way around the western side of the ridge and move northward. The question is how fast the ridge will move east, and whether a developing system will move into the eastern Gulf of Mexico before moving north, or stay east of Florida and move up the gulf stream towards the Carolinas. At this point, we’re dealing with the fine details of the timing of the ridge 5-7 days out which can’t be nailed down with confidence just yet, so either possibility could occur. We do, however, have the ECMWF and GFS both forecasting an upper trough axis just east of Florida through Day 7, which would imply a surface low under or just east of it, and this is decent support for that idea. Although the ECMWF has the low over Pensacola in 168 hours, I show in the video why this doesn’t make sense. The CMC and NAEFS ensembles, on the other hand, still have low pressure all confined to the Gulf of Mexico in 5-7 days, though I trust their solutions less than the combined GFS and Euro.

Regarding potential intensity of any subtropical development, it is always hard to get a handle on such messy systems, but there is potential for there to be a fairly potent low in the Florida area early next week. An upper trough will be diving down over the area and providing light and divergent winds aloft, promoting thunderstorm activity and lowering of pressures. This setup is forecasted to persist for several days, which could allow a decently-strong low pressure area to mature, possibly even becoming fully warm-core as it moves northward. The 06z GFS showed a sub-996mb low moving into the Carolinas, which actually made sense based on the upper-level pattern that it showed for that time. With a mid-latitude trough moving eastward next week, it could facilitate a conversion to a fully warm-core system between the mid-latitude jet and the subtropical jet, allowing a feedback that could lead to a very solid little storm. It is also possible that we will be dealing with just a very broad mess of things early next week with a large area of wind and rain for the Florida area, but with several days to work with over warm water and with an unstable atmosphere, it may be something to watch carefully.

We shall see what happens!


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