Tropical weather analysis - November 30, 2012
Unfitting considering the calender date, an area of low pressure has formed in the eastern Atlantic between the Cape Verde Islands and Bermuda. Like many lows in this part of the world this time of year, Invest 91L is of non-tropical origin.
The satellite signature is less than impressive, as the storm is experiencing westerly vertical wind shear. Consequently, nighttime infrared satellite pictures show an exposed circulation center near 25N 43W; this position is also consistent with an earlier AMSUB microwave overpass just after 0z, and also with a recent ASCAT pass.
Figure 1. Latest infrared satellite image of Invest 91L. Image credit: NOAA
The GFS develops a narrow area of weak shear over the system as it moves northward over the next couple of days. However, I should note that the denoted flow is not at all anticyclonic, and suggests only weak ridging aloft which will probably break down with any sudden increase in wind shear. The 0z SHIPS takes 91L to near hurricane strength prior to extratropical transition in about 72 hours, but this seems awfully suspect in light of the large-scale environment, and this model is not designed to handle non-tropical entities particularly well. That being said, some slow development of this disturbance is possible as it moves northward at 10 to 15 mph through Sunday. Thereafter, a turn to the north-northeast or northeast with some acceleration is expected as synoptic mid-level southwesterly flow increases over the system. This agrees well with the available model guidance.
It should be noted that the upper-level circulation appears less well-defined than the middle and lower-tropospheric circulations, which suggests that this low is on its way to working the associated circulation down to the surface. Notwithstanding, the aforementioned ASCAT pass showed only a sharp wind shift, not a closed circulation. Satellite imagery also supports this idea.
This low is forecast to become extratropical in about 72 hours, leaving only a short window for development.
Probability of development in 48 hours: 30%
Elsewhere in the tropics, dangerous Typhoon Bopha is heading toward the Philippines, and is likely to deliver a substantial blow to that nation. I will have a blog post on Bopha in a day or so, and will continue to follow it until dissipation.
Also, the global models, including the GFS and ECMWF, have been consistent over the last few days in developing another area of low pressure on the heels of Invest 91L. Such a system should be expected to take a northeastward path out to sea in response to a mid-level trough forecast to set up over the central Atlantic. The culprit for such a pattern is probably going to be the developing trough over the western United States/four corners region, which is showing up nicely on water vapor imagery. This low also has the potential for subtropical or tropical development.