Forecast Discussion 9/29

There is no potential for TC development in the Atlantic for the next 2 days, so discussion will focus on the Pacific.

NHC now has a medium potential for TC development associated with what is now Invest 91E. 91 does not yet have a well defined circulation, but it continues to be associated with a broad area of thunderstorms. Shear immediately around 91 is low, however an upper level trough and associated westerly jet is responsible for high shear to the north. SSTs are sufficiently warm for development, so whether or not this system develops may depend on if it can stay out of the shear. It is moving NW at around 10 kts, and a model consensus track keeps it along this track for the next 36 hours, bringing it to near 117W, 21N. At this point, the center of the storm would be right around the 20kt shear contour currently analyzed by CIMSS, which may be sufficiently low to at least not kill off the system. However, the GFS forecasts the jet to move slightly south over the next 2 days, which result in enough shear over the system to prevent any development and result in dissipation. After about 36-48 hours, the models diverge significantly, with some models showing an acceleration and curvature towards Mexico, while others slow it down and almost stall it or kill it off. This will depend heavily on the strength of the system, which in turn will depend on the shear it encounters.