Forecast Discussion 9/20

Primary areas of concern continue to be ex-Fred and 98L.  QuickSCAT missed both systems on its latest pass, but from visible imagery, both systems appear to be less organized today.  It is difficult to even find ex-Fred, and 98L also has a less clear circulation. 98L may also be associated with an upper-level low, as indicated by CIMSS upper level wind analysis.  Looking at the Atlantic as a whole, an upper level low persists W of the Bahamas, but the TUTT in the western Caribbean appears to be slightly weaker based on CIMSS upper level winds analysis.  The Gulf of Mexico is clear, and west of 98L, no systems appear to have any chance of development over the next 2 days.

98L still has the potential to develop into a TS within the next 72 hours.  Models appear to keep its intensity steady over the next 48 hours, then show gradual strengthening after that.  This is simmilar to yesterdays forecast, however 98L appears to have actually weakend since then.  It is worth noting, that multiple models show strengthening to a cat 1 hurricane by 120 hours, and the model consensus is a moderate tropical storm by that time.  Model tracks still show a northwest track with a turn to the north around 50-55W and 20-25N, which is fairly consistant with previous forecasts.  There is still a lot of spread on both intensity forecasts and track forecasts, but with the model consensus remaining fairly consistant, there still appears to be a decent chance of development- based on model output.  However, while  SST are warm, shear values have increased ahead of the storm, and an upper level jet is associated with the upper level low previously associated with Fred.  If the jet remains in place, development will almost certainly not occur.  Global models, including the GFS, suggest that it may weaken and split before 98L reaches it.  If this occurs, wind shear values would still be not particularly favorable for development, but not very unfavorable either, resulting in conditions that would allow for gradual organization and strengthening.  One interesting aspect of a split jet would be the potential for a coupled jet that could aid in cyclongenisys.  Marsupial tracking shows an increase in shear, but also an increase in strength of the system over time, so this could perhaps be one explanation for why.