Forecast Discussion 9/19

Ex-Fred and 98L continue to be the main areas of focus.  NHC has a low probability of development for ex-Fred and a high probability of development for 98L.  A low probability of development has also been issued for PG-28, but because it is so far from our target range, this system won't be discussed in detail.  A large upper level trough is located over the Atlantic, which will impact 98L.  While winds are generally easterly across the atlantic, the upper level ridge is contributing to westerly upper level winds just north of 98L, resulting in strong shear.  Outflow from 98L is likely contributing to the upper level flow in this region as well.  The center of the system is located in an area of low shear, so the environment of the system will likely be determined by how the upper level ridge contirbutes to the shear where the system is actually located.

Ex-Fred actually looks slightly healthier today, as convection appears more extensive and a circulation was apparant on QuickSCAT imagery.  Ex-Fred is in an area of amply warm SSTs and low wind shear values, which clearly are contributing to the increase in convection, and probably the development of a more coherent circulation.  HOwever, all Global models continue to kill Fred off.  The models have been doing this for almost a full week now, yet Fred is still around, so not much stock can be put into these forecasts, however it does suggest that while ex-Fred's circulation may stick around for a while longer, intensification seems pretty unlikely.

98L also appears more impressive on satellite imagery.  Convection has grown and now covers a much larger area, and is not getting signficantly sheared off, at least near the center of circulation.  Based on QuickSCAT imagery, it is difficult to pinpoint an exact center of circulation, and mulitiple small-scale centers may exist, however a larger-scale general cyclonic circulation is very clear.  The model consensus is that it will continue to move slowly to the northwest over the next several days, and gradually strengthen as it does so.  Even the GFS shows slight intensification of the system, before it kills it off, as it seems to do with all potentially developing systems.  The consensus forecast for track appears to be that it will be in the vicinity of 50W 20N in 72 hours.  At this point the models suggest that it could be a tropical storm, or at least a tropical depression.  It then begins to turn to the north, and eventually northeast, and the degree to which it turns depends on its strength.  For mesoscale models, the consensus appears to be that 98L will become a tropical storm in the 72-84 hour range, and could reach hurricane intensity another 24-48 hours after that.  The Global models have been fairly consistant run to run in handeling this system, while the GFDL and HWRF have been less so.  Both the GFDL and HWRF models have trended towards a stronger system over the past 3 runs (yeterdays 18Z through todays 6Z runs).  Marsupial tracking for this sytem shows strengthening in the UKMET and NGP, and weakening in the GFS.  One thing interesting to note is that all 3 models show an increase in mid level shear towards then end of the forecast period, which is probably a reflection of the shear ahead of the system currently.

Airport Forecasts:

No airpor forecasts today- most planes heading to St Croix, or not flying yet