Forecast Discussion 9/18

The Atlantic looks much more active in terms of potential TC activity today.  4 systems are located over the Atlantic, PG-24, ex-Fred, PG-26 (now 98L) and PG-28.  PG-27 is still over the African continent.  Discussion will focus mostly on PG-26, and some on ex-Fred.  PG-24 has yet to show low level organization, and it is not clear whether anything that could form would be PG-24 or a new entity.  A Global Hawk mission to the area was scrubbed.

Ex-Fred spun off an interesting low level low that diverged to the west from the upper level low and has produced some convection over the past 12 hours.  However, within the past hour or 2, convection has begun to die down and the circulation has become less defined.  The NHC has identified a low probability of development, but this may actually be generous based on recent satellite imagery.  While ex-Fred has lasted longer than most Global models predicted, development at this point continues to seem a moot point. 

PG-26 is the main target of concern, and likely will continue to be so over the next 3-5 days.  It has been identified as an invest area by NHC as is thus named 98L.  HWRF and GFDL are now being run for this system.  Currently, convection continues to be associated with the center of circulation, which is now well defined on QuickSCAT imagery.  It is in an area of relatively low shear (<20kts) and warm SST (29).  OHC is also fairly high, (>100 kg/cm^2).  Convection is not organized well enough to expect immediate development, but it is certainly possible within the next 48-72 hours.  The models do not all agree with 98L, but a mean forecast appears to be that it will develop into a TC.  The CMC and MM5 show the development of a clear TC, while the GFS appears to want to kill of the system entirely.  NGP takes the middle ground.  Both HWRF and GFDL strengthen the system into a tropical storm by around 72 hours from 18Z.  GFDL eventually strengthens it into a cat 1 by the end of its forecast.  I have more faith in GFDL b/c it handled ex-Fred better, essentially killing it, whereas HWRF reformed it into a tropical storm. in the 16 Sep-00Z runs of those models (the last time they were run in the Atlantic).  As the system strengthens, the models appear to slow it down, and curve it towards the north.  A large upper level trough is expected along the east coast of the US next week, which would result in steering 98L farther north, consistant with the models.

No other systems are significantly developed in any of the models, but it is worth noting that a low will be emerging off of Africa next week (consistent in all models) and this is likely PG-27.  That it looks so strong coming off of African compared to anything I have noticed recently, could be a good sign for development

Airport Forecasts:

No new missions planned, all planned flights for today scrubbed.  Likely will be forecasting for P-3 flights out of St Croix tomorrow, as well as potential Global Hawk flights to 98L.