Forecast Discussion 9/16

Several systems continue to dominate weather in the Atlantic.  The Bahamas low has developed farther north, and has begun to accelerate north faster than originally expected.  Shear continues to be high, particularly near the center of circulation (which is not particularly clear on satellite imagery or quickscat analysis).  Models agree on track that will keep it over water, but intensity still ranges from model to model.  At this point, tropical development appears to be completely out of the question.

An upper level circulation associated with PG-24 continues to persist south of Puerto Rico.  Shear is high here, and no real surface circulation exists, so probability of development of any tropical systems also appears to be zero with this system.

NHC has low probabilities of development for ex-Fred and PG-26.  While thunderstorms developed yesterday afternoon and overnight in association with Fred, the surface circulation appears to be weakening on visible satellite imagery.  Quickscat analysis shows no true center of circulation at this point.  This is despite the fact that convection was firing near the center of Fred and Fred was moving into a region more condusive towards development- lower shear and warmer SST.  All Models now kill Fred, with the interesting exception of the NAM, which continues to track a low begining 18th 00Z that should be ex-Fred.  Convection may again be firing up around the center this afternoon, but at this point it appears as though Fred is becoming completely disorganized.

PG-26 on the other hand continues to be associated with relatively unorganized showers, yet it shows a clear circulation on Quickscat imagery.  Several global models maintain this system and slowly move it westward, however, none show it strengthening very much, if at all.  For example, while NCEP Cyclone tracking reports that the 6Z GFS tracks a system for several days, it is not clearly visible in graphical output of sea level pressure throughout the period.  Conditions for development will be marginal, but with shear expected below 20kts and SST increasing as it moves westword, slow development can not be completely ruled out.

Finally, another wave is emerging off of the African Coast.  Models tend to strengthen this system more than PG-26 or ex-Fred, so it will be worth watching for future development.  The Montgomery page has listed another system as PG-27, but it is unclear whether this is PG-27 or a different intermediate system between 26 and 27.

Flight Forecasts:

Ft Lauderdale:  DC-8 Flights at 10Z Thursday, 10Z Friday, returning 18Z both days
Thursday 10Z: Expecting light (>10 kt) easterly winds Thursday morning, with no precip threat.
Thursday 18Z: Light Easterly environmental flow conitnues, trough axis expected to move towards east coast; could result in scattered T-storms over east Florida, which could impact DC-8
Friday 10Z: Easterly winds still expected- t-storm threat minimal due to time of day
Friday 18Z: Persistant Easterly wind, t-storm chance likely higher than Thursday b/c trough will continue to propagate west.