Forecast Discussion 9/21

Things have once again quieted in the tropics, to a fairly extreme degree.  In fact,  there are no tropical cyclones or even invest areas anywhere in the world at the moment.

Looking at the tropics as a whole, there are a few areas of convection, but nothing is very well organized at this time.  Ex-Fred actually looks the most impressive in terms of convection, but there is no low level circulation and the system is nothing more than a wave with no potential for TC development.  A low level weak vort max is located over the Puerto-Rico- Dominican Republic area, but very little convection is associated with it.  Shear is also >20kts so any convection that does form will likely have no chance to organize. 

Ex-98L is located farther west and appears to have fallen apart.  It is almost impossible to define a center based on QuickSCAT or IR/Visible Satellite imagery.  A low level wave/swirl feature is apparant around 45W,15N, about where we might have expected 98L to be, but there is essentially no convection here.  However there is convection to the northwest and north east (2 seperate and individually defined complexes).  CIMMS low level vort maps essentially show 2 different vort maxes, one which is associated with the NW convection, and an elongated max which stretches from the low level wave position to the NE convection.  Shear is also fairly high, as the system apparantly moved into the shear region instead of staying south of it.  (Or the shear region did not move out of the way as was suggested by some global models over the past few days).  It is far too early to declare ex-98L dead, as several of the models over the past few days have suggested it might weaken before eventually strengthening into a TS, but development into even an tropical depression seems very unlikely, at least for the next 48-72 hours.

Unsuprisingly, CMC still strengthens ex-98L into a TC.  What is slightly more suprising is that the GFS does not outright kill ex-98L and instead at least tracks a trough northward that eventually gets swallowed up by an extratropical cyclone.  NGP again takes the middle ground, but based on previous performance, I'm inclined to lean on the GFS for this forecast.  It is also worth noting that CMC develops a closed SLP low inthe vicinity of PG-30 in the 4-5 day range, however no other models support this forecast.  The MM5 meanwhile, is stubbornly continuing to develop a system in the Yukatan region, but no other models support this feature as well, and this could merely be a boundry proximity error.

Looking farther west, PG-27 and PG-28 have both died out, and are no longer being tracked.  PG-29 is located over Africa, and looks very unimpressive on satellites.  All 3 global models on the Montgomery site kill off or weaken PG-29 by the end of the forecast period. 

Airport forecasts:

Global Hawk scheduled to fly tomorrow, but mission likely to be scrubbed