Forecast Discussion 9/24

Tropical Atlantic continues to be fairly quiet, but the NHC has identified a low probability of development region associated with PG-29.  Looking at the entire North Atlantic Basin, only a few areas of positive relative vorticity exist at all.  Ex-98L has essentially disappeared, A broad and somewhat spotty vort max region is located over a good portion of Central America, but most is located over land, or will be shortly.  Some of the vorticity does reach into the Gulf of Mexico, and with conditions there being fairly favorable for development, it warrants watching in case any convection pops up in that region overnight.  The vort max east of Florida discussed yesterday continues to be associated with a baroclinic zone and an upper level low.  Interestingly, the GFS does not completely kill this off as it has with most systems in the Atlantic, however it is not tropical in nature and therefore is not a priority at this time.  PG-29 has moved over water and is now located just east of the Cape Verde Islands.  It is associated with some convection, which has begun to flare up over the past several hours- likely due to the diurnal cycle. There is not a lot of shear here at the center, but a large amount of westerly shear exists just to the north. SSTs are sufficiently warm for development, but OHC is fairly low, so conditions are not particularly favorable for development (but at this moment they aren't that unfavorable either)

The models do not particularly like PG-29.  In fact, not even the CMC develops it into anything.  All 3 models from the Montgomery page kill it before the end of the forecast period.  However, there may still be a chance for development.  All of the models depict PG-29 moving north into an area of large shear.  However if it remains relatively unorganized and continues to move westward, it may stay out of this shear region, allowing it to persist as a wave / vort max beyond the 36-54 hours suggested by the models.

In the Pacific, Nora continues to move to the WNW at around 4 kts.  It will shortly be moving into a very unfavorble environment, and has likely peaked in intensity at 45kts for this reason.  WIthin the next 24 hours, it will be experiencing increasing shear from the southwest and rapidly decreasing SST.  While a consensus model forecast is for Nora to remain a TS for 48-60 hours (beyond 12Z), this forecast seems very optomistic given the environment Nora is entering, and its fairly weak initial state.  The NHC forecast calls for Nora to weaken to a tropical depression in roughly half that time, which seems like a more reasonable forecast.