Tropical Storm Isaac is starting to become better organized this morning after struggling a lot last night with decoupled centers in the low and mid levels, and a more consolidated, vertically-stacked center appears to be forming south of Puerto Rico. Forward motion has slowed from 21mph yesterday to 13mph now, indicating that Isaac is approaching the western periphery of the subtropical ridge to the north, and this slowing should help the storm strengthen on approach to Hispaniola during the next 24-48 hours. Intensification into a moderate-strong tropical storm seems likely, but hurricane strength before Hispaniola seems a little bit too much to ask for. The intensity forecast remains a tad lower than the NHC in the short range, a theme which has verified very well so far. Interaction with the mountains of Haiti and eastern Cuba should weaken Isaac, but restrengthening should be quick to ensue north of Cuba due to improving upper-level conditions and very warm water in the storm’s path. Isaac could quickly become a hurricane very close to Florida if it gets any significant time over water on either side of the peninsula.
Isaac is beginning to move WNW around the periphery of the subtropical ridge, which currently extends into the Bahamas. Isaac will be gradually curving more towards the NW with time, taking it over Hispaniola and eastern Cuba, and eventually towards Florida. The models are in close agreement, and Florida is very likely to be significantly impacted by Isaac. The only exception is the ECMWF, which is still a western outlier, taking Isaac into Alabama in 7 days. This is a slight shift eastward from previous runs though, and may be the start of a correction trend eastward for the ECMWF towards the other models. The upper pattern ahead of Isaac features a weakness in the ridge that will be directly north of the Bahamas in 3 days, something that a strengthening storm coming off of Cuba may be more likely to take advantage of than some of the models show. The forecast track is still east of the model consensus at Days 4 and 5, very close to the eastern coast of Florida, similar to the 00z CMC and 06z GFDL solutions. The wildcard in the track forecast remains the mountains of Hispaniola and Cuba, which are notorious for jerking storms around, and could easily cause an unexpected shift in track at any time. A NOAA G-IV recon mission into Isaac tonight will hopefully transmit new data in time to be put into tonight’s 00z model cycle, which should result in more accurate track forecasts. If the model consensus remains on the west coast of Florida after they have received the G-IV data, my track may have to shift a bit to the west.
Overall, Florida is likely to get impacted directly by Isaac no matter what adjustments are made during the next few days. The average NHC forecast error for 5 days is 260 miles, and there is lots of room for adjustment while we are still 4-5 days away from a Florida landfall. Until Isaac clears the mountains of the greater Antilles, great inherent uncertainty with such a track will make nothing certain about the long-range forecast.
Elsewhere….Joyce has formed in the central Atlantic, and will be recurving near Bermuda in 5 days or so. This is no imminent threat to land, and can be mostly ignored until Isaac is out of our hair.
We shall see what happens!