POST-TROPICAL CYCLONE FLORENCE:
Hurricane Florence may have come ashore and transitioned to extratropical (post-tropical cyclone) but that doesn't mean she is done wreaking havoc on the east coast! Sure the skies have mostly cleared and the rains have largely stopped but with this new storm mode that Florence has a new threat. Tornadoes are now a big concern for Virginia. Currently, the SPC has a tornado watch out for portions of the state and several violent tornadoes have been spawned including one in Richmond, VA which has resulted in one fatality (CLICK HERE FOR THE VIDEO).
The flooding left over by Florence will also take a while to drain out to sea and could take a week to fully clear up, if not a bit longer in some places. If you look at the Carolinas and points just north you can see a very large number of the flood gauges in the area are at or above flood stage still and will be for quite some time! In fact, some rivers like the Neuse River at Kinston (click to see flood forecast and current level)
are still rising even though the rain has ended in this area. The Neuse River at Kinston is forecast to keep rising through the 22nd of September! Many other locations are currently at historic levels and even as they start to slowly drain down from that point they will stay above the flood stage for some time.
Florence is forecast to continue to move off to the northeast and will diminish further and further in threat as it does so but that does not mean the tropics are done with us yet. Taking a look out over the Atlantic at this time and you will see things have largely quietened down. The storms that are present are dying or not a threat to land, and one could easily be fooled into thinking the season is winding down. One storm could come along and slip under everyone's radar and pull a fast one on those who live along the Gulf Coast and that is Isaac.
TROPICAL DISTURBANCE ISAAC
Officially, Isaac dissipated at 5 am Eastern on Saturday and now is just an area of disturbed weather in the Caribbean by Jamaica. Normally this would be the end of a storm's life but last year one storm proved we cannot let our guard down. In the images below you will see the current path of Isaac (2018) and the path of Hurricane Harvey (2017). Harvey, much like Isaac struggled with shear when it came into the Caribbean and dissipated from a tropical cyclone to a disturbance (pink line along path) but once it made it into the Gulf of Mexico conditions came together to allow that disturbed area of weather that was Harvey to re-form into a tropical cyclone and from its passing into the Gulf off Mexico's Yucatan Peninsula to making landfall on Texas the storm was able to rapidly intensify into a category 4 hurricane.
Isaac currently is in much of the same state that Harvey was when it crossed the Caribbean. A victim of shear, Isaac has totally fallen apart and become nothing more than an area of disturbed showers and thunderstorms over Cuba and Jamaica. At the present time, wind shear over that part of the world is starting to decrease (pink circled area in Fig. 1 below) and conditions are slowly becoming more favorable for Isaac to become a tropical cyclone once more . Currently, the steering winds are forecast to take Isaac into the Gulf of Mexico near the loop current.
From there it is likely that the system will maintain its northwest track and move on to affect Texas or Louisiana by the coming weekend (Fig. 2 below). While not likely to be a repeat of Hurricane Harvey, the environment will likely allow the storm to organize some and I would not at all be surprised to see Isaac be reborn as a tropical storm before it comes ashore in the US. Some of the model guidance backs this up and has the system as a tropical storm as it nears the US Gulf Coast. Water temps will be more than favorable for development but the big question that remains is how long the wind shear will stay weak enough for Isaac to redevelop into a tropical cyclone (Fig. 3 below). Throughout the week there is just a small window for this but as we get closer to the weekend the current forecast is that shear ahead of Isaac will increase once more and halt any intensification of the system.
Despite what Isaac may or may not become, it will still bring gusty winds and rain to the Gulf Coast and I would not discount the possibility of some flash flooding associated with it. At this time the NHC gives it a low chance, only 10%, of reforming as they are pessimistic of the environment giving a long enough window of opportunity to Isaac to reform. While I agree that it is not the most likely to happen I think the real odds are closer to 20-30%. (It is worth noting that while some of the hurricane models show it becoming a tropical storm, the GFS and EURO do not do anything with Isaac).
ELSEWHERE IN THE NORTH ATLANTIC HURRICANE BASIN
Outside of that, the tropics in the Atlantic basin are to remain quiet and likely will for a week or two. My area of concern for the future would be the areas around the east coast of Central America where the annual monsoonal troughs can sometimes produce tropical cyclones that start near countries like Honduras and then move off straight north for the Yucatan, Gulf of Mexico, and the Greater Antilles and Bahamas. The GFS even hints at the possibility of tropical cyclones in this region way out in fantasy land (>180 hours on the run) and so it should be watched in the coming weeks. Climatologically speaking that is where most of your October storms happen.