Sep 24th: Active Atlantic for the end of September

We've been discussing the two invests in the Atlantic all week now with both newly designated Tropical Storm Humberto (eastern system) and 94L (western system) maintaining solid convection over the day today:

The NHC designated 93L as Tropical Storm Humberto at 5 pm on Wednesday and continues to give 94L a high chance to develop over the next several days. It is an interesting case that doesn't happen that often in the Atlantic, with two systems trying to develop so close to each other. Not only could their proximity determine how strong each one gets, it will also influence their paths. This phenomenon is something called the Fujiwhara effect. The two systems can remain separate and rotate around a common point but usually what happens is one system becomes the dominant one and limits the strength of the other system. 

It is interesting that most of our global forecast models do keep both as separate systems as we head into late this weekend and early next week. Below you see the operational GFS (left) and Euro (right) valid Monday morning:

Both models have two separate systems but the Euro has a stronger 94L (western system) whereas the GFS is much weaker. The Euro ensemble probabilities for a TD (left) also show two distinct systems:

It is also important to note that the hurricane models being ran on these systems have both intensifying into at least tropical storms, if not hurricanes and the official forecast from the NHC for Humberto is for a Hurricane by Saturday:

The biggest question is how far west does 94L get before it begins to feel the influence of a trough over the Southeast US and Humberto and gets pulled back out to the Atlantic. That will determine if a landfall actually happens on the Southeast coast or not:

  One thing we do want to mention is the threat for heavy rainfall in the Carolinas, regardless of whether the system comes onshore. Check out the moisture being transported between the tropical systems and upper-low over the Southeast:

The area needs the rain so the below forecast is mostly welcome. Still, if training occurs with heavy rain moving over the same area, that can lead to flash flooding so it is something to be aware of:

 
Each day will bring us closer to a solution for the complicated 94L and whatever impacts it brings to the East coast of the US. It could be as little as elevated surf or high rip current risk or it could bring strong winds to the coast.