Despite the flooding their would have been alot of preparation done in Raqqa (they were even using large cloth sheets to cover up their activity from satellite) and the additional combat experienced passed on from Mosel would make it difficult for the Kurds to crack (assuming ISIS will defend it) who don't necessary have the urban combat experience that the Syrian Army has acquired over the past few years.
Deir Ez-Ezor is the key to everything and ISIS is throwing the whole kitchen sink at the moment there to keep their deluded dreams alive. So much of ISIS strength is being used to destroy this pocket lead by the brave citizens, local tribes that would face extermination and governments forces that have had no reliefs from a siege that has lasted around 4 years now. The "mad" Druze that leads them has made the story of their survival against all odds truly inspiring and one for the history books. At the moment ISIS are using waves upon waves of human infantry and suicide car bombs (VEBID's) in a mad effort to crack into the city that continues to resist them. This should hopefully end like the attempts by the head choppers in lifting the Aleppo siege in weakening their dwindling strategic reserves and killing their most hardened fighters, but the task is enormous for the defenders given their lack of supply (food and weapons), combat exhaustion and lack of sufficient direct artillery support.
ISIS is essentially weakening their outer perimeter by letting the SDF advance on the outskirts Raqqa with little to no resistance due to a severe shortage of manpower and skilled fighters that have already gone to Deir Ez-Ezor which is now effectively the new capital of ISIS for communication and logistic purposes. The Syrian and Russian air force is not big enough to make sufficient attempts to interdict and have focused their efforts in the province of Aleppo and around Palmyra to secure the surrounding areas to form a safe jumping point to lift the siege. The coalition air force could do something around Raqqa but tends to be focused on killing civilians and infrastructure as it main priority. This is a deliberate policy by the Coalition to destroy the infrastructure to make it hard to rebuild for the Government if captured and helps to depopulate the areas making urban combat easier if they try to get their proxy forces in a successful land grab. It also allow the ISIS forces to be redirected under defacto air cover towards killing Government forces which is a bonus (like letting the bulk of ISIS manpower slip out of Mosel and accidentally bombing Iraqi forces when they closed the escape route).
However the Russians have been busy around Palmyra the past week there is dozens of videos of ISIS convoys being splattered across the desert so it makes sense that ISIS tighten their lines of communication by withdrawing from the fringes in delaying actions.
I recommend those that want to get a good understanding of the situation look at the Moon of Alabama website and Southfront (particularly their video updates) their is also some good work putout by Andrew Illingsworth of OZ Analysis that has his work posted at Al-Masdar news.
