The ensembles are projecting a low pressure system to dip south and sit there for a few days, before the high pressure system once again shifts east and reclaims the eastern US, bringing with it hot temperatures. If this were to happen, we could be looking at an extended period of unusually hot temperatures, possibly continuing into the rest of July.
Now, these ensembles are long range, and there is a lot of potential wiggle room involved here. However, this is what I find to be historic/unusual. Historic and/or unusual events are typically placed fairly few and far between. These events take special forces to somehow expel it from the region. This is a historic/unusual event, in the case that it has broken numerous daily high temperature records and is able to hold its ground. That said, I find the NCEP ensemble's case very believable. It will certainly have to be monitored.