The NCEP Ensembles have the same high pressure ridge that has been bringing record-smashing heat staying in place for at least the next 5 days. The image above is of the NCEP ensembles at about 7 days out. It is projecting a low pressure system to move south and interrupt the heat wave in the eastern portion of the heat wave, as well as push the ridge westward.
The ensembles have been fiddling with this idea for a while, but only last run these same ensembles were indicating that the ridge will be present through the next 16 days or more. So let's look at the MJO.
This is the MJO forecast from the European center that makes the ECMWF model and ensembles. This forecast is showing the MJO transitioning into Phase 2 and possibly a weak Phase 3. A Phase 2 MJO in the June-July-August timeframe involves cooler temperatures in the eastern US and warm temperatures in the western US. There is something that I think will get in the way of a full-on Phase 2 developing. This massive ridge has been smashing records left and right, and when we get something of a historic magnitude, it tends to not want to follow the rules. A ridge of this size and strength would fit nicely into that 'historic' category, which would explain the NCEP ensembles of what appears to be a partial Phase 2.