The 00Z GFS at 240 hours was not unlike the 00Z ECMWF in that time frame - the 06 and 12Z GFS runs were different, especially the 12Z GFS.
However, the difference is in the movement and development of the same trough, so all of the various outputs have this trough to a lesser or greater degree, but cannot decide on its motion.
Now, what would such a trough do *normally*? Or what have similar troughs done recently? Well, the answer to the latter seems to be that they have moved further south, and slowed up across the 4 Corners region, or somewhere in the vicinity. This tells me that the ECMWF is probably onto something - it has been fairly consistent over the last few runs - if today's 12Z shows something similar again, then we are perhaps getting towards a more likely scenario. If it doesn't then I guess we get a big of a kick in the teeth - however, this will be just 1 more deterministic run, and on their own, they are not especially great. The 00Z ensemble mean from the ECMWF also had the trough, and kept a mean trough over the Intermountain West for much of the last week of May, with a SW uppre flow across the Plains...this is the same set of ensembles which 1 week ago predicted a mean ridge over the Plains for the coming week, and from my experience of using it every day in the UK, I know it's a pretty good set of ensembles.
