It is a common misconception that there are only one set for each model: the GFS, Euro, UKMET, etc. There are acyually several various versions (per each model) which are run, so I thought this link may shed some light on the various GFS ensembles and their depiction of the progged UA (upper air) pattern:
http://www.meteo.psu.edu/~gadomski/ENSPRS_12z/ensloop.htmlCurrently, there is a shortwave trough over southern Canada and the upper Plains. The degree to which it amplifies (in response to upstream heights over western Canada and the Pacific Northwest trough) is questionable. Even if it strengthens to the effect shown on some of the ensembles, there is quite a bit of disagreement over the timing, intensity, and whether the overall synoptic pattern will allow the event to take place; certainly the GFS' cool bias (particularly around days three through six) could be a responsible player. The operational GFS seems to have one of the strongest biases, while the other GFS ensembles (and the Euro ensembles/other models) show a more or less weaker North American system.
On the other hand, the operational GFS could have a better grasp on the fluctuations of the Plains ridge, in addition to the thermal environment.
A lot (emphasis added) will depend on the forward motion of TD 4; the longwave pattern over North America (small changes can make or break it); the size/intensity of TD 4; and the possible Gulf system via 91L. It should be noted that there has been a persistent trough over the Canadian Maritimes, so I can see the justification for a more northward track. On the other hand, there are considerable uncertainties with respect to the extent of the low to mid-level ridge(s) over the Atlantic basin and southern United States. Many small fluctuations can make a huge difference. We will get a better handle over the next few days. In the meantime, the islands should closely monitor TD 4.
One additional note: it is entirely plausible that the first trough may not completely touch TD 4, and the big question concerns the ridging/North American pattern response after the progged trough (see the differences between the operational GFS and ensemble Euro on the upcoming pattern). The strength and amplitude of the trough will make some big differences, too. We'll see the factors over the next several days.
http://www.ghcc.msfc.nasa.gov/GOES/goeseastconus.htmlKeep a close eye on the whole picture.