The longer-range global models continue to have considerable differences. I've been able to take a look at the 144-hour, 500 mb ECMWF forecast from 00 UTC last evening, valid at 5 pm MST on Saturday, December 21st - above. The ECMWF forecasts a progressive short wave across the Southwest, with a weaker trailing wave. The same forecast from the GFS operational member - below - is similar, but with the leading short wave digging deeper into Mexico.
However, a number of the GFS ensemble members close the system off, dig it to south toward or west of Baja, and move it much more slowly. Forecast below is from one of those ensemble members and is quite different than the two shown above. At this time 5 of the GFS members forecast a progressive short wave and 7 members forecast a deep, closed and lingering low at 500 mb.
It has been a difficult fall and early winter for the long-range global forecasts with many cases of low predictability. The ECMWF has appeared to keep its advantage over the GFS, but not to a very high degree as has been the case in some winters. We will have to watch as this interesting scenario evolves this week.
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