Developing winter storm is evolving pretty much as forecast by both the ECMWF and GFS long range forecasts a few days ago. The 500 mb short wave that will dig southward down the West coast the next couple of days is over Washington state this morning. Above is 12 UTC 500 mb analysis from NCAR RAL this morning (Monday, December 29th). The wave appears to have closed off (note the south wind at Spokane) with temperatures colder than -36 C within its core. A very strong arctic cold front is accompanying the system southward.
By 5 pm MST on January 1st both models (ECMWF above and GFS below) forecast the low to be centered over the Southwest and weakening some. There are still differences in the details of these two forecasts - ECMWF is stronger, with the core of the low a bit further northeast. The key factor in how much precipitation the event produces will depend upon how much moisture the portion of the flow over the Pacific picks up. The most significant snowfall will be produced by upslope flow into the central and eastern mountains. It remains to be seen how low the snow level will drop to over eastern Pima County - probably snow on the saguaros on the east side of town, but iffy down here in the Rillito wash.
The early WRF NAM forecast this morning extends out to 11 pm MST on the 31st. The composite radar echoes forecast at that time (above) show a broad band of showers and mostly light rain moving across Pima County. Accumulated precipitation through that time (below) indicates very substantial amounts to the north over the Rim country. Regardless of the details, 2015 will start off with a significant weather event here in southeast Arizona.
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