An unsettled few days ahead before Christmas, as a closed low in the southern stream west of Baja interacts with features in the northern stream. The operational ECMWF 500 mb forecast from last evening (above - valid on Christmas Eve) has kicked away the southern low, while a strong short wave dug into the Southwest. The GFS operational forecast valid the same time still has the southern stream low over northern Mexico, with a much less amplified short wave to the north.
Only thing to do with such uncertainty is to watch the ensemble forecasts during the coming several days. The 06 UTC GEFS QPF plumes (above) are all over the place starting on the 22nd (Thursday) and ending up with forecasts for TUS of anywhere from zero to over an inch and a half by Christmas.
The short range ensemble forecasts (SREF below - from SPC) show even more spread for QPF begining on Tuesday and extending through Wednesday.
The 500 mb GEFS spaghetti plot for 500 mb (from 00 UTC last evening at bottom) shows greatest forecast uncertainty over the West and the Eastern Pacific. We'll just have to wait and watch as Christmas approaches.
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