Art Douglas sent the graphic above of SST from the 17th and pointed out that sea surface temperatures in the GoC remain quite warm for this time of year. Thus, the developing weather system may be able to pick up decent low-level moisture from the south. Indeed, the early WRF-NAM forecast from Atmo this morning indicates PW increasing to around an inch by 11 pm Thursday night the 21st (below). The model forecasts precipitation just beginning across much of Pima County at that time. A look at the GFS ensembles from last evening shows that 11 of 12 members forecast precipitation for southern Arizona during the Friday to Saturday time frame. So, it looks like the best chance for some meaningful precipitation in quite a long while.
The situation will likely be complicated by a strong cold front and high pressure that is forecast to plunge south over the Plains and Front Range. The forecasts above and below are the surface and thickness patterns from the ECMWF and GFS valid at 5 pm MST Friday afternoon. So it appears that this front will back-door into southeastern Arizona with easterly, downslope winds. This will add complications to the weather event and we'll just have to watch closely to see how all this evolves.
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