Showers For Next Saturday?

There is a middle and upper-level cyclone/short wave crossing Arizona today, but this system remains moisture-starved. Main weather of significance today appears to be the dense fog over the northeastern quarter of the state.

The global models forecast a stronger, 500 mb short wave to cross the West next weekend. The GFS operational forecast of 500 mb (above) is valid at 5 pm MST next Saturday. The strongest part of this wave crosses southern Arizona, within a southern branch of the flow. The northern portions of the short wave are poorly organized in the current model forecasts.

Below is GFS forecast, again the operational member, of precipitation through 5 pm on Saturday. The southern edge of the precipitation forecast runs along the border - this indicates that, due to coarse terrain resolution of this model, upslope flow in the model is related to the precipitation. So the main forecast problem will again be lower-level moisture available ahead of the system. If moisture remains low, as indicated by current PW forecasts, then precipitation will stay over higher elevations to the north of us. 

Currently the NWS forecasts a 30% probability of rainfall over the low elevations of metro Tucson. In the GFS ensemble forecast from 00 UTC last evening 10 of 12 members forecast precipitation over southern Arizona, but again with the southern edge close to the border. The ECMWF appears less bullish for precipitation - but my access to output from the model is quite restricted.