At 500 mb a vorticity maximum west of central Baja will swing northeastward across southern and eastern Arizona during the next 24-hours. Currently there is sparse moisture and no CAPE ahead of this system (above is TWC 12 UTC sounding from SPC). So development of convective showers this afternoon into early tomorrow will depend upon whether or not there are slight increases in low-level moisture, coupled with a bit of cooling at 500 mb. Regardless, precipitation with this system will likely be as sparse as the moisture. The 06 UTC run of the WRF-NAM at Atmo forecasts some very light precipitation on the mountains of eastern Pima County through 5:00 pm MST tomorrow afternoon. The model forecast may be a bit dry, since PW at TWC was already over 3 mm higher than the model forecast at 12 UTC. We continue with no measurable rainfall here at the house for April.
By Sunday morning a much stronger, closed low at 500 mb will be digging south-southeastward across the Great Basin - 500 mb forecast below is valid at 12 UTC on the 26th and is on the 5.4 km grid from the 00 UTC forecast run. Moisture with this continental system will be mostly what it drags along with it. Thus, it remains to be seen whether this system will bring wind and dust only, or if there will be a chance of some showers this far south.
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