The above image is from the NWS TUS radar a bit before 8 am MST this morning. The large area of terrain blockage to the southwest of the radar shows that it is not well-sited to monitor precipitation, especially in winter, across a large region south of Tucson. The radar is sited a bit above 5,000 ft msl in the Empire Mountains southeast of Tucson. The base scan (i.e., 0.5 degrees above) indicates significant blockage over many sectors. The small peak blocking the sector to the southwest is very close to the radar. The 3.5 degree elevation tilt, below, still has a bit of blockage in this sector.
The Tucson morning sounding today, above, shows a saturated layer from the surface to just below 700 mb and the precipitation is confined to this layer, as shown by the echo top map below. Most of the area southwest of the radar shows no echo tops, because the low tilts are blocked in that region.
The complicated terrain blockage pattern of the Tucson radar (identified as KEMX) is why composite reflectivity images are the preferred product for monitoring radar echoes. Interestingly, a much wider range of products for KEMX can be viewed at the College of Dupage weather page than are provided locally by the NWS.
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