For tonight the NWS has a hard freeze warning in effect (low of 28 F or colder). I checked the lows here at house and found that there have already been 8 hard freezes, during December and January this winter, here along the Rillito - not surprising since airport is routinely much warmer for morning lows than we are here.
This afternoon I checked the radar and found that there was very strong echo out along the western edge of Pima County (above is KEMX composite reflectivity at 2:40 pm). This was likely a chaff plume coming off the test ranges. These used to be quite common, but I have not noticed a chaff event for quite awhile (the streaks of weaker echo between Ajo and Tucson are also chaff).
The radar charts here are from KIWA (above) and KYUX (below) with data from the base scans (both at 22:17 UTC). Looking at higher scans (available on the CoD weather page) the echoes were essentially gone at the third scans. I checked radial velocities (also at CoD) and found that velocities ranged from about 15 to 60+ kts (outbound from KYUX and inbound at KEMX). Velocities were highest for the strongest reflectivies - meaning that the high reflectivites were at higher altitudes than the weaker trailing plumes. A check of the hydrometer products at the CoD site indicated that the echoes were from birds or insects - the algorithms don't or can't specifically identify chaff.
At bottom is visible satellite image at 22 UTC, showing basically clear skies out where the echoes were present.
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