Thursday, August 23, 2018

Some Spots With Severe Storms And Very Heavy Rains Yesterday

Heavy storms developed over parts of metro area during early afternoon yesterday. The view of Catalinas from campus (above and below) captured the very rapid storm formation near and west of the mountains in only 20 minutes.

The ALERT rainfall map (above for 24-hours ending at 7:00 am MST this morning) again shows a hit-and-miss pattern over the network. The very heavy storms over northwest metro produced rain amounts to over 3", damaging wet downbursts, and local flash flooding. Ron Holle reported over an inch and half in a short period yesterday afternoon, while here at house we had an extreme downpour that produced 0.69" in about 10 minutes (another storm around 2:00 am this morning added 0.14" for total 24-hour rain of 0.83". In contrast: Atmo had 0.11"; TUS 0.09"; and DM a Trace (also 24-hour amounts).

The plot of detected CG flashes (below, from Atmo and Vaisala for 24-hours ending at 7:00 am) indicates a very active day for much of south-central and southeast Arizona - but with large donut hole centered on Phoenix area.

At 500 mb (both of these 12 UTC charts from SPC) the cyclone over the northwest has weakened and shifted to Montana. Of interest to us is that the north end of a weak IT over Sonora has merged with the weak trough from Montana low just to our east. This feature does not extend up to 250 mb (below), where a significant cyclone is moving westward through the southern portions of the anticyclone. Another very complicated large-scale setting.

This morning's TWC 12 UTC sounding plot (above) continues very wet and with significant CAPE again present. Winds below 300 mb are very light, leaving little in way of steering winds and a continuing threat for heavy local rains and wet microbursts. However, today there is significant morning cloud cover - below is visible image from 7:30 am. Further complications are that: another MCV has been left behind by an early morning MCS - this feature is spinning in the above mentioned, merged-trough line near Douglas; and an area of thunderstorms has developed since sunrise northwest of Marana.

Another interesting day likely - given a bit of local heating.

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