Saturday, July 31, 2010

Very Heavy Rain Event Last Evening!

Exactly four years ago today extreme and record floods occurred in the Catalina Mountains, Sabino Creek, and the Rillito - it was nearly a repeat last night, with heavy rains across much of NWS Zone 33. Only 23 gauges in the ALERT network had had rainfall during the past 6-hours at 7 pm last evening. But this morning, 100% of the 93 gauges in the network have had rain, with some very heavy amounts. An amazing number of the gauges measured over an inch - 48, which is more than half the sites. More than 2" was reported at 16 sites; more than 3" at 3 sites; and more than 4" at 1 gauge (Altar Wash at Highway 286). Reports of serious weather were of the resulting flooding; I didn't find any reports of severe thunderstorms events. Curiously, for the second night in a row during the big storms, TUS at the airport did not report thunderstorms; whereas, DM AFB only a few km away reported thunder numerous times each night. Rainfall here at house was 1.27"; so 2.60" in two days; this will make the July rainfall total quite respectable.
Brief postmortem - this event far exceeded anything I expected. The boundary layer cooked up nicely during the day and grew deeper than I had expected, reaching almost to 700 mb by the time of the 00Z sounding. The SPC plot at top shows the Tv lifted surface parcel having moderate CAPE. The GPS PW data indicated that the sounding was too dry by about 5 mm - so there was more instability present than the RRS sounding data indicated. Strong storms moved across the Santa Cruz Valley south of Tucson around 6 to 7 pm. These storms generated a significant, mesoscale outflow boundary that passed TUS and DM between 7 and 8 pm, with wind gusts from the south to southwest of 30 to 35 mph. This outflow moved north across the metro area and onto the Catalina and Rincon mountains, forcing new storm developments (see TUS NWS radar middle image). A weaker outflow had moved, from storms to the north, southward across the metro area between 5 and 6 pm, so that colliding, interacting outflows produced the initial storm developments at lower elevations. The very heavy rains resulted in the nighttime flow in the Rillito that peaked above 7500 cfs as per USGS gauge data shown in bottom image. Thus, flow in the Rillito was six times as great as the night before (see earlier post), but nowhere near the record flow of 30,000+ CFS that occured on this day four years ago.

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