Thursday, August 02, 2018

August Starts Wet Here

August started out with storms and rain over western portions of Tucson metro area. Here at house we had loudest crash of thunder of entire summer so far and 0.33" of rain in less than 30 minutes - a nice break of the long dry spell here. The airport had severe wind gusts and 0.26"; DM had thunder but no rain; and Atmo had 0.29. Stroms developed out in Kitt Peak area around 5:00 pm MST (above), then a new line developed along the Tucson Mountains, and then a south to north line developed from airport north across Oro Valley. The ALERT map below (for 24-hours ending at 7:00 am) shows the parts of network that had rainfall - some amounts over half an inch. Second below shows detected CGs for 24-hours ending at 1:30 am this morning (from Atmo and Vaisala) - the eastern edge of activity is very distinct across the metro area.

There were several reports of severe thunderstorm wind damage with the line that developed over the airport - including the severe gusts at the airport. On my walk at sunrise this morning, I saw that a roof had blown off one of the greenhouses at the U of A research farm just southeast of the Campbell Ave. bridge over the Rillito (there were three buck-lifts in action with a number of workers repairing the damage).

The sounding plot above is for TWC at 00 UTC last evening (release time about 23 UTC) - sounding does not appear likely to support storm development! However, by 7:00 pm (see radar below from NCAR) a severe storm had developed over the airport. There had to have been significant moisture convergence occurring, as several gust fronts came into metro area from different directions. Indeed, the GPS time-series of PW on campus shows a very significant increase of about a quarter of an inch beginning about 4:30 pm, i.e., 2 hours before the airport storm developed. It was quite an interesting turn of events, and I was really surprised as storms developed overhead here with crashes of thunder.

This morning there seem to be two weak anticyclone centers at 500 mb - one west and one east of Tucson - leaving the wind field in a serious mess - see TWC morning sounding plot below from SPC. The SPC analysis really overdoes the CAPE, given the dry low-levels. The BL structures will be complicated by afternoon, but if storms develop, they will have potential both for local heavy rains and damaging downbursts. The 06 UTC runs of the WRF model at Atmo last night have different forecasts for the metro area this afternoon and evening. The GFS version forecasts a typical donut hole overhead, while the NAM version forecasts heavy storms. The NAM version has a strong outflow from the south forcing storm development just around dark. Flip a coin.

No comments:

Post a Comment