Finally, A Heavy Thunderstorm Here!

Photo above shows developing storm over west end of the Catalinas at about 3:30 pm MST yesterday afternoon (Sunday, 23 August). Once this storm moved out over lower elevations it quickly dissipated. The plot of detected CG flashes for 24-hours ending at 0830 UTC on the 24th (below, from Vaisala and Atmo) shows that thunderstorms mostly avoided the low elevation metro area, with the main activity from the north to west to south of Tucson. Across the ALERT network only 27 of 92 sites measured 0.04" or more of rain for 24-hours ending at 7:00 am MST this morning. The NWS had forecast highest POPs (50% at airport grid box) during period from 5:00 pm through 11:00 pm and this is indeed when most of rain occurred, with 25 sites having rainfall during that period. Amounts were generally very light, especially compared to Saturday's rains (compare the metro-west ALERT sector - second below, for the evening period - to the same sector for Saturday - earlier post). Only 4 sites had more than a quarter of inch. Heaviest amounts were 1.22" at Davidson Canyon in the far east part of the array and 0.94" at Avra Valley Air Park, on the far northwest side of the network.

But big news here was that a small storm developed just to the northeast at dark and moved slowly southwestward across our location. This was a very small radar echo and did not directly hit any ALERT site. Here it produced several crashes of thunder, wind gusts to about 50 mph, and very heavy rain. During a short period between 8:00 and 9:00 pm MST we received 0.63" - more than fell during the entire month of July. When I walked this morning, it appeared that rain was likely a bit heavier just to our west; regardless, a real rain was hugely appreciated here.

This morning's skewT plot for the TWC sounding (above from SPC) continues to show high PW, a much improved vertical wind profile, but with CAPE amounts only small to perhaps moderate for later today. Development of storms at lower elevations will require considerable sunshine, as well as substantial outflow convergence within the BL.

The 06 UTC forecast run of the WRF-NAM at Atmo is quite interesting. Most significant storm development is forecast from southwest to north-northwest of eastern Pima county (above forecast of composite radar echoes is valid at 5:00 pm). However, outflows from the western storms and from the Cochise County storms converge and produce evening storms over eastern Pima County - radar forecast below is valid at 8:00 pm. Will be very interesting to watch what evolves later today.