Tuesday, September 01, 2015

Heavy Storms And Rain As August Ends

The model-predicted upturn did indeed occur yesterday, although by mid-afternoon it was looking like it could be another doughnut hole day. Storms began developing just to the west of here, probably over the Tucson Mountains, around 5:00 to 6:00 pm MST. A heavy storm moved by just to the north of house (see photos previous post) producing considerable lightning and thunder, but only about 0.08" of rain. Another round of storms affected much more of the metro area around midnight to 2:00 am, resulting in some very heavy 24-hour rainfall amounts.

The plot of detected CG flashes above (from Vaisala and Atmo for 24-hours ending at 12 UTC this morning) shows most storm activity concentrated in the northern part of eastern Pima County and northward. The minimum area of flashes on the northern side of the Catalinas is quite puzzling, since heavy rains fell in that area. The MCS activity during the night mostly avoided us here also - my partitioning of the rain, based on nearby gauges, indicates that August ended with 0.10" and that September has begun with 0.26". There were strong west-to-east gradients in the rain amounts - for example, the ALERT gauge less than a mile to the east of had almost twice as much rain as the gauge here. 

The ALERT plots below are for the 24-hours ending at 5:00 am MST this morning - note 4 sites were over 2 inches in the Catalinas and Redington Pass areas, with Mt. Lemmon recording 4.72". Areal coverage across the network was almost 80%, with only southern and far western sites missing out on the rain.

The Tucson sounding for 00 UTC (skewT above from SPC) indicated moderate CAPE, but considerable lifting needed for low-elevation storms. However, the mountain storms that had been forming off and on several times, beginning during midmorning, did produce outflows that moved into lower elevations. The wind profile remained very light here in the lower half of atmosphere but was changing as 500 mb trough was inching eastward from the coast. The time series of T and Td and wind and gusts below from Atmo indicate that 4 or 5 outflows moved across campus, beginning a bit before 4:00 pm MST.

There were numerous severe wind reports up in the Phoenix area, with the SPC list showing 26 separate reports of wind damage. Only one of these was in our area - tree damage in Oro valley.

The 00 and 06 UTC runs of the WRF-NAM model at Atmo did quite well with the late night/early morning activity. Not surprisingly, the models forecast much diminished activity today, continuing the seesaw pattern of day-to-day storm activity for yet another day.

We are currently in a classic transition pattern, with westerly winds associated with the Pacific trough dominating above 600 mb, but with southerly flow and subtropical moisture remaining up to 700 mb or so. The trough is forecast to move little and tropical storm activity may continue the fetch of mT into parts of the Southwest, so an interesting few days ahead.

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