Edited to add: Photo above by Lee Carlaw, NWS TUS, about 7:00 pm MST yesterday from I-10 and Tangerine looking east toward Oro Valley. This is the storm that I mentioned in the text below.
Thunderstorm activity increased considerably yesterday, especially over the mountains and out in the Tohono O'odham highlands. There was a high-based storm off the west end of the Catalinas (at about 6:45 pm) that appeared as if it might be producing a dry microburst (see photo at top). There were 16 sites in the ALERT network that measured rainfall and these were mostly at the high elevation sites. Two sites on the Catalinas reported 1.02 and 0.55 inches of rain - this after two days with zero reports of rainfall in the network. High reached 110 F at the airport yesterday - so there was a seriously deep afternoon boundary layer.
The overall situation has changed dramatically during the night as higher PW air has moved into eastern Pima County from the west. Time series of GPS PW from the campus site is shown above and indicates PW over 1.70 inches. The morning sounding plot for TWC (below from SPC) exhibits northerly winds above 700 mb (as the midlevel anticyclone has continued its westward shift) and also moderate CAPE - so the situation today is much more favorable for storms.
The current morning forecast from NWS TUS second below continues to emphasize the heat with POPs at the airport of only 10% tonight.
The 06 UTC runs of the WRF model at Atmo are quite astounding forecasts - the most dramatic that I can remember for this summer - I am showing the WRF-NAM version here but the GFS version is somewhat similar. The model forecasts little storm activity during the afternoon today - apparently due to a substantial layer of convective inhibition (CIN) that develops. However, during the night PW increases again from the west and steering level winds become northeasterly. The CAPE forecast for 1:00 am MST early tomorrow (see above) is huge (GFS version even larger). Shortly after this time both models indicate a moist, absolutely unstable layer (MAUL) developing near 700 mb. Such a situation favors development of storms over eastern Pima County. The storms could be severe and would definitely produce very heavy rains (if the model forecasts is close to reality).
The NAM version forecasts an intense MCS to develop over eastern Pima County by 3:00 am tomorrow morning (above is the composite radar forecast for that time) and also forecasts heavy rains at lower elevations (below is total rainfall forecast through 6:00 am).
The model certainly forecasts the potential for very strong storms somewhere in our area, so that a careful metwatch is called for. If afternoon storms dominate, then the nighttime activity would probably not occur as forecast, but if little happens during the daytime hours, then the situation will be primed for a significant nocturnal storm event somewhere in southeastern Arizona.
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