Yesterday thunderstorms totally avoided eastern Pima County and also much of Cochise County. Plot above shows 24 hours of detected CG lightning flashes though 5 am MST this morning (September 27th). Across the ALERT network for past 24-hours only 8 stations measured light amounts of rain.
I don't know what I was distracted by when I showed the WRF-GFS model forecasts for yesterday afternoon - the forecast TUS sounding for afternoon was a classic one in which anvils from storms to the west cover the metro area and shut things down - which is what happened. Thunderstorms were widespread across south-central Arizona, western Arizona and the Rim Country. The bright colors in the plot indicate storms that were still active early this morning along the Rim Country and east of Phoenix. All of the severe thunderstorm reports logged at SPC for 24-hours ending at 12 UTC this morning were in the West (below). Two reports were logged in Arizona, near Green Valley and at the Nogales airport where a thunderstorm wind gust hit 59 mph.
Situation this morning is quite complicated across southern third of Arizona. The sounding above is from Empalme (aka Guaymas) this morning and it exhibits high PW and very large CAPE, even though the wind regime is westerly. So, there's abundant unstable air south of the border. But the Tucson morning sounding (below) has almost no CAPE, due to dry air at low-levels, which has come in from the southwest. It appears that drier low-level air has come through Yuma and has also affected the rest of Pima County. So a key question for southern Arizona is whether there will be enough moisture convergence ahead of the approaching Pacific front to support storm development this afternoon.
The early morning forecasts from WRF-NAM at Atmo seem to be reflecting the drying that has been occurring this morning, so I'll show a couple of graphics from that model. But note that the GFS version is much wetter this morning, and I may be zigging again with the wrong model. The WRF Nam forecast of PW above is valid at 3 pm MST this afternoon. The model forecast indicates a band of higher PW south-to-north from northern Sonora, through central Pima County and on to the Phoenix metro area. There is a second band of higher PW right ahead of the frontal zone.
The WRF-NAM forecast of accumulated rainfall through midnight tonight (below) again has a large donut -hole centered on the Tucson metro area, with heaviest rains to the south of here and also all across northwest to central Arizona.
The longer-term model forecasts indicate that the 500 mb low currently over California will be slow to move eastward because of the blocking pattern over the eastern 2/3rds of U.S. However, the dry air coming into western Arizona today gradually circulates around most of the system, drying out the Southwest. As for today, we'll just have to wait and watch.
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