Before I discuss the weather situation, it is important to note: The UA's spring undergraduate and master's commencement ceremony will take place at Arizona Stadium on Friday, May 10, at 7:30 p.m. The last time the event was held at the stadium was 1972. Organizers decided to move the event from McKale Memorial Center back to the football stadium this year to accommodate a growing crowd.
The weak 500 mb trough that has been stagnant over the West this week is being pushed southward now by the strengthening ridge over the Northwest. This morning's NAM forecast at 500 mb (above - valid 5 pm MST today) indicates the trough axis stretching from south Kansas across central Baja, with a vorticity maximum over Baja. The winds at 500 mb are forecast to be from the east during the afternoon. The composite radar echo forecast (below) from today's (Friday 10 May) early run of Atmo's WRF-GFS indicates widespread shower activity over over southeast Arizona at 4 pm this afternoon.
The forecast sounding for Tucson (also valid at 4 pm) remains very similar to yesterday's forecast from the WRF. There is a bit of CAPE within the very deep boundary layer. Steering winds are easterly, but light, in today's forecast. This remains a thermodynamic structure that would support strong downbursts, dry lightning strikes in the mountains, winds, dust, and etc - see yesterday's post. The WRF-GFS forecast for 10-m winds indicates several, organized outflows by 5 pm this afternoon (below), with the strongest forecast to be moving westward across the Tucson area.
The WRF forecast of rainfall at the surface remains very minimal through midnight tonight (below). The organized area of light rain east of Tucson appears to be the source for the significant outflow that is forecast across the metro area. Everything considered, today's high-based storms could be quite hazardous, even though they won't produce much rainfall.
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