Saturday, August 20, 2016

Upturn In Storm Activity Today

Another nice sunrise thanks to cumulus and some buildups drifting around the morning sky.

The unusual summer 500 mb pattern continues, with an anticyclonic circulation center off in northern California and another somewhere down in central Mexico (12 UTC analysis above from NCAR RAL/RAP). A weak trough line lies across southern Arizona. Certainly not a typical monsoon pattern and one without significant features to track. The stronger northeasterly winds over northern Arizona will try to move storms toward the lower Colorado Basin; a situation favorable for severe thunderstorms out there.

Locally, our morning sounding (skewT plot from SPC) above is more moist and definitely more unstable than was yesterday's. Winds in the lower troposphere are becoming light and variable again, increasing chances for local heavy rains and perhaps wet downbursts. There is also CAPE aloft from about 800 mb up to 600 mb, so earlier thunderstorm activity over the nearby mountains should be likely today.

The 06 UTC WRF forecasts from Atmo have a split personality for this afternoon. The WRF-NAM forecasts only isolated storms over eastern Pima County and keeps stronger storms off to south and southwest. However, the WRF-GFS forecasts a very active day for much of the state (forecast of composite radar echoes below valid at 4:00 pm), with very strong outflow winds over about the southwestern third of Arizona. Appears that today will be much more interesting than was yesterday.

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