Friday, August 12, 2016

Morning Storms Yesterday Followed By Severe Clear

Edited to add: The NWS PHX Office was interested enough in the strange "subsidence bomb" yesterday that they posted the following loop to twitter - see (thanks to Mike Leuthold for alerting me):

It was a very strange day yesterday, with the thunderstorms that produced the morning rains over the Catalinas and parts of Pinal County dissipating by midmorning. ALERT rainfall shown above occurred mostly before or a few hours after sunrise. Plot of detected CGs below (from Atmo and Vaisala) ends at 7:00 am MST this morning, but the blue tones indicate that the storms of interest occurred 21 to 24 hours ago. Outflows from these storms were weak and couldn't move south up the Santa Cruz Valley, and folks in the metro area were left looking at dark skies to the northwest during the morning.

When the thunderstorms dissipated, a huge circular area of subsidence and clearing expanded rapidly outward, covering almost the southeastern half of Arizona. The satellite image above is from 12:45 pm MST yesterday. It was intriguing to watch this "subsidence bomb" expand hour-by-hour in the visible imagery. I went back and looked at the 12 UTC WRF-NAM forecast of vertical velocity at 600 mb, and it actually seemed to forecast something like this happening. 

This morning the atmosphere over eastern Pima County remains moist and unstable below about 600 mb, but the winds below 400 mb remain very chopped up and disorganized (12 UTC TWC skewT above from SPC). Probably another strange day on tap with storms trying to develop mostly on the mountains. The 06 UTC WRF-NAM forecasts subsidence to dominate over much of Pima County this afternoon, with larger storms staying off in Cochise County and along the Borderlands. Both versions of the model suggest an uptick tomorrow, and then a serious dry-out coming in from the east on Sunday and Monday. Forecast below is from WRF-NAM for PW at 11:00 am on Monday morning.

A few mornings with lower humidity and cooler temperatures wuld actually be most welcome after the past week or so.

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