Wednesday, August 15, 2018

Some Slow-Moving Storms Metro Yesterday

Purple skies a bit before sunrise this morning, looking east toward Thimble Peak. Below, the Rillito was running this morning - which is the first time I can remember for many weeks. Second below is flow from the USGS gauge that is about a mile due east of here - there was a blip to over 300 cfs a bit after midnight.

ALERT data plotted above are from 24-hours ending at 7:00 am MST this morning. Reports of half an inch or more show where several of the heavy storms were drifting around the City. Two widely spaced rainfall amounts of over an inch. The airport had two storms move over, with a total of 0.91"; Atmo had 0.24"; DM had only 0.01"; while we may had a bit of a Trace here. My guess is that the Rillito flow resulted from the heavy cell that was a few miles upstream from here.

The plot of detected CG flashes below (from Atmo and Vaisala for 24-hours ending at 6:00 am) shows clearly the hit and miss character of the thunderstorms over eastern Pima County. Early morning storms continued off to our west.

The larger-scale pattern in the middle troposphere continues very chopped up - 12 UTC analysis above from SPC. It looks to me like an inverted trough (IT) extends from the northern GoC across Arizona up to Four Corners region. Another IT stretches from northern Sonora eastward and then northward into the Iowa closed low. Weak anticyclonic circulations are over eastern New Mexico and northwest Arizona. Looks like another day with no well defined steering flow (yesterday afternoon's sounding revealed that steering winds had become very light). The 250 mb analysis below is somewhat more coherent, with the anticyclonic center having shifted westward over central Sonora.

The TWC morning skew-T plot of upper-air data (above from SPC) remains moist but with a deep layer of cool air extending up to 700 mb, with a very strange elevated mixed layer above - perhaps having advected over us from somewhere off to the southeast. The SPC CAPE analysis is again a significant over-estimate of potential values for this afternoon. CAPE will be considerably less, with a significant inversion trying to suppress deep convection. The steering flow is weak from the south-southeast, while upper winds are fairly strong from the southwest - may be even more anvil shading today over metro area.

Both WRF runs from 06 UTC at Atmo forecast heavy storms to move northward across central Pima County, while storms over the Sky Islands stay mostly on the mountains. The central Pima County storms produce, in the forecasts, heavy anvil over the metro area. The chart below shows forecast rainfall amounts from the GFS version for period ending at midnight tonight.

Obviously, a very mixed outlook for the day.

No comments:

Post a Comment