Saturday, August 22, 2020

If Storms, Then Severe

Skies are still a bit hazy and dirty this morning. Yesterday, with a high of 96 F at the airport, was the first day of this record-breaking August below 100 F.

Plot above of 24-hours of detected CG flashes (ending at 01:30 am MST this morning - from Atmo and Vaisala), shows that not only was southeast Arizona totally suppressed yesterday,  as expected, but the entire southwest half of Arizona had no thunderstorm activity.

During the night a strong MCS has been moving west-northwestward across northern Sonora (below - IR image from 01:00 am). This system did push a weak outflow northward that moved past the airport around midnight last night, increasing low-level moisture across our area. The MCS has completely dissipated, leaving behind only a bit of residual cloudiness - second below, visible image from 07:45 am this morning.

The 12 UTC morning 500 mb analysis (above, from SPC) shows the anticyclone centered over the Four Corners, with strong west-northwestward winds across southern Arizona. Southeast Arizona is positioned to be in the region of strongest downward vertical motion associated with the anticyclone.

The 12 UTC TWC sounding below (also from SPC), is one of the most unstable, and favorable for severe thunderstorms, that I've seen in a long time. Cape is very high above the capping inversion at 700 mb. Vertical wind shear would support mesoscale organization, with strong easterly winds at middle-levels and westerly winds aloft at anvil levels.

So, it appears that we will watch a battle between large-scale subsidence and small-scale orographic lifting today - 06 UTC WRF-GFS forecast (at bottom - for 24-hour rainfall ending at midnight tonight) indicates that subsidence aloft dominates.

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