Monday, August 20, 2018

Chance For Thunderstorms In Metro Returns

Photo of Cb southwest of the Huachuca Mountains about 2:00 pm MST yesterday (Sunday) afternoon. Anvil is sheared back to north, but storm was nearly stationary. The plot of detected CG flashes below (from Atmo and Vaisala) is for 24-hours ending at 1:00 am this morning. Storm shown above produced the cluster of flashes in far southeast corner of Santa Cruz County.

The 500 mb analysis above (from SPC for 12 UTC this morning) shows that the main anticyclone over the Southwest is now centered over the Arizona/New Mexico border a bit northeast of Tucson. High has shifted eastward fairly quickly, as the cyclone in the westerlies moved out into the Central Plains (near St. Joseph, Missouri, this morning). Yesterday morning the center was near Death Valley, and then by evening it had shifted to near Blythe, California. Quite a rapid re-organization within a fairly blah synoptic pattern.

The morning sounding plot for TWC (above, also from SPC) exhibits significant potential CAPE for this afternoon. The depth to which the BL grows today will be crucial, since it will determine how much lift would be needed to trigger storms in the metro area. Yesterday - even with the 100 degree temperatures, the mixed BL reached only to about 750 mb and was substantially capped. The vertical wind profile is becoming much more favorable for storms, with an easterly steering component, however; speeds are fairly low.

I looked at the 06 UTC forecasts from the WRF model - both versions forecast thunderstorms across eastern Pima County after 5:00 pm this afternoon. Both models also forecast PW to increase during day due to the diurnal northwest flow, which has a fetch from the northern GoC. The forecast skew-T below (for TWC at 5:00 pm today from 06 UTC WRF-NAM), shows that model forecasts BL to grow to above 700 mb, allowing storms to develop into lower elevations. Mid-level wind speeds are also forecast to increase during the day.

At least there should be a bit more to watch today.

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