Sunday, July 31, 2016

Very Active Day Today?

There was enough heating yesterday for thunderstorms to move into the metro area at sunset last evening, as per above webcam view. The plot of detected CG flashes below (from Atmo and Vaisala for 24-hours through 5:00 am MST this morning) shows most activity in eastern Pima and northwestern Cochise Counties. Although winds gusted 35 to 45 mph rain amounts were fairly light in comparison to Friday's big event. 

There seem to have been serious problems with the ALERT network data and a large number of sites are missing this morning. Observations indicate a Trace at the airport, 0.06" here and at DM AFB, with Nogales picking up half an inch.

The sounding plot for TWC (above from SPC for 12 UTC) is VERY moist and quite unstable in low-levels. However, there are some downsides. The very moist low-levels are currently capped by a mesoscale subsidence layer (onion sounding aloft) left behind by the MCS of last evening. The spatial extent of this layer with considerable CIN is, of course, unknown. The winds aloft are light and variable below 250 mb - so storm movement may be slow and erratic - resulting in heavy rain and wet microburst threats.

The surface layer requires considerable heating to, and/or mesoscale outflows, to overcome the capping. This could be a problem, given the widespread cloudiness over southeastern Arizona and all of northwestern Mexico (associated with a large, subtropical inverted trough at 500 mb). My subjective appraisal, based on the morning observations, is that heating will be considerably restricted, so that propagation of storms into desert elevations would be dependent upon mesoscale outflows.

However, the forecasts from the 06 UTC runs of both versions of the WRF model are more optimistic  - both versions forecast a very active afternoon here in eastern Pima County.  The forecast below is of solar IR in W/m2 valid at 1:00 pm this afternoon from the WRF-GFS. The model keeps heavier cloud and early storms off to south and southeast and has strong heating for the metro area.

It appears that a 1:00 pm special sounding could be very useful today, if the situation remains murky at midday.

The model forecast sounding for TWC valid at 3:00 pm MST (above) is very unstable, with CIN layer gone, and would support heavy storms, as per noted above, for the metro area. The WRF-GFS forecast of accumulated rainfall through midnight tonight is below - definitely a big day in the model forecast. Close metwatch needed today.

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