Mesoscale Precipitation Discussion 0993
NWS Weather Prediction Center College Park MD
635 PM EDT Sun Oct 21 2018

Areas affected...Southern Arizona

Concerning...Heavy rainfall...Flash flooding possible

Valid 212234Z - 220234Z

Summary...Thunderstorms capable of producing hourly totals of 1-2"
will be possible across far southern Arizona through late
afternoon and early evening.

Discussion...In divergent flow aloft with an open wave across
southern California, combined with peak heating where instability
is around 1500-2000 J/kg, thunderstorms have developed across
southern Arizona. Recent analysis shows a tongue of higher PWATS
(1-1.1") across the area, which is in the 90th percentile and
roughly 1-2 SD above normal. Convection has been drifting
northeastward per latest KEMX returns, where hourly rates have
exceeded 1-2".

Hi-res models have not handled this activity well, but would
expect a slow progression northeast for the next few hours until
peak heating/instability wanes. Additional totals of 1-2", with
locally higher amounts, will be possible over areas with lower FFG
(1-hr 1" or less) which may lead to localized flash flooding.

Taylor

ATTN...WFO...PSR...TWC...

ATTN...RFC...CBRFC...

LAT...LON   33470983 32030947 30711000 30381073 31521201
            33041100