The southern Arizona sun was high in the sky, not far past solar noon and not long past summer solstice, when the Pinaleno Mountains lit up with thunderstorms. One of those became a multicell with this small but heavy core. As this was only the first week of any monsoonal convection at all for the season, the thin, rocky soil still was dry from baking behind the High Plains dryline all spring. So were the washes and gullies, meaning the water didn’t travel far before soaking in, and flash flooding was very small and localized at best. As they sometimes do, the monsoonal season kicked off not by intrusion of moisture from the Gulfs of Mexico or California around intervening mountain ranges, but instead, by a “back-door” process: a cold front that had moved down the High Plains in New Mexico fairly deeply, and brought post-frontal moisture that had originated over a week before in the Gulf of Mexico, then advected around a low and upslope through the Plains and the lower ranges and passes of New Mexico. To envision and predict this requires good conceptualization of fluid flow around fixed, jagged barriers of varying heights: the mountains themselves. During the solar-heating cycle, the higher terrain gets unstable faster, relative to physically closer cool temperatures aloft, than the deserts, even though they’re cooler in an absolute sense. That, and upslope lift, weaken the cap first over the mountains, and…away we go!
4 WSW Artesia AZ (5 Jul 21) Looking SW
32.6732, -109.7832