A line of storms formed over Eastern Pinal County yesterday evening and pushed WNW into the Phoenix area from 9-11 PM. Unfortunately the storms dissipated as the hit the East Valley, though the outflow did cause a few storms to regenerate over Central Phoenix (another good example blowing away the Heat Shield Theory – I’ll write up a post on that in the near future). The Phoenix Rainfall Index (PRI) came in at 0.00″ with just 11% coverage. Decidedly not a big day.
A note on what a “Big Day” might be. Using historical PRI data, on average there are only five days each summer where at least half of the Phoenix Metropolitan Area receives measurable rain and only one day where practically all (90% or more) of the Valley gets rain. By far, when it does rain the norm is to have hit-or-miss events (10-50% coverage) like the past few days – which happens on average 19 days each summer.
Turning to today…the mid-level anti-cyclone (sometimes referred to as the Monsoon High) is near the Four Corners this morning per RAOB data. One thing that jumps out at me right away is an area of cooler temperatures coming around the south side of the high (see below). Cooler mid-level temperatures promote steeper mid-level lapse rates and higher CAPE value.
Looking at the morning sounding, it is still very moist but there isn’t as much instability as was available yesterday (see below). One good thing though would be the incoming cooler mid-level temperatures, which would increase CAPE values. SPC Mesoanalysis page is showing good CAPE is already present though.
Satellite and radar images (below) this morning are showing convection has already developed, with the tops quickly blowing west. That may cut down on temperatures for the day (blocking the sun/solar insolation over the Valley) and inhibiting CAPE from developing.
A problem appears to be the continued influence of drier air over far Eastern Arizona and New Mexico which is inhibiting convection/thunderstorms from developing and reaching maturity during the afternoon hours. With all the moisture and instability over the deserts it just goes to waste if nothing can act on it. I’ll be watching upstream thunderstorm development through the day as I think that is a key missing feature right now.
High-resolution models run at the University of Arizona and other run by NOAA don’t point to an incredibly active day. SREF data also aren’t pointing to a big day. I’m going to go with a low activity (10% or less coverage) day as well due to the lack of upstream storms forming.
