After last evening’s surprisingly heavy rain, we have now met our average for May for Catalina, having received 0.47 inches of rain over the past 24 h, some 0.36 inches during some house-shaking thunderclaps last evening.
Below are the 24 h local totals, ending at 4 AM today from the Pima County ALERT gauges rolling archive , these totals pretty much capturing all of our beautiful storm:
—- —- —- —- —- —- —————– ———————
1010 0.63 Golder Ranch Horseshoe Bend Rd in Saddlebrooke
1020 0.83 Oracle Ranger Station approximately 0.5 mi SW of Oracle
1040 0.55 Dodge Tank Edwin Rd 1.3 mi E of Lago Del Oro Parkway
1050 0.75 Cherry Spring approximately 1.5 mi W of Charouleau Gap
1060 0.79 Pig Spring approximately 1.1 mi NE of Charouleau Gap
1070 0.39 Cargodera Canyon NE corner of Catalina State Park
1080 0.63 CDO @ Rancho Solano CDO Wash NE of Saddlebrooke
1100 0.35 CDO @ Golder Rd CSO Wash at Golder Ranch Dr
Santa Catalina Mountains
1030 1.18 Oracle Ridge Oracle Ridge, 1.5 mi N of Rice Peak
1090 0.35 Mt. Lemmon Mount Lemmon
1110 1.34 CDO @ Coronado Camp CDO Wash 0.3 mi S of Coronado, 1130 0.83 Samaniego Peak Samaniego Peak on Samaniego Ridge
1140 0.79 Dan Saddle Dan Saddle on Oracle Ridge
2150 0.24 White Tail Catalina Hwy 0.8 mi W of Palisade RS
2280 0.24 Green Mountain Green Mountain
2290 0.12 Marshall Gulch Sabino Creek 0.6 mi SSE of Marshall Gulch
The clouds and weather just ahead
A little cold morning rain, and even snow on The Lemmon, is looking likely for Saturday morning. Presently, the core of the jet stream at 500 millibars or around 18,000 feet associated with a mighty upper cold low that sits on Arizona on Saturday is forecast to be south of us (as was yesterday’s jet), a pretty black and white discriminator for cool season (Oct-May) rain here.
However, if that jet core around the low does not circumscribe TUS, you can forget rain. From IPS MeteoStar, this rendering of the upper level configuration for Saturday morning, showing that it WILL circumscribe TUS:
In the meantime, “troughiness” today, tomorrow and Thursday, with secondary jet stream to south of us, will give us some more photogenic high-based Cumulus, maybe with some with virga in the afternoons. Today, as our upper low says goodbye, subsiding air is supposed to keep clouds from attaining tops high and cold enough to form ice. So, no rain today.
Yesterday’s clouds (going deep, as in pedantically)
There were some great scenes yesterday, summer-like ones, odd for May here, with massive rainshafts as the cloud bases lowered, reflected a huge jump in surface dewpoints to summer-like values in the mid-50s. Cloud bases yesterday morning, riding the tops of Samaniego Ridge, were near 7 C, compared with -5 C the afternoon before.
This warming of cloud bases greases the precipitation “wheel” since clouds with warm bases are be able to rain easier than ones with cold bases (say, near or at below freezing temperatures). Droplet sizes have to be larger at any given level above cloud base compared to the clouds of the day before since more moisture is forming in those updrafts at the higher base temperature. And, oddly, the larger the droplets, the higher the temperature at which ice can begin forming in clouds. And when ice forms, snow, then rain, come out the bottom.
To go on too long on this in covering all rain possibilities for yesterday, a base temperature of 7 C here is on the edge of being able to produce droplets big enough so that some begin colliding with one another and sticking together so that drizzle, then raindrops can form, a couple to a few thousand feet above cloud base, and those sizes of drops can really accelerate the formation of ice and then rain out the bottom. Are there any readers left? I doubt it.
Let us go even deeper…. It was hazy, smoky looking yesterday most of the morning, even when some good thunderstorms formed. So what? Well, smoke is bad for storms. Remember when it was reported by Warner and and the U of Arizona’s own Sean Twomey (1967) that sugarcane burning made it stop raining downwind from those fires in Australia? That effect has been verified in satellite measurements by cloud seeding nemesis, Danny Rosenfeld2 of the HUJ in Science a few years ago.
Well, too much smoke can choke droplet sizes down and inhibit the formation of rain by collisions, and delay the formation of ice. And so we had that counter effect of smoke from somewhere, maybe LA this time since it was in the boundary layer, not aloft like that smoke layer from Asia was a couple of weeks ago.
So, cloud microstructurally-spekaing, it was an especially interesting day, one, if he were cloud maven person, wishes he would have had an aircraft to sample them.
But let us look now and see what all the fuss is about: