What a great storm, giving all it had to Catalina, 0.34 inches, when it looked like it might be considerably less. I really liked that storm, and will always remember it.
In case you missed the snow with flakes as big as pie plates (2-3 inches in diameter, small pies) because you work in an office with no windows, these photos below. They were equal to the biggest flakes1 I’ve ever seen, including that December 1990 Seattle snow and blizzard with all the lightning. The greatest momentary depth was about an inch on the rise above Catalina town. Within minutes it was down to half an inch it seemed.
The temperature fell no less than 10 degrees as rain began to fall and the cold front slammed Catalina about 11 AM, from 52 F to 42 F. It looked like the temperature might be recovering, the rain had pretty much quit (read, clouds tops lowered in height as well), when this second part of that band (deeper clouds again) showed up with something resembling the earlier arcus cloud that had earlier crossed the Oro Valley and rammed up the slopes of the Catalinas. You can see under the dark line that the visibility drops tremendously. Don’t need to look at the radar to see that something is upon you at this point.
I thought it was going to be just heavier rain after the first rain spell that dropped a few hundredths, but after the rain started, there was some ice in it immediately, and then the big flakes began to come down, drawing the snow level down with them. Yes, that’s right, when the clouds are loaded with snow as yesterday’s were, a heavy load of snow drops the freezing level. It takes so much heat for the atmosphere to try and melt that boatload of snow coming down that it can’t keep up. Recall from physics that when melting ice, it stays at the same temperature 0 C (32 F) until it has completely melted. This phenomenon is due the “latent heat of fusion”, in this case, taking heat out of the air to melt the snow, which causes the snow level to fall. I think it was discovered by Stanley Pons and Martin Fleischmann, at the University of Utah some years ago. Both of whom went on to become quite famous for their work, I might add. So, if the freezing level in light, itty-bitty snowflakes is 4,ooo feet, it might stay there, and with them, you sometimes even get a thin layer of 0 C (32 F) air (an “isothermal” layer) where the flakes are melting.
Now drop massive 1-3 inch flakes out the bottom of the clouds. Well, its easy to imagine how the freezing level would have to drop as the air being drained of heat as it tries to melt those massive flakes, ones staying at 0 C until they are completely melted. You may have noticed that during that heavy, heavy snowfall yesterday that the temperature dropped another whopping 9 degrees from the first cold plunge from 42 F to 33 F in response to that heavy precip.
That really heavy precip was not anticipated, and was the reason why I was caught off guard by the snow here at 2500 to 3000 feet elevation. Had we (spreading the blame around with “we”) been thinking a snow rate of around 1-3 inches an hour, we would have been thinking of sharply lowered snow levels and snow (!) in Catalina. And, as you would guess, once the heavy snow ended, the temperature was quickly on the rebound into the 40s in the afternoon.
But that thought of heavy precip never crossed my mind, nor did the thought early yesterday that a really strong rain band, the one that caused it, would erupt to the west of us out of the blue practically as it did yesterday morning before rolling into Catalina. In retrospect, it was like a fastball down the middle of the plate with a 2-0 count, but you didn’t take a swing. It coulda been such a great forecast day if brain fully intact! This is why you remember storms!
Below, next to the shot of the major rainband as it came in, is the more dramatic appearing arcus cloud (that undercutting shelf cloud below the main Cumulus and Stratocumulus clouds) that announced the approach of the cold front and wind shift line about 10:30 AM yesterday morning. Man, that was a great sight! SO dramatic as that colder air swept across Oro Valley and the Tortolita Mountains, giving a boost to the overall cloud updrafts above it. Enjoy.
More Catalina precip in the near future. Details when it gets here Thursday night.
1Weatherfolk call flakes, “aggregates”, because they are made up of dozens of individual ice crystals that are locked together.