“Recent memory” is, of course, only about three weeks, certainly not more than a month….
BTW, FROPA is occurring now (3:55 AM-4:10 AM, marked by the usual wind shift to the NW from the SW, heavy bursts of rain in this case, and a sharp drop in temperature–so far we’re down from 55-56° F to 45° F in about 20 minutes. Likely will hit the upper 30s before the cooling stops. The rain total yesterday morning by 7 AM from the first rain band, mostly fell from mid-level clouds, was but 0.02 inches. Today, the total is going to be over 0.25 inches by 7 AM. Excellent indeed.
(Update: 7 AM rain total here in Sutherland Heights is 0.51 inches, as measured in a NWS style 8-inch diameter gauge! Awesome. The U of AZ model really nailed this event, predicting 0.25 to 0.50 inches here.)
Let us begin the cloud marathon that led to “rain.”
12:52 PM, December 15. Altocumulus (perlucidus translucidus) announces the storm, sans the Altostratus that was actually expected.
4:36 PM. Now we’re getting serious with Cirrostratus on top of Altostratus and scattered Altocumulus clouds. Three separate layers whose appearance tells you that the atmosphere above is in gentle lift, the kind of thing that precedes the actual storm, though not so much in deserty climates like ours where such scenes may only lead to a pretty sunset.
Creeping toward the present….by yesterday morning, things looked serious. I hope you noticed how FAST the clouds were moving, ones just above Ms. Mt. Lemmon:
7:20 AM. Stratocumulus layer with undulations and concave bases zipping along at 50 kts or more from the SW. Did you notice the stationary clearing to the SW horizon? These clouds were forming by the lift provided by the Catalinas and other mountain ranges downwind from the lower SW desert areas. Sometimes steady rain falls for hours with this scene. Yesterday, only sprinkles around mid-day since tops at this time were marginal for ice formation.
Some sounding comments, this nice sounding plot from
The TUS sounding launched around 3:30 AM when rain was falling in Catalina, giving us eventually our 0.02 inches of rain. No doubt the clouds in the preceding photo had warmer tops than these did. The top of the lower, and mostly liquid layer was about -16°C, normally plenty cold enough for ice to form. The humidity element on these balloon soundings can measure the humidity with respect to liquid water quite well, and so when the temperature and dewpoint lines are on top of each other in these soundings (are the same temperature) you can bet that the balloon when through a cloud of mostly liquid water. When the lines spread apart, as higher up in the balloon sounding, but are still close, you can be almost sure its an ice cloud that the balloon went through. This doesn’t apply on partly cloudy days or broken layers when the balloon passes between clouds. But when the balloon went up yesterday, we had a solid overcast, likely in two separate layers, one being mostly liquid and the higher one mostly ice. The spacing between the two is so small, if it even existed, that ice likely fell into the lower cloud layer. Sometimes this is called the “seeder-feeder” situation, since the ice crystals have a chance to grow like mad and/or collide with droplets while falling through the lower layer.
Kind of a crummy diagram of a “seeder-feeder” cross section for the Seattle area. That lower cloud is CRITICAL for enhancing precip! Believe or not, most precip in winter storms is added in the last kilometer at the bottom of the clouds, essentially by what are Stratocumulus clouds below Nimbostratus decks, themselves considered “middle-level” clouds in our cloud nomenclature.
But what about those strange clouds later, undulations and cave-like cloud bottoms? For the person still reading this tedious blog, here are a few shots of those extraordinary scenes, ones you might only see with the ferocious winds aloft yesterday:
7:46 AM. Undulatus!
8:00 AM. Strange scene, concave cloud bases–personally would call this layer Stratocumulus, but with a base height of about 8,000 feet above ground, so could also be considered an Altocumulus opacus layer.
9:33 AM. Was taking photos every few seconds as sky seemed to change.
9:35 AM. While scary clouds were overhead, you could see that a clearing was going to move in before very long, given the horrific winds aloft from this direction. Note again “undulatus”, waves in the atmosphere showing up in clouds! But, it took hours for a clearing to occur because that was a standing zone of cloud formation to our SW yesterday, something like a gigantic lenticular cloud that stands in place downwind from mountains.
9:36 AM. Having taken three photos in the past minute, each one looks pretty much the same. But, this one has a bird of some kind on the side of that bird feeder at the bottom. Was small, yellowish; maybe a canary. Its important to note that each photo was taken at exactly the same spot, but rather tens of feet from the prior photos. Moving around helps provide perspective in photos….
9:51 AM. More undulatus. Looks like its going to clear up now. Nope, it didn’t as clouds kept being added to the upwind end of these.
9:53 AM. Waves began breaking out over or just downwind of the Catalina Mountains. Would want to be flying in them!
9:54 AM. Zooming in. What is going on here? Fortunately, for the sake of having to explain this to his three blog readers, Cloud Maven Person set up his video camera and starting a film of this phenomenon. Here’s the result: dry waves of air are causing the concave bottoms of these clouds. The air in the cloud layer was moving faster than the dry waves pushing up into them, and so clouds were forming on the right side of those concave bottoms (upwind side), and dissipating on the left side (the downwind side). So those dry waves that pushed up into the cloud layer, were in effect creating cap clouds at their highest point, the kind of cloud that tops a mountain for minutes to hours (not the same as a “lenticular” cloud which is almost always downwind of the peak). Without video, I would not have known this, might have put forward something crazy in the way of hand-waving….maybe.
11:55 AM. As you can see, that clearing is still out there. Here some ice began to form in these Altocumulus opacus clouds (I would call them) and a few drops made it to the ground about this time. Eventually, it did clear up; “the clearing before the storm” as we see so often here in Catalina and Arizona where dense bands of middle and high clouds, this time thick enough to provide us with a little measurable rain overnight (that 0.02 inches), is separate from the thick, lower-topped clouds that produce the actual storm such as this morning’s blast of rain accompanying the cold front.
3:55 PM. The evening closed with “partly cloudy”, producing those nice scenes on the Catalinas. These clouds are Stratocumulus, tending toward lenticulars if that bit smoother. You no longer see any ice or virga from these clouds. Since mid-day, the tops lowered to around -5 to -10° C, a little too warm for ice formation, and the upper atmosphere dried out, so no higher clouds around either, as the main front approached.
The TUS rawinsonde balloon sounding for yesterday afternoon. The arrows denote cloud bases and cloud tops. Yesterday, there were no overshooting tops, so they probably were at -9° C as shown in this sounding. Here, following up on the prior sounding discussion, the balloon passed between clouds since if it had gone in them (droplet clouds) the two lines would have indicated the same temperature (water saturated conditions).
Well, at 6:41 AM, the rain is over, with nothing definite now in sight, though there are close calls since we seem to be in or near the “mean trough” where individual troughs and fronts like to collect and reach their farthest points south. While rain is “iffy” in these passing troughs, fluctuating temperatures from pleasant days to cold days following dry cold fronts passing by is virtually certain.