“Hanging virga” materialized yesterday, starting from a cluster of late morning modest, but very cold, Cumulus clouds that transitioned to soft and small Cumulonimbus clouds as they approached the northern parts of Catalina, Charoleau Gap and Oracle yesterday.
How cold were those clouds?
Bases were at 10,000 feet, just above Mt. Sara Lemmon, at about -15 C (4 F), a real bottom temperature rarity for southern Arizona Cumulus clouds. The highest tops, probably only reached 15,000-16,000 feet above sea level and would have been close to -30 C (-27 F), also exceptionally cold for such a low top height. So, the clouds, for the most part, were less than 2 km (6,600 feet) thick. At times, they appeared to be miniature summer clouds with all the glaciation and “shafting” going on. Here are some shots:
Remarkably as cold as the bases were (-15 C), nature abhors starting an ice crystal until a liquid cloud drop has formed. So, the sequence goes like this; liquid droplet cloud forms (as in our smallest Cumulus yesterday, “humilis”), but then they must develop further to produce ice. There is a temperature AND drop size threshold requirement for ice formation, even in clouds this cold. As the clouds fatten upward, the drops in them get a little larger, and at the same time the temperature drops, too, and, voila, the ice-forming criteria for that day are met, and out pop the ice crystals. Those depth/cloud top temperature criteria change some from day to day.
And, as you likely noticed and wrote in your weather diary, those cold, but shallowest clouds yesterday did not produce ice, while ones that got a bit colder and fatter did. Most of the time, low-based clouds that reach just -10 C to -12 C begin to produce ice, and even at higher cloud top temperatures in the summer on occasion, the latter, a LOT of ice at cloud top temperatures warmer than -10 C.
There’s the enigma. How’s come yesterday’s tiniest clouds, with bases at -15 C, did not produce ice immediately? What is it about those itty bitty first formed drops that makes them so resistant to freeze? Surface tension? This kind of result for small cold clouds was found repeatedly in our aircraft studies at the U of WA.
11:29 AM. I looked to the sky for answers about ice formation, but it didn’t seem to know either.
Before the vast mid-afternoon clearing (associated with that passing trough /wind shift line above us), there was another complex of glaciating modest Cumulus and Cumulonimbus clouds with significant virga that went over the same area as shown in these photos. No doubt, someone got a flake or two, or more likely, a tiny ball of graupel (soft hail).
By mid-afternoon, it was “all over” as the Cumulus dwindled to tiny versions, with no ice, and ultimately disappeared within two hours.
And with the clearing skies late in the day, the plummeting temperature. Was 31.x F by 7 PM, but just after that, the “sliders” started, as they usually do here on a little hillside, and the temperature pretty much leveled out and has briefly hit 28 F. In the meantime, just down the road, its 21 F in the Black Horse subdivision! The CDO wash would be even colder if we had a measurement there.
You can see our regional temperatures here from the U of AZ, and the more local ones here from Weather Underground, now owned by The Weather Channel and they better not screw it up any more than they already have re radar depictions (they don’t work as good.)
The weather just ahead?
More little troughs like yesterday, such as one passing over us today, and later tomorrow, likely to again to be ones, especially tomorrow, bringing a few small Cumulus over us in the afternoon, some of them shedding ice. We already have some ice clouds, low Cirrus, today, and along with those, maybe a flake or two of Altocumulus. It’ll be pretty scenic again.
Then The Warming, a vast and an amazingly quick change in the flow pattern that warms us up during the middle of the coming week. And, no rain indicated in mods for next 15 days, though as always, there is hope in the final few days that it will be wrong. More on that way down toward the bottom.
Some newsworthy weather is farther ahead…
There is something that will happen that you’ll read about, extreme cold in the East, 8-12 days out. This happens as a gigantic storm-blocking ridge piles up along the West Coast, all the way into Alaska. In these situations, Pacific storms are diverted to Alaska where the folks up there think its comfy with all that marine air blasting into them from the ocean, but then, that air turns cold over the continent and streams down into the US akin liquid nitrogen rolling down the side of Mt. Lemmon.
Why even talk about this when its so far out in the models, since they are often a joke that far out?
You do it, stick your neck out, because of how POTENT the “signal” is in the NOAA “ensembles of spaghetti” for this to happen. Besides, you might be getting a “scoop” as well, if the other forecasters aren’t on their toes.
A pattern of extreme temperatures over ALL of North America is just about certain. Check this out below. I’ve added some text on to help you know what to think when you see it. That’s what I try to do here; tell people what to think. Its great!
The red arrow is up the shaft of a gigantic ridge, the one foreseen in the models lately. Note how special we are along the West Coast in this plot; there are no other protrusions of ridges anywhere in the whole northern hemisphere like this ours!
What is a ridge composed of? Deep WARM, comfy air. So a huge blob of warm air IS going to arise along the West Coast in 8 days. That translates to much warmer than normal temperatures practically from Alaska to here, probably a heat wave in southern Cal around this time.
At the same time, when the flow is disturbed like this, and has so much “amplitude” (goes north and south so much) like it shows here, that is, goes WAY to the north on one side of the ridge and then WAY to the south on the other, you get temperature extremes as you could EASILY guess. Extra warm over “there” somewhere means extra cold over “yonder” (in this case, the eastern half of the US.
Why do these coming temperature extremes have so much credibility?
Its because of the remarkably (to me) tight bunching of the lines (500 millibar contours), the way they are in the above graphic. This means the signal, the factors putting this pattern together are powerful, and have not been disturbed by the “noise” of the many small errors DELIBERATELY put into the model at the beginning of the run to get these differing plots. For 8-10 days out, these are about the “tightest bunching of lines” I have seen, meaning the forecast is robust; namely, is going to happen.
For us it means a further extension of droughty, but warm days that follow soon on the heels of our cold spell, into the 20-25th of January.
As robust as the forecast is for 8-10 days out shown above, the models are pretty much clueless about how this pattern falls apart (not too surprisingly). To experience model cluelessness hereabouts, check this plot out below for 15 days away from the same computer run and notice the “out of phase” pattern being indicated. The gray lines show a trough in our region (maybe storms and cool), and the yellow lines, from a model run just 12 h later, last evening, shows a ridge over the West (warm, sunny weather indicated here). A forecaster, looking at this, and covering all the bases might say:
“Continued cool with variable clouds and showers today, otherwise mostly sunny and warm.”
That’s about what you get out of this last plot. Not much confidence.