Jet stream passes overhead today and tonight, drifts to south; Cumulus and Stratocumulus clouds deepen upward during day and by afternoon tops reach the glaciation level (below -10 C, 14 F) and begin spewing snow virga which melts to rain at lower elevations.

I guess I don’t need to blog after that title…   We’ll be lucky to get 0.15 inches, and a few hundredths is the most likely amount, and that will fall later in the afternoon to late evening hours according to me and my (U of A Weather Department) model.

How much the U of A model thinks will fall regionally is seen for this event are seen in the panel below ending at 7 AM AST tomorrow morning, but first a caveat:  I just saw this run for the first time now, and did not at first notice that it was OLD, from the night before last, not based on data from last night at 11 PM AST as usual.  This lack of a model run might well be due to the effect of budget cuts noted on the link above…  Dammitall, budget cuts!

Anyway, check the times of the run to make sure you’re not looking at something old today.  I used it anyway because things have been well predicted in our incoming trough and jet stream for a couple of days now.

You can also see that with the jet over and just to the south of us later today through tomorrow morning, that most of the precip is to the north of the jet, as per usual in the interior of the SW1.

Here’s what the jet stream does in the next 18 h, starting with the forecast for 5 AM AST (these are from the latest data!).  The winds are very strong over us now, but a core of strong winds is dropping down from California toward southern Arizona and reaches us later today and then drifts south.  As that happens, the air aloft is really cooling off.  The temperature at 18,000 feet (500 millibars) drops almost 10 F between 5 AM this morning and 11 AM!

As that happens, the air will be getting more moist from the bottom up.  Small Cumulus will form later this morning and their tops will be rising and getting colder by the minute, eventually reaching our ice-forming temperatures around here of -10 C to -12 C, but tops to -20 C are likely late in the day.   When ice begins form in these “supercooled” clouds, those crystals grow at the expense of the liquid droplets and fall out.

This cloud drama, if you will, is what makes today extremely exciting; watching for that first ice to form in these lower, relatively shallow clouds.  They can be shallow and still develop ice because its so damn cold aloft later this morning and this afternoon.

So, a pretty if cold day, with lots of virga around later this afternoon, and with that, a great sunset is likely, too.







Yesterday’s clouds

10:03 AM. Your really cold Altocumulus clouds; transitioned to ice shortly after this. Probably colder than -25 C.
1:32 PM. CIrrus spissatus, maybe it could be called an Altostratus translucidus since its quite thick toward the horizon.
1:32 PM. You got yer Altocumulus lenticulars (brighter white clouds below patchy Cirrus.
5:25 PM. The major band of high and some middle clouds exits south of Catalina.
No high clouds today, only low and maybe some Altocumulus/Stratocumulus formed by the spreading out of Cumulus tops.

More rain in our future after later today and tonight?

Oh, yeah, baby.  Its in the bag. A “virtual” certainty (which actually means its not absolutely certain at all, but that’s because its a weather forecast based on imperfect models and humans.  Remember that great metaphor about why there’s an eraser at the top of a pencil?  Profound.)

OK, when?  Let’s start by looking to Canada for rain in Arizona:  Here’s a panel from Enviro Can for Monday morning, December 31st at 5 AM AST.  Its got rain in it, rain that would have fallen in the prior 12 h leading up to 5 AM.  I like this model because it has more SE AZ rain in it than the US ones….

See lower right hand panel for rain in Arizona by the morning of the 31st.

There is a fly in the oinkment.  The US models have a big “anti-cyclonic” bulge in the flow over us as our the trough/low approaches us on the 30-31st.  These bulges in flow, toward the inner portion of the jet stream, can have devastating effects on clouds and precip formation.  A bulge like that is like a high in a low; the effect is to weaken all the upward motions in the atmosphere.  The Canadian model has less of that; hence, shows more rain here.


1Of course, if you have a copy Willis and Rangno (1971),  “Final Report to the Bureau of Reclamation, 1970-71 Season, Colorado River Basin Pilot Project, Season 1”) you already know that.