Upper low passing to south; clouds to wrap around overhead from the southeast and east this evening

Backdoor rain?   Looks like any chance of rain will happen later this afternoon through overnight as mainly mid-level clouds twist around our low from the east.  That low is now over Yuma, AZ, and the center will pass to the south of us tonight.  We don’t see that happen too often.  Here’s a nice loop of the circulation around it, also showing the radar echoes–very handy.  (Some cloud shots at the bottom, way down there.)

Right now, our low is looking pretty dry, not much going on in it, or around it right now, and so any rain falling from mid-level cloud bands, like Altocumulus (with virga) and a likely deep band of Altostratus (also with virga),  will be pretty light; sprinkles to maybe a hundredth or two.  The better part of this is that with mid-level clouds coming from the east, they won’t be much dissipated by the air going downhill from the Catalina Mountains as you would expect with low clouds.  However, in Mexico, since it is so cold in the center of this low, there will likely be reports of snow in unusual places, as in the last storm.  The U of AZ Beowulf Cluster model, the best around for us, sees only a brief, fairly close call for rain tonight.

Still, while this low passage will be a little disappointing as far as rain goes, the skies will be great today with scattered Altocumulus, likely with a little virga, and scattered Cirrus, with a great sunset.  Both of these cloud types can be very “photogenic” on days like this.  Likely those mid-level clouds will clear out tomorrow morning, so get’em while you can.  If you don’t, of course,  this compulsive cloud photographer will.

Full cold ahead

Get ready for some terribly cold days just ahead, likely some snow in Catalina still, though the more moist Canadian model prediction for this storm has dried out overnight–tending to be more in line the with US models which have always had this cold wave as a light precip event.  Precip is pretty much guaranteed here, and some snow probable in Catalina, but max and least precip totals from this storm have to be revised downward in view of the latest Canadian results.    Minimum  amount, 0.10 inches, max, 0.50 inches (was an inch due to how much offshore flow the prior Canadian models had the night before last).  So, most likely amount is between those two extremes, or about 0.30 inches with precip beginning during the day on the 11th.

A further disappointment is that the mods now see this storm as a quickly moving event, and the precip is over by evening on the 12th, so it ends up as just a 24 h period of rain and snow chances, most coming, of course, in the first segment, a line of rain changing to snow with the frontal cloud band and wind shift line on Friday, the 11th.  Dang.

What about the second cold blast on the 15th-16th?

Still coming, but this wiggle in the jet stream shooting down at us from the northwest, has a trajectory toward us that is farther east than it was shown in the models earlier, and the farther east and the further away the trajectory is from the coast,  the drier these cold pushes will be.  So, that second blast of cold air, while still looking very cold, is also looking pretty dry right now; may only get a passing snow flurry, or we’ll just see scattered small Cumulus with some virga.

In these latest model runs the jet stream pattern that has led to our “trough bowl”, the favored location where storms have been collecting in our region for the past month, begins shifting to the east at mid-month, and what’s more, the amplitude of the north-south oscillations in the jet stream fade to a more west to east flow.

This very different than what was depicted just the night before last.  Here’s what I mean.  Shown below is the first forecast panel, high “amplitude” pattern in the jet stream–always associated with temperature extremes, cold where the jet dips down, like HERE, and warmer than usual where it shoots up from the southwest,for example,  there in Alaska.

Valid for the evening of January 15th.  An example of "high amplitude jet stream configuration associated with temperature extremes.
Valid for the evening of January 15th. An example of “high amplitude jet stream configuration associated with temperature extremes.

This is a very common pattern.  You probably remember how warm it was in Alaska during the 1962-63, and the 1976-1977 winters, but how friggin’ cold it was back East when this kind of high amplitude pattern was pretty extreme and persisted for weeks: the jet racing into Alaska from the mid-Pacific, and then shooting south into the US.   Really horrible times for the East in those winters.

But look at what the model sees for the end of the 15-day forecast period, shown below!  The jet hardly has any amplitude, just shoots in from the Pacific in a west to east flow.  That means no temperature anomalies to speak of, and a moist West Coast regime, sometimes with precip getting this far south, too.

Valid the evening of January 22nd, a Tuesday.
Valid the evening of January 22nd, a Tuesday.

What does spaghetti say about all of these changes?

Its pretty clueless, that is, slight changes in the observations make a big difference in what happens, and that’s why its so wild looking in the Pacific and the US (shown below).  This means you probably can’t count on the above pattern a lot, except that the amplitudes have gone down, that seems to be a pretty solid expectation.   That jet surging into AZ could just as well be intruding into Washington State in a west to east pattern.  That would mean that our 30-days of below normal temperatures here in AZ, beginning in mid-December (shown here by the NWS, lower right panel),  are about to end after about a week to ten days, and with that, a long dry spell likely to set in.

valid 21 January 2013 spag_f360_nhbg-1
























Yesterday’s clouds

Mostly Altostratus, thinning at times to Cirrus, and late, a few Altocumulus/Cirrocumulus patches.  Here are a couple of shots.

7:22 AM.  Under lit Altostratus at sunrise.  Those little pouches are regions of light snowfall (virga).
7:22 AM. Under lit Altostratus at sunrise. Those little pouches are regions of light snowfall (virga).
1:49 PM. Boring, generic Altostratus translucidus, an ice cloud. Sometimes droplet clouds like Altocumulus are embedded in them, but none can be seen here (they would be dark, sharp-edged flecks). The massive clearing that occurred in the late afternoon is visible on the horizon.  If you saw that clearing, it would have been a great time to tell your friends that, “Oh, I think it will be sunny in 3 hours.”  It would have been quite a magical moment for you.
5:36 PM.  This beauty of a patch of Cirrocumulus (tiny granulation) and a lump of Altocumulus.
5:36 PM. This beauty of a patch of Cirrocumulus undulatus (tiny granulations in waves) and a lump of Altocumulus (lower left). TUcson sounding indicates they were about 15,000 feet above the ground at -20 C (-4 F).  No ice apparent.  It happens.