Author: Art Rangno

Today, as everyone knows, will be the last pleasant day for quite awhile, so we’d better get out and enjoy it if you can, maybe call in sick.  Likely to be a couple of AZ low temperature records set over the next week.

The skies will be great today, as they always are with some clouds present, and for a few days afterwards in the cold air with those deep blue skies along with passing Cumulus clouds, that at times and even though they are shallow, will send some virga down as the colder parts of the troughs go by.   Should provide for some nice late afternoon and evening photo ops in the days ahead.

Today the satelllite imagery plus looking out the window, shows lots of Cirrus clouds today, probably devolving into dense, shady Altostratus at times.  And a scenic,  Altocumulus lenticularis cloud¹ downstream from the Cat Mountains is pretty much a lock.

Also, in the progression of clouds today, we will probably see that clear slot that so often separates the middle and high clouds from the low, frontal clouds go by.  If the timing of that clear slot is right, could be an extra special sunset.

Some Cirrus from yesterday, another one of those rarer days with virtually no contrails in the Cirrus, followed by a nice Catalina sunset:

Rain, from the U of AZ mod run at 11 PM AST,  has the rain beginning tonight after 10 PM AST and lasting but a couple of hours.   Amounts here in Catalina, between 0.10 and about 0.40 inches, average of 0.25 inches, virtually no change from what was predicted 24 h ago.

Just about everything mentioned yesterday is the same today in the model, marginal cloud top temperatures for precip at the TUS site for most of the time the front goes by (in the model), but  cloud tops will be colder over us and more likely to precip.

Seems temperatures will  be marginal for ice-in-rain drops at the ground here since the much colder air will not arrive with the front’s very narrow rain band but encroach as it departs.

Also of some interest, the jet core at 500 mb is shown to become bifurcated with one branch overhead S as the rain moves in (another branch over NW AZ).  This would be compatible with a rule of thumb about the rain and the jet at 500 mb.  Rain, with extremely rare exceptions (<5% of the time), does not fall here on the southeast side of a jet stream racing to towards the NE, as we will have over us today.  Will be curious to see if this “rule” holds up this time.

Tomorrow will be one of those cold days with spectacular small Cumulus clouds contrasted against the deep, dark blue of the winter sky.  Should be some great scenes of light and shadows on the Catalina Mountains.

Snow ahead?

Snow falls here later in this cold, almost week-long episode, as a series of troughs plunge southward along the West Coast to AZ.  Most likely day for some snowfall in Catalina, is now on the 14th².  Here’s the map for that, valid for 11 AM AST, Monday, from IPS MeteoStar:

The weather way ahead–Dr. Jeckyl or Mr. Hyde?

Check out these bowl of rubber bands from “flapping butterfly wings” (slight perturbations put into the initial data ingested into  the WRF-GFS model after which its re-run a number of times to see what slight differences do). The first one below is for the evening of January 20th, a real laugher considering its only 10-11 days away now and the mods are still clueless.

Note the yellow lines, a “control” run from last night’s 5 PM global data, and see how they BULGE toward the north over the western half of the US.  Then look at the hard-to-see gray lines representing a “control” run from just 12 h earlier, that from yesterday’s 5 AM AST global data.  They BULGE southward, the opposite way the yellow lines do.

The yellow lines indicate a huge ridge over the West, with little precip and seasonal temperatures in the SW US.  On the other hand, the 5 AM control run shows a continuation of our present storm pattern and continued injections of cold air down the West Coast.  Check out that gray line over northern Cal, for example.

So, from one model run to the next lately, our weather toward the end of the month in Catalina has gone from “yawn” to “yikes” (the latter  blurted out yesterday when I saw that 5 AM output and all the storms it had).  But blurted out a “yawn” when viewing the output from last evening.  No precip after the cold week.

However, in deciding which of those two outcomes is most likely you have to dwell on the predominance of those blue lines that also bulge northward and are mainly located in southern Canada.  Those strongly indicate that the “highs” , the  bulges to the north over the western US, will prevail on the 20th, not the storms and cold weather with them.

But how about after the 20th, at the very end of last night’s model run 15 days out?  Now you see BOTH the yellow and the gray lines are bulging to the south (creating a trough bowl), AND, more importantly, those blue lines are not constrained to southern Canada, but are all over the West as well.  Remembering that the atmosphere remembers suggests that the models are remembering, too, trying to regenerate the kind of flow pattern we’ve already been experiencing after a significant, quite pleasant, really, break from the cold week.

While this last panel  is also a real laugher in many ways, due to all that uncertainty that’s indicated, the wildness in all those lines, you might want to hop off the fence in a longer term forecast toward the end of the month by thinking that what we’ve been having will return, a sort of “Back to the Future.”

Re-inforcing this view is how the red lines (570 dm height contours), usually on the periphery of the jet stream, become more compacted in the LATER, second plot compared with the first!  This is a little remarkable since that would suggest the models have a better handle on the circulation pattern at 15 days over 11 days.  Odd.  Note, too, that those lines at 15 days are FAR to the south of AZ, supporting the idea of a cold trough in the SW.

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¹These are the almond shaped clouds composed of droplets with smooth, sharp edges that hover over the same spot, expanding and shrinking, sometimes for hours at a time, as the grade of moisture changes in the air being lifted behind the mountains.

²Personal aside:  Have friend, former WA State Climatologist who also worked at the U of WA as I did, arriving here with his wife on the 12th for a vacation from the dark days of wintertime Seattle.  They will be here for the whole cold week and possible snow as it turns out, and then go back.  Having looked at the progs,  he now wants to take a “vacation from his vacation”, maybe head off to Costa Rica after arriving in AZ!

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.

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.

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.

Yesterday’s clouds

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