Category: Altocumulus clouds

Summary statement:  Begins in 5-6 days in the northern US, then expands southward; goes on and on, like the discussion below,  after that. Cloud pics WAY below the “novella” on spag plots.

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Our docile weather in the West for the past few months is about to end, as well as for those in the Rockies and Plains States.  Wasn’t gonna blog on TG day, but looking at mods, and realized that I am the SAME person that I was as a 6-year old in Reseda, California, on January 10, 1949,  that ran up and down Nestle Avenue knocking on doors to tell people it was starting to snow that afternoon (!), I realized that same “gotta tell ya” impulse lives on.

The trigger for THIS “gotta tell ya” is how bad the cold, snow, rain, and wind look for the western half of the US starting in about 5-6 days from now as cold air and storminess works its way south from the Pacific Northwest and Rockies at that time.  I am sure you have heard something about this developing pattern already from your favorite media weathercaster, but I’ll try to take it a bit farther out in time, and tell you why I think you can do that in this case.

I haven’t looked at the models per se with the exception of the Enviro Can one, one in which the lasted posted output is at the start of this episode, but rather the excitement for Mr. cloud maven person was triggered by those chaotic looking, “errorful” plots we call spaghetti plots, “Lorenz plots”, if you will, posted by NOAA that tell us how sensitive a pattern is to small errors.

It seemed, too,  like there was something to be learned from them, as well demonstrating a high confidence pattern of a severe weather pattern more than a week away.  Many forecaster, maybe most, shy away from forecasts beyond a week because we know how often they are faulty.   But there are exceptions and this is one coming up.

OK, here we go.

Above I have added boxes in this plot to show you where the forecast is highly reliable and in another one, where its not.   This is indicated by the bunching of those lines, height contours, the same ones, from many model runs starting with the introduction of slight errors.  At first in these plots, with errors being tiny, there is no difference in them in the first day or two.  But, as time goes on, the errors have greater impact.  A metaphor:  when you hit a ball off the tee, the error in the first inch of travel is nil in magnitude.  But 5 seconds later? Oh my.

Here, the bunching of lines in most of the US is what got me going.  Continuing the metaphor above, after 5 seconds and 300 yards of travel in this case, its analogous to 2 yards from the hole!  In other words, the were essentially no effect of errors in the model runs; you slugged that golf ball perfectly.

But what does it mean, in terms of weather?  That trough (the curved area where the “high confidence zone” is located, means a tremendous plunge of cold air into the West and Plains States.  Don’t need to look at future maps to know this.  You all know that a trough is a tongue, a wedge, of DEEP cold air that drags cold air at the surface southward on the west half of it, and drags warm air northward on the east side (in this case, toward the eastern US.  The size of this wedge indicates a gigantic area of high pressure from the Arctic will be pushing DEEP into the West and Plains States as this pattern develops in the few days before December 5th.

Once established this pattern lasts for several days, a huge, deep and cold trough dominating weather throughout the US.  And where the air masses clash at the ground presents ripe conditions for low centers to spin up, given a trigger aloft, like a traveling, much smaller wave in the jet stream where the lines are bunched.

Below, farther along in the sequence, these plots each one day later than the one above that illustrate how a confident pattern begins to erode.  In this case, “uncertainty” in the central and eastern Pacific begins to spread eastward into our confident pattern; the blue lines start to go goofy (highlighted by boxes):

Last, here is the plot for 15 days (360 h) out in which those little errors have had their biggest effect, really done a number (haha) on the forecast confidence game, everything’s pretty unreliable except maybe in eastern Asia and the extreme western Pacific, and along the East Coast.

But, even with all of this chaos below, we can see that the model still thinks a trough (a bend in the contours to the south) will still be present in the mid and western sections of the US.  Since we know that weather, once changing into a new pattern likes to stay in that pattern for weeks at a time (with brief interruptions),  a reasonable forecast for December would be colder than normal in the Southwest and West overall, and in the central US, while its warmer than normal in the East, particularly the southeast US.

Precip?  Always more dicey than temperatures, but CM is going with above normal in the interior of the West and in the Southwest, near normal to above normal here in SE AZ.  Remember while reading this, Mr. Cloud Maven person is NOT an expert in long range forecasting, like for a month, and, he likes to see precipitation in the desert, and those wildflowers that follow.  (“Truth-in-packaging” clause.)

In a couple of days, the Big Boys at the CPC, that is, the NOAA Climate Prediction Center, will be issuing their temperature and precip forecasts for December.  It will be interesting to see what they make of these patterns, combined with other factors like sea surface temperature anomalies and northern hemisphere snow cover.

BTW, with a pattern like the one coming up, snow that falls during the storms is going to remain on the ground for long periods due to the lower than normal temperatures, those that snow cover helps to maintain (strong feedback loop, as we would say).

Your clouds of yesterday

If anyone is still with me, you had your Altostratus, your Altocumulus, and some Cirrus.  Here they are: