Intriguing jet stream pattern arises in computer models

First, your sunset for last evening:

5:38 PM. Altostratus with nearly straight lines of lower hanging virga in the distance. One cannot help but wonder if the passage of an aircraft enhanced the ice falling out and that’s why they are so linear.  There were quite a few aircraft-induced lines of ice yesterday, but all developed in cold Altocumulus clouds.  There’s one over there to the north….
5:33 PM. Lppking over there to the N and you can see a rivulet of ice falling below that patch of Altocumulus. one of many yesterday, though this is the prettiest one.  You can also see that there is something “wrong” here.  The trails below appearing to be slanting toward the west and the flow was from the east.  This indicates that the flow was stronger at lower elevations, an oddity.

Brain’s been pretty empty lately, not much to say except “same old same old”, as here.

But then some wild computer forecasts came out last evening that were worth mentioning in light of the fires that have plagued southern California.


Because they suggest that a belt of tropical flow will break through under the gigantic ridge that has blocked storms from the entire West Coast over the past weeks.

Sometimes, as most weathermen know, such persistent ridges get too big for their britches, that is, over-extend to the north, and then fracture, letting through moist tropical air from  lower latitudes of the Pacific ram into the West Coast.  Often the very greatest rains in California are associated with such patterns, as you could guess since there is so much water with those lower latitude-originated jet streams that strike the coastal mountains head on.  Here’s  the concern, this output valid for Christmas Day:

This from last night’s 5 PM GFS global model run, valid for 5 PM AST Christmas Day. What is shown is that the tropical input is about to combine with a trough over the Bay Area. When this happens both are energized.

Rain is forecast to have fallen in southern Arizona before this point, however;   our measly 0.01 inches so far here in Sutherland Heights so far  in December will likely be added to along with a switch to uncomfortably cold weather.

Of course, we look for support in these longer range forecasts by having some spaghetti–that is, take a look at what we call “spaghetti”, those crazy plots in which the model starting conditions are tweaked that bit to see how the model runs change.  Here are some plots, also from last night’s model.

As you can see in this first plot for the evening of December 22nd AST,  the clustering of blue and red lines off the West Coast, that our blocking ridge (composed of a deep mass of warmer air)  is extruding all the way almost to where Santa Claus lives, yes, that far to the north.  In fact,  so far that it will be unsustainable  over that distance fromt the deep tropics to almost the North Pole.  At this point,  cold air is pouring  down the east side of that ridge and into the Pac NW and eventually down toward us.  Does that extremely cold air make it here?  Not sure yet, but its something to watch out for before that ridge fractures and allows warmer, moist Pacific air to reach us.  So much uncertainty, so much fun!  I am really pumped up about all this uncertainty!

Valid at 5 PM December 22nd. This is really strong support for a major trough in the SW, and likely rain in southern AZ, Sutherland Heights around this time.

Below is the spaghetti dish for the “breakthrough” flow situation shown in the prog output valid for Christmas Day with some annotation on it.

Looking at the above, I think we can count on a breakthrough flow pattern from the Pac.  Where it barges into the West Coast will be subject to question over the next week or so.  That really can’t be determined right now.

I am sure those in southern Cal  hope, if there is a breakthrough from the Pac,  it comes in farther north than shown in the model run here today!  Patterns like the one shown in that  run  can routinely produce 10 inches in a day once they get going  should something burst through at low latitudes.

The brighter side would be that the chances of a significant rain here droughty southern AZ would at last  increase.

The End

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.


  1. Okay, Art. I’m ready for another change- whatever it is. The stretch of “dry” weather here was nice (I put quotations around the word because the fog didn’t make it feel dry) but we need to get back to real winter weather. I’m seeing shrubs and other plants starting to bud, as if spring is coming.

    1. I hear you, Roland, from my decades in Seattle, the blocked rain systems leading to colder weather with dense fog! Those were tough days, all right. Was up looking for Geminid meteors at 2 AM AST, so starting late today!


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