Roar of the jet

But where will it be?  While our little baby low center spins and almost dies before it gets here on Sunday-Monday with only the possibility of a few brief light showers now, the bigger question is where exactly will the jet stream break through and slam the Pac coast in about a week?   The model runs have been jerking that West Coast location around, but HAVE  been shifting it southward in the past 36 h of runs (hmmm, doesn’t sound good)  to the point where, in some results, Pac storms riding this roaring Pacific jet stream are far enough south to impact Arizona in a major way.  Take a look at these various predictions of where the “breakthrough” jet strikes the Pac coast, the first one from yesterday morning’s model run, and which I was just beside myself at how far south it was impacting the West Coast. In a day or so, major precip moves into Arizona, even our area (second panel). This was not the case in the model run just 6 h before this one.  Finally, the next model run just 6 h later, based on data taken at 11 AM LST, even MORE rain was predicted in AZ, and a substantial rain here.  Look at the 3rd panel and that darker green region over us! I was so energized!

I think you can tell, given the background of model eractic-ness, where this is going.  Its now like a Hallmark movie where everything seems to be just fine in the beginning, but then there’s the problem phase in the middle that must be resolved, followed by the satisfying ending where everything is fine.

Well, after these model runs below, we have entered the problem phase of our “movie” since the very latest model run, from last night at 11 PM LST, had no precip whatsoever in Arizona and the Pac jet slamming the coast much farther north again when it breaks through in a week or so.   If you read this blog, you will know that to get winter rain in AZ, you must have the jet stream wind maximum over or to the south of your location.  This is also true of most of the interior of the Southwest in winter.

I say to HELL with last night’s model run!  Not even going to show it.  It is true that the La Nina Southwest Rain Repeller, will try to do exactly that, push jet streams coming into the West Coast farther north.   The Climate Prediction Center  (“expert assessments”) further alleges, based on many different models, that the La Nina will even strengthen in January, and will likely persist all spring.  So, the tendency to push our incoming Pac jet streams north will always be there from here on out, dammitall.  So, you have to give, in spite of my cussing, the model output that showed no rain from last night credibility; that we will only be dry, and maybe a bit windy at times as storms skirt Arizona in the 1-2 week period from now.

But does an La Nina always “win” in pushing storms to the north in the late winter and spring?  Not only no, but HELL no, to cuss that bit more.  The presence of a La Nina, while tending to produce dry conditions, is not all powerful and wet conditions can occur in some La Nina years.

Idle speculation:   One wild card out there is that in the very La Nina-El Nino region where the water is cooler today than normal (about 1 degree C) there has been a persistent region of clouds and convection, quite contrary to expectations.  Its been feeding heat and moisture into the mid-latitudes since November and remains in that region, shifting very slowly eastward.  The CPC did not mention this feature in their global ocean expert assessments for some reason, though a giant blob of colder than normal cloud top temperatures is quite evident from this feature in their plots.   But what does it mean?   I don’t know.  But, since this anomaly (SE of the Hawaiian Islands near the Equator) is acting, in fact, like a little El Nino was present, perhaps it will lead to a less dominant La Nina drought tendency here.

You can see this anomaly here, in the University of Washington’s 24 h satellite loop for the western northern hemisphere.  (Hit the higher loop speed button for a nicer presentation, on the left side.)  The actual cloud producing region is about 1000 miles SE of HI now.  Quite nice.  You can see that it is also feeding clouds into our little baby low off southern California.

OK, REALLY idle speculation above.   But, based on only a gut feeling, I think the Pac jet slam will shift farther S again in model runs, and that AZ will benefit from the incoming storms and not be completely dry as the last model run showed from last night.  Recall, too, however, that I am biased toward precip, and feeling a little chastened by the disappointing run.

OK, enough!


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.