Return of the cut offs followed by a Pac blast

While waiting for the to the 10-15 day forecast to resolve itself, most reliably experienced by having those days that used to be two weeks away get here,  we can get a more reliable fix on the next few days to a week.

Take a look at these WRF-GFS model forecasts based on last evening’s global measurements of 500 mb flow rendered by IPS Meteostar:

In looking at those, its pretty remarkable that the “weather” has remembered to place some cut off lows down in the SW as it was doing off and on throughout November and December, ones that helped our rain and snow totals zoom above average!  Like a sad love affair you can’t get out of your mind, one that keeps coming back to remind you how bad it was, the weather, too, in something we weatherfolk call “persistence,” tends to keep coming back to the same pattern, even after long breaks like our recent three week sunny malaise, in which I thought the cut off pattern had been forgotten about.  In the first panel for tomorrow, an elongated bend in the winds is passing over Arizona.  Then part of that “trough” shears off into a circular whirl over eastern New Mexico and west Texas.  Cut off magic!

This is followed by another dollop of the jet stream that breaks away from over the Pacific NW (that bend in the winds in the second panel up there), shears off from the main current, and ends up as a whirl over southern Arizona by next Friday. the 13th.   As you know, ones in that position have a lot of promise for providing a decent rain, such as the 0.25 to half inch we’ve gotten out of several of the prior cut offs in that position.    The first trough is too dry and goes by too fast (tomorrow the 8th) to produce rain except well east of us.  We’ll only see the temperature plummet.

So,  here we are again in the cut off regime, at least in the week ahead.

Pac blast

Beyond our little cut off low spell, the models are starting to rev up the central Pacific flow that breaks into northern and central California, flow that would be associated with tremendous rains (ones you will read about it in the newspapers) if it happens like this.   And in this latest model run, some of that rain dribbles into Arizona (on Friday, the 19th) with SEVERAL more chances for AZ rain in quick succession after that during this Pacific blast that rips into the West Coast in the days ahead.  The change in circulation is stunning!  Below is an example of what is predicted to happen by Sunday, the 21st of January,  of one of my favorite patterns whilst growing up in southern California.  Overall, the rains begin to move into northern California on the 15th.

However, from the prior harangues you also know that these model forecasts have had an unusual degree of “eracticity”;  can’t be relied on too much, especially with regard to the placement of that incoming jet into the West Coast.  It will be interesting to see how this turns out (as always!)

However, it was a nice model gesture to show rain in AZ from this Pac blast this time around.

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.