Early December storms on the horizon

Been away from you for a couple of days, wanting to see how you do on your own, perhaps see you grow in your cloud watching obsession, namely, that you want to name everything you see, as though you were Luke Howard himself.  Hope you logged all cloud genera, varieties and species, 27 all told,  over the past 48 h.  You can bring your photo log books to the next meeting and CM will go over them with you.

Measurable rain chance still pretty reasonable for the window of Nov. 20-21st, too, as we have purported for some time here, but it will be pretty minimal.

The weather way ahead; a promise of substantial rains

However, as often happens, on the horizon is a substantial storm for Arizona, ones that have a habit of disappearing it seems as the foretold event gets closer.  Here it is depicted below in plots from our cherished NOAA spaghetti factory:

Valid at 5 pm AST, December 2nd.  Cool, eh?
Valid at 5 pm AST, December 2nd. Pretty cool, huh?
Annotated same spaghetti plot.
Same spaghetti plot, annotated. Recall that these plots are ones where the model input has been deliberately errorized to see how big little errors make in the outcome of the model. Why do that? Because we know at the outset that our measurements are not perfect, and have all kinds of actual little errors in them. These plots are a way of seeing how robust a predicted pattern is, and those areas where the forecast is pretty reliable, is indicated by bunched lines (key contours of the airflow at 500 millibars, or around 18,000 feet above sea level in the mid-latitudes, lower near the poles, higher near the equator.  Why do we use a pressure level instead of the pressure at a height?  Go here.  CM would like to see as well, in the 21st century,  maps of constant altitude (3 km, 5 km, etc.) with the high and low pressures on them as we have on our sea level maps!  Is anybody listening? (A google search just now could not locate, constant altitude pressure maps….)

While the above forecast of contours is two weeks away, and numerical models are often  unreliable at those long horizons, we see that the red lines (not to be confused with political markers) have dipped down in great bunches over the extreme eastern Pac 12 Ocean, and continue all bunched up across Baja Cal and  thence into Texas.  In the plot above, the red lines represent a 500 mb height of 5760 meters, one that’s on the southern periphery of the jet stream.  So, when its well south of us, our chances of rain are engorged.  Recall, too, that the 5640 meter contour, just that bit lower in height, is associated with leading edge of rains in the central and southern California area–remember Brier and Panofsky, Some Applications of Statistics to Meteorology?  Well, in that book you will learn that contour  is what the LA weather service used to use as crib for when rain would occur in southern California.

I am sure you remember two things, maybe more than two.  We have an El Nino in progress, not a great one, but an OK one, AND that El Niños strengthen the southern jet stream in the eastern Pac and across the southern latitudes to the east.  So, we expect to see this pattern, one of a stronger jet stream in the sub-tropics carrying stronger disturbances as a result,  evolving as the winter develops, that is, more disturbances in the lower latitude band of the jet stream (sub-tropical part).  The plot above is a classic one for predicting that kind of regime, maybe with a bit of a split in the J-stream with northern and southern branches being pretty vigorous at this map time, and before, for that matter.

Valid at 5 AM AST, Monday, December 1st. Colored regions are those in which the model has calculated that precipitation has fallen in the prior 12 h.

The plot above, it has laid a foundation of credibility for what is show below, from IPS Meteostar’s rendering of our WRF-GFS forecast model output based on the 5 PM AST global data.  It shows 24 h of substantial Arizona rains, including here in Catalinaland, at the beginning of December.  You’d be pretty cool to inform your friends about this, ones who might be heading back somewhere after a TG visit to sunland1.    (Besides, they won’t remember what you said anyway by the time December gets here.)

Valid at 5 PM AST, Monday, December 1st. Colored regions are those in which the model has calculated that precipitation has fallen in the prior 12 h.

 Today’s clouds

Lots of pretty Cirrus.

The End


1By the way, the Great Lakes are freezing over already, a month earlier than normal due to some astounding cold back there, so TG visitors from back East, upper Midwest,  may look for reasons to stay longer.  Well, at least until the rain and cold hit here.