Measurable rain to fall in Catalina at last!

The chance of measurable between this afternoon and Saturday morning isn’t 10%, or even 50%, but 100% as seen from THIS keyboard.  If you want a better forecast than that, go to the NWS, Tucson, or see Bob’s expert take.  I like Bob.   But we go for it here; don’t mess around.  The last measurable rain, only 0.10 inches,  fell WAY back on November 27th.  I wonder if you remember it?  Probably not.  I don’t.

Rain hits after midnight tonight after a day of heavy middle cloud overcast (that would mostly be Altostratus opacus, probably with virga later, maybe some Altocumulus embedded or at the bottom or below that As layer, with mild temperatures.  Don’t expect sunset color tonight since the cloud deck is likely to extend too far to the west for that.

These middle and upper clouds, whose bases will lower as the day goes on and during the night when the rain hits, are associated with a surge of moist air from the tropical Pacific.  This chapter of rain will followed by an extremely sharp cold front passage (“FROPA” in weather speak) late Friday night or early Saturday morning with another round of light rain.

AZCat model output foresees quarter to half inch here in Catalina.

Wind will be a problem.

This situation will be accompanied by a really intense low developing to the north of us and the wind by tomorrow mid-day through early Saturday morning will be ferocious.  We’re likely to see gusts here in Sutherland Heights to well over 50 mph from the SW as the wind bunches up against the Catalina Mountains, helping to increase speeds here in The Heights.  CMP suggests putting loose stuff in the yard away somewhere.  He might even do that himself instead of trying to find where all his baseball caps, left outside on the front porch,  went the next day.  Some composition shingles likely to come off, too.

The Weather WAY ahead

The good news here, if you like below normal temperatures and chances of rain or snow, is that the forecast models are indicating we’re in the mean trough position.  Periodic fronts and troughs will affect the SW over the next two weeks, bringing with them at least a CHANCE of rain, while cooler than normal spells are virtually certain.

Check out the spaghetti:

From last night's computers, valid two weeks from now, Dec 30th at 5 PM AST.
From last night’s computers, valid two weeks from now, Dec 29th at 5 PM AST.

Notice the broad dip to the south in those red lines beginning in the eastern Pacific Ocean then down to Baja and then back toward the east-northeast into the southern Plains States.  The 5700 meter height contour (red lines) is pretty much on the edge of the jet stream, and a dip like this would be considered a weakness, a vulnerability for troughs in these plots.  Remember these are initial starting conditions in last evening’s global data with DELIBERATE slight errors put into those data to see how slight errors affect the outcomes.   There are almost no differences at the beginning, the errors are so slight.

And there are ALWAYS slight errors in our measurements, of course, so spaghetti helps pin down what those errors might do to embarrass us forecasters.   When those red (or blue lines) cluster somewhere into a band, it means that the errors introduced have little effect on the position of the troughs at this level (500 mb).

For example, over there off SE Asia, the cold continent and the warm ocean constrain the jet stream into a tight bundle of contours.  Errors don’t have too much influence on where that jet stream will be off Asia;  its pretty much locked into place.

Down stream, things are more of a mess, but you can still see, in this case, where there is kind of a bunching to the south in the red lines in our SW domain.  So, there would be, oh, moderate confidence of passing upper level troughs during these latter days of December. At least that’s what I see from this from here.

Worried about wildflowers now, due to the paucity of fall rains….so really hope December can come through with some major rains!

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.