Good timing

Those disappointingly dry days of the weekend and yesterday with only isolated Cumulonimbus clouds (Cbs) are gone.  Today, a wall of clouds will develop to the west and southwest of us and, with luck, we’ll get at least a third of an inch to, if we are REALLY lucky (dump spot of Cb passes overhead), an inch of rain here in Catalina.  If one had to estimate the top and bottom amounts of rain today, those limits would be quite wide, say 10% chance of less than 0.20 inches, and a 10% chance of more than 0.90 inches.  In many cases such limits would be far narrower.  Also, as we found during a pilot project in the 1970s, averaging these two values was often close to the amount that occurred.  In this case, the average of the top and bottom amounts that the storm COULD bring would be 0.55 inches.   Well, let’s see what happens!  This is fun now.

Why is so much rain possible today?

Because the upper air pattern organizing this wall of cloud will be passing over us in the later afternoon and evening so those clouds will have a chance to be humped up into larger Cbs than they might otherwise be due to our afternoon warmth.  If this same pattern passed over, at say, 8 AM tomorrow, we’d probably be lucky to get a quarter of an inch here in Catalina.  Also, I am guessing we will see a nice arcus cloud as this hits this afternoon, a shelf cloud in a long arc ahead of the rainband and windshift line expected today as well.  Should be a dramatic site.  Get cameras ready!

Example of nice skies with building Cumulus over the Catalina Mountains and a Cb dump spot (densest portion of rainshaft) from yesterday afternoon.  Note how small the “dump spot” is compared to the whole cloud in the second photo at right, but a thin strand many times smaller than the highest turret.   This structure is not well understood, since its not possible for measurements to be made even in such narrow chutes within a cloud, perhaps only 10s of yards wide.  However, it is thought that a few “lucky” first ice crystals forming in the upper portions of the cloud fall must fall through equally narrow zones where there is an extra amount of condensed water and therefore they grow to bigger hail/graupel particles, melting into the largest rain drops falling through the updraft of the cloud.  Need a drawing here.   Will look for one… More later when I find the diagram I am looking for…