Just yesterday I pooh-poohed the chance of rain toward the end of the month based on spaghetti, the NOAA spaghetti. In fact, I was afraid to be too happy yesterday when I saw that rain was foretold here after the long dry spell we are now in. So I looked around for some reasons to mash down my happiness, happiness that might well be mashed anyway by reality when the end of the month got here. Maybe, in the event of getting too happy about foretold rain that didn’t materialize again, and then being let down one more time, I wouldn’t feel like blogging anymore, and both of you might disappointed in that turn of events.
But this morning I looked again at the rain prediction map for the end of the month, based on our best model, the US WRF-GFS. As if to personally humiliate me, there was even MORE rain for AZ, and the very epicenter of that rain was forecast to fall on my house on the 28th of September! That model KNEW I had said rain here was unlikely (though not impossible).
Take a look at the small purple blob in the map below from the WRF-GFS model indicating 1.50 to 2 inches on top of Catalina that day! Imagine! What a wonderful rain that would be! The whole wonderful sequence is here. And below the rain map is the map for the winds and contours the day before the storm hits. This is the BEST weather map I have ever seen for AZ! Look at that tropical storm center over Hermosillo. Its incredible, still amost a hurricane at the 500 mb level (around 18,000 feet). Never seen one like this before, and it couldn’t be a better map if I drew it myself!
OK, so we got us a possible great storm in the works.
But, if you’re like me, you should keep your rainy hopes in control. Its still an unlikely event, even with the global data taken last evening at 5 PM AST, crunched into millions/billions of calculations, has come out with this “solution.” It won’t take much for this to disappear entirely with the vagaries in solutions shown in yesterday’s spaghetti plot.
In addition to all of this excitement, yesterday’s sunset, featuring Altocumulus clouds with no ice virga. They were likely less than 500 feet thick. Tops were about -9 C (18 F) at 17,000 feet MSL, bases about -7 C (21 F). Such thin clouds, and those in a hazy layer like yesterday’s, have higher concentrations of cloud drops in them, causing them to be quite small, and in that attribute, resistant to freezing. The larger the drops in the clouds, the higher the temperatures in which ice forms, as a rule.
Also, at high levels, there is likely to be a dearth of those special aerosol particles we call ice nuclei. The most active natural ones come from soil particles. So the higher a cloud, the less likely will be a soil particle. There is some evidence that desert dust is an especially good ice nuclei.
So, it is not unusual to see Altocumulus layer clouds sit at low temperatures (-10 to -20 C) and not produce ice (which you would see as virga trailing down from them). There was a bit of virga visible in a spot or two in this morning’s Altocumulus clouds, ones that were a tad colder at top, at -10 to -11 C.
Will be polishing raingage surfaces every day now so I will be ready.