Snow day February 25th; “webby” Cirrus

Remember, whether it happens or not, you heard about it FIRST here!  Tell your friends.

Was pretty excited to see this 500 millibar map (about 15,000 to 20,000 feet above sea level) for the morning of February 25th below from our friends at IPS Meteostar.  Pretty cool, eh?  This from the model run based on global data taken at 5 PM AST yesterday.

Note on that map, we are encircled by the jet stream, indicated by the brownish orange regions at the outskirts of this behemoth of a trough, a requirement for winter precip here.  How “be-a- moth-ian” is it?

Check out how abnormal this pattern is in the panel below this one, marked by the dark blue bulls-eye here in Arizona!  So, its really an unusual pattern that is being calculated by the computer.

An aside:  Oddly, we use contours of the height above the ground of a pressure surface for our upper level maps1, and the LOWER that height is (such as over AZ in the top panel), the COLDER the air must be overall below that height.  Low sea level pressure also adds to this height “deficit”, but mainly its the density of the cold air that does it.  The more dense the air is, the more rapidly you reach above you any particular pressure level.  (It really would be so much better to have pressure maps with highs and lows at a constant level above us than having to divert attention for this explanation.)

So, in the panel below, its the LOW HEIGHT of at which the 500 millibar pressure was reached (i. e., 5340 meters) that tells you this is a cold, cold, cold, cold system.  (They say that redundancy is the key to remembering things.  Remember, “534” (decameters) is COLD).

What DOES that the huge anomaly from normal in the bottom panel tell us weatherfolk?

The forecast map for February 25th is a real outlier model forecast, and so we shouldn’t be proclaiming a snow day or anything like that here 15 days in advance because it is such an extreme prediction and likely to go wrong.  So, that’s what I have not done here.

HOWEVER, this outlier prediction shown below, is a part of a jet stream pattern that is developing RIGHT NOW in which low pressure systems and cold fronts will come zooming down into the Southwest from the northwest, one that is likely to go on for  a couple of weeks or more.  I would guess there might well be a hard freeze at some point, though not in the immediate future.  Be ready!

This developing pattern also means more chances for rain here in Catalina over the next few weeks, and with the cool air ahead, holding our late winter vegetation together better even if there is not much precip because it won’t get burned out.  So, overall, good news unless you came to AZ for consistently warm days.  Ain’t gonna happen so enjoy the warmth we have now!

 Webby Cirrus clouds

Yesterday, moving rapidly out of the north, were some “webby” looking Cirrus clouds.  These are always seen only right after they have formed, maybe 10-20 minutes or so after that.   They start out as tiny flecks (which for a moment might be termed, Cirrocumulus clouds), and, possibly, for the briefest moment, may be comprised of liquid.  They then convert to ice and as the individual crystals grow and fall out,  or are dispersed by turbulence,  the tiny flecks become larger and larger and some of the ice falls out in strands.

After about a half an hour to an hour, they are usually just masses of tangled looking Cirrus without much cellular structure.  Here’s what they looked like yesterday in that younger formative stage.  At most upwind end (lower part of photo), the newest flecks have formed, while the older Cirrus elements are broadening and becoming “webby” looking.  The likely ice crystals in these older Cirrus, for some additional annoying trivia, “bullet rosettes”, spikey-looking crystals having columns jutting out from the original “germ” ice particle.  Nice images of bullet rosettes here at the beginning of a long article…






















1In the olden days, weatherfolk liked to look at “isentropic surfaces” which helped them figure out where the air was sliding upward and likely to form clouds and precipitation before there were computer models. These areas were well represented on constant pressure maps where the cold and warm air was being pushed around.