Author: Art Rangno

Hundredths, that is, right here in Catalina.  It was so nice also being “back in Seattle” for a few hours of low hanging clouds and barely enough rain falling for an umbrella.  Yep, that’s the way it is in Seattle, just what we saw two days ago.   Being despondent over the prior rain “bust” and really nothing else whatever I tell you, I lapsed a storm day.  OK, here’s retrospective, about as valuable as yesterday’s newspaper.  The morning Stratocumulus topping the Catalina Mountains, always a nice sight, followed by the “storm”.  Of course, I did not expect as much as 0.05 inches and we got 0.22 inches here (wrong again–maybe I should change my name to Wrongno…), and a little more up toward uppity Saddlebrook (no gravel roads there!) where they got about 0.3 inches.  Southerland Heights, that strange area above Catalina, got 0.26 inches.  In the Cat Mountains, up to half an inch fell!

Second photo shows clouds moving in, with “crepsucular” rays (haha).   The diverging rays show clearly that the sun is much closer to the earth than is generally believed by astronomers.

Third photo, THE STORM as it would appear in Seattle.

Lastly, the gift of living close to those gorgeous Catalina Mountains;  those golden quilted scenes as the clouds shallow out and begin to clear as the storm departs.

No rain in sight now until the numerical mirage of another tropical storm remnant moves into Arizony on the 16th of October.   That from a NCEP model run last night (“WRF-GFS”).   Don’t even think about it.   You’ll be disappointed, but there it is and it is my duty to report it.  Not even going to show a gif of that event since they don’t ever materialize do they?   Remember Hurricane Hilary, its remnants,  and how they were supposed to come up here on quite a few model runs and give us a good rain?

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…

First, let us examine the National Center for Environmental Prediction (NCEP) model that came out based on this morning’s data from around the globe.  This first panel is for the winds around 500 mb (millibars, or “hectopascals”) and is at about 16,ooo to 20,ooo feet in the atmosphere, depending on how high or low the point at which a pressure of 500 is reached.  The winds blow along the lines, and the strength is noted by colors; red  and purple are real strong.  The colors tell you where the jet stream is, and storms are steered  by and develop/dissipate along it.

This panel below is for 144 hours from now or is valid Tuesday, October 4th in normal speak.  Note the area off southern California, where the winds are light, come on to the coast and then turn to the NE over Arizona.  Where the winds turn like that is called a “trough.”   The air tends to rise on east side of a trough, and descend on the west side.   This particular “trough” over southern California would be considered rather weak.  However, some tropical moisture is still around and some spotty showers are forecast for eastern Arizona (not shown) with this pattern.

This is not a particular “good” model output; perhaps it could be considered even quite “bad” since we need a lot of rain, and this trough, according to THIS model, is quite weak, and can’t do the job.

So, we start looking around to see if there is a “good” model run that for this same forecast hour, 144 h from now, on Tuesday morning, October 4th, which shows that there will be a LOT of rain.

Sure enough, Environment Canada has supplied what we are looking for based on the SAME data taken around the globe, but using a version of the European Center for Medium Range Forecasting model.  The next figure shows the “classic” four panel projection for that time from the Canadians.  Here, the jet stream part of the forecast is the upper left hand corner, and LOOK at the difference along the West Coast!  Instead of a jet stream ramming into Oregony and northern Califiornia as it does in the US model, its going into Juneau, AK!  And more importantly, that Canadian jet stream map shows a whole low pressure center along the California coast!  And with a low center there, and tropical air to the east of that low, it acts like a pully system to bring a gush of that tropical air across Arizona with substantial rain, as predicted as noted by the green areas in the panel in the lower right.  Not shown is the rain even more substantial rain that PRECEDES this map, also substantial for the 24 h prior to this map.  The heavier rain is due to the low center along the California coast being a stronger, better organizer of the storms in the tropical air over us.

Thus, the Environment Canada model output for 144 h,  would be called a “good” model prediction since we need rain and this model run “delivers.”

So, this is what forecasters deal with a lot of the time, though the differences shown here would be considered a little out of the ordinary as great as they are in this “model divergence.”

In real life the models are perturbed slightly at the beginning of the prediction period to assess how different the results could be, and a whole group of predictions is obtained from the SAME model having slightly different starting conditions.   The resulting outputs are called “ensembles”.   The more likely a situation will be observed in the future is in how LITTLE the ensembles vary.  Here, to be serious for a second, this :model divergence” for the situation in the SW means that neither model output can be relied upon heavily.

Darn.

The End.

These, and lots more of them yesterday.  I think one of the lightning strikes associated with photo No. 2 down there was about 8 inches from the house!  Also, there one particularly dangerous stroke from a thin anvil cloud just after 12 Noon yesterday as the first thunder began to be heard from a Cumulonimbus cloud raining on Mt. Lemmon (where they received over an inch of rain).  However the strike was a near vertical cloud to ground strike many miles from the rain area, and out of a fairly innocuous anvil cloud.  See photo number 3 below.  The cloud-to-ground stroke came down about a mile away, perhaps in or near the Sutherland Heights development east of Lago del Oro road,  and behind the mesquite tree humped up in the center of the foreground area of this photo.  I just could not believe it and it does tell you to be cautious about lightning.  I might well have been outside up there in the Sutherland Heights area being a little too non-chalant about where lightning will hit next when looking up at that anvil cloud.

However, the 0.13 inches received was a little disappointing considering the behemoth of a storm that was approaching about 4 PM.  Actually, only about an hour earlier I had told a neighbor, after looking at the mostly glaciated skies, that it probably wasn’t going to rain here after all, so maybe I should be happy with a few “crumbs” of rain!

The End.