Low forming in AZ. Jet stream (at 500 millibars) goes by/strengthens to the SE during day; therefore, expect rain to begin before 5 PM, but not before 11 AM. It should continue off and on for about 24 h afterwards (in case you don’t watch TEEVEE where YOUR weatherman/gal will be telling you much of what is below. But to make it that much more interesting for you, this: (Cloud pics way at bottom in case you want to skip all this.)
Let’s play, “Beat the Model!”
Kind of a “fun” day for us amateur and highly paid media forecasters since rain will develop where there is none upstream of us; clouds will thicken downward from the Cirrostratus/Altostratus with Altocumulus as the day progresses, with light rain beginning from that process, heavy layer clouds with virga that eventually reaches the ground as the atmosphere moistens up. For example, at this hour (4:50 AM AST) there are NO radar echoes from precip in all of Arizona, somewhat surprisingly.
As part of the “game”, and AFTER laying this out this rain onset timing (rain beginning after 11 AM and before 5 PM in Catalina, that is, best guess, first drops in mid-afternoon), its then fun to see what the very latest super Beowulf Cluster model of the U of AZ says, the best of the best. Its a test of how well you and me are “learning the territory”; “don’t need no model” if you truly understand our SE AZ patterns (when rain occurs relative to air flows aloft like jet streams), know the local and upwind terrain effects because you’ve lived here long enough, can read satellite imagery, etc.
Of course, you can’t really do this except when the weather is at hand. The models are just too good when it comes to beyond a day or so.
Here’s how to make an educated guess from “patterns and such”:
First, determine the outliers, those least and greatest amounts “possible”: bottom of precip amount (if things really go bad) here now looks like 0.20 inches (estimated 10% chance of less), top amount (if things really go well) up to 0.80 inches, (estimated only 10% chance of more than that, i. e., everything appears to be falling into place for a substantial desert rain here. The best guess, now that you’ve figured out the top and bottom amounts in kind of a mental “ensemble”, the average of those two outlier forecasts, 0.50 inches by mid-day tomorrow. That’s it.
Since I have busybeelabored these weather points for some time now for the two of you that read this blog, we should all be making the same interpretations today sans model outputs; just eyeballing stuff, doing our own thing, getting a handle on this storm by really THINKING about it, not just soaking in some model output quite yet. Don’t peak yet! This is better than Sudoku, Angry Birds, bajan, facebook, etc.
NOW let’s peak at the U of AZ model from 11 PM AST to get the latest, best prediction for Catalina and see how it matches up with our own forecast based on perusal of a few simple model charts, sat imagery, dewpoints, etc.
Here are the U of AZ Beowulf Cluster outputs, run from 11 PM AST data. The rain arrival in Catalina is shown in the panel below and its between 5 and 6 PM AST, moved up from 10 PM on the run 24 h ago, but a few hours later than our SOP (Seat of the Pants) forecast above.
This is so great! I love this competition! Its weather sports! Its great to tweak the model’s nose, the model with its billions of calculations when maybe you’re own grades in calculus weren’t so great! I am pumped about rain before 5-6 PM!
Now for a comparison of our amounts….
Beowulf predicted amount: between 0.10 and 0.50 inches from this map below by noon tomorrow (when most of it should be over), which shows the heavier rains to the SE of us, ones up to 0.7 inches.
These Beowulf model amounts are often on the HIGH side, and so this is of some concern for us and our prediction, since this model foretells a tenth to a quarter inch or so by 1 PM AST tomorrow when the precip is over except for spotty light passing showers. If that is an amount pushed to the high side, egad!
So, we’re (I’ve roped you into this) a bit on the high side of a model prediction that is known to push rainfall amounts already toward higher than observed. Hmmmmm.
Those lower predicted amounts in the model are probably due to all the low dewpoints we have to overcome with a nice Gulf of Cal/subtorpical influx, and that influx, the model thinks, will end up arriving to the southeast of us. Uh oh. Too late to back off rain amount guess, will ride out the storm; Riders of the Storm Amount, you might say to break up the boredom here a bit.
Now, if you were going on TEEVEE with a forecast after all this investigating, you might, after your educated guess, tweak our own forecast some; somewhere between the original one, which was pretty original, and these model ones. Works out for the best that way. We used to call this, the “man-machine mix”. We still do it.
What can we learn from this exercise?
If those green areas (those having more than 0.50 inches) are farther to the west and over us, we might be detecting a slight eastward bias in the model in these kinds of situations when the moisture roars out of the south-southwest as it will today. No doubt about it, we will be on the edge of the major rains! Fingers crosssed.
I have to admit that due to an internal pro rain bias, I am often on the high side of rain amount predicting.
Wouldn’t it be great if newspaper stories about political events had bylines like, Joe Blow, Democrat, “disclosures” so we might read between the lines some, etc?
In a word, phenomenal. So many gorgeous patterns up there in those Cirrus and Cirrocumulus clouds yesterday afternoon, I could hardly take enough photos of them. Like etched glass, this beauty: