When the jet goes by, the clouds, lower ones, roll in

And that’s pretty much what happened yesterday.   Here are some maps showing what happened as the jet stream in the middle troposphere (500 millibars or about 18,000 feet above sea level) went overhead while deploying to the east and south of us.  The sky change was pretty dramatic as you may have noticed.

First, how the forecast model had it timed, then the sky pictures as it was happening.  These panels, from IPS Meteostar,  are for 2 PM, 5 PM and 8 PM AST (these panels look almost identical, but believe me, that reddish area, indicating the strongest winds, is shifting eastward over southern Arizona!)  By 8 PM AST, the jet is completely past us (third panel).

The last panel, from the University of Washington, is the actual observations and “contour” map for 500 mb at 5 PM AST yesterday, that time when the sounding balloons (“rawinsondes”) went up.   That flag and four and half barbs at Tucson tell us that the wind was over 100 mph at 18000 feet above sea level over us at that time, likely the heart of the jet at 500 mb.  Its pretty unusual to see winds that strong so low.

You can also see in that contour map with satellite images of clouds that the clouds pretty much end south of that wind maximum at Tucson.  At the same time,  you can also see clouds puddled around inside the low in northern Arizona, encircled by a jet stream.  This sight, no clouds or just high and middle clouds, on the outside of the jet core, and low clouds with precipitation, is a common occurrence in the Southwest into the southern Rockies.  Scattered light snow showers were common in northern Arizona yesterday.  It is virtually required before any precip occurs in SW in the wintertime, that you have to be circumscribed (“inside”) the 500 mb jet.   BTW, this “rule” does not hold in coastal regions, such as southern California, or very far east of the front range of the Rockies, or in the summer months,  of course.  But its pretty solid here storm after storm.

And, of course#2, the sophisticated models of today know all about this “rule”, incorporating it in their outputs, and so we weatherfolk don’t really need to look for where the jet max is anymore like we did in the olden days of forecasting.  Still, its simplicity is appealing.

On some occasions, such as yesterday, when only brief virga accompanied those lower clouds, it is a “necessary” condition for rain here, but not always “sufficient.”  It was just too dry, even with this low’s little puddles of lower clouds filling its center.




















The cloud sequence:  1)  1:38 PM:  “nice weather we’re having.”  2) 2:18 PM:  wha’ happened?  Yesterday afternoon’s band of clouds accompanied the jet core passage overhead.  Cool, and it got even cooler.  Was hoping for a sprinkle, but didn’t get it.  There was a brief radar echo north of Catalina about this time.  That was it!

Since we’re still inside this jet/low this morning, there’ll be some lower clouds, Cumulus here and their, likely with a little ice, but too high and too sparse to have rain at the ground.  Most of the dust should be gone now, so a great looking day is ahead!

The weather farther ahead?  Some model fiction below, for March 19th.  Nice scoop jet rises up into southern AZ after scooping water out of the Pacific off California.  Would be a nice rain, if real.