Rain follows the jet

0.02 inches of it, anyway, as the core of the jet stream at 18,000 feet or so passed by Catalina yesterday afternoon.  Keep your eye on the orange and reddish streak in these progs from IPS MeteoStar yesterday morning beginning at 5 AM AST and how it slides over us as the clouds began to ice up:2016042812_CON_GFS_500_HGT_WINDS_0005 AM yesterday.  Jet at this level races across central AZ.
2016042812_CON_GFS_500_HGT_WINDS_00611 AM yesterday.  Maximum winds getting closer!  Tiny Cumulus clouds begin to appear over the Catalinas and on the west to north horizon.

11:40 AM.
11:40 AM.

The jet separates deep cold air on the left side, looking downwind, and deep warm air on the south side.  The deep warm air prevents Cumulus clouds from getting very deep due to inversions and stable layers where the temperature does not change much with increasing height, or even rises.  The temperature at 500 millibars or 18,000 feet above sea level dropped from -17.7 °C to -21.1° C over TUS yesterday between 5 AM and 5 PM, while the temperature about which ice begins to form in our clouds dropped about 400 meters during that time.  With the temperatures at the ground rising into the mid-70s as the colder air moved over us, Cumulus clouds deepened, reaching the ice-forming level between 1 and 2 PM.

Also with patterns like this, the cyclonic rotation (vorticity) in the air above us is increasing like mad, and that leads to a gentle upglide motion in the atmosphere, one that also helps cool the air aloft and usually produces sheets of clouds like Cirrus, Altostratus, Altocumulus and NImbostratus.  But yesterday the air was too dry for sheet clouds to form.

First ice was noted just after 1 PM.  Can you find it?

1:11 PM. Looking N toward the Charouleau Gap.
1:11 PM. Looking N toward the Charouleau Gap.  Tiny puff of ice ejects from a Cumulus humilis cloud based at about 8 thousand feet above ground level.  Bases were running about -5 °C

Keep you eye on the brown and yellow streak.

2:31 PM. Cumulus and Stratocumulus clouds launched off Pusch Ridge and the Tucson Mountains stream toward Catalina. The sky begins to fill in rapidly.
3:44 PM.
3:49 PM.
3:57 PM. A horse eating as it clouds up.
4:33 PM.
5:09 PM. Light rain falls in Catalina/Sutherland Heights.
6:04 PM. RW- (light rain showers) continue in Oro Valley.

5 PM yesterday.  Just passed!  B y this time, Sutherland Heights had 0.02 inches as  the tops of Cumulus and Stratocumulus complexes continued to cool and ascend.   The sounding from TUS at 5 PM AST (launched about 3:30 PM AST) indicated the coldest tops had reached -20 °C or so, plenty cold enough for ice, virga, and light rain showers.  Too bad the bases were so high since we could have had some real rain if they had been lower.

But, we were “lucky” to get that.  Even the great U of AZ model had no rain anywhere near us late yesterday afternoon when it fell!  THAT does not happen very often.

Looking ahead….today:

Nice Cu, ice, too.

Farther out:

Substantial rains, maybe half an inch or so,  still on tap between May 6th-8th as previously foretold here.  Yay!  May averages 0.38 inches here in Catalina.  More rain likely after that episode, too.  So an above normal May in rain is pretty much in the bag now.   Could be an especially great May, too.

The End.

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.