Heavy Rains over southern California

February 28th, 2017 |

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

The GOES-15 Water Vapor animation, above, shows a potent cold front moving through southern California late on 27 February. This front that passed through San Diego at 0500 UTC on 28 February (9 PM PST) was accompanied by abundant precipitation, the heaviest rainfall in 13 years at the San Diego airport (link), with widespread 2+-inch rains that caused power outages and flooding. The image below (from this site), shows the 24-hours precipitation ending at 1200 UTC on 28 February 2017. Values in excess of 6″ occurred in the mountains east of San Diego.

Accumulated Precipitation for 24 hours ending 1200 UTC on 28 February 2017 [click to enlarge]

Accumulated Precipitation for 24 hours ending 1200 UTC on 28 February 2017 [click to play animation]

Hourly MIMIC Total Precipitable Water estimates for the 72 hours ending 1400 UTC on 28 February 2017 [click to enlarge]

Hourly MIMIC Total Precipitable Water estimates for the 72 hours ending 1400 UTC on 28 February 2017 [click to play animation]

Satellite estimates of Total Precipitable Water (TPW) suggested that heavy rains were likely. MIMIC total precipitable water plots, above (source), show a moisture source that tapped the rich moisture of the Intertropical Convergence Zone. NOAA/NESDIS Blended Precipitable Water Percent-of-Normal plots (source, at this site), shown below, show values exceeding 200% of normal over southern California. Both MIMIC and Blended TPW products offer excellent situational awareness.

NOAA/NESDIS Blended Total Precipitable Water Percent-of-Normal, times as indicated [click to play animation]

NOAA/NESDIS Blended Total Precipitable Water Percent-of-Normal, times as indicated

An interesting aspect of the GOES-15 Water Vapor animation, at the top of this post, is the appearance of land features. The spine of the mountains over Baja California appears throughout the animation, for example, as does the Front Range of the Rockies from Colorado southward to New Mexico. Should land features be visible in water vapor imagery? An answer to that lies in computed weighting functions, shown below (from this site), that describe from where in the atmosphere energy at a particular wavelength is being detected by the satellite.

At the start of the water vapor animation, near 0000 UTC, thick clouds cover southern California (and the sounding from San Diego shows saturated conditions); dry layers in the sounding appear by 1200 UTC. The 7.4 µm weighting function shows that information is detected by the satellite from lower down in the atmosphere; energy detected at 6.5 µm comes from higher in the atmosphere. This difference arises because of the better absorptive qualities of water vapor gas for 6.5 µm radiation vs. 7.4 µm radiation. By 1200 UTC, sufficient drying has occurred that the 7.4 µm Sounder Channel is detecting radiation that emanates from sea level. Note also at 1200 UTC that each individual moist layer influences the weighting function — but there is insufficient moisture at 1200 UTC in those moist layers that they are opaque to energy at either 6.5 µm or 7.4 µm.

Note: GOES-R Series satellites, including GOES-16, have ‘water vapor’ channels at 6.2 µm, 6.9 µm and 7.3 µm.

Water Vapor Weighting Functions at 72293 (San Diego) for GOES Imager (6.5 µm) (Black Line) and GOES Sounder (7.4 µm) (Red Line) at 0000 UTC 27 February (Left) and 1200 UTC 28 February (Right). The Sounding for San Diego is also indicated [click to enlarge]

Water Vapor Weighting Functions at 72293 (San Diego) for GOES Imager (6.5 µm) (Black Line) and GOES Sounder (7.4 µm) (Red Line) at 0000 UTC 27 February (Left) and 1200 UTC 28 February (Right). The Sounding for San Diego is also indicated [click to enlarge]