Tuesday, May 12, 2020

Hazardous Convective Storms

Yesterday, hazardous convective storms affected locations in Utah, resulting in at least one fatality.

Northern Utah

In northern Utah, convective storms produced strong microburst winds in several areas, resulting in a few downed tree branches and a lot of blowing dust and pollen.  The tweet below includes a video showing dust and probably pollen kicked up by microburst winds moving through downtown Salt Lake City.

There were some downed tree branches reported near Liberty Park according to NWS reports.

Microbursts are produced by strong downdrafts that fan out when they contact the ground, as depicted below.
Source: Fujita (1981)
Yesterday's was a traveling microburst similar to the schematic above, initiated by virga (precipitation aloft) falling from shallow mid-level clouds near the Oquirrh Mountains.  The afternoon sounding from the Salt Lake City airport, collected around the time of the microburst, shows a remarkably dry lower atmosphere with what is known as an "inverted-V" sounding.  Clouds are based just below 500 mb (probably about 8500 feet above ground level) and below the cloud the sounding is remarkably dry, with a 58˚F dewpoint depression at the surface (equating to a relative humidity of only 12%).

Source: SPC
Thus, precipitation from aloft falling into this dry airmass let to dramatic cooling and the formation of a downdraft and microburst winds.

Microburst winds can be very dangerous for aviation and can produce damaging straight-line winds.  In the future, they will be a serious concern for drones, which are being used increasingly for deliveries of all sorts of products.  For example, yesterday's winds would have been above the recommended operating threshold of some (maybe all) drones that are currently being developed for for commercial deliveries of packages of 5 pounds or less.

Southern Utah

At least one person is dead due to flash flooding produced by a thunderstorm near Goblin Valley in the San Rafael Swell south of I-70 and north of Hanksville.  Below is a radar image showing what I believe is the storm at upper right, just northwest of HVE, the abbreviated identifier for Hanksville.

Source: NCAR/RAL
Radar imagery shows storm initiation over the 11,000 foot high Thousand Lake Mountain Plateau north of Torrey and then proceeding to the Goblin Valley area.  Apologies for the crappy video below (source: NCAR/RAL), but I'm working on borrowed time right now. 

There are several popular slot canyons in that area, including Little Wildhorse.  The video below was shot by Atmospheric Sciences graduate student Tom Gowan who was in the area.

Media reports suggest that in addition to one confirmed fatality, some vehicles remained at the trailhead overnight.  Hoping for the best as search-and-rescue operations resume this morning.

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