Saturday, May 6, 2017

DCAPE, Microbursts, Pollen, and Wind Damage

This morning's sounding showed classic conditions for strong microburst winds with an "inverted V" profile created by the temperature (red) and dewpoint (green) traces above the shallow surface-based nocturnal inversion.

Source: SPC
Microbursts are are sudden strong winds caused by downdrafts induced by precipitation and may be "wet" or "dry", with the latter occurring when the precipitation evaporates before reaching the ground.
Source: Wikipedia
One measure of the potential for strong downdrafts and microburst winds is what is known as the downward convective available potential energy or DCAPE.  DCAPE is maximum energy available for a negatively buoyant parcel of air cooled by evaporation assuming no mixing with the environment.  Values above 1000 Joules/kg are considered high and significant.  This morning, the DCAPE at Salt Lake City international airport was 1223 Joules/kg.

Not surprisingly, we've had some strong winds in northern Utah.  This morning, I observed a modest microburst drive an impressive "pollen front" into the Avenues (sorry, no pictures, but I did sneeze). Subsequently, several valley locations have reported microburst wind gusts in excess of 60 mph including the Kennecott Tailings Pile (61), Lake Point (63), Vernon Hill/Tooele Valley (65), Syracuse (66), and Pleasant Grove (68).

The biggest gust so far was recorded on Fremont Island in the Great Salt Lake, with a gust of 91 mph.  The NWS just tweeted that there is wind damage on nearby Antelope Island.

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