Mesoscale Precipitation Discussion 0962
NWS Weather Prediction Center College Park MD
224 PM EDT Tue Aug 22 2023
Areas affected...northwestern AZ, UT into western WY
Concerning...Heavy rainfall...Flash flooding possible
Valid 221822Z - 230020Z
SUMMARY...The expected development of thunderstorms during the
afternoon will increase the threat for flash flooding from
portions of northwestern AZ into central UT and far southwestern
WY. Rainfall rates of 1-2 inches in an hour or less are expected
with the stronger storms that develop.
DISCUSSION...Water vapor and layered PW imagery at 18Z showed UT
beneath deep-layered moisture sandwiched between upper troughing
over the West Coast and a large ridge over the central U.S. The
12Z sounding from SLC showed relatively high relative humidity
throughout the column with a PW value of 1.09 inches, between 2 to
3 standard deviations above the mean. Flow was unidirectional from
the south with sufficient speed shear for at least some
organization of thunderstorms. Eastern portions of the Great Basin
were also located within the right entrance region of a 70-80 kt
jet max centered near 300 mb via GOES West DMVs, favoring ascent
through divergence aloft. One missing ingredient for robust
thunderstorm development is instability with a fair amount of
cloud cover in place west of the Wasatch Mountains and the 18Z SPC
mesoanalysis indicating MLCAPE was generally less than 500 J/kg
north of the AZ/UT border. However, visible satellite imagery
showed partly cloudy to mostly clear skies along and east of the
Wasatch Range along with isolated thunderstorms over the past few
hours over the higher terrain.
Continued solar heating within areas of reduced cloud cover, from
northwestern AZ into central UT...focused around the Wasatch...and
far southwestern WY is expected to increase MLCAPE into the
500-1000 J/kg range by 21Z. Thunderstorms are expected to develop
by 21Z as low level inhibition is overcome with increasing
coverage of thunderstorms into the early evening. The environment
is expected to support rainfall rates of 1 to 2 inches in an hour
or less with some S to N or SW to NE training/repeating segments,
which could generate some areas of flash flooding. Flash flood
potential will be greatest where overlap of heavy rain occurs with
vulnerable slot canyons and dry washes.