Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 221009

Area Forecast Discussion
National Weather Service Salt Lake City UT
409 AM MDT Tue May 22 2018

.SYNOPSIS...A large storm system will enter the area beginning
tonight, bringing unsettled weather through Tuesday and
Wednesday. A ridge will build in for the end of the week.


.SHORT TERM (Through 00Z Friday)...An upper level low evident in
early morning water vapor imagery is currently spinning across the
lower Colorado River Valley, and will weaken as it slowly lifts
northeast across the forecast area today through tonight. Cool mid
level temperatures will allow for modest instability resulting in
scattered to numerous showers and thunderstorms across the
forecast area. With light flow and relatively weak forcing these
storms will likely be largely terrain driven, but will likely
spread into adjacent valleys late this afternoon and evening,
particularly  across northern Utah. Have increased PoPs a bit
across the the higher terrain of northern and central Utah while
leaving PoPs in the scattered/chance category across the valleys
as well as southwest Utah.

As this low continues to lift northward overnight into Wednesday
drier air/subsidence will spread across the area from south to
north, with a threat of convection remaining across the northern
third of the area Wednesday. By Thursday dry southerly flow is
forecast to be in place across Utah and southwest Wyoming
resulting in a warming trend with little to no threat of
convection expect perhaps across the higher terrain of the far
north. As such have lowered PoPs for the Thursday period.

.LONG TERM (After 00z Friday)...A continuing trend in guidance vs.
24 hours ago points towards cutting off the Memorial Weekend low
overhead allowing for a potentially more prolonged period of
somewhat unsettled conditions lasting late weekend through Tuesday.
Previous solutions from days past filled and lifted this through the
area more rapidly, by late Monday. Being a closed low (and now a cut
off closed low for a period) late in the forecast period, confidence
in trough evolution is somewhat subdued even with the model to model
continuity that currently exists.

Mid level ridging overhead Friday will begin shifting downstream
with inland translation of the low Saturday. Temps some 10-15
degrees above climo Friday are expected to get knocked down a bit
Saturday with this, but anticipating a more noticeable cooldown back
to seasonal norms Sunday with increasing precip chances as the upper
low settles overhead. Expecting increasing showers/storms with this,
largely diurnal aided by steepening lapse rates, but enough weak
forcing should be present for some maintenance of showers
nocturnally as well (especially along the northern/western flanks of
the low where convergent flow will be maximized). Have maintained
slight chance/chance PoPs focused across central/northern portions
thereafter through day 7 as minimal change looks to exist


.AVIATION...Southerly surface winds currently in place at the KSLC
terminal are expected to switch to the northwest between 17-19z, but
gusty and erratic outflow winds will become an increasing threat
shortly thereafter. Scattered showers/storms are expected to develop
mid afternoon, with potential impact to the terminal greatest in the
20-03z timeframe. Operational impacts will most likely remain tied
to these gusty and erratic outflow winds (most likely southerly),
but a slight chance of lightning within 5 miles exists as well.





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