Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 211043

Area Forecast Discussion
National Weather Service Salt Lake City UT
443 AM MDT Wed Mar 21 2018

.SYNOPSIS...A moist southwest flow aloft focused on California
will spill into the Great Basin late in the week. A weak
disturbance embedded in this moist southwest flow will cross Utah
Thursday night through early Friday. A second cold upper level
trough off the Pacific Northwest coast will move east across the
region this weekend.


.SHORT TERM (Until 00z Sunday)...The upper ridge building along
the spine of the Rockies combined with the deepening upper trough
moving south along the BritCol coast will leave the Great Basin
under an increasingly strong southwest flow aloft the next couple
of days. The leading edge of the atmospheric river (AR) over
central/southern California will begin to spill over into Great
Basin tonight/Thursday. PWAT values will be running between 3 and
4 standard deviations above normal at the peak of the ARs
influence over Utah Thursday afternoon through early Friday.

A weak shortwave lifting northeast across northern/eastern Nevada
overnight will cross northern Utah this morning. Light showers are
already indicated in radar imagery across the far north. This
activity will increase in areal coverage early this morning, with
a second trailing shortwave maintaining some weak convective
showers across the far north through this afternoon/early evening.

Winds will become an issue across the western valleys on Thursday.
The increasing southwest flow aloft between the upper ridge to
the east and cold core trough off the BritCol/Pacific Northwest
coasts will create near 700mb winds approaching 50 knots across
western Utah Thursday afternoon through early evening. The surface
cold front moving east across Nevada will settle into northwest
Utah Thursday afternoon. The close proximity of the front along
with the supporting 700mb winds will produce very strong southerly
winds across the western valleys. The strongest winds will likely
extend from near I-80 west of SLC southward across west-central
through southwest Utah. Currently, an high wind watch is in effect
for the west-central and southwest valleys Thursday. Somewhat
concerned that scattered showers and cloud cover will inhibit
winds enough that high wind criteria won`t be met. May end up
issuing a wind advisory versus an high wind warning in subsequent
forecast packages.

Anticipating widespread, and occasionally heavy, precip Thursday
night through early Friday. Two areas to watch will be in the
upglide areas of extreme southwest Utah, and along the near 700mb
baroclinic boundary moving slowly east across northern Utah.

For southwest Utah a persistent and quite moist southwest flow
into the higher terrain could focus steady moderate/locally heavy
rain late Thursday afternoon/night. A vorticity lobe embedded in
the southwest flow along with low-level warm advection should
generate sufficient synoptic-scale lift enhanced by the terrain
upglide to support moderate/heavy rain. The relatively short
duration of this event would likely preclude any significant river
flooding. Flash flood-prone areas would be another issue as the
intensity and duration of this event could generate local flash
flood issue Thursday night/early Friday.

Up north weak frontogenesis along the near 700mb baroclinic zone
combined with dynamic support from passing mid-level shortwaves, a
favorably positioned upper jet and the exceptionally moist air
mass should generate deep synoptic-scale lift and widespread
precipitation. Anticipating significant amounts of valley rain
and high elevation snows near the low-level boundary across
northern Utah Thursday night, then into the northern portion of
central Utah and northeast Utah late Thursday night/Friday morning.
Winter weather highlight may be become necessary as snow levels
dip down to the higher passes later in this event.

Looking at a bit of a lull in precip Friday afternoon/night ahead
of the arrival of the coastal upper low this weekend. Shortwaves
ejecting out ahead of the main low will spawn scattered convective
precip across mainly northern/western Utah Saturday. Not
anticipating any heavy precip until the main upper low moves into
the area beyond the current short term forecast period.

.LONG TERM (After 00Z Sunday)...The main storm system is expected to
be parked along the Pacific Northwest coast Saturday night. Global
models all bring it across the western United States during the
early part of the upcoming week, and in the latest guidance EC/GFS
eventually close the system off over Arizona on Tuesday. However,
the models continue to show relatively poor agreement and run-to-run
consistency with respect to exactly how that evolves.

Latest EC is now the model that shows the system as a more cohesive
unit, bringing it and the associated front into northern Utah during
the day Sunday and through the remainder of western Utah on Monday.
The GFS shows similar timing but brings the system through in
pieces, with the initial piece accompanying the cold front into
northern Utah during the day Sunday before the second piece catches
up with the first, moving the front across the rest of the forecast
area on Monday.

As previously mentioned, global models now agree that the system
will slide south into Arizona by Tuesday as it closes off. This will
shift the focus of associated precip to southern Utah, with the
precip gradually tapering off by day seven as the system continues
southeast. Temperature are expected to cool noticeably Sunday with
the arrival of the front, bringing maxes down to values up to 10F
below seasonal normals. As the system slowly exits, temperatures are
expected to gradually rise as the airmass dries and warms.


.AVIATION...VFR conditions are expected at the SLC terminal
throughout the day. Cigs are expected to lower to or below 6000 feet
AGL at times, primarily mid to late morning, with rain showers
developing in the vicinity of the terminal. Southeast winds should
continue through the daytime hours.


UT...High Wind Watch from Thursday morning through Thursday evening
     for UTZ013-016.




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