Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 191633

Area Forecast Discussion
National Weather Service Salt Lake City UT
1033 AM MDT Sat Aug 19 2017

.SYNOPSIS...Moisture will slowly increase again from the south
this weekend and continue into early next week, bringing a general
increase in showers/thunderstorms and possibly hampering eclipse
viewing Monday. The chance of storms then lingers next week.


.Discussion...Water vapor satellite imagery this morning shows a
circulation center off the SoCal coast, with an east-west
deformation axis upstream stretching from the northern Sierra to
the Colorado Rockies. A cumulus field accompanies this deformation
axis across central Utah currently, and is slowly drifting
northward. Area radars show a few light returns, while webcams
suggest the precipitation is high-based and likely only reaching
the ground over higher terrain. This makes sense given the fairly
dry current conditions of surface dewpoints, mainly in the 30s to
around 40.

Instability looks to be limited again today per model soundings.
As a result, convective-allowing models are not generating much
activity across Utah and southwest Wyoming, and what little they
are producing is mostly limited to the higher terrain. Thinking
today will be another round of cumulus more than anything, with
scattered coverage of showers and thunderstorms over the
mountain. Previous forecast has this well-covered, so no forecast
changes this morning.

The latest eclipse forecast looks largely unchanged from the last
few days, namely partly cloudy skies with mostly mid and high
clouds over northern Utah, with increasing clouds farther south to
the point of being overcast from around Bryce Canyon southward
associated with deeper moisture moving up from the south. To our
north, clearing skies from west to east are expected Monday
morning over Idaho as a trough axis moves through, while partly
cloudy skies linger in Wyoming in advance of the trough. Westerly
flow associated with the aforementioned trough passage may bring
in smoke from fires in NorCal and southern Oregon, so hazy
conditions are a possibility from around the UT/ID border

Previous discussion...Issued 435 AM MDT

.SHORT TERM (Through 12Z Monday)...The deformation axis is
expected to dissipate Sunday as the southern Rockies ridge expands
northwestward across the forecast area. This will likely suppress
convection for the most part, although isolated storms will
remain possible over the terrain. A tongue of higher PW is
expected to spread into the lower Co River Valley later Sunday
into Sunday night, and may impact southwestern Utah by Sunday
evening. As such have maintained slight chance PoPs for this area.

.LONG TERM (After 12Z Monday)...A trough progressing across the
US/Canada border has trended faster with the last few model runs.
The net impact for portions of the CWA with this outcome will be
dry air advection and less cloud cover across northern Utah during
the solar eclipse period Monday morning into early Monday
afternoon. Decreased cloud cover from Logan south to about Salt
Lake City to coincide with these changes in the models. Reduced
pops in the northern mountains as well for similar reasons.

A weak wave/convergence zone will gradually move north across the
state from the southeast Tuesday morning through Wednesday
afternoon. This should help to provide sufficient lift for
convection both afternoons, even outside the typical favored
terrain areas.

Another round of convection Thursday is expected to be followed
by a drier southwesterly flow Friday. There is a significant model
to model discrepancy regarding the depth/timing of a trough
crossing the northern Intermountain Region. The EC favors a weaker
solution and thus mid-level moisture remains more robust over the
CWA Friday, whereas the GFS favors a stronger, faster wave and
more drying across the CWA Friday. Leaned toward the GFS solution
for now Friday. Drier conditions continue into the weekend.


.AVIATION...Winds at the SLC terminal are expected to switch to the
northwest around 18-20Z. There is a 10 percent chance of the
terminal being impacted by a thunderstorm this afternoon, with gusty
winds from storms to the south as the most likely impact.


.FIRE WEATHER...A weak weather disturbance coupled with mid level
moisture will bring a small threat of high based showers and
thunderstorms to the area through this evening. High pressure
will strengthen a bit on Sunday resulting in a bit of a drying
trend, and maintaining above normal temperatures and low daytime
RH. By Monday this ridge will shift east, with southerly flow
allowing for an increase in moisture, which is expected to
linger through at least mid-week.




UPDATE...Van Cleave

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