Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 211615 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Salt Lake City UT
1015 AM MDT Mon Aug 21 2017

.SYNOPSIS...Southerly flow aloft will draw moisture north across
the forecast area early in the week. Somewhat drier air will
return to the south midweek.


Water Vapor Satellite shows a closed low off the southern
California coast, with ridging over Utah. 400mb-200mb MDCARS
wind observations place a 40kt-55kt southwesterly jet from
southern California into the Central Rockies, along with a 90kt-
125kt anticyclonic jet over western Canada. GOES/SLC 12Z
RAOB/HRRR indicate that precipitable water values vary from
0.25"-0.50" mountains, 0.75"-1.15" across most valleys.

Due to the eclipse updated 3-hourly sky grids today based on
satellite observations and best performing guidance. Also in the
diurnal temperature curve show a cooling trend during this
normally rapidly warming period of time around 18Z.

Overcast conditions with rain over southwest Utah due to a wave
propagating northwest around the ridge. However tonight expect
ridge to move east and allow this activity to propagate northeast.

Southwest Utah heavy rain concerns should be dampened by overcast
sky and stratiform rain. However cannot rule out some heavy
rainers especially Kanab eastward where dewpoints and warm cloud
layer on BUFKIT soundings are marginally supportive. Thunderstorms
will also develop along the spine of the mountains given decently
strong destabilization per SREF.

Dewpoint depressions especially across south central Utah could
see dry microbursts in the order of mainly 35-45kt range. As
everything eases north late day and tonight, this threat will
extend into north central Utah.


The upper low still parked off the southern California coast will
continue push moisture north through the Great Basin the next
couple of days. PWAT values remain the highest over extreme
southern Utah this morning. This moisture will begin to work north
into central Utah later today, then continue north into northern
Utah Tuesday.

Cloud cover for this mornings eclipse remains issue one at this
time. The most extensive cloud cover will remain at the core of
the moisture plume through south-central and central Utah, then
arcing northeast into northeast Utah. Some early mixing may
briefly thin out some of this cloud cover, though many areas of
south-central and central Utah could see a persistent 50%+
coverage of opaque clouds today. Southwest and southeast Utah
which are on the fringe of the moisture plume will see clouds thin
a bit early this morning, with a gradual increase returning late
morning. The visibility of the eclipse will be somewhat
restricted, with the best viewing likely near the Nevada border.

Up north a weak deformation axis extending west-to-east from near
Wendover to Vernal will focus scattered cloud cover early this
morning. Anticipating that early mixing will thin out some of the
clouds, with cloud development confined mainly to higher terrain
mid-morning. Overall, viewing of the eclipse looks to be good
across the north.

Now back to the precipitation forecast. Scattered loosely
organized convection today will center on the moisture plume
across southern/central Utah, with a secondary maxima across the
northeast mountains later today as the moisture profile improves.
A shortwave ejecting north from the coastal southern California
low will move slowly north across the forecast area beginning
Tuesday afternoon for the south, then reaching central/northern
Utah Tuesday evening. This convection will remain active
overnight, with the greatest areal coverage tied to the shortwave
across the northern third of the state

Models in good agreement with timing of upper level disturbance
lifting northward through northern reaches of forecast area at
start of period, and in bringing one last weak disturbance across
northern sections of eastern Great Basin Wednesday afternoon and
evening. With residual moisture still in place, precipitable water
values near three quarters of an inch, will continue chance
wording for showers and thunderstorms northern Utah and extreme
southwest Wyoming, with just slight chance wording for central and
southern Utah, where have scaled chances for precipitation down a

With continued drying on Thursday, potential for showers and
thunderstorms will primarily be limited to mountains.

GFS indicates a weak upper level disturbance will graze northern
sections of forecast area Friday afternoon and evening, while
Canadian and ECMWF are much weaker with this feature.  Given low
confidence, will limit chances for precipitation to high terrain of
forecast area, with best potential over northern sections.

Globals models in excellent agreement with building a ridge of high
pressure over the Great Basin over the weekend, with a generally
west to northwest flow promoting a very dry air mass over region.
With 700 millibar temperatures warming to 16 degrees Celsius, should
see readings several degrees above normal.


VFR conditions will continue through the TAF period at the KSLC
terminal. Light and variable winds will shift to the Northwest
around 17-18Z.


ERC values are below the 50th percentile across southern and
portions of eastern Utah. Elsewhere values are higher, varying
from 50th through 79th percentile most locations. Near Dugway ERC
values are in the 80th through 96th percentile.

A wave propagating northward over Utah tonight and tomorrow will
bring increased cloud cover and chances for wetting rains. Across
southern Utah today some may be locally heavy, while across mainly
central Utah dry microbursts mainly in the 35-45 mph range are

Periods of clouds and cooling temperatures next couple of days
through remaining dry. Will have to watch for new lightning
starts where fuels are particularly dry. The coverage of showers
and storms will diminish through the week. A ridge builds in next
weekend drying out the state with very warm temperatures.





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