Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 231134

Area Forecast Discussion
National Weather Service Salt Lake City UT
534 AM MDT Sat Sep 23 2017

.SYNOPSIS...An upper low over Nevada will slowly track across
Utah through the weekend, keeping cool and wet weather in place.
Another low may impact Utah midweek.


.SHORT TERM (THROUGH 12Z TUESDAY)...The closed low upstream of
the area is noted on satellite imagery centered over Nevada this
morning. Meanwhile, a northwesterly MSLP gradient remains in place
over Utah behind the cold front which finally made it through the
entire area. Skies are currently mostly clear across a good portion
of the area except for some patches of stratus or fog across some
northern, western, and central valleys.

The upper low will slowly approach the area today, helped by an
embedded vort lobe currently diving along the Sierra Nevada and
which will swing across Utah this afternoon and evening. This will
help to reinforce the cold front across central Utah which will
serve as a focus for precipitation today. Temperatures down to -5C
or even -6C is forecast to spread into west-central Utah by this
evening. As a result, snow levels across central Utah could fall to
6000ft or lower tonight.

Meanwhile, as the low rotates into the area, moisture will wrap
around the low across northern Utah and back down through western
Utah. As a result, significant precipitation will be possible for
favored upslope regions such as the Bear River valley side of the
northern Wasatch mountains and the central Utah mountains along the
I-15 corridor, as well as southwest Wyoming and some western Utah
valleys. Although widespread precipitation is expected through the
weekend for all but the far southern and southeastern part of Utah,
the above areas are expected to see the longest duration of

Have issued a Winter Weather Advisory for the northern Wasatch and
central Utah mountains, areas with the highest confidence for
significant snow. Southwest Wyoming may eventually need to be added,
but higher snow levels initially are expected to limit accumulations
until late tonight. The Uintas and southern Wasatch could see
periods of heavy snow depending on the flow, but given the
uncertainty regarding duration and impacts, will let the day shift
consider further.

Cool and wet weather will continue through Sunday as wraparound
moisture continues to affect the area with the low slowly lifting
into northeast Utah. Precipitation will gradually wind down Sunday
evening as the low continues into Wyoming, although lingering
stratus could remain in place across northern Utah and southwest
Wyoming through the nighttime hours. For Monday, although Utah will
remain under a cool cyclonic flow aloft, there does not look to be
much in the way of dynamics to produce any significant weather at
this time.

.LONG TERM (After 12Z Tuesday)...Drier northerly flow aloft sets up
Tuesday as the closed low from earlier in the week fills and pulls
eastward. This will allow for warmer temperatures and generally
quiet weather conditions for Utah and southwest Wyoming. However,
the lingering baggy trough over the Intermountain West appears to
still have more tricks up its sleeve, as models depict a piece of
energy sliding down the backside of the trough and eventually
carving out another closed low over AZ by Wednesday. This low then
eventually tracks northeastward, perhaps impacting central and
southern Utah by late in the work week into the first part of the
weekend. Forecast certainty is limited in this timeframe as
evidenced by ensembles showing a wide spread in the amplitude and
track of the low. That said, nearly all of the GEFS members show a
closed low in vicinity, so it appears likely that such a feature
will develop, the question is how strong and where.

For now, have the kept the forecast largely unchanged for the latter
part of next week, which means a chance of showers and perhaps
thunderstorms from around the central mountains of Utah eastward.
Northern Utah would be more of a longshot to see showers out of this
disturbance, but it`s not out of the question. Otherwise, expect
temperatures to warm in general to just below normal for the middle
to end of the workweek for northern Utah, with warming less certain
over southern Utah due to the possible closed low in vicinity and
related precipitation chances.

Peeking further ahead, the GFS and ECMWF models are in good
agreement that West Coast ridging is on the way for at least the 7-
10 day forecast period, meaning warmer temperatures and quiet
weather are probable for next weekend and into the following week.


.AVIATION...VFR conditions will continue this morning at the SLC
terminal, with a 40% chance of occasional ceilings below 7000 feet
AGL through mid-morning. Thicker clouds will develop by late today,
with rain moving in by 0Z-3Z this evening along with cigs under
7,000 ft AGL, possibly reaching MVFR conditions. Periods of rain and
are expected tonight with possible MVFR cigs.


.FIRE WEATHER...An upper low will slowly cross the fire district
through the weekend. This will keep cooler temperatures in place
along with a moist and unsettled airmass. Widespread valley rain and
mountain snow will develop by early this evening with the bulk of
the precipitation expected across the northern and central Utah
mountains and western valleys through Sunday. Wetting rains will be
likely, and significant snow accumulations can be expected above
around 7000ft in these areas. Otherwise, there will be periods of
breezy northwest winds this afternoon and evening as a reinforcing
cold front crosses the area.

The storm will exit the area Sunday night with a warming and drying
trend expected for Monday and Tuesday. However, another upper low
developing across southern Nevada and Arizona midweek could start to
spread moisture back into the area from the south beginning
Wednesday or Thursday.


UT...Winter Weather Advisory from 6 PM this evening to midnight MDT
     Sunday night for UTZ007.

     Winter Weather Advisory from 6 PM this evening to 4 PM MDT
     Sunday for UTZ517.



Cheng/Van Cleave

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