Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 202150
Area Forecast Discussion
National Weather Service Salt Lake City UT
350 PM MDT Tue Sep 20 2016
.SYNOPSIS...A weather disturbance will bring moisture into far
southern and eastern Utah tonight. A stronger and colder storm
system will impact most of the region Wednesday through the end of
.DISCUSSION...Mid and high level moisture continues to stream
across about the sern half of the CWA this aftn as the low off
srn CA draws it northward from the dying tropical storm Paine.
A cold trof continues deepening over the PacNW and the increasing
swly flow ahead of this feature will draw the CA low newd across
srn UT late tonight into Wed as a weakening open wave. The
remains of the tropical storm should not be a factor for our CWA
other than the moisture it will supply.
The low levels have been starting to moisten up this aftn across
srn UT and light rain has been reported at Bryce Canyon NP and at
Page, AZ. The rain should stay mostly east of a line from around
Kanab to Hanksville and will taper off during the night. Most of
this will be stratiform but there is a potential for some
convective precip tonight along the northern fringes of the
stratiform rain as the remains of the CA low approach srn UT late
The PacNW trof is moisture starved at this time but it is forecast
to deepen far enough west for it to entrain moisture from the srn
branch features. This starts late tonight as the CA low opens up
and slides into srn UT with moisture spreading nwd thru the day
Wed. Expect more convective precip to develop on Wed as the trof
dynamics begin to come into play increasing Wed night.
The heaviest precip looks to occur Wed night and Thu in the warm
sector as deep moisture arrives under increasing dynamic lift. The
frontal boundary enters nwrn UT late Wed night but does not
advance much only reaching about an Evanston to Delta line by the
end of the day Thu. However precip will remain active along and
west of this front as well.
Temps will stay warm again tomorrow altho we will shave a few
degrees from todays highs due to the increased clouds and showers.
Temps will remain elevated Wed night in the pre frontal
environment and not rise much Thu with the widespread rain.
In summary, be prepared for major changes in the weather coming this
week, and keep an eye on latest forecasts as details become clearer
in the coming days.
.LONG TERM (after 00Z Friday)...A significant early-season winter-
like storm is expected to continue impacting the area Thursday
evening and into first part of the weekend. 12Z model cycles are in
good agreement now with the track of a potent closed low swinging
through the Great Basin, as well as pulling a subtropical moisture
tap into our area in advance. Model anomalies indicate that moisture
transport Thursday into early Friday will be unusually strong, on
the order of a 1-in-10-year event. Meanwhile, a surface cold front
late Thursday and cold air-advection aloft associated with the core
of the upper low will combine to quickly lower snow levels Thursday
night to as low as around 7000 feet. Additionally, locally windy
conditions are likely at times as strong southerly winds aloft move
into southern Utah, and gusty northwest winds develop along the cold
front over western and northwestern Utah.
Current model consensus brings the cold core of the low through
central Utah around noon Friday, with 700MB temps around -3C. While
a dry slot moves into southern Utah around this time and decrease
precipitation chances, ample wrap-around precip looks to slide into
northern Utah. This strengthens later Friday as a backside jet and
associated vortmax suggest plenty of synoptic lift in the already-
moist environment over northern Utah. Temperatures will be
drastically colder Friday, with highs 15-20 degrees lower than
Thursday (and around 25 degrees below normal).
Potential impacts Thursday night into Friday for northern Utah: raw
conditions are expected for those with outdoor plans, particularly
those at higher elevations such as hikers, hunters, etc. Snow levels
are expected to lower around 7,000`, with significant snow
accumulations likely over 8,000`. This could impact seasonal passes
such as Monte Cristo Pass. For central and southern Utah, much
colder temperatures will be the main impact. However, there is still
some uncertainty as to how far south wrap-around precip will reach;
some models do indicate the wrap around could affect central Utah
higher elevations with the same raw conditions.
Wrap-around precip will likely continue into the first part of
Saturday for the mountains of northern Utah, especially the Uintas.
Precipitation eventually wanes for all areas into Sunday, with the
GFS as the outlier holding onto to mountain showers the longest
(towards mid-day Sunday). Meanwhile, a tightening north-south
pressure gradient over southern Utah could lead to strong winds in
that area Saturday and Sunday.
Forecast confidence wanes going into next week, as models struggle
with the evolution of the upper low as it shifts east of us. There
is some hint that a general trough could linger into the
Intermountain West, with the Canadian suggesting the closed low
could retrograde back into Utah. For now, have gone with a
climatological forecast of near-average temperatures and slight
chances in the mountains given the uncertainty.
.AVIATION...Winds at the SLC terminal are expected to switch back
to the southeast around 02-03Z, and will likely be gusty at
times overnight. Otherwise, VFR conditions with ceilings well
above 7000 feet will prevail through at least 12Z Wednesday.
.FIRE WEATHER...Moisture spreading into southern Utah has brought
rain to the far south and this will continue over southeastern
Utah tonight. Gusty south winds have developed over much of the
area. These winds will increase across the north tonight where
a dry airmass remains in place potentially keeping active burning
conditions going on current fires overnight. The winds will remain
gusty across most areas on Wednesday but RH will be rising. Rain
showers will increase in coverage on Wednesday as a cold Pacific
storm approaches the area with rain becoming widespread Wednesday
night into Thursday as the storm settles into the Great Basin.
Gusty south winds will increase on Thursday as well. Rain showers
taper off across southern Utah Thursday night into Friday but will
continue across the north through Saturday. The snow levels will
lower to around 7000 feet in the showers Friday and Saturday but
significant accumulation looks to be limited to elevations around
8000 feet and higher. Wetting rain is likely in most areas at some
time between Wednesday night and late Saturday and this could be a
season ending event at the higher elevations.
LONG TERM...Van Cleave
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