Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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948
FXUS65 KSLC 121549
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
949 AM MDT Thu Oct 12 2017

.SYNOPSIS...A dry cold front will cross northern and western Utah
today then stall. The front will retreat north on Friday then
push south again Friday night and Saturday as a colder storm
system crosses the north.

&&

.DISCUSSION...
Water Vapor Satellite shows a trough over the western CONUS, with
one wave lifting northeast into Montana as the next one drops
southward along the Pacific Northwest coast. 400-200mb MDCARS
wind observations place a 110-140kt cyclonic jet from central
California across northern Utah and the northern Rockies.
GOES/SLC 12Z RAOB/HRRR indicate that precipitable water values
vary from 0.10"-0.20" mountains to 0.25"-0.50" valleys.

Progression of the cold front, currently near Cedar City to Price,
will slow with the upper level support lifting into Montana. The
mid level shear axis is the back-edge delineation for mid level
clouds, as well as the mechanism for bringing in some of the smoke
aloft from the California wildfires. Early morning gusty
northwesterlies have subsided with high pressure building in
across northern Utah.

In between waves, height rises are expected beginning this
afternoon. Going forecast is in great shape. Did lower
temperatures a few degrees across the north post-frontal and tweak
sky cover based on latest satellite imagery. Otherwise updated
the hourly diurnal curves based on latest observation and
guidance.

&&

.PREVIOUS DISCUSSION...
A broad cold upper trof extends from the nrn Plains to the PacNW
states early this morning. This system is moisture starved and the
associated cold front is dry as it crosses nrn UT with just a
band of mid and high clouds.

The front will stall across the south this afternoon with cooler
temps and lighter winds north of the front while it stays breezy
and mild south of it. The clouds across the north should move out
of the CWA by afternoon.

The broad upper trof remains in place thru Fri with a colder short
wave dropping out of wrn Canada into the PacNW. The flow over UT
backs somewhat Fri and increases as the trof deepens to our west.
This sends the front back northward Fri aftn with the winds aloft
surfacing in most areas, and warmer temps.

The colder short wave rotates thru the base of the trof and across
nrn UT late Fri thru early Sat sending the cold front back south
again and bringing scattered showers to the north. The airmass
with this wave will be cold enough for snow to the valley floors
but do not expect much if any valley accumulation. The mountains
could pick up a couple inches, mainly north of I-80.

The short wave moves quickly east Sat morning with drying in the
afternoon and temps aloft already starting to warm back up. Sat
will be a cool day however as the warming aloft is not expected
to mix down. Sat night could be chilly in most areas in spite of
the warming aloft due to the low level cold air in place and the
expected clear calm conditions. We are forecasting the first
freeze at SLC Sun morning that could finish off any gardens in the
Salt Lake valley that have not yet frozen.

Rising heights, a warming airmass and clear skies will bring
temps back up on Sun altho warming will be sluggish due to poor
mixing.

Tranquil weather is expected across the forecast area for the
majority of the long term period, as a broad mid level ridge
becomes established across the region Monday through Tuesday. This
will yield a warming trend with temperatures rebounding to near
climo across the north by Tuesday, and slightly above climo across
the south.

The medium range models are in agreement weakening this ridge on
Wednesday and allowing a predominantly zonal flow to become
established across the eastern Pacific/western CONUS. However
differences between the ECMWF and GFS become obvious later
Wednesday into Thursday, as the GFS is much more aggressive
punching a strong Pacific jet into the Pacific Northwest/northern
Great Basin, potentially bringing precipitation to northern Utah
late Wednesday night into Thursday. Meanwhile the ECMWF is
considerably slower and more amplified with this feature, which
would yield a drier and more mild Thursday. For now have leaned on
the EC and kept the forecast dry through day 7.

&&

.AVIATION...
Northwest winds with VFR conditions will prevail at KSLC through
the day today.

&&

.FIRE WEATHER...
A dry cold front is pushing through northern Utah early this
morning and will cross the northern and western portions of the
district today before it stalls out over the south. This front
will bring cooler temperatures, higher humidity and lower clearing
indices in the stable airmass behind the front. A westerly flow
over the area Friday will bring stronger transport winds. The
front will retreat north allowing the winds to surface in the
afternoon and raising the clearing index.

A stronger cold front will sweep south Friday night into early
Saturday with a chance of snow showers over the north while the
front remains dry across the rest of the area. In the wake of
this system a dry westerly flow will bring a warming trend Sunday
through the middle of next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

$$

PUBLIC...Rogowski/Wilensky/Seaman
AVIATION...Kruse
FIRE WEATHER...Wilensky

For more information from NOAA`s National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php



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