Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 142258
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
358 PM MST Tue Nov 14 2017

.SYNOPSIS...A weak ridge will build briefly over the area tonight
into early Wednesday. A stronger and colder storm system will
cross the region Thursday into Friday.

&&

.SHORT TERM (Thru 12Z Friday)...A low amplitude upper trof
extends from the nrn Plains to wrn WY this aftn with the
associated cold front pushing thru srn UT. Other than lingering
flat cumulus over the far nrn CWA, skies have cleared this aftn.
The cold front pushes thru the rest of the CWA by this eve, as a
weak dry front.

A low amplitude short wave ridge crosses the CWA early tonight and
is along the ern border by 12z Wed. High clouds will start to
increase tonight in the wake of the ridge and are expected to
thicken and lower thru the day Wed as southwest flow picks up.

The airmass warms tomorrow and expect the southwest flow to get
strong enough by aftn to at least partially mix the valleys
sending temps several degrees warmer than today. However the high
clouds are forecast to get fairly thick by aftn and this will
probably hold maxes down somewhat.

The southwest flow is developing ahead of a deep cold trof
starting to move into the PacNW and swrn Canada on Wed and
continues to increase thru Thu as the trof moves inland.

This trof is forecast to weaken and become more positively tilted
as it moves inland but it is advertised to have a tap into
tropical moisture for a time starting tomorrow. This is what
produces the high clouds Wed and deepens enough on Thu for precip
to develop. This occurs on the warm side of the trof so snow
levels will be quite high, 9000 ft or higher, and precip will be
tied to orographics due to the strong swly flow. Temps Thu stay
mild, and could be even warmer than Wed due to thorough mixing
from gusty southerly winds.

The cold front associated with this trof starts to move into the
CWA thu night accompanied by widespread precip. Timing
differences between the GFS and EC have gotten a little less with
the new 18z GFS slowing down and trending towards the slower EC.
However, they are still great enough between the models to make
the difference between snow reaching the valley floors across the
far nrn CWA by 12z Fri, and not. Have trended the forecast towards
the EC and kept the mention of valley snow out of the forecast
thru 12z Fri.

.LONG TERM (After 12Z Friday)... There are still some
discrepancies among guidance regarding the timing of the frontal
passage on Friday. The GFS is the fastest solution placing the
front over Central Utah by 12Z Friday and pushing it through the
remainder of the state by Friday afternoon. The EC is about 6
hours slower and also keeps the upper level trough and associated
dynamics more intact over Utah keeping the front more energetic
from a precipitation standpoint. The Canadian is similar to the
GFS regarding the timing of the initial frontal push through
northern Utah but ultimately slows down to come more in line with
the EC later in the day. These discrepancies in frontal passage
timing make it difficult to time when snow levels would drop to
valley floors and consequently whether or not northern valleys
would see any accumulations. At this time went with a blend
slightly weighted towards the slower EC for PoPs and snow levels.
The current forecast would imply that although most valleys will
probably see snow at some point on Friday, any valley
accumulations/impacts would be limited due to the short window
when precipitation is possible and temperatures are cold enough to
support snow.

The aforementioned timing discrepancies also make forecasting the
exact end of precipitation difficult although once the trough axis
passes and heights build in its wake (Friday afternoon in the
GFS, and Friday night in the EC) precip should taper off quite
quickly. Saturday morning will be on the cool side across most of
the state and given clearing skies and a cold airmass went on the
low side of guidance for overnight mins. Temps begin to warm aloft
Saturday/Sunday with shortwave ridging building in, however,
valley inversions will likely limit maxes to near or just below
climo in northern/central valleys. Mixing and associated valley
warming improve slightly on Monday as southwesterly flow increases
ahead of the next storm system. Guidance is currently in
disagreement on the amplitude of this system (the EC being more
amplified at this time), consequently increased PoPs a bit on
Tuesday but made little other changes to previous forecast due to
higher than normal uncertainty.

&&

.AVIATION...Conditions at the KSLC terminal are expected to remain
VFR through the TAF period. Northwest winds will persist through
the day with a shift to the southeast expected by 03z this
evening.

&&

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

$$

SHORT TERM...Wilensky
LONG TERM...Carr
AVIATION...Dewey

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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