Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 161547
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
947 AM MDT Tue May 16 2017

.SYNOPSIS...A cold Pacific storm system will push a strong cold
front through northern Utah tonight, then into southern Utah by
Wednesday morning. This system will then gradually settle into the
area through the remainder of the week.

&&

.DISCUSSION...Morning water vapor and H5 analysis depict a closed
low over NW Arizona, with a weak deformation axis extending
northeast into southwestern Wyoming. A deeper closed low over
Washington continues to encroach on the area from the NW, and will
be the focus of weather to come tonight through late week.

Scattered shower activity has broken out along the northern flank
of the deformation axis across northern Utah this morning, with
showers predominantly focused north of I-80 attm. As the
deformation axis slowly shifts east today coincident with
downstream progression of the parent low across Arizona, expect
the focus of this shower activity to pivot south and east settling
over the central/southern Wasatch front and points east.
Meanwhile, upper diffluence in advance of the Pacific closed low
will gradually nose into extreme NW Utah this afternoon increasing
the potential for scattered showers and isolated thunderstorms
there. Updated earlier to increase PoPs north of Salt lake City
and along the UT/ID border this morning to follow recent trends,
but the afternoon looks to be in good shape.

Regarding the approaching upper low from the NW, sensible weather
evolution over the next 24 hours is coming more into form as time
nears. Passage of the strong cold front tonight (in the north),
tomorrow morning (southwest), and afternoon (southeast) will
bring a focused band of heavy precip for 2-4 hours before waning.
Snow levels in the north and west could mix down very briefly to
lower valley floors around dawn Wednesday, but expecting bulk of
precip to fall as rain overnight. Accumulating snow is likely to
remain above 6000ft (with the front), but can`t rule out a spotty
dusting down to higher bench levels of the central/northern
Wasatch Front if local intensities drive stronger diabatic
cooling for a brief time. Strong subsidence in wake of fropa
coupled with a mid level dry slot intrusion into the parent low
will likely limit precip potential in the coldest part of the
airmass Wednesday morning, this until the cold core upper
circulation drops south Wednesday afternoon. Much more detail on
storm evolution thereafter exists in the previous discussion
below if curious.

Strong post frontal winds remain a potential across the desert NW
and potentially Utah Valley early Wednesday morning, aided by the
aforementioned post frontal subsidence/dry slot. Currently have a
High Wind Watch for the NW desert, and will be addressing any
upgrade by the afternoon package.

The Winter Weather Advisory in effect for the mountains looks on
track, though the bulk of accumulating snow should come from the
frontal passage. Snow is still expected to impact the morning
commute over the mountain passes of northern Utah Wednesday
morning.

PREVIOUS DISCUSSION...A much stronger surface front will surge
through northern Utah overnight, coupled with strong diffluence
and jet support, should bring a transitory band of heavy
precipitation to the area after midnight, before settling into
central and southwest Utah by sunrise Wednesday. Should see a
burst of valley rain and heavy mountain snow with this band, and
given precip intensities may see snow mix down to the valley
floors late, although any accumulation would likely be confined to
areas above 5000 feet (and likely above 6000 feet). Given the
tight frontal circulation and strong drying in the models in the
wake of this feature, don`t expect heavy precipitation to linger
much more than 2-3 hours in any one location.

Once the frontal band passes into southern Utah and weakens
Wednesday morning, will have to wait for the upper low to slowly
settle in across the forecast area, which will occur Wednesday
afternoon through Wednesday night and into Thursday. With the
forecast track advertised in the 00z suite of models, it appears
organized precipitation will likely remain confined to areas west
of I-15 Wednesday into Thursday, while the remainder of the area
likely relies on convective precip owing to surface heating
underneath a core of -25C air at 500mb.

Primary change this forecast package is the issuance of a High
Wind Watch for late tonight into Wednesday morning for post
frontal winds across northwest Utah. May eventually need wind
headlines for downslope areas of northern Utah County as well.
Left the going Winter Weather Advisory in effect, which looks in
good shape at this time.

Given the cold temperatures aloft with this system, freezing
temperatures across the valleys will also be a concern Wednesday
and Thursday nights. However, given the cyclonic flow and
anticipated cloud cover, it appears at this time most valleys will
escape a hard freeze with the exception of the Wasatch Back and
perhaps a few outlying western Valleys. At this time have held off
on any sort of Hard Freeze products given uncertainty with cloud
cover.

&&

.AVIATION...South winds are expected to remain in place at the SLC
terminal this morning prior to shifting to the west- northwest
this afternoon between 19-21z. This said, there is a 30 percent
chance that they remain southerly. CIGS are expected to remain
above 7000ft this morning and may even scatter out briefly late
this morning into the early afternoon hours. A few showers may
develop near the terminal this afternoon which could then bring
CIGS near or below 7000ft and erratic winds. Otherwise, showers
are expected to become a bit more prevalent after 02-03z and then
widespread after 07z with possible MVFR or lower conditions and
gusty northwest winds as a strong cold front crosses the area.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Winter Weather Advisory from midnight tonight to midnight MDT
     Wednesday night for UTZ007>010-517.

     High Wind Watch from late tonight through Wednesday morning for
     UTZ005.

WY...None.
&&

$$

Merrill/Seaman

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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