Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 181158

Area Forecast Discussion
National Weather Service Salt Lake City UT
458 AM MST Sat Feb 18 2017

.SYNOPSIS...The upper level trough along the California coast
will work slowly east across the desert southwest and Great Basin
through the weekend. High pressure aloft will return early next
week, followed by a new and rather cold storm system arriving


.SHORT TERM (Until 00Z Wednesday)...The upper trough currently
along the west coast will continue to split as it moves slowly
east this weekend. The northern split will lift northeast and
have little or no impact northern Utah. The southern split will be
far more energetic with strong vorticity and thermal advection
across southern California today, then extending into western
Arizona this afternoon and evening.

Utah, especially southern Utah, sits under an exceptionally moist
southerly flow. What is lacking is any real substantial dynamic
and/or thermally-driven synoptic-scale lift. Precip so far is
light with little organization outside of the extreme south where
terrain upglide and weak diffluence aloft are supporting steady
light to locally moderate precip.

Anticipating a gradual increase in areal coverage of valley rain
and high elevation snow today, with the focus still on the
southern portion of the state. By tonight increasing thermal
advection ahead of the advancing trough will likely generate
better organized and somewhat heavier precip across extreme
southern through east-central Utah late tonight/early Sunday.
Snow levels will be lowering a bit late tonight, though
accumulating snow will still be confined to southern mountains
with limit impacts on travel outside of the highest passes.

A fast-moving shortwave ejecting inland through the Pacific
Northwest early Sunday will cut across the northern Rockies and
northern Great Basin Sunday afternoon. This feature will turn the
flow westerly across the north Sunday afternoon with deep layer
cold advection setting up for the afternoon and evening hours.
This cold advection along with dynamic lift from the shortwave
and the exceptionally moist air mass will generate widespread
valley rain/mountain snow across northern and portions of central
Utah. Significant accumulations of snow are possible for Wasatch
range which may lead to winter weather highlights in later
forecasts. Snow levels will lower enough that snow or a rain/snow
mix will develop in the valleys. Any accumulations look to be
minimal with little or no impacts on travel in the lower valleys.

Building heights will end most if not all precip across Utah
Monday. This break looks to be short-lived as heights lower again
ahead of the next cold trough deepening over eastern Pacific. A
near 700 mb baroclinic zone will sag into extreme northwest Utah
Tuesday. This boundary will serve as a focus for precip generated
by shortwaves ejecting out ahead of the eastern Pacific trough
by Tuesday afternoon.

.LONG TERM (After 00z Wednesday)...The long wave pattern across the
western CONUS will reload early next week as the next trough begins
an inland trajectory. Said trough will begin to impact the forecast
area Wednesday packing deep moisture and modest forcing.

Amplification of the trough Wednesday will aid to tighten mid level
baroclinicity over the north, surging a cold front south across the
area during the day. Expect precip to be most tied to this boundary,
and the post frontal environment through axis passage Wednesday
night. Net drying/decreased forcing post axis passage will limit
precip potential outside of orographically favored areas (NW flow)
Thursday. As per norm subtle differences in detail exist in globals,
but the big picture trough evolution is quite similar. Opted to
boost PoPs a bit during this period, though previous package handled
quite well for now. At this time, breadth of cold advection looks
suitable for at least of mix of snow to lower valley areas across
central/northern Utah in the post frontal environment, if not a full
changeover to snow. Do not foresee significant lower elevation
accumulation potential however.

Thereafter a mean longwave trough will settle over the northern
Rockies/Great Basin region late week maintaining cooler than normal
temps. This said, a relatively dry continental airmass looks to
limit precip potential significantly.


.AVIATION...Southerly winds, increasingly gusty by mid morning, will
persist at the KSLC terminal through the valid TAF period. Cigs are
expected to remain above 6000ft AGL, though mountain obscuration
remains likely at times this afternoon.


.HYDROLOGY...A persistent and quite moist southerly flow into
southern Utah will maintain an extended period of light to locally
moderate precip into southern Utah through early Sunday. The
heaviest precip will likely concentrate in the upslope zones in
Washington and Kane counties.

Fairly high freezing levels will keep precip as rain below roughly
7000 feet into early Sunday. This persistent steady rain will lead
to increasing flows in the Virgin river drainage along the
numerous slot canyons and dry washes in eastern Washington and
western Kane counties. At this point it appears that elevated
flows will occur, but the flash flooding threat appears minimal.
The caveat would be if rainfall rates come in a bit higher than
expected. For now will hold off on a flash flood watch, but
monitor closely the flood prone areas in and around Zion NP, the
Virgin river drainage and western Kane county.





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