Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 141003

Area Forecast Discussion
National Weather Service Salt Lake City UT
403 AM MDT Wed Mar 14 2018

.SYNOPSIS...A cold storm system will move into the area for the
latter half of the week and upcoming weekend. This storm will
bring a threat of windy conditions and a few thunderstorms today,
followed by cooler temperatures, lowering snow levels, and showery
periods into the weekend.


.SHORT TERM...An active weather pattern is in store for the area
through the weekend, with multiple weather hazards possible. A
longwave trough axis will gradually work its way inland through
Sunday, with multiple shortwave troughs swinging through the Great
Basin and Intermountain West (3 main trough passages to be
specific). Each trough is expected to be incrementally colder,
bringing temperatures back to normal and then below normal by the
weekend. Friday should be the one break in the action for most of
the area, though precipitation with the next system may begin as
soon as Friday afternoon over the higher terrain of northern Utah.
Here is a breakdown by hazard type:

Spring is upon us, or at least according to the meteorological
setup today. A negatively-tilted shortwave trough is expected to
slide through our area by the afternoon, following by a weakening
mid- level baroclinic zone nudging into the NW corner of Utah.
Strong mid and high-level winds will provide plenty of shear,
while still-warm temps and increasing moisture in the boundary
layer will add some instability to profiles across much of the
area. The SPC has placed an axis of marginal risk across the
center of Utah, extending into southwest Wyoming, for a risk of
winds and hail. Taking a look at BUFR soundings and various
convective fields, it currently appears the best chance would be
this afternoon in a corridor from around Dugway to Utah County.
Cloud bases should be low, along with relatively-shallow
instability given the early spring setup. Straight-line hodographs
support multicell convection with a few stronger cells. Shear
looks to be too strong in comparison to instability to support
anything particularly organized however. The one possibility for
a more organized line would be far NW Utah where the mid-level
baroclinic zone reaches to around the Raft River Range, but that
is outside of the axis of best instability. Current thinking is
hazards would be gusty winds and small hail. 12Z models arriving
later this morning should give us a better idea.

An axis of elevated 700MB winds is progged to slide into western
Utah by mid-morning, then shift eastward tonight. Widespread gusty
winds and hazardous cross-winds for E-W routes are expected for
west and southwest Utah deserts, where the winds aloft line up
well with surface pressure gradients. For the remainder of the
I-15 corridor and northwest Utah, winds will be breezy, but likely
not as consistently gusty. However, the aforementioned
thunderstorm threat could help mix down stronger winds aloft in
those areas, so the current wind advisory is merited there as

Winter Weather...
As mentioned, each trough should be progressively colder. 700MB
temps are expected to fall to around -5C tonight, sink further to
the -8 to -10 range Thursday night, then finally fall below -10
across the entire area Saturday night. As such, not expecting
winter weather impacts for valley locations until perhaps the
weekend trough (snow levels do approach valley floors Friday
morning, but that is around the same time the atmosphere is drying
out). In the mountains, the best chance of road snow would be
Thursday night. At this point, we don`t have enough confidence in
specifics to highlight anything, but we`ll need to keep this
hazard in mind, especially for mountain locations favored by
southwest flow.

Temperature swings will not be as wild as they can be this time of
year since we`re not looking at an organized cold front passage.
Instead, temperatures will approximately stair-step their way
down with each trough passage, finally bringing widespread
freezing temps again by the weekend.

.LONG TERM (After 00z Saturday)...Global models feature a trough of
low pressure, centered over northern California, at the start of the
period. The core of this through will slide eastward, crossing
the Great Basin Saturday and Saturday night, ultimately sliding a
cold front, though weakly defined, across the forecast area. A
strong prefrontal southerly flow will advect enough moisture in
advance of this system to support chances for precipitation Friday
night and Saturday morning, especially across northwest Utah and
in areas where precipitation is favored by southwesterly upslope.
Snow levels should remain above the valley floor Friday night and
Saturday morning. The bulk of precipitation with this system will
occur Saturday afternoon and night as the 700 millibar baroclinic
zone pushes through the forecast area. Although the system is only
at best weakly frontogenetic, enough upper level support and
moisture will be present to support a period of moderate
precipitation along and behind the boundary, particularly in areas
favored by westerly upslope flow. With exception of Dixie, a quick
transition from rain to snow will take place, as 700 millibar
temperatures fall to minus 10 degrees Celsius, perhaps even a bit
cooler, as progged by ECMWF.

With a somewhat moist and unstable air mass in place on Sunday,
chance probabilities for snow look good for areas favored by west to
northwest upslope. Temperatures will be fairly cool on Sunday,
with maxes running about 10 degrees below normal over much of
forecast area.

ECMWF, and GFS too some degree, hint at a weak upper level
disturbance scraping the northern portions of forecast area on
Monday, so existing slight chance for snow across mountains of
northern sections of forecast area look good.

Should see generally dry conditions and a warm trending on Tuesday
as low-amplitude ridge builds over region.

GFS and ECMWF indicate a series of upper level disturbances will
eject out of an eastern Pacific trough, with initial disturbances
impacting the region Tuesday night or Wednesday. An impressive
moisture tap is indicated with this system, with precipitable
water values increasing to near three quarters of an inch by
Wednesday evening. Giving timing differences, will indicate chance
wording for now.


.AVIATION...VFR conditions will generally prevail at KSLC through the
valid TAF period, though a period of MVFR conditions will be
possible with showers this afternoon and evening.  There is a small
chance for a thunderstorm to impact KSLC as well.  Southerly winds
will increase significantly as the morning progresses, then continue
through the afternoon hours, with a possible switch to a northwest
direction this evening.


UT...Wind Advisory from 9 AM this morning to 9 PM MDT this evening
     for UTZ015-016.

     Wind Advisory from 9 AM this morning to 6 PM MDT this evening
     for UTZ003>005.



Van Cleave/Barjenbruch

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