Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 201202
Area Forecast Discussion
National Weather Service Salt Lake City UT
502 AM MST Mon Feb 20 2017
.SYNOPSIS...A mild southwest flow will develop over Utah today and
continue through Tuesday. A cold front Tuesday night will usher in
a period of cold unsettled weather through the end of the week.
.SHORT TERM (Thru 12Z Thursday)...A short wave ridge axis is
overhead this morning with a mild and very moist southwest flow
upstream extending from central NV off the CA coast. A weak warm
front is crossing nrn UT at this time with a few showers on radar.
Expect this moisture to increase across our CWA today as a short
wave ejects into the nrn Rockies but do not expect any heavy
precip out of it as it lacks organization and a lot of precip will
have already fallen out across the mtns of CA and NV. However the
mild airmass will keep temps up well above normal and this
combined with some rain will exacerbate the snow melt flooding
already occurring across far nrn UT. Will leave the current Flood
Advisory in effect.
The main ern Pac trof begins to nose inland Mon night and it
gradually sags into the Great Basin on Tue. The associated cold
front moves into nrn UT Tue aftn then stalls across central UT Tue
night into Wed. Expect a band of precip to accompany the front
with snow levels lowering to the valley floors by Wed morning.
Strong south winds will develop across the CWA ahead of this cold
front Tue aftn with speeds possibly reaching advisory levels in
the wrn valleys.
The main upper trof axis moves into the Great Basin on Wed with
increasing dynamics over UT. This should make precip with the
front more active Wed aftn with it becoming more widespread Wed
night. Med range models disagree somewhat on just where this will
occur so have kept pops fairly high across much of the CWA.
Significant snow amounts are likely in the nrn mtns starting Tue
night shifting into central UT Wed. Altho snow is expected in the
valleys Tue night into Wed, the threat of travel impacts looks
small during this time. The snow will have a greater impact Wed
night at all elevations but the uncertainty on the location
prevents issuing any highlights at this time.
.LONG TERM (After 12Z Thursday)... The extended period starts off
with a cold 500mb trough transitioning southeastward across the
CWA. The fairly moist northwest flow with/behind the trough along
with the unstable airmass of 18-20C spread between 700mb and
500mb will produce snow across mainly the northern and central
mountains and in the valleys along the Wasatch Front Thursday. The
coldest 500mb air across southern Utah will occur Thursday but
the moisture is not as plentiful there so scattered snow showers
due to orographics are more likely. Both the GFS and EC show a
weak shortwave ridge building in across the southwest Thursday
night while instability snow showers expected to continue across
the central and northern portions of the CWA.
By Friday modest warm advection develops over Nevada and spreads
eastward across northern Utah and southwest Wyoming according to
the GFS but this appears to be somewhat of an outlier as the
trend by the GFS itself over the past three runs has been backing
off on the strength/depth of the shortwave dropping down the
central West coast which was causing the stronger backing winds
across Utah. Therefore leaned toward the EC solution of minor warm
advection with more of a westerly flow developing over Nevada due
to the shortwave stretching farther west off the coast and
keeping a broader zonal flow in across the Great Basin.
By Saturday this trough enters the western Great Basin and moves
southeast towards southern Nevada. The 700mb convergence and
300mb diffluence begin to develop late Saturday over northern Utah
as this trough approaches but the best dynamics move into
southwest utah Saturday evening according to the EC with the GFS
about 6 hrs slower. Have increased the PoPs Saturday night over
southwest Utah and kept some PoPs into Sunday in case the GFS
slower solution pans out. A shortwave ridge is expected to develop
behind this trough but timing is somewhat questionable so only
went with one period of generally dry conditions for the CWA and
then leaned toward climo PoPs for Monday but increased them by
Monday night as the active pattern brings another trough into the
region. Temperatures should remain below average through the
.AVIATION...South winds will increase at the SLC terminal through
the mid morning and afternoon time frame with gusts over 30 kts
expected. Showers are expected to develop late this afternoon with
only a 30 percent chance of the cigs lowering below 7Kft AGL
between 00z and 07Z.
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