Area Forecast Discussion
Issued by NWS Salt Lake City, UT
FXUS65 KSLC 162300
Area Forecast Discussion
National Weather Service Salt Lake City UT
400 PM MST Mon Jan 16 2017
.SYNOPSIS...High pressure aloft will settle over the Great Basin
for the first half of the week. A return to an active weather
pattern will occur late in the week.
.SHORT TERM (Through 00Z Friday)...High pressure is building over
the Great Basin this afternoon, as the trough over the desert
southwest continues to slowly exit the region. A northeasterly
gradient is in place, which is producing some gusty winds in far
southwest Utah, though these winds should generally decrease
through the night. Low level inversion conditions continue to
strengthen across Utah, with stratus lingering over several
valleys across western Utah.
The short-term fate of this stratus is low confidence. Some of
this stratus has dissipated through the daytime hours, including
over parts of the Wasatch Front. For now, the forecast has the
idea that the stratus may fill in again across portions of west
central Utah and the Salt Lake/Utah Valleys overnight, but not
expand much more beyond this area. Have continued the mention of
flurries under the stratus deck, and have also kept fog in the
forecast tonight for many valleys across northern Utah that do not
currently have cloud cover. Similarly, the minimum temperature
forecast is lower confidence than usual. Have lowered some
temperatures in places that are currently without cloud cover,
but some of these temps may be too cold if stratus fills back in
faster or more thoroughly than expected.
The ridge will continue to build through much of Tuesday and
Wednesday, leading to warming temperatures in the mountains,
strengthening low level inversions, and deteriorating air quality
in many valleys. Improved mixing is not expected until Wednesday
night or Thursday, when the next storm system pushes into the
This next storm is somewhat warm, with snow levels likely
settling around 4500-5500 feet once conditions are fully mixed.
Models have generally slowed down this storm, with the best
precipitation across the CWA currently expected late morning
through afternoon Thursday. There is some concern that the best
forcing is trending further south into Arizona, but there looks to
be enough instability to allow for at least some light
precipitation across much of the area Thursday.
.LONG TERM (After 00Z Friday)...The global models are in sync with
the storm system which is acting like the sacrificial lamb in
breaking down the ridge. What initially had good dynamics is
weakening as it bumps into the ridge. The EC and GFS both have a
700mb wind shift moving across northern Utah Thursday evening while
a jet is overhead so have increased PoPs to likely across the
Wasatch Front and likely/categorical over the northern mountains.
Farther south the dynamics are weaker so am not as bullish with PoPs
The wave length between this shortwave and the incoming stronger
system aiming for the Desert Southwest is rather short and does not
allow for much drying across the north. This second system is fully
loaded with moisture and will spread into southwest Utah late Friday
afternoon, although the latest 18Z GFS shows it arriving a little
later. Kept the PoPs the same over the south but decreased them a
little across the northern portion of the CWA. The negatively tilted
700mb trough axis marches northeast across Utah Friday night into
early Saturday which should bring the heaviest precip to southwest
Utah under the prefrontal southwest flow. However, significant
precipitation is possible farther north with the passage of the
trough axis and the flow becoming northwest favoring the west facing
slopes of the central and northern mountains Saturday. Nudged PoPs
up a little from previous forecast.
A sharp shortwave ridge is forecast by both the GFS and EC
operational runs to move in across the CWA late Saturday night
through Sunday. Confidence was boosted that we will see a dry 24-
30hr period across the CWA when taking a look at the GFS ensemble
mean which indicated there was less than a 30 meter 500mb height
difference between members at 18Z Sunday indicating very good
agreement between members.
Moisture spreads back in quickly Sunday night through Monday as the
next trough comes ripping into the western CONUS. Have gone with
well above climo PoPs Sunday night through Monday night with
slightly below normal temperatures.
.AVIATION...VFR conditions will prevail at the SLC terminal
through about 04-05z. MVFR vsbys in haze are expected to develop
after that with MVFR cigs expected to move back in after midnight.
There is a chance these cigs may not develop with a 10 percent
chance of LIFR in fog if they do not. Northerly winds will likely
shift to light southeasterly around 04-05z but there is a 40 percent
chance of remaining northerly through the TAF period.
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