Scientific Forecaster Discussion
fxus65 kslc 132308
Area forecast discussion
National Weather Service Salt Lake City Utah
408 PM MST Wed Dec 13 2017
Synopsis...the strong and persistent upper ridge will retreat
off the West Coast over the next couple of days, allowing weather
disturbances to begin eroding the strong valley inversions, with
the strongest of these storms expected on Saturday.
Short term (through 00z sunday)...a shortwave trough is dropping
into Utah and southwest Wyoming from the north this afternoon,
bringing increased cloud cover and some snow showers. Most of
this precipitation is not reaching the valley floors, but light
accumulating snowfall is possible in the higher terrain. The best
moisture and instability with this disturbance is expected to
remain to the east of the forecast area as the wave passes
Though this system has begun to weaken low level inversions
across northern Utah, it should not be strong enough to really mix
the valleys. Many locations have had some dissipation of fog and
stratus through today, but it is likely that some of the low level
clouds re-form after midnight.
The other primary expected impact of this shortwave trough and
the associated cold front is increased winds across the southern
half of Utah, especially in north-northwesterly downslope and
canyon areas. Have increased winds tonight across Castle country,
and even moreso in Washington County. Tonight looks like a
particularly good set-up for north-northeasterly Washington County
gap winds, with a 7 mb mslp gradient from Cedar City to Las
Vegas, 700mb flow of 40-50 knots, and good cold advection. As
such, have issued a High Wind Warning on this shift, with winds
expected to peak during the early morning hours Thursday.
After the system exits to the south, high pressure re-builds over
the region through Thursday and Friday, with inversions
strengthening again. The next shortwave trough is still forecast
to push into the intermountain west on Friday night and Saturday,
though there is some uncertainty about how much the storm will
split before it reaches Utah. While a more splitty system would be
less efficient at scouring out the valleys, even the less
aggressive GFS solution would still dissipate the inversion fairly
effectively, with increased cloud cover, a chance of snowfall,
and 700mb temperatures eventually falling to about -12 celsius by
Saturday evening. Hopefully the models will return to better
agreement about the details soon, but at least some light
accumulation of snow is likely in the northern mountains, with a
chance of showers in the valleys as well through the day Saturday.
Long term (after 00z sunday)...regardless of the amplitude of the
trough overhead heading into Saturday night, models continue to
remain consistent in downstream propagation and decreasing precip
potential (very light snowfall) into Sunday morning.
In wake of the trough passage low amplitude ridging will build into
the region, with a somewhat moist anticyclonic northwest flow aloft
clipping the Utah/Idaho border region early next week. Outside of
periodic cloud cover in the north, do not foresee much precip
potential as the jet will remain well north of the area.
Of more note is the developing long wave pattern across the western
Continental U.S. Beginning later next week. Global ensemble mean progs are in
agreement that retrogression of the Pacific Ridge will become more
notable, this potentially allowing for a higher likelihood of short
wave energy rolling over the ridge and impacting the intermountain
region. Deterministic output in the globals suggest this, but not
buying off on their details at this point (low confidence in short
wave evolution timing attm). It's more of a big picture idea
currently, and the good news points towards a minimal threat of any
redevelopment of strong inversions, and the potential for more
frequent precip (even if on the light side with clipper like
systems). Time will tell.
Aviation...redevelopment of low stratus and the potential of
dense fog will continue to be the primary operational weather
concern at kslc this taf period. Prevailing IFR conditions are
expected through tonight in haze/fog, with redevelopment of stratus
in the 005-010 range. It's anticipated that the potential for dense
fog formation will hold off until after 07z, then a 60% chance
exists for LIFR/vlifr fog into tomorrow morning.
Light snow fall out remains a potential at times overnight into
tomorrow morning, with any fall out creating light accumulations on
untreated surfaces. Light winds will prevail.
Utah...High Wind Warning from 10 PM this evening to 9 am MST Thursday
For more information from noaa's National Weather Service visit...