Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 162201

Area Forecast Discussion
National Weather Service Salt Lake City UT
301 PM MST Sun Dec 16 2018

High pressure over the area this weekend will give way to a
weakening storm system that will transit the state on Monday.
Strong high pressure aloft will settle across the region by the
middle of the week.


.SHORT TERM (Through 00Z Thursday)...
Mainly quiescent weather will prevail through mid-week, with the
main focus being the valley inversions that will become noticeably
worse with time as the week progresses. Otherwise, a weakening
storm system with limited moisture will cross the region Monday
bringing some minor mountain snow accumulations, and otherwise
mainly cloudy skies with perhaps a few brief rain showers making
it to the valley floors.

Today the ridge has rebounded ahead of a splitting trough coming
ashore in California later tonight. The strengthening ridge has
resulted in a strengthening inversion today with increasing haze.

Monday and Monday night, the splitting wave will transit Utah,
the weak dynamics and limited moisture will result in mainly
increased cloudiness with some mountain snow showers. A fresh 1-3
inches of mountain snow is not out of the question with this wave
passage. Otherwise, this will mainly result in overcast skies across
the valleys with the chance for a few rain showers breaking through
to the valley floors. The weak flow and limited cold pool with the
weak trough will likely not be sufficient to clear out the valley
inversions that developed over the weekend, although there is a
small chance for a relatively minor improvement. Any precipitation
falling into the inversion may only help to strengthen it through
evaporative cooling, and help result in some fog and low stratus for
Tuesday and Wednesday. Behind the splitting wave, the ridge will
strongly rebound into the middle of the week, which will further
lock in the valley inversions with continually worsening
haze/fog/stratus likely.

.LONG TERM (After 00Z Thursday)...
The western CONUS ridge will amplify while shifting downstream
Thursday, with axis poised to position itself overhead by days
end. Backing flow aloft will aid another round of warm advection
(H7 temps rising to +2-+4C), with further warming of the lower
levels aided by the increasingly subsident environment. All this
points to a rapid strengthening of valley inversions, likely
enhancing what will have remained in place from days previous. The
subtleties of inversions are challenging (ie surface based vs.
slightly elevated etc.), but the big picture points towards net

By next weekend, the upstream pattern appears to transition lower in
amplitude and more progressive once again, with several short wave
troughs translating across the intermountain region through early
next week (at least). Not buying off on model resolve of any
individual feature during the days 5-7 period at this time, but
continue to have growing confidence that the progressive nature will
allow for boundary layer mixing and deteriorating inversion
conditions at some point during that window. Have maintained largely
slight chance PoPs focused over the north days 5-7 to account for
wave passages, though timing of each remains suspect.


A low level inversion will maintain haze at the KSLC terminal
through the TAF period with slant-wise visibility restrictions
aided by the lowering sun angle through sunset. After sunset,
surface vis is expected to deteriorate to at least MVFR levels by
03z, with a 20 percent chance of lowering further to IFR levels
after 06z. Light northwesterly surface winds will continue to
prevail through this evening, but will most likely become variable
through the middle of the night, and largely a light southeast by





LONG TERM...Merrill

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