Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 061627

Area Forecast Discussion
National Weather Service Pocatello ID
927 AM MST Wed Dec 6 2017

.UPDATE...Another quick update this morning to adjust Sky and Wx
for current trends. Area web cams and surface observations
indicate patches of fog, so have broadly spread the coverage
through the morning. Otherwise have modified Sky to better fit
satellite imagery, with clearing this afternoon. With strength of
high pressure ridge and continuing inversions, expect this to be
situation going forward through the week. DMH


.PREV DISCUSSION... /issued 312 AM MST Wed Dec 6 2017/

SHORT TERM...Today through Fri night. High pressure is dominating
western states weather. This leaves the major forecast problems
limited to what temperatures will be and whether or not fog or low
clouds will develop. Unlike yesterday, the low clouds aren`t very
low, mostly at or slightly above 2000ft AGL. Indications are that
once it warms up that these low clouds will dissipate, and that
should happen around noon. However, shaded areas may continue to
be cloudy and foggy. The rest of the days are repeats, with little
change in the weather pattern expected until perhaps the 14th-a
long time from now, weather-wise.


Saturday Through Wednesday. No change in our expectation of a large
ridge of high pressure to remain anchored over the Pacific Coast
this weekend and throughout the upcoming week. This system will
serve to block storms from moving onshore and impacting SE Idaho,
and we continue a moderate to high confidence forecast of dry and
partly sunny weather for everyone right through and likely beyond
the end of the forecast period, with no major changes made with this
morning`s forecast package. The possible weak front advertised by
the ECMWF/GFS for Sun eve or Mon morning now looks to be positioned
further to the NE of our area. Maintained slightly higher cloud
cover during this period due to the proximity of this feature, but
even this only gives us scattered cloud cover. This pattern is
conducive to strong temperature inversions setting up with warmer
low temps in our higher terrain and colder lows in the valleys. This
may lead to periodic low stratus decks and/or fog for some
valleys/plains, especially midnight to noon each day. This is
currently not reflected in the forecast as it is difficult to pin
down exactly who will be most affected more than a couple days out.
For snow lovers, your enemy is the ridge, with little to quench your
thirst until it starts to erode/break down. The CPC outlook for SE
Idaho for Dec 16-29 is for above average temperatures, but
potentially above average precip. We shall see. - KSmith/Messick

The persistent ridge of high pressure forecast for the next 1-2
weeks will continue to create favorable conditions for strong
nighttime temp inversions in our valleys and plains, with an
associated risk of fog and/or low stratus, and this remains our only
aviation concern for the foreseeable future. Stratus, not fog, seems
to be our theme again this morning, with decks hovering around 1,500
to 2,000 feet affecting KSUN/KPIH/KIDA/KDIJ. Generally we expect
these conditions to continue as late as 17-20Z. Based on
observational trends so far this morning and model guidance, think
MVFR conditions will be predominant, with chances of dropping into
IFR fairly low, although this will continue to be monitored. Winds
today will be quite light, generally 4-9 knots. Expect the potential
for low stratus to return to at least KPIH/KIDA/KDIJ late tonight
into Thurs morning. - KSmith/Messick

With high pressure in place and associated nighttime inversions
likely, there is the potential for some air quality issues to slowly
develop over the course of the next 7 days, particularly for valley
locales and portions of the Snake Plain. These inversions will tend
to trap smoke from fires, fireplaces, and wood stoves near the
surface, with industrial sites and vehicular traffic potentially
adding some additional particulates/condensation nuclei. Everyone is
encouraged to monitor air quality forecasts, especially toward the
end of the week and this weekend. - KSmith/Messick



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