Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 150359
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
859 PM MST Thu Dec 14 2017

.UPDATE...Based on observational trends and webcam imagery this
evening, fairly widespread, dense fog has developed across the
eastern Magic Valley and much of the Snake River Plain, with ASOS
sites frequently dropping to 1/4 mile. Short-term computer model
guidance suggests dense fog will continue in these areas
throughout the night and well into Friday morning. Guidance also
suggests that fog may increase across lower elevations/valleys in the
Upper Snake Highlands and Caribou Highlands over the course of the
night. With temperatures well below freezing, freezing fog is
also expected. Based on this, we have issued a Dense Fog Advisory
effective until 12:00 noon Friday for the areas outlined above.
Made a quick forecast update to increase fog/freezing fog in the
weather grids for these areas as well. - Messick/KSmith

&&

.PREV DISCUSSION... /issued 147 PM MST Thu Dec 14 2017/

SHORT TERM...Tonight through Sunday. Early afternoon satellite
imagery was showing a broad area of low clouds and fog throughout
the Snake River Plain, Magic Valley, and a number of the CNTRL
mountain valleys. Observations and webcams were indicating reduced
visibility of 1/2 to 2 miles in fog with some areas occasionally
lower. Early afternoon visible satellite imagery trends were not
showing any appreciable dissipation and thus have continued a
mention of low clouds and fog through the night. Following shifts
may need to issue a dense fog advisory if conditions deteriorate.
Looking further NW on the satellite, the next upstream Pacific storm
system was noted advancing through the Gulf of Alaska. Numerical
models remain consistent with the idea that this system will top the
coastal ridge and dig SE into the CNTRL mountains Friday evening and
across the remainder of SE Idaho Friday night through Saturday
evening. Both the NAM and GFS are showing a Snake Plain Convergence
event developing across INL Saturday morning and then sweeping SE
through the Snake River Plain and into the ERN Magic Valley by late
Saturday afternoon. At this time, snow totals look to fall in the 1
to 3" range for the Snake River Plain and upwards of 3 to 7" across
the ERN mountains through Saturday evening. Given these
accumulations, I would anticipate a Winter Weather Advisory being
issued either late tonight or Friday for the incoming storm. The
system exits east of the region by Sunday morning with light
overrunning moisture spilling over the flat ridge along the coast
into the CNTRL mountains and/or Montana divide region by Sunday
afternoon. The statistical temperature guidance looks reasonable
with some warming anticipated ahead of the storm Friday followed by
colder temperatures behind the passing system Sunday. Huston

LONG TERM...Sunday evening through Thursday. The end of the
weekend is expected to be between weather systems; with one exiting
our region from earlier in the weekend and another approaching by
early Monday. The next wave early Monday is still looking to be
relatively mild, with little snow accumulation in the valleys,
perhaps a few inches at mountains tops through Tuesday. Since we are
still a few days out and have another system to account for before
this one, confidence in this system and its impacts remain low at
this time. Temperatures Monday morning are still expected to be
quite chilly though, behind a cold front from earlier in the
weekend. A gradual warming trend is expected Tuesday morning. As we
look toward the middle portion of the week, yet another storm system
is working its way eastward through Idaho. This one is looking more
substantial at the moment, but again, we will place a low confidence
in its impacts and forecast snow amounts. It is important to be
aware of the potential for heavy snow and blowing snow for both
higher elevations and the valleys next week though. This particular
system begins to clear out by Thursday with lingering moisture in
the eastern portion of the state. NP/MH

AVIATION...A parallel forecast from yesterday is expected for today
with lowered ceilings/visibility due to mist/fog at BYI/PIH/IDA this
afternoon/evening and again tomorrow morning. Expect the fog/mist to
reappear at DIJ this evening as well. SUN should stay out of these
conditions and continue at VFR. Light winds are expected at all
sites. NP/MH

AIR QUALITY...Poor air quality continues for Franklin county
because of temperature inversions. There may not be any significant
improvement in that area until late Friday or Saturday when a
stronger cold front arrives. Valle

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until noon MST Friday for IDZ017-019>021-023.

&&

$$


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