Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 141812

Area Forecast Discussion
National Weather Service Pocatello ID
1112 AM MST Thu Dec 14 2017

.SHORT TERM...Through Saturday Night. A quick update this morning to
allow the Freezing Fog Advisory to expire. Visibility has improved
slightly (1/2 to 2 miles in most areas) allowing for a near normal
flow of traffic. Although some ice accretion was noted on
meteorological instruments, it has not been substantial enough to
cause traffic issues per the ITD road reports. See the previous
discussion below. Huston

Primary issue this Morning is the expanding area of Freezing Fog
across the Snake Plain and Arco Desert, with visibility down around
300ft per ITD RWIS sensors situated along I-15 and HWY 20 north of
Idaho Falls. With recent Precipitation only adding to an already
moist boundary layer and strengthening inversions through Friday,
see little reason why Fog won`t continue through the Morning hours.
Thus, have issued a Freezing Fog Advisory for the Snake Plain and
Upper Snake Highlands, which includes the I-15 corridor from
Pocatello to Montana, HWY 20 north of Rexburg, I-15 north of Camas
and the Highway 33 corridor east of Rexburg into the Teton Valley as
well as the I-86 corridor from Pocatello to Raft River. High-
resolution model guidance suggests Fog will continue to expand
through the morning, perhaps impacting areas west towards Burley.
Confidence not high enough to issue Advisory for the Burley area
but will need to monitor trends this morning for possible
expansion of Advisories into these areas. High-resolution
guidance indicates some potential for improvement this Afternoon,
but with strengthening inversions, improvements to visibilities
may be limited. With a continuation of the stagnant airmass, odds
favor Fog continuing/re-expanding tonight across the same areas as
this morning. Fog likely doesn`t improve until Friday night as an
approaching disturbance promotes deeper vertical mixing.

Above-mentioned disturbance is currently taking shape across the
Gulf of Alaska and is forecast to begin spreading Snow Showers
across the Sawtooths, Pashimeroi and adjacent valleys Friday
evening with Snow Showers expanding across the Island Park, Teton
Valley, Pine Creek pass and Wyoming border region south to Utah
late Friday night into Saturday morning. Snow showers expected to
come to an end across the Sawtooth region Saturday, while Snow
Shower activity diminishes Saturday evening across the Wyoming
border region. Snowfall amounts are forecast to be highest across
the Wyoming Border and adjacent Highland region, with amounts
generally ranging from 3-6". Confidence on timing and location is
Medium to High across the Higher elevation areas. However, confidence
is lower on amounts and impacts across the Snake Plain/Interstate
corridor as models are struggling with the usual Mesoscale
details. Currently forecasting amounts under 1" across the Snake
Plain, however there is some potential for a convergence band to
develop which would result in higher amounts. As it stands now,
impactful Snow accumulations are possible across the Interstate
corridor and likely across Passes and Higher terrain areas.

.LONG TERM...Sunday through Thursday. Models continue to show little
run to run consistency in the extended forecast. Sunday remains dry,
but now GFS and ECMWF show the moist northwest flow that was to
remain north of the region in yesterday`s runs, now over our region
in today`s solutions. In addition, we are monitoring a potent cold
front which could arrive on Wednesday bringing light snow to the
region along with cold temperatures for the second half of next
week. Confidence in the long range forecast remains low given the
lack of run to run consistency. Valle

.AVIATION...Lots of low stratus and fog over the Magic Valley,
Teton Valley and Snake Plain this morning. Models are indicating
that the fog/stratus may break up this afternoon. This could be a
little optimistic but will continue the forecast for improving
conditions for now. Expect additional fog/stratus again tonight.

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


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