Area Forecast Discussion
Issued by NWS Pocatello, ID

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000
FXUS65 KPIH 161132
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
432 AM MST Fri Nov 16 2018

.SHORT TERM...Today Through Monday Afternoon.

...MINOR WINTER STORM TO IMPACT SOUTHEAST IDAHO THIS AFTERNOON INTO
SATURDAY AFTERNOON WITH SLICK ROAD CONDITIONS POSSIBLE...

Synoptic circulations around a large ridge of high pressure located
over the Pacific Ocean and stagnant longwave troughing centered over
central and eastern Canada have set up a NW flow over the northern
Rockies. An embedded shortwave trough (storm system) over southern
British Columbia and Alberta early this morning will track S-SE
following the environmental flow, crossing MT, ID, and WY this
afternoon into Sat afternoon and bringing our SE Idaho forecast area
our next chance of precipitation. This is our only organized storm
system in the forecast between now and Thanksgiving.

WHAT HAS CHANGED: Very few changes have been noted over the past 24
hours in how the models are handling this storm system, and thus,
our overall forecast confidence remains moderate to high. Model
consensus including high-res HREF ensemble members are slightly
slower to bring initial shower activity into far northern portions
of our forecast area this morning, a slight trend that began about
36 hours ago. The ensemble members seem to reasonably capture where
current precip is located on radar up across northern ID and western
MT, and radar trends seem in line with what models are projecting,
so nudged the onset of precip back just a bit this morning and this
afternoon in the forecast. Guidance also has trended just slightly
earlier on clearing precip out of our forecast area from north to
south Sat afternoon, so made this nudge in the forecast as well.
This likely isn`t a sign of a faster or weaker system, but rather
the models better tuning into how the system will behave as we get
closer to the event. Finally, a very slight increase in storm total
QPF projections was derived this morning from a blend of
NBM/NAM12/GFS/Wnmm guidance, including up to one-tenth of an inch in
portions of the Central Mntns and up to one-quarter inch in our
mntns south of Driggs and the Caribou Highlands. We emphasize
SLIGHT, as expected snow amounts have changed little. Overall, the
forecast remains very similar to what we advertised 24 hours ago.

TIMING: Initial, isolated snow showers may nudge into the far
northern Central Mntns north of Stanley and Challis, the Upper Snake
Highlands including Spencer and Island Park, and the Teton Valley
including Ashton and Driggs as we head into the late morning hours.
Shower activity will increase and nudge as far south as Galena
Summit, Willow Creek Summit, Dubois, St. Anthony, and Palisades by
dinnertime, but dry conditions are expected to prevail to the south
of this activity until after dark today, making for another fairly
nice day for most of SE Idaho. Temps will support predominantly
snow, with some valley rain mixing in. Activity should be light
through this time, with perhaps moderate snow showers impacting
Island Park. Accumulations will be an inch or less through sunset.

From after sunset tonight into daybreak Sat, shower activity will
become more widespread and make a more progressive push to the
south, spreading across the entire forecast area from north to
south. It is during this period that the heaviest activity and
greatest accumulations are expected. As this coincides with our
coldest diurnal temperatures, precip should be all snow, perhaps
still mixing with rain in the eastern Magic Valley and parts of
Cassia County, especially during the first part of the night. Models
continue to key in on the greatest impacts targeting areas along and
east of US-20 from West Yellowstone to Idaho Falls, and east of I-15
from Idaho Falls to the UT border, affecting the Island Park region,
Teton Valley, Palisades, the Caribou Highlands, and eventually the
Bear Lake region toward morning. We carry 50-90% PoPs in this zone,
with 20-40% PoPs across the rest of the forecast area. As has been
the case for the last 2 days, models still seem a little too
progressive with clearing shower activity out to our SE Sat morning
as potential vorticity analysis indicates the cold front associated
with this storm system will not cross the forecast area until Sat
afternoon, and daytime heating may provide a bit more instability.
So while we trended precip a bit faster to clear out over the
previous forecast, still undercut guidance and held on to some snow
showers through early afternoon in the southern Central Mntns,
Southern Snake Plain, and Teton Valley area, with the heaviest
precip sinking south of the I-86 and US-26 corridors by lunchtime.
Finally, lingering showers will continue across the southern
Highlands, Cache Valley, Bear Lake area, and Caribou Highlands into
the later afternoon, diminishing and shifting SE out of our forecast
area by sunset. Used a blend of the previous forecast, NAM, GFS,
NBM, and some high-res guidance for PoPs and QPF, with some manual
edits to bump up PoPs where models lined up the best.

ACCUMULATIONS: Storm total QPF of 0.05 to 0.10 inches is expected
across the Central Mntns, falling as all snow outside of a few of
the warmer valleys including Stanley, Challis, and parts of the US-
93 corridor. This will translate into a dusting to 1.5 inches of
accumulation, greatest overnight Fri night into early Sat morning.
Across the Snake Plain, a dusting to 0.75 of an inch of snow is
expected during the same general time frame as showers make it into
the plain during our colder overnight temps. This includes American
Falls, Pocatello, Blackfoot, Idaho Falls, and Rexburg. Further east
in the highest-impact zone as outlined above, QPF of 0.10 to 0.25 of
an inch is expected, yielding storm totals of 1-4 inches, again
especially overnight Fri night into Sat morning, greatest at higher
elevations. Locations that may pick up 2 inches or more include but
are not limited to Raynolds and Targhee Passes, Island Park, Ashton
Hill, Driggs, Victor, Pine Creek Pass, most of the Caribou Highlands
including Wayan, Georgetown Summit, Geneva and Border Summits, and
Emigration Pass. A coating to 2 inches is expected for Montpelier
and the immediate Bear Lake area. Finally, less than 0.10 inch of
QPF will produce a dusting to 1.5 inches of snow across the Southern
Highlands, with the highest amounts again at higher elevations.

IMPACTS: Slightly breezy conditions are possible in the Snake Plain
and eastern Central Mntns up toward the MT border overnight into Sat
morning with speeds of 10-20 MPH, so this may cause some localized
blowing snow across roads with reduced visibilities. However, winds
elsewhere are expected to run 5-10 MPH, so blowing snow is not
expected to be a widespread, major threat with this system. The
greatest impact will be snow-covered and slippery roads as the snow
shower activity works through, especially (again) along and east of
US-20 from West Yellowstone to Idaho Falls, and east of I-15 from
Idaho Falls to the UT border. Overall snowfall intensities will be
light to moderate, but brief 1/2 inch per hour rates will be
possible in the heavier showers. The timing of the heaviest activity
suggests travel will be especially impacted Sat morning. Drivers
will want to slow down, leave plenty of extra room between you and
other vehicles, make sure emergency supplies in your vehicle are
stocked, and let family or friends know where you are going and what
time you plan to arrive at your destination. Accumulations with this
system do not meet criteria for issuance of a Winter Weather
Advisory, and considering only limited anticipated impacts from wind
and the fact that this is occurring on a weekend, we do not have
plans to issue advisories at this time. We will, of course, continue
to monitor the situation closely for any changes.

BEYOND THE STORM: High pressure will quickly build back in by early
Sun morning and continue into the upcoming holiday week, with
clear/sunny skies and colder temps behind the front. A
combination of very light winds and clear skies Sun morning are
likely to support excellent radiational cooling, and anticipated
morning lows have trended colder, likely falling to near zero
across portions of the central Mntns, Upper Snake Highlands, and
WY border, and upper single digits to lower teens elsewhere. Any
untreated roads that are wet or snow-covered will freeze, and this
may result in continued icy and slick travel conditions overnight
Sat night into Sun, especially in eastern sections of the
forecast area. Most guidance including the blends continue to run
far too warm for morning lows, so nudged them down anywhere from
3-10 degrees Sat, Sun, and Mon mornings closer to MAV/MET
guidance, with the greatest reductions Sun morning. - KSmith/RS

.LONG TERM...Monday Evening Through Next Thursday.
Models show good agreement on high pressure ridge and fair
weather dominating Monday evening through Thursday morning. A
chance of rain and snow during the day Thursday with snow levels
around 4400-4900 feet elevation, then lowering to valley floors
Thursday evening. The initial round of precipitation may favor the
mountains early on, then with increasing moisture and forcing,
precipitation lowers to valley floors. Looks like once this
pattern changes, the weekend following Thanks Giving will be
rather unsettled. - RS

&&

.AVIATION...
Middle level clouds will increase and gradually lower through the
day at KDIJ, KIDA, and KPIH. Ceilings are not expected to drop
below VFR until the front moving southward reaches KDIJ/KIDA about
17/06Z-11Z, and KPIH 17/09-15Z. May be just enough north wind
with front, 10-15 kts to create some slant wise visibility
lowering. Precipitation should be all snow. Clearing expected
behind the front and return to VFR. - RS

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


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