Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 230246
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
746 PM MST Sun Jan 22 2017

.UPDATE...Updated forecast to add Winter Storm Watch late Monday
morning through Monday night for zone 21. Expect heavy snow in
Pocatello to Blackfoot region behind cold frontal passage as all
indications show it to be heavy snow pattern post frontal passage
and local convergence pattern in the lower Snake River Plain.
Expect 4 to 8 inches from 11 am through Monday night.
GK

&&

.PREV DISCUSSION... /issued 216 PM MST Sun Jan 22 2017/

SHORT TERM...Tonight through Tuesday...First band of light
snow is making its way across the Magic Valley this afternoon.
Areas like Sun Valley are seeing snow lowering visibility as
well. As the evening continues, expect this band to progress
northeastward rather quickly, approaching Island Park by about
9 p.m. MST. Meanwhile this afternoon, southerly winds have been
gusting in excess of 30 mph from Pocatello to Twin Falls along the
I-86 corridor. Stronger wind gusts are noted at mountain peaks.
We`ll look for these winds to persist through sunset in these same
areas, therefore if you need to travel be cautious of cross-winds
and any drifting snow already on the ground.

Going into the overnight hours into Monday morning is when we
expect the heaviest snow to fall. In places like the Central
Mountains, Caribou Highlands, Wasatch Mountains and Southern
Highlands we will continue to see gusty winds. A blowing snow risk
is likely in these areas, making travel difficult due to reduced
visibility. There is a forecasting concern starting in this period
regarding precipitation type/amount and timing. At the present
time, we are expecting a small sliver of a rain/snow mix from
Burley to Blackfoot, thereby creating a difficult snowfall amount
forecast. Our best solution is that precipitation is expected
overnight with a potential break early Monday morning followed by
another possible round of snow Monday during the day. This
secondary round of snowfall is not yet covered in the current
round of Winter Weather Advisories/Warning and may need to be
considered for extensions if it does indeed occur. The band would
begin in the Southern Highland area and progress northeastward
toward Idaho Falls and encompass Pocatello to Mackay during the
afternoon and evening Monday. For our advisories and warning
currently in effect, we are forecasting various amounts of
snowfall this afternoon through Monday with the heaviest snow and
greatest confidence in snow occurring in the Caribou Highlands and
Wasatch Mountain range. NP/Keyes

LONG TERM...Tuesday night through next Sunday. We will continue
with light snow especially along the interstate corridor and into
the highlands. This is a combination of northerly upslope flow and a
weak wave dropping south over Idaho. This northerly flow continues
until the end of the week, keeping some mention of light snow
through then mainly along the interstate corridor and into the
adjacent highlands. After that, high pressure builds overhead for
next weekend. Expect colder conditions to remain especially under
stronger inversions. We should see the slopes warming up by
Saturday.  Keyes

AVIATION...The initial wave of moisture is ongoing across eastern
Idaho. We will continue to see moisture increasing through this
evening across eastern Idaho. KSUN is already dropping to IFR with
some snow moving through. As moisture increases through this
evening, expect conditions to eventually deteriorate at all TAF
sites. We are expecting snow at KSUN, KIDA and KDIJ and IFR/LIFR
weather. KBYI and PIH should actually start out as rain due to warm
temperatures but eventually mix with and change to snow. That snow
may not happen until the tail end of this first round. Strong
southerly winds will continue through this evening as well. 20-35kt
winds are expected at KPIH (where they already persist) and
eventually at KBYI. We have wind shear included at KBYI until winds
switch. We should see precipitation ending from west to east
starting late this evening. Current trends have conditions drying
out at KBYI and KSUN by 7-8z, 8-9z at KPIH and KIDA...eventually for
KDIJ 10-11z. For Monday, forecast confidence for our TAFs remain
pretty low. The models still struggle on what to do with a band of
snow across the area. We are either looking at a band that basically
lingers across southern areas and only affecting KBYI and KPIH, or
it moves just far enough north to hit KSUN, KIDA and KDIJ. Whatever
happens with this band, it should begin affect more southern
airports around 12-14z Monday. It would be a few hours later for
others. Expect IFR weather where get snow to fall.  Keyes

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Monday morning through late Monday night
for IDZ021.

Winter Weather Advisory until 6 PM MST Monday for IDZ019-020.

Winter Storm Warning until 6 PM MST Monday for IDZ023-025.

Winter Weather Advisory until noon MST Monday for IDZ017-018-022-
024-031-032.

&&

$$



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