FXUS65 KPIH 192037
Area Forecast Discussion
National Weather Service Pocatello ID
237 PM MDT Thu Oct 19 2017
.SHORT TERM...Today Through Saturday Night.
SE Idaho can enjoy one last day of gorgeous, mild fall weather as
a departing ridge of high pressure provides dry weather, mostly
clear skies, and breezy conditions through early Fri morning.
A wave of low pressure and associated cold front is on track to
impact SE Idaho Fri into early Sat. This system will bring
widespread rain and snow showers, starting early Fri AM in the
Central Mntns, overspreading the entire region during the late
morning, and winding down Fri night into Sat morning. Once again,
only minor adjustments were made to the ongoing forecast.
STRONG WINDS: We have 2 primary concerns with this system, and the
first is wind. Sustained winds of 25-35 MPH with gusts of 45-55 MPH
are expected Fri afternoon and eve along and behind the cold
front across the eastern Magic Valley and up the Snake Plain. This
forecast represents a good consensus of the majority of our
computer guidance. The GFS and associated MAV guidance is a
notable outlier, suggesting winds a solid 10 MPH higher than
currently forecast. The rest of the guidance has NOT been trending
toward the GFS during the last several runs, and thus the
forecast largely disregards the GFS. This is looking like a solid
advisory event, and the previous issued Wind Advisory remains in
effect from 10 AM to 8 PM Fri. Please refer to the advisory for
expected impacts. If guidance were to trend toward GFS/MAV
values, upgrade to a High Wind Warning would need to be
considered. Again, this is looking unlikely at this time.
ACCUMULATING SNOW: Our second concern is snow. Snow levels start
at 6,500-7,000 feet Fri morning, falling rapidly behind the front
to 4,500 feet by Fri eve. Accumulations of 2-5 inches are
expected across the Central Mntns above about 5,000 feet including
the mountain passes, while 5-8 inches will be possible above
about 7,500 feet including peaks and ridge lines, especially
near/west of Ketchum and Stanley. Combined with gusty winds,
blowing snow is also possible. This will create hazardous driving
conditions due to snow-covered and slippery roads along higher-
elevation highways including Galena Summit and Banner Summit. Have
continued the SPS for now highlighting these impacts, although an
upgrade to a Winter Weather Advisory is possible. Elsewhere, 1-2
inches of snow will be possible in the higher elevations of the
Upper Snake Highlands and along the WY border, with a few light
snow showers possible to valley floors.
A few rain/snow showers may linger Sat/Sat night across the
Central Mntns and along the WY border, with another couple inches
of snow possible in the higher elevations. - KSmith/DMH
.LONG TERM...Sunday Through Thursday.
Some lingering Pacific moisture is expected to bring showers to
central mountains and Upper Snake Highlands Sunday and Sunday night.
Will have some lingering slight chance of showers Monday along
the Wyoming border. Dry conditions expected Tuesday through
Thursday. European and GFS models sliding weak system to north on
Wednesday hinting at perhaps showers again along eastern border
with Wyoming. For now left any chance out of grids. Temperatures
are expected to be above normal in the long term period with
breezy conditions Sunday and Monday. Snow level Sunday at around 9
thousand feet so other than mountain passes event should be
mainly rain. - GK
Main feature will be strong cold frontal passage on Fri with
sustained wind speeds of 25-35 knots expected at BYI, PIH and IDA.
Expect those speeds to start between 14 and 18Z Fri morning.
Gusts at those three sites will likely surpass 40 knots. Along
with the strong winds rain showers are likely especially at SUN
and DIJ with MVFR ceilings expected and rain could change to snow
at those locations in the afternoon. Could also see showers Fri at
the low elevation sites but kept ceilings VFR for now. - GK
Wind Advisory from 10 AM to 8 PM MDT Friday for IDZ017-020-021.
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