Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 201950

Area Forecast Discussion
National Weather Service Pocatello ID
150 PM MDT Thu Apr 20 2017

.SHORT TERM...Tonight through Sunday. The latest satellite and radar
imagery was showing a negatively tilted trough and attending
precipitation working east through the region. Numerical models show
the trough advancing into Wyoming later tonight with drying shifting
from west to east across the region. The remains of the upper trough
shear SE along the divide Friday supporting ongoing shower activity
across the NE highlands and a portion of the Snake River Plain. The
upper ridge builds into the area Saturday morning with dry
conditions expected throughout the day. The drying will allow
morning lows to drop below freezing in most areas resulting in
frost. Warming temps are expected in the afternoon under an
increasing SW flow and plenty of sunshine. The next upstream Pac low
begins to eject energy inland Saturday night and Sunday with
accompanying precipitation impacting mainly the CNTRL mountains and
possibly the Snake River Plain and SE Highlands depending on which
model run you entertain. So expect more resolution on that item as
we get closer to the event. Huston

.LONG TERM...Sunday night through next Thursday. Active period of
weather remains in store for SE Idaho through the period as an
energetic Pacific wave train sends several moisture-rich
disturbances through the region. One such wave is expected to be
exiting the area Sunday night before the next round of precipitation
spreads from west to east across the region on Monday. This flow
regime favors west-facing slopes of the Sawtooths and Eastern
Highlands while the Arco Desert is somewhat shadowed. Snow levels
for Monday hover around or just under 6,000 feet elevation across
the Central Mountains, so this may mean snow down to valley floors
and the potential for slushy driving conditions at pass level. After
a brief break Tuesday, a robust wave surges into the region Tuesday
evening with precipitation persisting thorugh Wednesday. Snow levels
are forecast between 6,000-6,500 feet elevation, with the potential
for significant snowfall above these elevations. For elevations
below 6,000 feet, rainfall amounts from around 0.10" to 0.30" with
locally higher amount possible. The flow configuration with this
wave is more favorable for widespread precipitation across all of SE
Idaho. Showers are then expected to persist Wednesday night through
Thursday across the region. With flow turning slightly
northwesterly, snow levels may drop a few hundred feet. Temperatures
are expected to remain below climatological averages through the
period. In fact, Pocatello has not recorded a 70 degree high
temperature yet in April, and this has only occurred 5 other times
since 1939. Strong westerly winds are possible Tuesday through
Thursday, which may make for some difficult driving conditions,
particularly on north/south oriented routes. AD/Huston &&

.AVIATION...Aviation concerns are plentiful through the period.
First off, a band of moderate to heavy precipitation is impacting
KIDA early this afternoon with IFR CIGS/VIS. This should exit the
terminal by late afternoon while shifting eastward and impacting DIJ
from roughly 20-02Z. However, do not expect CIGS/VIS to be quite as
low as what was observed over the Snake Plain today as a downslope
component my limit the fall of CIG/VIS to MVFR. May also see some SN
mix in at times at DIJ through this evening. Have VCTS in at all TAF
sites, with the greatest potential for TS at PIH, IDA and DIJ from
about 22Z-02Z. There is some uncertainty on overall coverage of TS
however as earlier showers have limited destabilization potential.
Gusty west-southwest winds of 15-25kt have developed at BYI and
expect these to develop at PIH by about 22Z and IDA by around 00Z.
Winds should persist through the night at BYI and PIH and back more
northwesterly at IDA while decreasing. Model time-height products
indicate the potential for stratus development at all sites with the
exception of SUN overnight. Not confident to add this in a
predominant group at this time, but have hinted at the potential.
Best window for stratus appears to be 10-15Z. Lighter winds and
drier conditions expected for Friday with the exception of DIJ where
showers continue. CIGS should oscillate between MVFR (in showers)
and VFR (outside of showers) at DIJ. AD/Huston

.HYDROLOGY...The combination of widespread showers today and
warming temperatures heading into the weekend will support forecast
rises on rivers into the weekend. The Portneuf at Pocatello will
remain in flood stage throughout the weekend and a flood warning has
been extended to cover the expected flows. The Snake at Blackfoot is
expected to remain below flood stage due to curtailed releases out
of Palisades Reservoir. The Bear River at the Wyoming border is
forecast to remain below the official flood stage of 8.6 feet
through the weekend but we did notice issues near Dingle and Pegram
when the stage reached 7.38ft back on March 20th. The stage as of
noon Thursday was 7.45 feet. A flood advisory has been extended for
that area to cover the low land flooding. Huston


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