Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 192004

Area Forecast Discussion
National Weather Service Pocatello ID
204 PM MDT Fri Apr 19 2019

.SHORT TERM...Tonight through Sunday night. Upper ridge in place
over Eastern portions of the state this afternoon, slowly shifting
east into the Plains tonight into Saturday. Southwest flow aloft
remains in place ahead of encroaching upper trough reaching the
coast tonight, which absorbs weakly closed low shifting inland
over southern/central California. Moisture begins to spread
across the region tonight into Saturday, allowing showers and
thunderstorms to develop very late tonight and through the day
Saturday. Lapse rates and instability favorable for convection but
weak shear should keep thunderstorms from being organized,
therefore expecting generally gusty subsevere winds and very small
hail. The steep lapse rates are favorable for gusty winds in
general and have nudged winds up slightly Saturday over forecast
guidance despite weak surface gradient.

Upper trough closes over Nevada Saturday night into Sunday, with
steepening surface gradient across the region Sunday, as well as
better chances for more consolidated precipitation shield to
spread across the region. Cooler air wrapping into the region
along with precipitation drops temperatures significantly from
todays and Saturday highs. Snow levels over 9000 ft Saturday drop
to about 7500 ft in the central mountains Sunday with 1-3 inches
accumulations above those levels. Every where else just expected
to see rain through Sunday night. DMH

.LONG TERM...Monday Through Next Friday.
A relatively drier although still progressive wx pattern is expected
for the upcoming week. Showers may be slow to clear the southern and
eastern highlands Mon, but dry weather is expected everywhere Mon
night-Tues night as a tongue of ridging extends NE across the
forecast area. Ridging really flattens out by Wed, and this is where
significant forecast uncertainty begins as the GFS and Canadian send
a weak shortwave toward our region by Thurs afternoon continuing Fri
with increasing chances of showers, while the EC is drier and
maintains better ridging. All of the long-term models are also a bit
messy in depicting moisture/showers underneath an otherwise "ridgy"
and weakly forced environment. For now we carry light broadbrushed
PoPs across the region Thurs and Fri as we watch for better model
agreement. Significant precip amounts don`t look likely over the
course of the week at this time, so warm spring temps will likely
remain the driving factor behind snow melt and runoff rates.


A ridge of high pressure will maintain high confidence in widespread
VFR conditions this afternoon/eve, with winds generally under 12 kts
and skies nearly 100% clear! Gradually increasing clouds are
reflected in the TAFs starting late this eve as a new trough of low
pressure starts to approach the region, with an introduction of VCSH
between 08Z and 13Z based on projections from the high-resolution
HREF ensemble members and HRRR. Any development looks quite isolated
and would occur during a diurnally unfavorable time when
heating/instability is lacking, so confidence in this activity is
lower with most locations likely to remain dry. Next threat to watch
will be at least VCTS beyond the end of the TAF period Sat afternoon
(except reflected at 19Z in KSUN`s 30 hour TAF), with some potential
for gusty/erratic outflow winds and small hail. Still, none of our
guidance is projecting cigs dropping below VFR at this time through
late Sat afternoon, so outside of the upcoming t-storm risk,
confidence remains high in few if any aviation impacts. -


.HYDROLOGY...Spring snowmelt ongoing with warm temperatures
continuing into Saturday across the region. Several rivers across
the region seeing rises due to runoff, and ponding/seeping
occurring many areas as well along with near bankfull conditions
many waterways. A low pressure system will pass through the state
this weekend, bringing rain and high elevation snows to most
areas. Isolated to scattered showers and thunderstorms are
forecast Saturday through Sunday night with localized moderate to
heavy rainfall mainly Southern Idaho. Cooler temperatures are
forecast Sunday but expected to rebound by midweek. Snowmelt
runoff will continue into East Idaho waterways, which may be
enhanced by the weekend precipitation. The Portneuf River at
Pocatello is expected to reach minor flood stage this weekend, and
the Big Wood River at Hailey is expected to produce minor impacts
along the river. The Snake River is forecast to remain high and
fast with elevated flows, but remain below flood stage through the
weekend. People planning recreational activities along or near
the rivers are urged to be aware of the potential for high flows
and keep children/pets away from the river banks. DMH



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