Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 210015

Area Forecast Discussion
National Weather Service Pocatello ID
615 PM MDT Tue Mar 20 2018

.UPDATE...Updated northern Custer County in coordination with
National Weather Service Missoula, who came back on line this
evening after an outage.



.PREV DISCUSSION... /issued 153 PM MDT Tue Mar 20 2018/

SHORT TERM...Tonight through Friday.
Expect nothing more than light rain or snowfall tonight or
Wednesday mainly in the mountains. Main impacts will be Thursday
as very moist Pacific storm system will move through eastern
Idaho. It will be very warm ahead of the cold front and snow
levels are expected to rise to 8500 feet by Thursday afternoon.
The rising snow levels and expected heavy precipitation have led
to flood watches being issued for most of the mountain regions in
southeast Idaho with extensive runoff expected. Low lying areas
and streams and creeks may experience flooding mainly on Thursday.
Mountain passes above 9000 feet may see heavy snow but most areas
will see rain with this system. A cold front will work through
the region Thursday night with cooling conditions expected Friday
so that should mitigate any flooding that occurs Thursday and
Thursday night. This system is still over 24 hours from full
impact. Confidence in snow melt and precipitation is high but
still low confidence in exact amounts of precipitation.

LONG TERM...Friday night through next Tuesday. Fair agreement
between models beginning the long term period. ECMWF and GFS
guidance are generally in phase with upper trough progressing across
the Great Basin through Sunday, through ECMWF continues to be a
deeper solution and begins lagging Sunday night. Best confidence
remains Saturday and Sunday with continued moisture transport ahead
of the upper trough. ECMWF is developing a cutoff low over Idaho
Monday and dropping it into southern NV by Tuesday while the GFS
develops a cutoff over the Four Corners area by Tuesday. Both models
are showing a trend towards drier weather as the low cuts off, so
Monday and Tuesday should be dry. Hinsberger

AVIATION...Weak upper trough is filling and progressing through
Idaho this afternoon, providing just enough lift for some showers
over the area. The showers should subside late this afternoon/early
evening as the trough moves to the east. A second wave of moisture
is poised to reach southeast Idaho tomorrow morning with showers
beginning at KBYI and KSUN as early as 7am tomorrow. KSUN is seeing
early LIFR ceilings with the upper trough moving through, but should
improve later this afternoon to VFR. Hinsberger

HYDROLOGY...Models continue to heat things up through Thursday as
the mountain snowpack undergoes a rapid melt and runoff. This is
causing some concerns for flooding in low-lying areas and small
creeks. In addition to snowmelt, a significant amount of QPF will
fall over southeast Idaho as an atmospheric river makes landfall and
penetrates inland. Temperatures will cool on Friday, which should
reduce the runoff potential from snowmelt. Hinsberger


Flood Watch from Wednesday evening through late Thursday night
for IDZ018-031-032.

Flood Watch from late Wednesday night through late Thursday
night for IDZ019-022-023-025.


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