270
FXUS65 KPIH 030959
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
359 AM MDT Mon Jun 3 2024

.SHORT TERM...Today and Tuesday.
Looking at current satellite imagery, southeast Idaho has zonal
flow aloft with a very moist, atmospheric river system starting to
move into our area. The leading edge is just hitting our
northwest areas with the back end of the moisture just off the
Pacific coast of Oregon. There is a dry slot off just off the
Washington coast and an upper level low just off the coast of
British Columbia. Farther off the Pacific coast there is high
pressure ridging behind this system.

For today, Monday, an atmospheric river system is just starting
to hit our area in zonal flow hitting mainly the western Central
mountains (mainly the Sawtooths) where 1 to 2 inches (with
isolated areas up to 3 inches) of rainfall in a 18 to 24 hour
timeframe. Galena Summit will get around 1 inch of rainfall with
Ketchum receiving around 0.3 inch. The Pioneer mountains will get
around 0.85 inch of rainfall. Currently the western Central
mountains are in a slight risk in the Excessive Rainfall outlook
by the Weather Prediction Center. This area is also highlighted by
the ECMWF model Extreme Forecast Index product. Snow levels will
start around 11k feet dropping to around 10k feet this evening.
Temperatures will hold above freezing throughout the event
(including AM lows this morning right up to near mtn peaks) will
result in an additional 1 to 2 inches of liquid melting out of the
high elevation snowpack east of Stanley and Ketchum, and as much
as 3 to 4 inches west of Stanley and Ketchum, as modeled by
NOHRSC. There is a flood watch out for the Sawtooths and Sun
Valley regions for today. The impacts will mostly be flooding of
small streams in the mountains and chances for rock and
landslides. See hydro and fire weather sections below for more
details. Currently, the Salmon river and Big Wood river only look
to rise around a 1/2 to 1 foot. Look for 0.10 to 0.50 inch of rain
today for most other lower elevations across Southeast Idaho. For
the Eastern Highlands along the Wyoming border, including the
Wasatch mountains and the Big Hole mountains, expect around 0.50
to 1 inch of rainfall in the mountains. It will be breezy to windy
today. Currently look at gusts of 25 to 45 mph across the area.
Blowing dust is possible but is not expected to be widespread.
Wind advisories are out for the Arco desert and the Upper and
Lower Snake Plain. High temperatures today will be cooler than
normal, around 8 to 12 degrees below normal. Expect 40s/50s in the
mountains and 60s in lower elevations for daytime highs today.

On Tuesday models continue to show high pressure build into the
area bringing overall dry conditions, mostly clear skies, breezy
winds, and near normal temperatures. Look for 50s/60s in the
mountains and 70s in lower elevations for daytime highs on
Tuesday.
Wyatt

.LONG TERM...Wednesday through next Monday.
High pressure will begin building in over the Southwest U.S. mid-
week, allowing for a continuous warming trend through the end of the
week. Widespread daytime highs will reach 15 degrees above
climatological norms by Friday or Saturday, with portions of the
eastern Magic Valley and Snake River Plain reaching into the low 90s
for the first time this year. Winds will be a touch breezy over the
Snake Plain and eastern Magic Valley Wednesday afternoon before
becoming light through the remainder of the extended period. At this
time, sustained winds over American Falls Reservoir look to be at
the very low end of Lake Wind Advisory criteria. The National Blend
introduces slight chances for precipitation beginning Thursday,
becoming more widespread throughout the weekend into Monday, as
models show weak signals for moisture filtering northward into Idaho
in the upper level flow.
Cropp

&&

.AVIATION...
Rain will spread eastward to all terminals during the
overnight/early morning hours. VFR conditions will likely give way
to MVFR CIGs and VIS as precipitation and any potential
thunderstorms are realized, with the possibility for further
degradation during periods of heavier rain and gusty winds. Winds
will increase again during the afternoon, with sustained SW to W
winds reaching around 25 kts with gusts to around 40 kts. Conditions
will turn dry on Tuesday, but winds will remain breezy.
Cropp

&&

.FIRE WEATHER...
Radar shows precipitation moving into the central mountains.
Widespread wetting rains will overspread the region early this
morning with chances for isolated thunderstorms during the afternoon
and evening. The Weather Prediction Center has included portions of
Fire Weather Zones 475, 476, and 422 in their Excessive Rain
Outlook. The most accumulated rainfall will be over the Sawtooths
and Stanley region, where 1 inch to 3 inches of QPF is forecast to
fall through tonight. During the 24-hour or so timeframe, 1" to
1.50" is forecast over the eastern half of the Ross Fork burn scar,
with 1.50" to 2" over the western half. Elsewhere, generally 0.40"
or less is forecast with locally higher totals of 0.50" to 1" in the
high terrain of the eastern mountains. Min RH will continue to
increase today, ranging 40 to 70 percent. Elevated winds aloft will
support breezy conditions at the surface each afternoon and evening
through Tuesday or Wednesday. The strongest SW to W winds are
expected across Fire Weather Zone 410, where SW winds will range 25
to 35 mph with gusts 40 to 50 mph. The pattern turns dry and warmer
beginning Tuesday as a ridge of high pressure builds in from the
Southwest. Temperatures will even reach into the 90s for some
portions of the Magic Valley and Snake River Plain beginning
Thursday through late week. Minimum humidities will widely range 15
to 40 percent during this timeframe, but winds should remain light.
Cropp

&&

.HYDROLOGY...
To review from the SHORT TERM section above...Expect a general 1
to 2 inches (with locally higher amounts) of rainfall, coupled
with 1-3 inches of Snow Water Equivalent (SWE) melt from
remaining high-elevation snowpack, is expected with today`s
system. This combination of moderate to heavy rain and melting
snow will likely result in strong rises and localized flooding
along smaller creeks and streams, as well as localized rockfalls
and landslides. Some backcountry roads and trails may become
impassable. The period of greatest potential impacts is expected
from 2 AM early this morning through 8 PM this evening (Monday). A
flood watch is effect today for the Sawtooth/Stanley and Sun
Valley regions. Anyone with plans in the backcountry and mountains
in the Ketchum, Galena Summit, and Stanley corridor should
monitor this situation closely and be prepared for hazardous
conditions. Consider altering your plans to avoid being in the
mountains or backcountry during this period.

While this is a lot of water, in general this event is expected
to be more of a smaller creeks and streams issue with flooding
generally not expected along the larger rivers. The Salmon and Big
Wood rivers are currently expected to rise around 1/2 to 1 foot,
and Valley Creek in Stanley could reach bankful, if not higher.
Currently action stage on the Big Wood at Hailey is not forecast,
but action stage for the Big Wood at Hailey is possible depending
on actual snowmelt and rainfall. Looking east toward the WY
border, currently only small rises are expected on mainstem rivers
for Monday and Tuesday. Some streams in the mountains could
approach bankful.
Wyatt/KSmith

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 9 AM this morning to 9 PM MDT this evening
for IDZ052>054.

Flood Watch through this evening for IDZ072-073.

&&

$$