793
FXUS65 KPIH 022100
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
300 PM MDT Sun Jun 2 2024

...STORM SYSTEM TO BRING SOME FLOODING AND WIND HAZARDS TO
SOUTHEAST IDAHO MONDAY...

.SHORT TERM...TODAY THROUGH MONDAY NIGHT...
This afternoon remains "the calm before the storm" as weak troughing
supports breezy conditions over much of the CWA and perhaps a couple
showers or an isolated t-storm across the NE and E-cntrl highlands,
while the rest of the region holds dry. Winds may approach WIND
ADVISORY criteria across the Arco Desert/Mud Lake region this
afternoon, and may also approach LAKE WIND ADVISORY criteria for
American Falls Reservoir, but in both cases the scenario looks
borderline with not all guidance reaching criteria and stronger
winds are enroute for Mon, so we continue to let the forecast speak
for itself today. Highs will reach the upper 60s to upper 70s.

Now for the main event...later tonight through Monday, a low
pressure storm system and associated atmospheric river of moisture
remains progged to bring a period of moderate to heavy rainfall to
the mountains west of Ketchum, Galena Summit, and Stanley. This is
our main corridor of concern hydrologically. 1 to 2 inches of
rainfall is expected, with locally higher amounts along and west of
the spine of the Sawtooths, and lower amounts east of ID-75. A
couple thunderstorms are also possible, especially Mon afternoon.
Our official forecast (which leans on WPC calibrated QPF guidance)
continues to run near the 99th percentile of the NBM which has been
toward the lower end of our various guidance sources, and has
changed little compared to 24 hours ago except QPF totals have
decreased slightly in the valleys adjacent to the Sawtooths such as
the Sawtooth and Wood River Valleys (no real change up along the
ridgeline of the Sawtooths). Now that this event has drifted into
the period of the HREF, the ensemble mean is advertising 1.50" to
2.00" QPF in the Sawtooths which certainly supports the WPC
guidance, and the ensemble max even exceeds 3.00"! In addition,
temps holding above freezing throughout the event (including AM lows
Mon right up to near mntn 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. This combination of moderate to
heavy rain and melting snow may result in strong rises and localized
flooding along smaller creeks and streams, as well as localized
rockfalls and landslides. We can easily envision some backcountry
roads and trails may becoming impassable, and we`ll need to keep an
eye on the Ross Fork burn scar west of Galena Summit as well. The
period of greatest potential impacts up along this corridor of
greatest concern is expected from 2 AM early Monday morning through
8 PM Monday evening. Still a touch of overall uncertainty on whether
the current, much-higher-than-NBM QPF forecast will fully verify as
well as exactly how much heavy rainfall will be able to spill over
the Sawtooths on a westerly flow, and in general mainstem rivers are
not forecast to reach flood stage with this relatively limited 12-18
hour event. Thus, we continue to run with an SPS for messaging
(added some additional detail and timing information to the
statement) as opposed to issuing a FLOOD WATCH. Elsewhere across the
rest of SE Idaho, fairly widespread rain and/or shower activity is
expected from mid-AM Mon through at least early Mon afternoon,
transitioning west to east, before the regime becomes a bit more
convective with isolated t-storms possible, ending by sunset. On the
QPF side of things, the region should be able to easily handle a
widespread 0.10 to 0.50 inches of rain, with a few locally higher
amounts the Centennials, Big Holes, and Bear River Range. While
overall afternoon instability will be a bit modest, strong 700mb
flow as high as 50kts may assist a few storms in mixing strong gusty
winds down to the sfc.

The other main impact from this system will be moderate SW winds
ramping up by mid-late Mon AM, and continuing through about sunset.
Over the Arco/Mud Lake Deserts and Snake Plain...where speeds may
reach 25-35 MPH with gusts to 55 MPH...a WIND ADVISORY has been
issued from 9 AM to 9 PM. Our NBM-led forecast has trended just
slightly higher on peak winds over the last 24 hours, as has the
HREF and projected 700mb winds on the GFS, but at this time we still
feel this will be a solid advisory-level event vs. a high wind
event. Still, there is perhaps a 20% chance that at least the
Arco/Mud Lake Desert zone may be considered for an upgrade to a HIGH
WIND WARNING if guidance continues to trend higher. One concern is
modest low-level lapse rates of 6.5-7.5 degC/km advertised on
NAMNest and RAP forecast soundings even "out in the clear" behind
the steadier rain exiting eastward Mon afternoon which could damper
full mixing potential, although system dynamics could try to
overcome this (and certainly any stronger t-storm should be able to
tap into this flow). Can`t rule out localized blowing dust, but the
overall blowing dust risk may be mitigated by the rain and shower
activity moving through over the course of the day. Unless
guidance trends higher, main wind impacts are likely to be
difficult driving for high-profile vehicles and unsecured outdoor
objects blowing around. All impacts from this storm system are
expected to settle down by around sunset Mon eve. High temps Mon
will also hold about 10 degrees cooler compared to
today...generally upper 50s to upper 60s. - KSmith

.LONG TERM...TUESDAY THROUGH NEXT SUNDAY...
Generally held with NBM guidance for the rest of the forecast as
main focus is on Monday`s system. Synoptically, a building ridge of
high pressure will take control and support drier conditions with a
solid warming trend resuming, although at least isolated diurnally-
driven showers and t-storms return to the forecast starting Thu
afternoon and beyond. Winds will still be breezy Tue afternoon, and
to a lesser degree Wed afternoon, before becoming light through the
remainder of the extended period. Forecast high temps have actually
nudged just a few degrees cooler, although still approaching 90 for
many lower elevation population centers each day starting Thu. - 01

&&

.AVIATION...
A wet system works into East Idaho beginning tonight. There is
still a very low probability of seeing a shower or thunderstorm
develop this afternoon around DIJ, otherwise rain shifts into the
state after midnight and spreads across all terminals by/around
sunrise. Expect CIGS to trend down from VFR to MVFR with
occasional IFR in heavier showers/potential thunderstorms during
the day. Gusty winds today weaken slightly overnight but return
with a vengeance during the day Monday. Expect the strongest winds
Monday afternoon from KPIH-KIDA and across the Arco Desert, 25-
30kts sustained with gusts around 45kts. - DMH

&&

.FIRE WEATHER...
The next system arrives this evening with precipitation reaching
the central mountains tonight. Widespread wetting rains will
overspread the region early Monday 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 2.5 inches of QPF is forecast to fall
between this evening and Monday night. During this 24-hour or so
timeframe, 1.25" to 1.75" is forecast over the eastern half of the
Ross Fork burn scar, with 1.75" to 2.25" over the western half.
Elsewhere, generally 0.50" or less is forecast with higher totals
of 0.75" to 1" in the high terrain of the eastern mountains. Min
RH will continue to increase through Monday, ranging 20 to 35
percent for much of the region today before rising to 50 percent
and higher. Elevated winds aloft will support breezy conditions at
the surface each afternoon and evening beginning today and
continuing 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 45 to 55 MPH. The pattern
turns dry and warmer beginning Tuesday as a ridge of high pressure
builds in from the Southwest. Temperatures may even reach into
the 90s for some portions of the Magic Valley and Snake River
Plain beginning Thursday into next weekend. Widespread minimum
humidities will begin to drop below 25 percent during this
timeframe, but winds should remain light. - Cropp

&&

.HYDROLOGY...
To review from the SHORT TERM section above...a general 1 to 2
inches of rainfall, coupled with 1-3 inches of SWE melt from
remaining high-elevation snowpack, is expected with Monday`s
system. This combination of moderate to heavy rain and melting
snow may 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 Monday morning through 8 PM Monday evening. 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 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 to 9 PM MDT Monday for IDZ052>054.

&&

$$