River Flood Warning 4, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
941 PM MDT Sun Jun 12 2022

.UPDATE...

The heavy rain showers have exited the central mountains so will
update to remove the areal flood watch and flash flood watch.
River flooding is ongoing in some locations and additional river
flood warnings are likely on Monday as the combination of snowmelt
and rainfall from this system makes its way down area streams and
rivers. Otherwise, forecast update includes minor adjustment to
rain chances, amounts and winds for Monday based on 00Z model
guidance and evening trends.

13

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.PREV DISCUSSION... /issued 238 PM MDT Sun Jun 12 2022/

SHORT TERM...Tonight through Tue night. A negatively-tilted
upper level trough trough has enough entrained moisture and
instability to put together a marginal risk of severe
thunderstorms and a threat of excessive rainfall for the central
Idaho mountains, which, along with the Island Park area, will be
bearing the brunt of the heavy rains and general bad weather.
During the overnight hours, the available moisture in the air will
decrease, but there is still a threat of showers and thunderstorms
on Monday as well. The upper level trough will close off and
become an upper level low, slowing its eastward progress and thus
lingering around the northern rockies for all of Monday. With a
northeastward track, by Tue mid-day the low tracks into Canada,
reducing the threat for the southern half of the forecast area,
but a continuing threat for showers and thunderstorms to the
northern half and the ID-WY border. The closed low then rotates a
trough through Wed evening which will keep the thunderstorms going
Tue evening until late.

This trough-turning-into-a-low is also bring strong wind aloft
already today and continuing Mon. Local study indicates wind in
the entire Snake River plain starting early Mon morning will start
reaching 30 mph, with the area growing substantially by afternoon.
Gusts to 55 mph area also supported. This low has a number of
features going for it: a 100KT jet max aloft over eastern Idaho,
strong cold air advection, 500mb and 700mb wind very strong in the
morning, a gradient across southern Idaho in excess of 10mb the
entire time, and the surface low slowly moving through Montana
and Wyoming. Expect this to subside sometime after 14/00Z.

The cloud cover will bring afternoon temperatures down strongly,
with a 12 to 18 degree drop Sun compared to Sat having already
occurred. Expect another 10 to 14 degree F decrease in the
afternoon high for Mon. The decreased clouds for Tue means the
start of a warming trend, but just a minor start, perhaps up 2 to
8 deg F. Messick

LONG TERM...WEDNESDAY THROUGH NEXT SUNDAY...
Our sprawling longwave low pressure center finally starts to move
east Wed with a shortwave ridge bringing dry conditions Wed/Thu and
a STRONG/FAST warming trend. Highs should make a run at 65-70 Wed,
then 80-90 Thu, and 85-95 Fri! As the previous forecaster noted, NBM
v4.1 probabilities of reaching or exceeding 90F have increased from
24 hours ago, now running at 60-75% from Pocatello through the
Burley area and in some of the valleys along the UT border. A fresh
low pressure system is forecast to bring a renewed chance for
showers/t-storms starting Fri eve and continuing into the weekend,
with much cooler temps again for the weekend as well. WPC 500mb
height cluster analysis suggests some forecast uncertainty with the
exact timing of this system, which will ultimately determine whether
or not we will squeak out a third consecutive dry day Fri. 01

AVIATION...
Few overall changes were made to the TAFs with the 18z/noon
package...utilized the latest high-res CAMs to refine start time for
VCTS this afternoon (generally 20-21z/2-3pm) as well as end time.
Continued to hold predominant cigs at VFR...cannot rule out brief
dips to MVFR in t-storms along with gusty/erratic winds over 40kts
and small hail, but will need to watch radar trends for any
potential direct hits to a TAF terminal and amend accordingly.
Degree of destabilization/instability is the main limiting factor
today for sev
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.SHORT TERM...

.LONG TERM...

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.AVIATION...ere t-storms with KBYI, KPIH, KIDA, and KDIJ all
sitting right along the edge of better clearing to the SE where
instability may end up being greatest. Watch for a SW crosswind at
KSUN this afternoon/eve. Regional winds should generally be WSW at
the Snake Plain terminals this afternoon/eve (KBYI, KPIH, KIDA), but
VWPs are a bit messy with some suggestion of a period of NNW winds,
and the HRRR is super contaminated with convective outflows, so
confidence in wind trends today is low with potential for winds to
get pretty variable depending on how t-storms evolve. -SHRA or -RA
linger overnight tonight from the Snake Plain south/east, as
regional winds start to ramp up once again by 08z/2am with an
additional increasing trend after sunrise. Can't rule out some
additional isolated showers/t-storms Mon afternoon for all terminals
except KBYI, but biggest impact should be the winds potentially
exceeding 20g35kts out of the WSW through just after sunset. As
usual, KSUN and KDIJ will remain a bit more sheltered. 01

HYDROLOGY...The moist air mass has brought the potential for
flash flooding if thunderstorms develop this evening, with only
some partial lowering of the moisture content overnight and
Monday. The moderate to heavy rain that has fallen already, as
much as one inch already today, will also increase snow melt. This
combination has led to areal flood watches and flash flood watches
in the central Idaho mountains, and main stem river flood warnings
for the Big Wood River at Hailey and the Big Lost River at Howell
Ranch (near Chilly). Minor flood stage is expected to be reached
during the overnight hours, plus or minus an hour from midnight.
With the additional rainfall possible Monday, do not expect a drop
below flood stage until Tuesday morning at the earliest. Messick

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.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 6 AM to 9 PM MDT Monday for IDZ051>055.

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