797
FXUS65 KPIH 281956
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
156 PM MDT Sun Sep 28 2025

.KEY MESSAGES...

- Unsettled conditions already onset with showers and thunderstorms
across the south and east of East Idaho.

- Increasing risk of showers and thunderstorms across the entire
forecast area on Monday, peaking on Tuesday.

- Much cooler temperatures for Tuesday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 156 PM MDT Sun Sep 28 2025

Satellite imagery shows upper low over the desert southwest opening
up allowing moisture to shift north across East Idaho. A few light
showers have been ongoing today and further development expected
this afternoon and evening. Winds aloft fairly light so a few storms
may be efficient and able to produce locally heavy downpours.
Portions of the eastern highlands have been upgraded to a MARGINAL
RISK for Excessive Rainfall for the remainder of today. A few
showers may continue into the overnight hours, otherwise most of the
region expected to be on a brief break with dry conditions. Further
upstream, amplified Pacific trough off coast projected to spin next
shortwave feature into Idaho early Monday. Showers and thunderstorms
expected to redevelop across the western zones during the afternoon.
Rainfall could become moderate to heavy at times associated with
this feature, and MARGINAL RISK for Excessive Rainfall remains in
place. Heaviest rainfall across the central mountains could be
exacerbated by embedded thunderstorm activity, and recent burn scars
remain a concern. NBM probability of reach 0.50" of QPF for the
Monday evening hours ranges from 15-40% within the vicinity of the
Wapiti scar. Those probabilities have trended slightly upward from
previous runs. A Flood Watch may be necessary if projections
continue current trend. Models shift the heaviest precipitation east
across the remainder of the forecast area for during the day
Tuesday, and the MARGINAL RISK for Excessive Rainfall shifts
eastward accordingly. Temperatures likely to be coolest on Tuesday,
with widespread daytime highs dropping into the 60s for most
areas.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Issued at 156 PM MDT Sun Sep 28 2025

Southwest flow aloft continues with upper low churning off
Seattle/Vancouver coast. Ensembles begin to shift the low inland
Wednesday, opening it up and ejecting a weak shortwave across the
region. Showers and a few thunderstorms remain possible Wednesday,
but the main trough passage appears to be Thursday to Friday. There
are significant differences in how the ensemble clusters portray
passage of the trough. NBM blend brings the greatest precipitation
chances Friday, but spread the chances out from Thursday night into
Saturday. Breezy conditions accompany another cooldown, this time
lasting into the weekend with deterministic NBM reaching only into
the 50s for lower elevations both Saturday and Sunday. It is worth
nothing that there is roughly a 15 degree spread between low-end and
high-end temperatures, so not a lot of confidence yet. That said,
the late week frontal system does look more robust and amplified
than the previous features ejecting across the region.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1116 AM MDT Sun Sep 28 2025

Main impact this afternoon will be some moisture pushing
northward into southeast Idaho as southerly flow east of an
upper low in southern Nevada. Have prob 30 thunderstorm groups
in after 21Z at all the TAF sites outside of SUN which should
stay north of the convective activity today. VFR conditions will
continue through Monday. May see a return to shower and
thunderstorm activity after 18Z on Monday.

&&

.FIRE WEATHER...
Issued at 156 PM MDT Sun Sep 28 2025

Southern zones will have showers and thunderstorms this evening
mainly isolated to scattered coverage.  An upper level trof will
move east Monday and expect more widespread showers and
thunderstorms Monday and Tuesday.  Will have drier conditions
Wednesday with a slight chance in 475, 476 and 422.  Will see a more
widespread chance Thursday and Friday with the next Pacific system.
Temperatures will cool off significantly Friday through next
weekend.  Will see highs in the 40s mountains and 50s valleys
possibly at the end of the weekend.  So main impact is expected to
be wetter conditions this week and a significant cool down late week
through next weekend.

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...DMH
LONG TERM...DMH
AVIATION...GK
FIRE WEATHER...GK