197
FXUS65 KPIH 190123
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
623 PM MST Sun Jan 18 2026

.KEY MESSAGES...

- Fog and low clouds will continue to affect some areas through
  Tuesday, especially during the overnight and morning periods,
  amid a continued quiet weather pattern for the region.

- A pattern shift is then expected later this week, with
  potential for (finally) some rain and snow after Wednesday,
  especially in mountain areas.

&&

.UPDATE...
Issued at 620 PM MST Sun Jan 18 2026

Idaho Transportation Department weather stations are showing
some very dense fog is out there, so have issued a Dense Fog
Advisory. The fog bank is currently into the American Falls area
and continues to propagate eastward, so have included the lower
Snake River plain, besides the eastern Magic Valley.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 1244 PM MST Sun Jan 18 2026

Fog and stratus continues to be an issue across the Magic
Valley. Based on the latest forecast, we will likely see it
hanging around in some form into this evening. At that point, we
expect it to expand and make a run up into the Snake Plain and
surrounding adjacent valleys. We MAY see some pockets in other
valleys across the area, but not as widespread as across the
Plain. We expect to thin out a bit tomorrow afternoon, but
likely not 100% gone before redeveloping tomorrow night and
Tuesday morning. Coverage will likely extend up to the Montana
border tomorrow night, along with some potential in other
valleys. The question will be the extent of any dense fog. We
did let the one from this morning expire, but fog is still there
and locally dense especially back toward Twin Falls and Jerome.
We wouldn`t be shocked if another advisory is needed for tonight
and tomorrow, and again tomorrow night and Tuesday. Believe it
or not, the stratus deck along with a "wave" dropping along the
Divide may kick off some flurries later Tuesday around Island
Park. It isn`t a slam dunk but something different in the
forecast. Temperatures will remain in the 20s and 30s where
inversion and lingering fog/stratus hold temperatures down,
otherwise we will see some lower 40s around. That said, the
trend is overall slightly cooler than past days.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 1244 PM MST Sun Jan 18 2026

We are still looking at a pattern change, BUT it appears to more
of another shift in where the low/high pattern sets up. The
ridge breaks down, but now very briefly and ends up
reestablishing itself farther out in the Pacific. There is a
closed low still expected to drop into the southwest U.S. As
this happens, we still active weather with the overall low
pressure pattern over the northern U.S. shifting west. There is
some moisture embedded within the pattern, so precipitation
eventually returns, likely later Thursday or Friday now. It
still looks like the mountains will see most of what falls, but
with the flow now remaining more northerly...that means moisture
will be much less than when it looked like we might get more a
direct shot from the Pacific. Any moisture we see is likely not
going to put any dent into our lack of precipitation issues of
late, especially at lower elevations. Temperatures do warm up
just a bit later in the week, but that is likely due just losing
the ridge and some mixing takes place.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 450 PM MST Sun Jan 18 2026

Despite such a quiet weather pattern for most areas, THE MOIST is
coming. Swath of entrenched LIFR moisture manifesting as both dense
fog and low stratus remains very clearly visible on satellite down
the Treasure Valley into the eastern Magic Valley. Models
overestimated how much erosion would occur this afternoon and how
cold sfc temps would stay in that area, and the swath is already
expanding northeast on the satellite loop with a very light up-
valley/up-plain sfc flow now observed on many weather stations. Even
with models starting out too optimistic as of the issuance time of
this discussion, strong agreement exists between the NBM and HRRR in
advecting this moisture northeast up the Snake Plain tonight, posing
major impacts for several of our terminals.

Our TAF for KBYI now carries LIFR conditions below airfield minimums
from now straight through to 18z/11am Monday due to fog/low stratus,
and our neighboring office out of Boise has just issued a Dense Fog
Advisory through the same time covering the nearby terminals of
KTWF, KJER, and KGNG. Confidence in impacts is highest in this Magic
Valley region, with those impacts already occurring and expected to
slightly worsen as the sun sets and things cool further this
evening. Further northeast up the Snake Plain, confidence is growing
in the fog/low stratus deck reaching KPIH between 07-09z/12-2am
tonight, which will quickly send them into LIFR and very close to
airfield minimums from 09-17z/2-10am. Confidence in onset timing is
moderate to high, but a bit lower for exactly when things will
dissipate Monday, although overall model agreement so far is decent
on that end too. We will need to watch upstream trends this evening
to see if cigs/vsbys need to be lowered even further at KPIH.
Confidence is lowest in the action reaching up to KIDA, but both the
NBM and HRRR do support it just barely reaching the terminal, and
nearly all model VWPs as well as HRRR guidance lightly switch
(mostly variable) sfc winds into the SW late tonight which would
assist in the advection as well (whereas a light flow out of the
north holding on longer would keep drier air in place). Thus, we now
forecast KIDA to reach LIFR and approach airfield minimums as well
starting around 11z/4am, lasting until around 17z/10am. Meanwhile,
VFR conditions will reign at KSUN (normal diurnal wind cycle with
light speeds) and at KDIJ, with guidance keeping cig/vsby impacts
out of those two terminals. We may be contending with yet another
round of fog/low stratus Monday night/Tuesday as well, at least at
KBYI and KPIH.

&&

.AIR QUALITY...
Issued at 1244 PM MST Sun Jan 18 2026

We have extended the AIR STAGNATION ADVISORY out through Tuesday
for now. We really haven`t seen any movement of poor air across
the Pocatello area as well as across the Cache Valley, even with
the flow more northerly and no longer directly under the
ridge. This pattern should hold through Tuesday. We do see the
pattern change, or more of a shift farther west again, by
midweek. The issue that we will see increase in wind and
precipitation, but moisture potential is lower than before and
the pattern shift may not be strong or different enough to
completely scour things out especially down near the Utah
border. Stay tuned.

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until noon MST Monday for IDZ051-054-055.
Air Stagnation Advisory until noon MST Tuesday for IDZ054-059.

&&

$$

UPDATE...Messick
SHORT TERM...Keyes
LONG TERM...Keyes
AVIATION...01
AIR QUALITY...Keyes