Mackay, Idaho Weather Forecast Discussion

560
FXUS65 KPIH 280827
AFDPIH
Area Forecast Discussion
National Weather Service Pocatello ID
227 AM MDT Sun Sep 28 2025
.KEY MESSAGES...
- Isolated showers develop in the south this morning. Isolated
  thunderstorms in the south and east this afternoon.
- Increasing risk of showers and thunderstorms across the entire
  forecast area on Monday, peaking on Tuesday.
- Afternoon temperatures plummeting 10 to 15 deg F from today
  to Tuesday.
&&
.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 124 AM MDT Sun Sep 28 2025
A coastal trough is slowly moving eastward toward the northern
Rockies. South to southwesterly air flow will bring in a moist
air mass for Mon and Tue. Today, there is enough moisture and
instability to generate isolated showers and thunderstorms, but
a much bigger change occurs starting Mon afternoon, when the
probability of rainfall surges for most locations at 70 percent
or higher during a 12 hour period, and thunderstorm risk gets to
the chance level. Showers will see occasional breaks with timing
dependent on location. The precipitation amounts in the
deterministic published model runs show timing has a high level
of uncertainty. Amounts also show a wide range of values for any
6 or 12 hour period. The central Idaho mountains appear to be
the main recipient, with some potentially heavy rain at times.
For example the latest NAM has 0.46 of an inch of liquid water
at Stanley Ranger Station for a 6 hour period on Mon evening.
But other guidance is much lower, with the NBM registering only
0.19 of an inch. It can be said that most locations in the
central Idaho mountains, especially the Sawtooths, can expect
0.60 to 0.75 storm total amounts from Mon through Tue night.
Thunderstorm intensity will peak on Mon, then the extensive
cloud cover for a prolonged period will still mean thunderstorms
on Tue, but not as intense as the air stays more stable.
Afternoon temperatures will be similar to the previous Sat, then
on Mon the eastern half of the forecast area does not get as
direct a hit as the western side, so highs will be much cooler
on Mon than Sun, while the eastern half is still not much
cooler. This all changes on Tue with high temperatures cooled
down to climatic averages for this time of year. Overnight lows
stay very mild with all the cloud cover, in the middle 40s or
higher for locations like Pocatello, while Stanley stays right
around climatic average for lows, which is right around
freezing.
Wind is strengthening aloft on Mon evening, which will allow
for windier conditions at the surface and stronger outflow for
thunderstorms starting at that time. Tue should be the windiest
day for surface wind, with gusts approaching 30 mph.
&&
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Issued at 124 AM MDT Sun Sep 28 2025
The trough that brings all the rain in the near term forecast
starts to wane. Wed and Thu still continue to feature a very
deep, but filling low at the coast and continued moderate
southwest flow over the Gem State. Finally on Fri the low moves
on shore and almost immediately changes into an open wave. 22
percent of the solutions for a cluster than has the trough over
eastern Idaho by Fri. The others keep it west until Sat. The
placement is all over the place, some north in MT, others
centered to the south in UT/NV. This means the precipitation
threat is not going to zero any time soon, with Wed and Thu
having the best chance for a break. Compared to the near-term,
though, it may feel like one.
The mild overnight lows continue with no threat of Frost in the
Snake River plain until perhaps Fri night. Afternoon
temperatures gain back some warmth for the less cloudy Wed and
Thu, then cool down again Fri and Sat. Sat has the coolest
temperatures for the entire week.
Wind stays elevated during this period, reminding us what fall
in Idaho is all about. However, see nothing strong enough to
meet Wind Advisory criteria at this time. Mostly gusting around
30 mph rather than sustained wind at that strength.
&&
.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1047 PM MDT Sat Sep 27 2025
Clouds are forecast to gradually increase from south to north
overnight tonight with most terminals VFR but BKN by
11-13z/5-7am Sunday...courtesy of a low pressure system to our
south over the Four Corners region. We continue to evaluate and
tweak timing in the TAFs for this push of moisture based on the
latest NBM and HREF trends. KSUN will reside in the northwest
fringes of the cloud cover and may be able to hold SCT. Some of
the high-res CAMs do develop isolated light shower activity
Sunday morning (last two runs now), but confidence remains very
low on occurrence, coverage, and placement, thus will not add
VCSH to any of the TAF sites with the 06z forecast package (but
this will continue to be evaluated as well). By Sunday
afternoon, moisture continues to trend higher with a general
increase in diurnal convective development noted across the
suite of CAMs (supported by NBM PoP trends as well)...it`s just
enough that we`ve introduced PROB30 groups for -TSRA everywhere
but KSUN starting between 19-21z/1-3pm. Looking at model
forecast soundings, traditional instability/CAPE as well as
DCAPE are marginal (and DCAPE less than 1,000 J/kg is
accompanied by PWATs increasing above 0.8 inches and pretty
modest resultant HREF ensemble max wind gusts), suggesting
thunderstorms should stay fairly weak with not much wind
potential (possible gusts only to 25kts). With sfc winds
otherwise generally holding 10kts or less and cigs remaining
VFR, and modest winds aloft as well, this thunderstorm potential
will be the main aviation threat to watch for the afternoon and
early evening. Shower/t-storm potential wanes for a period by
late Sunday evening leading to a quiet night, but more
precipitation is on the way for Monday and Tuesday as a
shortwave trough pushes through from the west.
&&
.FIRE WEATHER...
Issued at 124 AM MDT Sun Sep 28 2025
Today and Mon feature the breakdown of the upper level ridge
with isolated showers and thunderstorms today, mainly in the
south and east. This continues with more coverage on Mon, which
covers the entire forecast area of responsibility. On Mon night
through Tue night moderate to heavy rain is expected over the
entire forecast area. This is coming from a trough that sets up
on the OR-WA coast and keeps pumping moist air in from the
southwest. Rain looks especially heavy for the Salmon-Challis
and northern Sawtooth NFs. Wind will be stronger, but the
temperatures will be falling closer to normal for the time of
year and humidity will eventually staying above 50 percent for
most locations during the period, at least for a while. A
warming and drying out develops on Wed as the new fiscal year
kicks in, possibly working as a fire season-ender, if the
forecast pans out.
&&
.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
SHORT TERM...Messick
LONG TERM...Messick
AVIATION...01
FIRE WEATHER...Messick

NWS PIH Office Area Forecast Discussion