Barrymore, Idaho Weather Forecast Discussion

306
FXUS65 KBOI 281514
AFDBOI
Area Forecast Discussion
National Weather Service Boise ID
914 AM MDT Sun Sep 28 2025
.DISCUSSION...Forecast details in better focus now as models
have come into agreement. We still have a 10-20 percent chance
of afternoon/evening thunderstorms today in the Twin Falls BLM
due to a weakening upper low in Nevada. The main Pacific cold
front is delayed a few hours according to latest models, and
the delay will allow warmer temperatures through Monday on the
Idaho side. The front is now expected inland as far as Harney
County/OR Monday afternoon, to the OR/ID border 8 PM MDT Monday
evening, and south-central ID 3 AM MDT Tuesday. A 15 to 30
percent chance of thunderstorms is forecast as the front
passes, with the better chances in southern areas, followed by
several hours of steady light to moderate stratiform rain
behind the eastward-moving front. WPC has most of our CWA
under a marginal risk of excessive rainfall, even though latest
models continue to decrease total rainfall from previous runs.
Plenty of rain is still forecast, however, with NBM showing
about .40 inch Monday night in the Snake Basin and .60 to .80
inch in the Boise Mountains. This rain should account for most
of the total through Wednesday, and therefore only about half as
much as forecast last Friday. The rain should end from the west
in OR around midnight Monday night, western ID around 9 AM
Tuesday, and south-central ID noon Tuesday. Drying and cooler
later Tuesday, except a 20 to 30 percent chance of rain showers
continuing in northern mountains through Tuesday night into
Wednesday.
Current forecast is on track. Will adjust the details in the
afternoon package.
&&
.AVIATION...VFR. Isolated showers/thunderstorms in S-central ID
(Magic Valley) and eastern Oregon this afternoon after 18z.
Thunderstorms capable of 30kt outflows. Surface winds: Light
and variable, except E-SE 5-10kt across the Snake River Plain
through late morning/early afternoon. Winds aloft at 10kft MSL:
S-SW 10-20 kt.
KBOI...VFR with clear skies. Surface winds: E-SE 4-7 kt this
morning, becoming NW up to 10 kt this afternoon around 20z.
&&
.PREV DISCUSSION...
SHORT TERM...Today through Tuesday night...Warm and mostly dry
through much of today with the last day of high pressure before
a deep Alaskan low. Temps will be about 10 degrees above normal,
with partly cloudy skies. Isolated storms and showers in the
Magic Valley this afternoon are hints of the changing pattern,
as just enough moisture will enter our area to kick off
convection as a closed low provides dynamics for s-central
Idaho. The slight chance of precip continues overnight into
Monday afternoon, before precip chances begin to drastically
rise as the core of the moisture flow begins to move overhead.
Monday afternoon precip chances across the area are around 50%,
rising to 80-90% by Monday night through Tuesday morning. This
will be the peak of expected rainfall through the period, with
chances of most places getting 0.1" are around 70% and the
forecast rainfall totals are 0.2-0.4" for lower elevations and
0.5-1" for mountains and mountain valleys. While this continues
the slightly later trend exhibited by yesterday`s afternoon
forecast, models seem a little more locked in on this solution.
The orography in terrain gives us higher confidence in rainfall
totals for these areas. While shadowing reduces confidence in
rainfall forecasts for valleys, convective enhancement with
CAPE ~500 J/kg may allow for efficient precipitation in these
shadowed areas. The atmosphere will be very moist with PWs near
to just above the 95th percentile, it will just be a battle to
get the favorable dynamics to take advantage of that. In the
SW to W Idaho, high PWs, orography, and convective enhancement
could support showers with heavy rainfall rates, and WPC has
forecasted a marginal risk for excessive rainfall causing flash
flooding.
As the trough axis moves in with the moisture core, temps drop
to near normal Monday and Tuesday. Strong flow aloft will carry
showers and storms quickly, and while Sunday and Monday see
fairly light winds outside SE Oregon, Tuesday afternoon 25-35
mph gusts cover most of s-central and SE Oregon as well as
ridgetops across Oregon and Idaho. After the bulk of the
moisture passes Tuesday morning, precip chances move mostly to
Idaho and drop to 20-40% Tuesday afternoon and evening. With
some remaining CAPE at the same time, there`s roughly a 20%
chance of thunderstorms. The unsettled weather continues into
the long term.
LONG TERM...Wednesday through Sunday...An unsettled pattern
continues as a deep, slow moving, upper-level low continues to
infringe upon on our area. This system will allow for widespread
precipitation through Saturday morning. As this system moves
east, precipitation chances will taper off and be reserved to
the higher terrain in southwest Idaho Saturday afternoon through
Sunday. Temperatures will stay 5-10 degrees below normal
throughout the long-term, dropping a couple degrees each day.
With these cooling temperatures (and therefore decreasing
1000-500mb thicknesses), snow levels will drop to 7500-8500 ft
MSL by Sunday. This means that higher peaks may receive
precipitation in the form of a wintry mix, however little to
now snow accumulation is expected.
All of that being said, there is considerable uncertainty in
this long term forecast. The GFS members build up a ridge off
the Gulf of Alaska, allowing the main trough to close into a
low and remain quasi-stationary over our area through most of
the long-term. While normally this would result in that being
the cooler solution, both the Euro and Canadian show another
push of energy moving across our area, vacating the trough
sooner, but also bringing with it a much cooler airmass. Either
way, there is confidence in below normal temperatures Thursday-
Friday. In terms of precipitation, almost all of the ensemble
members show our area under the influence of troughing through
Saturday, before having the trough shift east late Saturday/
early Sunday. This increases confidence in the unsettled pattern
prevailing through the coming week. Looking at multi-run 24
hour QPF in both the GFS ensemble and the ECMWF AI ensemble,
they have been pretty locked in on another 0.10-0.20 inches of
rain falling through the day Friday in lower valley locations.
Which is the strongest signal for another round of soaking rain
in the long-term.
&&
.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.
OR...None.
&&
$$
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DISCUSSION...LC
AVIATION.....JY
SHORT TERM...JM
LONG TERM....NF

NWS BOI Office Area Forecast Discussion