234
FXUS65 KBOI 201601
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1001 AM MDT Mon May 20 2024

.DISCUSSION...As of 9:30am MDT, there is already some light
cumulus development over terrain, indicating instability and
some moisture availability for shower and thunderstorm
development later today. Isolated showers and thunderstorms look
to begin around 1pm MDT, with the best chance for convection in
the Lower Snake River Plain and southern portions of the Boise
Mountains. Winds will be gusty and erratic, with outflows up to
40 mph. Strong northwest winds will persist later this afternoon
throughout most of the region, but are already starting across
the Lower Treasure Valley, with KONO gusting to 30 mph.
Unsettled conditions will continue through the week with a much
wetter and colder system still expected on Wednesday-Thursday.
The forecast still looks on track with the latest data, so no
updates are necessary.

&&

.AVIATION...Mainly VFR. Scattered showers and isolated thunderstorms
this afternoon across SW Idaho and higher terrain of SE Oregon.
Mountains obscured in showers with snow levels of 5500-6500 feet.
Thunderstorms capable of small hail and outflow gusts to 35 kt.
Surface winds: W-NW 5-15 kt, becoming 10-20 kt with gusts to 25-30
kt this afternoon. Winds aloft at 10k ft MSL: N-NW 10-25 kt.

KBOI...VFR. Showers developing in the vicinity after 18Z with a 20%
chance of showers hitting the terminal between 19Z and Tue/01Z.
Nearby shower and thunderstorm activity will be capable of producing
outflow wind gusts 30-40 kt that could impact KBOI. W-NW winds
becoming gusty by late morning.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Wednesday night...An active weather
pattern will persist in the short term, bringing early spring
conditions. An upper-level trough today will be followed by a
much stronger closed low-pressure system on Wednesday. Overall,
temperatures will remain well below normal.

This morning, the upper-level trough will gradually shift
southeastward across eastern Washington and Oregon, reaching
south-central Idaho by the afternoon. This will result in the
highest instability and moisture in central Idaho zones (West
Central and Boise Mountains), offering the best chance for
convective showers and thunderstorms. There is a possibility of
convective initiation extending further west over the higher
terrain of Baker County and northern Malheur and Harney
counties, although confidence in this is low. Some showers and
thunderstorms may also reach the Upper Treasure and Western
Magic Valleys later this afternoon. Despite weak overall
convective parameters, with CAPEs around 200 J/KG, conditions
are favorable for gusty outflow winds from mountain convection.
The most favorable outflow winds are expected across the Western
Magic Valley and Eastern Upper Treasure Valley, with gusts up
to 50 MPH potentially causing localized blowing dust.

A transient ridge is forecast for Tuesday as the region remains
between systems. Temperatures are expected to rise
approximately 6 degrees above todays highs but will still be
about 4 degrees below normal.

A much stronger low-pressure system is anticipated to reach the
Pacific Northwest late Tuesday evening, pushing a cold front
through the forecast area on Wednesday morning. Widespread
precipitation will precede the front on Wednesday morning,
followed by gusty westerly winds in the afternoon as the upper-
level low center reaches northern zones. Winds have increased
significantly from previous model runs, approaching wind
advisory levels across Southeast Oregon. Additionally, a strong
upper-level jet will promote orographic lift across the Central
Idaho mountains, leading to prolonged precipitation into
Wednesday evening. The mountains could receive up to 0.75 inches
of rain, while the Western Snake River Plain and areas south
and west could see between 0.10 and 0.20 inches. Snow levels
will start around 7500 feet on Tuesday, gradually lowering to
around 5500 feet by Wednesday evening. Current guidance suggests
a total of 2-4 inches of snow above 6500 feet from Tuesday night
through Wednesday night, with about a 45% chance of McCall
receiving 0.1 inches of snow or more (mainly Wednesday night).

LONG TERM...Thursday through Monday...As a closed low tracks
eastward, cool northerly flow and wrap-around moisture will continue
over the forecast area into Thursday. The best chance for lingering
precipitation will be over northeast Oregon and southwest Idaho,
where a 50-75% chance is forecast. The coldest air, and therefore
the lowest snow levels of 4500-5000 ft MSL, are expected early
Thursday morning...resulting in the potential for light snow to
accumulate down to these elevations. Several inches of snow is
likely over the high mountain terrain of east Oregon and southwest
Idaho through Thursday, with greater than 50% chance of seeing at
least 6 inches of snow above 7000 ft MSL for the west-central Idaho
mountains. Precipitation should taper off throughout the day on
Thursday as the low continues east, but breezy to windy conditions
can be expected in south-central Idaho in the wake of the low.
Thereafter, slightly warmer temperatures are anticipated Friday with
the return of brief ridging. The next low pressure system is
expected to arrive Friday night through Saturday night, supporting a
chance (15-35%) of showers to return to the area. Models still
diverge on how far south this trough will progress, and therefore
the temperatures, wind speeds, and shower extent is still a bit
uncertain. Warmer, drier weather is on track to return Sunday/Monday
as high pressure builds.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.
OR...None.

&&

$$

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DISCUSSION...SA
AVIATION.....DG
SHORT TERM...JDS
LONG TERM....SH