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000
FXUS65 KBOI 121556
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
956 AM MDT Fri Apr 12 2024

.DISCUSSION...A warm and somewhat moist/unstable south-southwest
flow aloft exists over the region through this evening with an
upper low poised off the coast. Temperatures are expected to
climb well into the 70s to near 80F this afternoon which will
destabilize the atmosphere and allow for showers and
thunderstorms to develop by early-mid afternoon. The parameters
(including more favorable instability and shear) are most
supportive in SE Oregon, but it does extend into SW Idaho.
Therefore, there remains a marginal risk for severe
thunderstorms, capable of producing strong winds and hail. The
threat persists into the early evening hours then diminishes
after sunset. Forecast is on track for now and thus no updates
are planned.

&&

.AVIATION...Mainly VFR. Scattered showers and thunderstorms
will develop this afternoon and evening. Thunderstorms may
become strong to severe. Storms will produce brief heavy rain,
along with the potential for outflow wind gusts to 50 kt and
large hail. MVFR and brief IFR in storms, along with mountain
obscuration at times. Surface winds outside of storms: SW-SE
10-20 kt, with afternoon gusts 25-35 kt. Winds aloft at 10k ft
MSL: southerly 20-35 kt.

KBOI...Mainly VFR. Scattered showers and thunderstorms will
enter the Treasure Valley generally between 21Z this afternoon
and 06Z this evening. Some storms are expected to become strong
to severe, with potential for large hail and outflow winds to 50
kt. Thunderstorms are more favored west of the terminal, but
outflow winds from nearby storms are expected to reach KBOI at
times. Currently, there is a 40% chance of showers, and 20%
chance of thunderstorms, directly hitting KBOI terminal. Outside
of storms, surface winds will be SE 10-15 kt with gusts 20-25
kt. Confidence in wind direction in the TAF this evening is very
low due to nearby storm activity.

Weekend Outlook...Generally VFR on Saturday. Scattered
thunderstorms in central OR Saturday afternoon, mainly west of
KBNO. Surface winds E-SE 10-20 kt Saturday with gusts 20-30 kt
during the afternoon. Mountain obscuration with MVFR to IFR in
scattered showers and thunderstorms on Sunday. Surface winds SW-
SE 10-20 kt Sunday with gusts 20-30 kt during afternoon. Snow
levels above 8000 ft MSL Saturday, lowering to 6000-7000 ft MSL
Sunday.

&&

.HYDROLOGY...The Owyhee River below Owyhee Dam is expected to
remain above flood stage through early next week, then lowering
below flood stage on Wednesday.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Sunday night...At 2 AM MDT, a
distinct low-pressure system looms near 137 W and 45 N, poised
to drive the development of strong to severe thunderstorms
today. As we progress into the morning, expect a surge in
southwesterly flow aloft, intensifying high-level moisture
across our forecast region. By this afternoon, the low center
will approach the central California coast, ushering in upper-
level diffluence over southeast Oregon. Coupled with shear of
around 50 knots and surface-based CAPE ranging from 500-900
J/kg, conditions appear conducive for the formation of a few
robust, potentially rotating thunderstorms later in the day.

With a dry boundary layer and seasonably low freezing levels,
the primary threats include severe winds and hail. The epicenter
for these strong to severe thunderstorms is forecast to be
across southeast Oregon, where the most favorable parameters
align. While the potential for stronger thunderstorms extends
into parts of southwest Idaho later in the evening, convective
parameters are somewhat less favorable. Nonetheless, model
soundings indicate inverted-V profiles across southwest Idaho,
suggesting outflow winds could impact the lower valleys,
particularly the Treasure Valley, emanating from convection in
southeast Oregon.

Considering these factors, the Storm Prediction Center (SPC)
has extended the outline of a marginal risk for severe
thunderstorms, with a 5% chance of severe wind and hail, into
parts of southwest Idaho. Expect convective activity to wane
late tonight with the diminishing solar heating.

Temperatures are forecast to soar into the upper 70s in the
lower valleys today, with a high likelihood (75%) of surpassing
80 degrees.

The upper-level low will progress inland across central
California and into west-central Nevada on Saturday. This will
sustain warm southeasterly flow across most of our forecast
area, resulting in temperatures roughly 15 degrees above normal.
Depending on cloud cover associated with the approaching low-
pressure system, parts of the western Snake River Plain have a
75-85% chance of reaching or exceeding 80 degrees. Southeast
Oregon may experience slightly cooler conditions and a slight
chance of precipitation, given its proximity to the northeast
flank of the upper low. Presently, the most favorable dynamics
and instability remain west of southeast Oregon, with Harney
County having the highest likelihood for thunderstorms.

Most guidance indicates the low center expanding across all of
Nevada by Sunday, with its northern flank pushing across the
northern Nevada border. This will usher in cooler temperatures
along with showers and thunderstorms, particularly on the
northern edge of the low center across southeast Oregon and into
the West Central Mountains.

LONG TERM...Monday through Friday...The center of an upper
level low is expected to move east into Utah on Monday. This
will decrease shower activity over the forecast area, while
introducing breezy northwest winds in its wake. While
temperatures will be a little cooler on Monday, they are
expected to remain slightly above normal. A large, colder trough
is forecast to traverse through western Canada and reach the
Pacific Northwest by Tuesday. Models widely disagree on how far
west and south this trough will go, which significantly lowers
forecast confidence in wind, temperature, and precipitation
Tuesday and Wednesday. Currently, a little more than half the
ensemble members favor bringing the trough right through
southeast Oregon and southwest Idaho, which would dramatically
lower temperatures to well below normal and bring windy and
showery conditions to the area. This solution will also lower
snow levels to near/at valley floors. The other solutions keep
the trough farther north and east, yielding warmer and drier
conditions with breezy winds at times. Current forecast leans
toward the cooler, showery, windy solution, with the chance for
precipitation reaching 10-30%. Thereafter, model divergence is
high, but are trending toward a warming and drying period toward
the end of the week.

&&

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

&&

$$

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DISCUSSION...BW
AVIATION.....ST
HYDROLOGY...BW
SHORT TERM...JDS
LONG TERM....SH