934
FXUS65 KBOI 220356
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
856 PM MST Thu Nov 21 2024

.DISCUSSION...A large-scale upper-level trough remains
positioned off the Pacific Northwest coast this evening, with
an atmospheric river continuing to target the forecast area
through Friday. Despite strong winds aloft, shadowing effects in
the Treasure Valley are less pronounced this evening as
precipitable water (PW) values climb to the 98th percentile.
Precipitation is further enhanced by an embedded closed low,
visible on water vapor imagery near 131W and 43N at 8:00 PM
MST, moving north-northeast along the coast.

This moisture plume, combined with the closed low, will sustain
widespread precipitation through Friday, with the heaviest
rainfall expected across southeast Oregon, the Lower Treasure
Valley, and into the West Central and Boise Mountains. Snow
levels remain elevated, ranging between 7,500 and 8,500 feet, so
no snow is anticipated in mountain valleys below 7,000 feet.

Expected rainfall totals remain unchanged in the guidance
through Friday morning of 0.20 to 0.30 inches across the lower
valleys of southeast Oregon and the Lower Treasure Valley. The
mountains are expected to receive between 0.50 and 1 inch of
rain below 7,500 feet. The trend of observed precipitation over
the past six hours remains aligned with the forecasted
Quantitative Precipitation Forecast (QPF), affirming the
anticipated total amounts.

As the embedded low tracks northward, the upper-level flow will
shift southward, ushering in warmer air and providing a brief
lull in precipitation for much of the area on Friday afternoon.
Temperatures are forecast to rise 10 to 15 degrees above normal,
with the western Magic Valley seeing the greatest warming with
about a 50% chance of reaching 60F or higher. The current
forecast already accounts for these changes, and no updates are
necessary at this time.

&&

.AVIATION...Widespread rain/snow creating MVFR/IFR at times,
especially in the mountains. Mountain obscuration. Snow levels
5500- 7500 ft MSL. Surface winds: SW to SE 10-20 kt, gusts 25-35
kt, decreasing overnight but continuing still stronger than
normal. Winds aloft at 10kft MSL: SW 35-50 kt, decreasing to 25
to 40 overnight.

.KBOI...Periods of light rain to moderate rain with MVFR ceilings.
Surface winds: SE 10 to 15 kts with gusts up to 25 kt.

WEEKEND OUTLOOK...Periods of MVFR/IFR in precipitation Friday,
with best chance from KBNO-KBOI-KSUN and north. Mountains
obscured. Snow levels around 7000ft MSL. Surface winds SE-SW
10-20 kt, with gusts 25-40 kt. Cold front Fri night thru Sat,
bringing widespread rain/snow and MVFR to LIFR conditions at
times. Lowering snow levels to 4500-5500ft MSL Saturday. Surface
winds becoming SW-NW 5-15 kt, gusts to 20-30 kt. Drier and VFR
Sunday, except in patchy fog.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Saturday night...A slow moving deep
trough off the Pac NW coast will continue to funnel moisture
into the region on a southwest flow. The focus of precipitation
will stay across SE Oregon and portions of SW Idaho through
tonight while remaining north of the western Magic Valley. This
will continue to be a mostly orographic event with the bulk of
precipitation falling across higher terrain while lower
elevations see periods of light rain. As warmer air builds, snow
levels will push above 7000 across the east-central Oregon and
w-central Idaho mtns, and to above 8000 feet along the NV
border. A ripple in the flow on Friday will bring warmer
southerly winds into the region. Most areas will see a break in
the precipitation with possibly some breaks in the clouds across
lower elevations. Temperatures will warm to 10-15 degrees above
normal. The western Magic Valley will be the warmest spot,
seeing a 40% chance of reaching 60 or warmer.

The trough moves onto the coast Friday night, bringing a return
of rain and high mountain snow. A frontal passage early
Saturday will bring a wind shift and initiate a drop in snow
levels while also cutting off the main moisture tap. So while
mtn valleys will likely see a changeover to snow, accumulations
will be light as precipitation intensity decreases. Instability
Saturday afternoon will support showers and possibly a
thunderstorm mostly across the central and northern portions.
Temperatures are 5-10 degrees cooler on Saturday, though still
slightly above normal. Snow accumulation will be heaviest at
7kft MSL and above, where 1-2` of additional snow is expected
through Saturday. This will translate to 1-2" of rain or liquid
equivalent in the mountains while lower elevations could see up
to 0.75".


LONG TERM...Sunday through Thursday...After a weak ridge briefly
passes over the region Sunday, SW flow aloft will redevelop
ahead of an upper level low off the coast. The low is forecast
to open up and move east, moving over our CWA Tue or Tue night.
Relatively dry NW flow will then dominate our weather Wed and
Thanksgiving Day. Overall, this keeps a mention of precipitation
in the forecast through most of the long term, with snow levels
3500-4500 ft AGL Sunday through Tuesday, falling to 2500-3500
Wed and Thanksgiving Day. Although it will be cold enough for
snow on most valley floors Wed and Thu, very little if any
precipitation is forecast in the valleys. Precipitation will
even be light in the mountains, especially compared to what they
are experiencing with the current atmospheric river event. Only
1 to 3 inches of new snow are forecast for the mountains in the
long term period, with only a trace currently forecast for
lower elevations.

&&

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

&&

$$

www.weather.gov/Boise

Interact with us via social media:
www.facebook.com/NWSBoise
www.twitter.com/NWSBoise

DISCUSSION...JDS
AVIATION.....JB
SHORT TERM...DG
LONG TERM....SP