480
FXUS65 KBOI 230316
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
816 PM MST Fri Nov 22 2024

.DISCUSSION...A persistent upper-level trough remains anchored
off the Pacific Northwest coast this evening, with several
embedded low-pressure systems rotating within it. As of 8:00 PM
MST, water vapor imagery reveals at least five distinct low-
pressure circulations, including a significant low near 128W
and 40N, just off the Oregon coast. This system is expected to
traverse the forecast area late tonight through Saturday
morning, bringing a return of atmospheric river moisture and
widespread precipitation.

Radar imagery has shown increasing activity over the past few
hours, with further intensification anticipated within the next
four hours. Unlike last night, when precipitation was driven
primarily by shortwave energy and orographic lift, tonights
activity appears supported by a jet streak moving across
northern California. This feature is fostering embedded
convective bands, a few of which have already produced
lightning. Additionally, a slow-moving cold front approaching
from the west may enhance precipitation across southeast Oregon
and the West Central Mountains by Saturday morning.

Snow levels currently hover around 7,500 feet but will
gradually lower to about 4,500 feet by Saturday afternoon with
the passage of the cold front. Brief periods of heavy rain, and
heavy snow above 7,500 feet, are likely overnight as the system
and cold front progress across the region. Mountain areas could
receive another 0.50 to 1 inch of rainfall overnight, while
lower valleys in southeast Oregon and the Lower Treasure Valley
may see 0.25 to 0.50 inches. The Upper Treasure Valley will
likely receive less precipitation (0.10 to 0.20 inches) due to
shadowing, except where convective showers occur.

While thunderstorms are unlikely overnight (5% chance), the
likelihood increases to 20% on Saturday afternoon following the
frontal passage, as the upper-level trough moves inland. A few
low-topped thunderstorms are possible, with the strongest storms
capable of producing small hail. The current forecast covers
these developments comprehensively, so no updates are required
at this time.

&&

.AVIATION...Mostly VFR outside of precipitation. However, areas
of rain/snow will cause MVFR/IFR at times, especially in higher
terrain where Mountains will be obscured. Snow levels 8000 feet
tonight will come down to around 4500 feet on Saturday. Surface
winds: S-SE 5-15 kt with gusts to 40 kts south of KBNO. Winds
aloft at 10kft MSL: SW 35 to 45 kts.

.KBOI...VFR to occasionally MVFR ceilings in periods of rain
tonight. Surface winds: SE 10 to 15 kts with gusts to 25 kts.

WEEKEND OUTLOOK...A cold front will move through on Saturday
bringing widespread precipitation. Cloud ceilings will be VFR to
MVFR in valley locations and MVFR to IFR over higher terrain
with mountains obscured. Snow levels with fall to 4500 feet by
Saturday afternoon. Precipitation decreasing on Sunday. Surface
winds will gradually shift to W-NW at 5-15 kt.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Sunday night...At 2 PM MST radar
sees only scattered light rain showers in the Boise Mountains.
The rest of our CWA is cloudy, breezy, and mild, except very
windy at higher elevations in southern Harney County. Winds
will decrease there this evening. Cold front is breaking up in
eastern Oregon but the portion in ne California should redevelop
tonight as a vort center in the Pacific near 40/130 approaches.
A new warm front is expected to form in southeast Oregon later
tonight and move northeastward through our CWA overnight and
Saturday morning, followed by the cold front moving eastward
later Saturday. Instability near the cold front looks strong
enough for a slight chance of thunderstorms on the Idaho side
Saturday afternoon. Models continue to show moderate to heavy
pcpn (1-2 inches total liquid equivalent) tonight through
Saturday in eastern Oregon and west central Idaho. Snow level
will stay above 6000 feet MSL until Saturday morning so most
pcpn will be rain. Rivers will rise but not to flood stage.
Rain will change to snow down to 5000 feet Saturday afternoon
behind the cold front, then 4000 feet Saturday evening, and
down to valley floors Sunday morning when pcpn is nearly over.
Total snowfall will be 12-18 inches where pcpn stays all snow,
but only a couple inches near the 5000 foot level. Most of the
cold air has been mixed out of the lower valleys so we don`t
expect a surprise snowfall at low elevations as occurred with
the last storm.  Rain and snow showers will continue Sunday
on the Idaho side as the upper trough moves inland and through
the area. Brief upper ridging will pass through late Sunday
followed by another Pacific trough and chance of light snow in
eastern Oregon Sunday night. Temperatures will continue to
slowly lower through Sunday night but only to near normal for
late November.

LONG TERM...Monday through Friday...Ensembles are in excellent
agreement that an upper low centered off the coast of Oregon
Monday morning will move east and open up Tuesday, then move
through our area Tuesday night. This will keep relatively high
chances of precipitation (30-80%) going through the early part
of the long term period. Models remain in above-normal agreement
on the flow all the way through Friday, so this is a high-
confidence long term forecast. As the trough moves east
Wednesday, fairly dry air will move into the area on NW flow
aloft. This will lower our chance of precipitation to just a
slight chance 10-25%) in the higher terrain Wed through Friday.
Temps will start the week a few degrees above normal, but then
fall to near normal Tuesday and remain a few degrees below
normal through the end of the period. Thanksgiving Day is
expected to be dry and cool with light winds and partly cloudy
skies.

&&

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

&&

$$

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