Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 281658
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
958 AM MST Wed Feb 28 2024

.DISCUSSION...Light snow showers continue across Baker County,
Lower Treasure Valley, Upper Weiser Valley and the West Central
Mountains of Idaho this morning. These will continue through the
afternoon but change to rain below 4500 feet as temperatures
increase ahead of the next cold front.

The next winter storm system will approach quickly from the
west-northwest Thursday afternoon. Surface winds increase to
30-40 mph ahead of the cold front, with gusts to 60 mph across
Harney and Malheur Counties in southeast Oregon and Owyhee
County in southwest Idaho (70-90 percent chance of wind gusts
exceeding 55 mph). ECMWF Extreme Forecast Index (EFI) values of
0.95-0.99 further support current forecast wind magnitudes,
which is based on reforecasts from 2003-2024. Given the
magnitude of the wind across a large area (most of southeast
Oregon as well as Owyhee County, Treasure Valley and the West
Central Mountains of Idaho), that power outages and downed trees
will be likely. In addition, conditions will be favorable for
convective segments along the front, especially across southeast
Oregon, for lightning and brief heavy snow/sleet/graupel
reducing visibilities with up to an inch of accumulation.
Guidance suggests the front will be less convective in southwest
Idaho given the later passage of around 03z. However, do expect
heavy rain with the front that could briefly switch to snow or
graupel for little to no accumulation. Temperatures will be
quite warm ahead of the front reaching the low to mid 60s in the
valley. Current forecast and headlines look good for now. Will
add thunderstorm chances in the afternoon package.

&&

.AVIATION...Mainly VFR with broken low/mid level clouds.
Scattered snow showers will continue over e-central Oregon and
w- central Idaho today, impacting KMYL. Rain and snow will
expand across the central Idaho mountains after Thu/06z. Periods
of MVFR/IFR conditions and mountain obscuration in precip. Snow
levels: 2500-3500 ft MSL today, rising to 4500-5500 ft MSL
tonight into Thursday. Surface winds: SE-SW 10-20 kt with gusts
up to 25 kt. Winds aloft at 10kft MSL: SW 20-35 kt.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Friday night...Conditions will
generally be dry this morning before warm overrunning will
enhance snow over the central ID mountains and eastern OR
mountains, especially Baker County this afternoon. Snow will
change over to rain this afternoon below 4000-5000 feet MSL. As
a strong trough digs south out of the Gulf of Alaska, the flow
will shift to the southwest and increase significantly on
Thursday. Warm air will be advected into the area Thursday with
temperatures in the low to mid 60s in the Snake Basin. The NBM
forecast confidence is low on the max temperatures on Thursday
as the 25th/50th/75th percentile has 63/66/68 respectively at
BOI, while the NBM has 61. Also of note will be the strong
surface winds on Thursday, with ECMWF Extreme Forecast Index
(EFI) values of 0.95-0.99 across southeast OR and southwest ID
for wind and wind gusts, showing the magnitude of the event,
which is based on reforecasts from 2003-2024. Surface winds
increase to 30-40 mph on Thursday ahead of the front, with gusts
to 60 mph across Harney and Malheur Counties in SE OR and
Owyhee County in SW Idaho (60-90 percent chance of wind gusts
exceeding 55 mph). 700mb winds of 60-80 kt in eastern OR also
support this. Thus a high wind watch has been upgraded to a
warning for these areas, with a wind advisory for Baker County
OR, the West Central ID Mountains, southern Twin Falls County,
and the Treasure Valley in Idaho. The strong surface winds aloft
will limit any precipitation development to the west central ID
mountains and Sawtooths on Thursday. The cold front with this
system will move across eastern OR late Thursday afternoon and
southwest ID on Thursday night into Friday morning.
Precipitation will increase to widespread (70-95 percent
chance) on Thursday evening along the cold front. Snow levels
will be around 6000-7000 feet MSL ahead of the cold front in SW
Idaho, and around 3000-4000 feet MSL behind the front on
Thursday night west of Burns and Baker City OR. After the cold
front moves through, winds should subside. Snow levels will fall
to the valley floors but there is still some uncertainty as to
whether or not valley floors will see any snow accumulation.
Significant snow expected for the West Central and Boise
Mountains, where a winter weather advisory has been issued
through Saturday. Did not issue a winter storm warning due to
the prolonged event. The snow will come at a relatively slow
pace with 6 to 11 inches of snow in the mountain valleys, and 8
to 17 inches over the mountains from 8pm MST Thursday through
5PM MST Saturday. Temperatures will be around normal today
rising to around 10-15 degrees above normal on Thursday and then
falling back to normal on Friday. Areas with existing snow
cover will see less pronounced warming on today and Thursday.

LONG TERM...Saturday through Wednesday...A large upper level trough
over the Northwest U.S. will continue to promote snow showers over
southeast Oregon and southwest Idaho over the weekend. Snow levels
will lower near/at valley floors early Saturday morning, allowing
the opportunity for light snow accumulations or a mix of rain and
snow below 3000 ft MSL. Moderate snow amounts will persist for the
mountains of southwest Idaho, continuing the long duration snowfall
event. A very brief period of drier conditions will move in behind
the eastward progressing trough late Sunday. Meanwhile, moisture
associated with the next series of troughs will quickly follow,
impacting the forecast area by Monday. This active pattern will
promote gusty southerly winds, along with additional snowfall for
east-central Oregon and west-central Idaho (currently at 25-45%
chance).

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory from 8 AM to 8 PM MST Thursday IDZ011.
     Winter Weather Advisory from 8 PM Thursday to 5 PM MST
     Saturday IDZ011-013.
     Wind Advisory from 8 AM to 11 PM MST Thursday
     IDZ012-014-030-033.
     High Wind Warning from 8 AM to 11 PM MST Thursday IDZ015-029.
OR...Wind Advisory from 7 AM to 10 PM PST Thursday ORZ062.
     High Wind Warning from 8 AM MST /7 AM PST/ to 11 PM MST /10 PM
     PST/ Thursday ORZ061-063-064.

&&

$$

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DISCUSSION...JDS
AVIATION.....ST
SHORT TERM...KA
LONG TERM....SH


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