811
FXUS65 KBOI 021553
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
953 AM MDT Thu May 2 2024

.DISCUSSION...Today, another shortwave is sweeping through the
region, maintaining the risk of showers and thunderstorms. This
morning, it has already reached central Oregon and is slated to
progress east-southeast across the forecast area until this
evening. Observations indicate snowfall at around 4000 feet in
Burns, Oregon, with mainly rain at Drinkwater Pass (3500 feet).
While snow is visibly accumulating, it seems to be less than a
quarter-inch deep, causing slushy conditions on the roads.

High-resolution models from this morning consistently depict
convective showers and thunderstorms forming later today across
Southeast Oregon and along the Nevada/Idaho border. While
significant Convective Available Potential Energy (CAPE) is
present in the area, there`s also expected to be partial
clearing and sufficient forcing from the shortwave to trigger
more showers and thunderstorms in the afternoon. Some of these
showers might be intense enough to produce lightning strikes and
graupel. Light snowfall could occur above 4500 feet. The
forecast has been updated accordingly to include the possibility
of thunderstorms this afternoon.

Looking ahead, a more notable low-pressure system is
anticipated on Saturday, potentially bringing heavy rainfall to
the forecast area. Further details will be provided in the
afternoon forecast package.

&&

.AVIATION...A mix of overcast VFR and MVFR/IFR in rain and snow
showers. Local LIFR possible in snow showers above ~4000 ft MSL snow
levels. Mountain obscuration in precipitation and low clouds.
Isolated thunderstorm development expected this afternoon mostly
west and south of the Snake Plain, although a stray storm could
wander into the Magic Valley. Showers will dissipate by Fri/06z.

Surface Winds: W 10-20 kt with gusts to 35 kt in terrain west of the
Owyhee Mtns, otherwise S-SE 5-15 kt in the Snake River Valley,
becoming W-NW 5-15 kts after Thu/23z. Winds aloft at 10kft MSL: W-NW
20-30 kt south of the Snake Plain, variable 5-10 kt north of the
Snake Plain.

KBOI...VFR with scattered/broken clouds 6000-9000 ft AGL.
Showers are moving in this morning near KBOI, with a 30% chance of
rain showers hitting terminal between Thu/16Z-Thu/22Z, and again
Fri/04Z-06Z. There`s a low chance (<10% chance) in a thunderstorm
near the terminal Thursday afternoon, but most of the thunderstorm
potential exists well south and west of the area. Surface winds: SE
5-15 kt this morning, shifting to NW 5-15 kt around Thu/22z.

Weekend Outlook...Friday will be mostly VFR for the area, but
conditions become unsettled going into Sat/Sun. As the next system
moves in Saturday, skies become overcast. Precipitation moves in
west to east through Saturday, as mostly rain with snow levels at
6000-7000 ft MSL. The core of a colder airmass moves overhead on
Sunday, dropping snow levels to ~5000 ft MSL with consistent
rain/snow through the day (60-80% chance of precip area wide).
Surface winds increase significantly Saturday morning and stay
elevated through Sunday. SE 15-25 kt with gusts 25-35 kt Saturday
afternoon, W 5-15 kt with gusts to 25 kt Sunday.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Saturday night...Precipitation will
expand across the area today as a compact low tracks from the
Oregon coast into SW Idaho. An initial shield of rain/snow will
focus oriented along a NW-SE warm front that lines up over SE
Oregon and far SW Idaho this morning. This area of more
stratiform precipitation isn`t expected to extend beyond the
I-84 corridor this morning before dissipating. Snow levels this
morning will range between 3500-4500 feet and accumulations of
of to 2 inches above 4500 feet are possible through this
morning. Showers will become more widespread this afternoon as
the upper low center passes overhead. Snow levels will rise to
between 4500-6000 feet and any accumulation will be limited to
the higher mtn peaks. While the probability of a thunderstorm
isn`t zero, instability parameters are marginal, and less
impressive than yesterday, so have opted to keep mention of
thunder out of the forecast. Flow aloft will bring wind gusts to
30 mph across open and elevated terrain near the northern NV
border this afternoon otherwise we`ll see lighter winds than
yesterday. Friday brings a break in the active weather as dry
and mild conditions accompany a transiting shortwave ridge.

The next storm will take the form of a deep closed that will
push into SW Oregon by Saturday night. Precipitation will form
along the cold front at the leading edge of the low, spreading
across SE Oregon early Saturday and into SW Idaho through the
evening. There is good agreement among the deterministic models
on the track of the low and development of precipitation along
the cold front. While differences in amounts are notable they
are all fairly robust and support a 70-90% chance of 0.10" or
more of liquid across much of the area. Winds will ramp up on
Saturday as well, first out of the east as air is pulled into
the deep surface low, then shifting to the NW Saturday night
behind the cold front. This will bring sustained winds of 20-30
and gusts of 30-40 ahead of the front on Saturday along the I-84
corridor from Twin Falls to Baker City as well as the over
higher open terrain of SE Oregon. Warmer air ahead of the low
will raise snow levels to 7-9kft MSL for the bulk of the
precipitation through Saturday evening.

LONG TERM...Sunday through Thursday...A quick moving upper
level low will move inland on Sunday, bringing a very cold air
mass along with widespread precipitation to the region.
Temperatures will be about 10-20 degrees below normal through
Monday as the low center moves to our southwest and then lifts
out of the southeast by late Monday. Cold northwest flow will
build in over the region, keeping similar weather for the area
through Tuesday. High terrain will see the heaviest
precipitation, with anywhere from 10-12 inches of snow
anticipated above about 7000 feet by Tuesday morning. Mountain
valleys are currently forecast to see about 1-2 inches by
Tuesday morning, but there is a chance that forecast snowfall
totals will increase as we get closer to the day. Current model
blends show about a 5% chance of the Snake River Valley seeing
snowfall. However, based on the depth of the upper level low and
the forecast trend from the past few nights, there is medium
confidence from this forecaster (50% chance) in at least a trace
of snowfall in most locations on Monday and Tuesday morning.
Snow levels will be around 4000-5000 feet through Monday, with
the potential for lowering to valley floors each night through
Tuesday.

As this low tracks to our east, cold northwest flow will keep
temperatures below normal and a chance for showers each day
through the long term period. The best chance for precipitation
will continue to be over high terrain (40-50% chance) on
Wednesday and Thursday, with low chances over the Snake River
Valley (10-20% chance). Model agreement remains high through
Wednesday with regards to the storm track, but after Thursday,
there`s differences in the deterministic solutions, but better
agreement in the ensemble clusters. A few members show another
deepening low over the region on Thursday, although not quite as
strong as the low this weekend. The majority of members show
agreement in a ridge building in by the end of the week.
Therefore, forecast uncertainty is high after Wednesday.

&&

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

&&

$$

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