320
FXUS65 KBOI 041528
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
928 AM MDT Sat May 4 2024

.DISCUSSION...Major north Pacific storm now moving onshore
in Oregon. Ahead of it, warm, dry air in our CWA with gusty
southeast to south winds today. Wind Advisory remains in
effect for the Upper Treasure Valley, Western Magic Valley,
and Harney County. Winds will decrease before the rain moves
in. Rainfall will be substantial as the storm`s cold front
moves through: .15 to .50 inch in the Oregon valleys, .50 to
1.00 inch in the Oregon mountains, .40 to 1.00 inch in the
Idaho valleys, and .80 to 1.75 inch in the Idaho mountains.
That is, wetter in Idaho than in Oregon. Forecast also has
a 20 percent chance of thunderstorms in south-central Idaho
this afternoon. Much colder behind the cold front overnight
and Sunday, with a little wet snow down to the valley floors
in eastern Oregon early Sunday morning, and as low as 4000
feet MSL in western Idaho. Current forecast is on track.
No updates at this time.

&&

.AVIATION...VFR today. Rain and high mtn snow moving into
SE OR this morning and into SW ID this afternoon. Isolated
thunderstorms across SW ID this afternoon. Local MVFR/IFR
conditions limited to mountains through this afternoon,
increasing across lower elevations overnight. Mountain
obscuration in precipitation becoming widespread tonight.
Snow levels 7-9kft MSL, lowering to 3-4kft SE OR and 4-6kft
SW ID Sunday morning. Areas of LLWS this morning near KONO
with winds S-SE 35-45 kt at 2000 ft AGL and light winds at
the surface. Surface winds: S-SE 10-25 kt, with gusts 25-40
kt becoming W-NW 10- 20 kt this evening. Winds aloft at 10kft
MSL: S 20-40 kt.

KBOI...VFR today with gusty SE surface winds. Rain this
evening will bring wind shift to NW and periods of MVFR
conditions. After 03Z probability of ceilings below 2kft
increases to 30-50% with a 20-30% chance of IFR conditions
developing.

Sunday Outlook...Rain changing to snow in the mountains will
produce widespread IFR and obscure terrain across w-central
Idaho and e- central Oregon. Snow levels 3500-5500 feet,
lowest in SE Oregon Sunday morning. Elsewhere low VFR-MVFR
with showers continuing through the day. Surface winds:
W 10-20 kt with gusts 20-30 kt.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Monday night...A deep closed low
positioned off the Oregon coast will come onshore near the
OR/CA border this evening. Showers will break out today in the
increasingly unstable airmass ahead of the low center. A
minimal threat for thunderstorms exists over s-central Idaho
where instability and surface heating overlap with a strongly
sheared environment. Gusty easterly winds will strengthen
through the morning, channeling through the Snake Plain and
Baker Valley while a strong southerly flow aloft (30-40kt at
~5kft MSL) mixes to the surface across higher terrain of SE
Oregon. The focus of strongest winds will be across Malheur
County in Oregon and portions of the Treasure Valley where
sustained winds of 25-35 mph and gusts of 40-50 mph are
expected. These areas remain covered by a Wind Advisory.

A band of heavier, steady precipitation will develop along
a cold front which will track into SE Oregon early this
afternoon and SW Idaho by this evening. Precipitation rates
of 0.10-0.20"/hr are expected along this line which will be
accompanied by a wind shift to the W-NW. Snow levels of
7-9kft MSL ahead of the front today will drop to 3-4kft
across SE Oregon and 4-6kft over SW Idaho by Sunday morning.
Areas of Harney county and the western Magic Valley could
see precipitation mix with or change to snow, though surface
temperatures will likely be too warm for any accumulation.
Widespread showers will continue in the mountains through
Monday night as the main upper low is followed by a weaker
trailing trough. Lower valleys catch a brief break Sunday
night into early Monday, before the trailing wave brings
another round of rain/snow.

The axis of heavier precipitation totals through Sunday
night arcs from Baker County into the w-central ID mtns
south into lower Snake Plain. These areas will see totals
of 0.5-1.0" in the valleys and 0.75-1.5" in the mountains.
Otherwise widespread 0.2-0.5" of liquid is expected through
Sunday night. The heaviest snow will be above 5500 feet where
total accumulation of 4-8 inches with locally higher amounts
possible.

LONG TERM...Tuesday through Saturday...Showers continue
Tuesday as the low gradually shifts eastward. An upper ridge
builds along the West Coast on Wednesday while the recently
departed low will circle over the upper Great Plains through the
remainder of the week. Models show some variability in how far
west the low will retrograde and introduce the possibility of a
piece splitting off and resettling over the eastern Great Basin
late in the week. While much of the area dries out starting
Wednesday, a slight chance of showers (~20%) remains over
w-central Idaho through Thursday. Temperatures will slowly
warm through the end of the week.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Wind Advisory until 6 PM MDT this evening IDZ014-016.
OR...Wind Advisory until 5 PM PDT this afternoon ORZ061.

&&

$$

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DISCUSSION...LC
AVIATION.....SP
SHORT TERM...DG
LONG TERM....DG