411
FXUS65 KBOI 300223
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
823 PM MDT Mon Apr 29 2024

.DISCUSSION...A cold front moved across southwestern Idaho late
this afternoon and south-central Idaho this evening. The front
brought wind gusts to near 60 mph in the Lower Treasure Valley,
and blowing dust that briefly reduced visibility to a quarter
mile, but the front gradually weakened as it passed through
the Upper Treasure Valley and into the Magic Valley. A Wind
Advisory expired at 7 PM MDT. Gusts to 40 mph and blowing dust
were still being reported in the Magic Valley this evening.
Conditions will improve the rest of tonight, but temperatures
will be colder in all areas overnight through Wednesday as an
upper low passes directly overhead. Scattered instability rain
and snow showers will develop Tuesday, and Wednesday morning
will be cold enough for snow showers down to valley floors
although moisture will be limited. Current forecast describes
things well. No updates.

&&

.AVIATION...VFR. Scattered snow showers in the north and isolated
rain showers elsewhere. Mountains obscured. Snow levels lowering to
3000-4000 ft MSL by Tue/12Z. Surface winds: W 15-25 kt with gusts to
30 kt this evening, decreasing to 5-15 kt by Tue/12Z. Winds aloft at
10kft MSL: W 15-25 kt.

KBOI...VFR with mid clouds. 20% chance of -SHRA from 02-04Z, but
most likely virga. Surface winds: NW 10-15 kt, decreasing after 06Z,
then becoming light and variable after 09Z.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Wednesday night...An unseasonably
cool upper level trough is moving across the Pacific NW today.
This system is bringing a cold front through the area, and has
reached Ontario, OR as of 2pm MDT. This front will continue to
track southeast through the afternoon, and temperatures will
slowly fall behind the front, with rain, snow and graupel
showers mainly north of a line from Burns OR to Boise ID.
Isolated thunderstorms, too. The front is forecast to generate
windy conditions in the upper Treasure and Magic Valleys and
Camas Prairie through this evening where a wind advisory remains
in effect. The upper level trough will remain over the area on
Tuesday, keeping temperatures around 10 to 15 degrees below
normal. The coldest air aloft will reach -30C at 500mb and
extend from a line north of Burns to Boise on Tuesday. Slightly
warmer 500mb temperatures near the NV border will lessen the
chance of showers. Scattered showers will redevelop on Tuesday
afternoon with a slight chance of thunderstorms, where the
coldest air aloft is. Another shortwave moves south into the
upper level trough on Wednesday morning, enhancing the threat of
snow showers over the area. 500mb temperatures reach -33C on
Wednesday morning, which is near record cold for this time of
year. Light snow possible down to the valley floors, but
precipitation will be showery in nature, so some places may see
snow, while others will not. Snow showers will linger over the
mountains on Wednesday afternoon, but warmer air aloft and
ridging will bring drier and more stable conditions by Wednesday
evening.

LONG TERM...Thursday through Monday...Long-term models are
showing poor agreement that gets worse with time. As a result,
most aspects of the forecast have a high degree of uncertainty.
A compact low will move in from the northwest on Thursday,
bringing precipitation through Thursday night with breezy
afternoon winds. Chance of precip is fairly low, 25-35% in
higher terrain and ~10% in lower valleys. Most precipitation
will be rain, but above 5000 ft MSL snow becomes possible since
some colder air will linger from the low earlier in the week.
Instability in central Oregon will support a slight chance of
thunderstorms. The track of this low varies a little model to
model, but most of the disagreement is in the coverage of the
precipitation, hence the mediocre precipitation chances.

A brief calm period will persist Friday into Saturday morning
thanks to a ridge of high pressure. At the same time, a deep
closed low from the Gulf of Alaska will track southeast along
the Canadian west coast. While models are beginning to show more
agreement with each run, they are still not quite there. The
GFS continues to show the low making landfall in California,
and then putting us under a deformation zone as it moves east,
while the 12z EC now shows the core of the low moving directly
overhead, putting us under the coldest part of the airmass. This
is a fairly big change from the 06z EC that showed the low
becoming a longwave trough leading to still unsettled but a
notably weaker weather pattern. The National Blend consensus
has area-wide 30-50% precip chances beginning Saturday afternoon,
dropping to 20-40% Sunday afternoon through the rest of the
long-term. Saturday onward each afternoon will have a chance of
thunderstorms and breeziness. The spread in maximum temperature
forecast is 10-20 degrees each day, with the consensus being
normal temps.

&&

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

&&

$$

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DISCUSSION...LC
AVIATION.....SP
SHORT TERM...KA
LONG TERM....JM