844
FXUS65 KBOI 152028
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
228 PM MDT Sun Sep 15 2024

.SHORT TERM...Tonight through Tuesday night...

KEY POINTS

*This afternoon scattered showers in southeast Oregon,
 thunderstorms in southeast Oregon and south-central Idaho with
 heavy rain and strong winds (30-40 mph gusts) the main concern.

*Heavy rain in showers and thunderstorms could bring localized
 flooding on Monday afternoon-evening, mainly in steep
 drainages and burn scars.

*Temperatures will cool, with highs about 10-20 degrees below
 normal by Tuesday.

A deep low becomes close on the PacNW coast today, and begins to
move directly south of our area. This will bring scattered
showers to SE OR and isolated thunderstorms to SE OR and
S-central ID. Instability is marginal, so if storms develop
gusts to 30-40 mph are in the upper end of what would be
possible in outflows. Winds will also be breezy tonight in the
Lower Snake Plain, with gusts of 20-30 mph. Cloud cover
increases to mostly overcast heading into Monday.

With the low south of us Monday afternoon and evening, moisture
flow and favorable dynamics will bring a stark increase in
precipitation chances. While scattered shower coverage
decreases in SE OR, coverage in SW Idaho increases to
widespread. Decent instability over the West Central Mountains
is supportive of thunderstorm development, with most convective
allowing models showing a cluster of storms developing Monday
afternoon. Gusty winds and heavy rain are again the main concern
with storms. Heavy rain over burn scars, especially those in
steep terrain, poses a localized flash flood risk. This begins a
period of widespread rainfall that lasts through Tuesday
evening. Updated rainfall accumulations by the end of Tuesday
are 0.5-0.8 inches of rain in mountainous areas, up to 1 inch on
ridges, and 0.3-0.5 inches in lower elevations. This continues
a trend of accumulation forecasts decreasing, though as the
event begins to move into range of convective allowing models we
have increasing confidence in our totals. There is still room
for the forecast to come down, given the downward trend each
run, but unless the position of the low changes major rainfall
updates are unlikely.

This system will bring a cooling trend as well as increases to
surface relative humidities. Highs in the Snake Plain on Tuesday
are in the 60s, generally about 15 degrees below normal in the
latest forecast. Breezy conditions are expected each afternoon,
especially Tuesday as the low exits eastward and another fills
in quickly behind it for the long term forecast.

.LONG TERM...Wednesday through Sunday...An upper level low will move
south and east across northern CA and the Great Basin Wednesday and
Thursday. The bulk of the moisture associated with this system is
expected to remain to our south. However, there is a chance of
showers, especially across southern areas closest to the low on
Wednesday. Highest pops Wednesday (around 60 percent) will be near
the Nevada border and generally 15-40 percent chance elsewhere.
Instability is marginal with this system, but enough to justify a
slight chance (less than 20 percent) of thunderstorms mainly near
the Nevada border. Models generally favor a drying trend from west
to east as the upper low moves east Thursday followed by a weak
ridge on Friday. Temperatures will remain below normal but trend
slightly warmer each day. Forecast confidence is low next weekend
but models are showing a weak trough impacting the area in NW flow
resulting in low pops (15-30 percent) with northern areas having the
best chance of precipitation. Temperatures remain 5 to 10 degrees
below normal.

&&

.AVIATION...VFR and mostly cloudy. Scattered showers in SE
Oregon along with a slight chance of afternoon thunderstorms.
Showers and thunderstorms spreading into SW Idaho after Sun/22Z.
Showers continue overnight in SE Oregon. Surface winds: NW 5-15
kt with gusts up to 25 kt in the Snake Plain this evening. Winds
Calm to NW 5-10 kt early Tuesday morning. Winds at 10kft: S
10-20 kt, becoming W 10-20 kt around Mon/06Z.

KBOI...VFR. A 30 percent chance of showers after Mon/06Z.
Surface winds: NW 5-10 kt this afternoon, increasing to 10-15
kt with gusts to 22 kt this evening.

&&

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

&&

$$

www.weather.gov/Boise

Interact with us via social media:
www.facebook.com/NWSBoise
www.twitter.com/NWSBoise

SHORT TERM...JM
LONG TERM....TL
AVIATION.....JM