383
FXUS65 KBOI 160229
AFDBOI

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

.DISCUSSION...Strong wind gusts and thunderstorms this evening.
Gusts up to 60 mph from the north recorded near Farewell Bend,
OR, with blowing dust at Ontario. Isolated thunderstorms
developed randomly throughout the CWA this evening, still
ongoing and moving rapidly north. Associated wind gusts near
50 mph at Twin Falls and east of McCall. Current forecast is
in good shape with showers and isolated thunderstorms gradually
increasing through Monday, then heavier and more numerous
showers and much cooler Monday night and Tuesday in our Idaho
zones, as a moist upper low moves from northern Nevada across
southeast Idaho. No updates needed.

&&

.AVIATION...Mostly VFR. Areas of MVFR in scattered showers
and isolated thunderstorms through the night. Surface winds:
generally N to NW 10-20 with gusts to 30 kt, becoming variable
5-15 kt after 08Z. Winds at 10kft: E through S 10-25 kt.

KBOI...VFR. A 30 percent chance of showers after Mon/06Z.
Surface winds: NW 10-15 kt with gusts to 25 kt through 05Z,
then decreasing to less than 10 kt after 08Z.

&&

.PREV DISCUSSION...
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 closed 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.

&&

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

&&

$$

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