371
FXUS65 KBOI 022049
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
249 PM MDT Sun Jun 2 2024

.SHORT TERM...Tonight through Tuesday night...A strong surge of
moisture will flow into our area this evening. It is still
forecast to qualify as a moderate atmospheric river. While lots
of the moisture will struggle to make it over the Cascades, we
are still forecasting near record amounts of PW in our area and
QPF for the Mountains. PWs of just under an inch throughout the
event and orographic effect support rainfall amounts of near to
just under an inch in mountain valleys, and 2-3 inches on
mountain ridges. There will be some terrain shadowing,
especially NE of the Steens and the Owyhees, but valley
locations near the foothills will see amounts of 0.2-0.6 inches,
strongly correlated to proximity to the terrain. Shadowed
regions and portions of far SE Oregon will still see some rain,
but closer to 0.05 inches. As mentioned in the morning
discussion QPFs did come down a hair with this latest forecast,
especially as early observations from the coast are on the lower
end of the forecasted range, but storm total amounts didn`t
actually change that much even taking these into account. While
the trend of underachieving could continue leading to a wide
range of outcomes, the current forecast is still the most likely
scenario. Steady rainfall will last from about Sunday evening
to Monday evening, though most Oregon locations will end closer
to noon on Monday.

Monday afternoon and evening, in the mountains, some low-level
instability will aid in the development of banded terrain driven
storms and heavier showers. High resolution models are
consistently resolving these types of training showers in
portions of the West Central Mountains. These storms will cause
locally much higher rainfall as they train over the same areas
downstream of terrain. The current QPFs, banded storms, and
snowmelt augmentation has warranted the issuance of a Flood
Watch. The timing of the Flood Watch is offset from the
beginning and ending of rain as it may take a few hours for
water to travel to areas of elevated flood potential. Mud or
rockslides are also possible in the watch area, especially under
banded storms.

Monday afternoon will also be breezy for SE Oregon, far SW
Idaho, and the Magic Valley. Gusts of 35-45 mph are possible
with sustained winds near 20-25 mph. Some clearing Tuesday
morning could allow for fog and low stratus clouds to form in
valleys, especially in areas with poor drainage where standing
water and saturated surface soils will contribute to fog
development. As the low pressure and moisture move east,
temperatures increase on Tuesday as high pressure builds and a
sharp warming trend begins.

.LONG TERM...Wednesday through Sunday...Models remain consistent in
showing an upper ridge over the southern U.S. expanding northward
and persisting into next weekend. The expansion of the ridge results
in unseasonably hot temperatures developing by Thursday and lasting
into the weekend. There remains a high probability (60-90 percent
chance) of high temperatures exceeding 90F in the lower valleys
(including the Treasure Valley) and about a 30 percent chance the
Ontario area in the Lower Treasure Valley could reach 100F (which
would be near record territory). Models also remain consistent in
showing enough moisture available for a chance (20-30 percent) of
showers and afternoon/evening thunderstorms each day beginning
Thursday.

&&

.AVIATION...VFR the rest of the afternoon with deteriorating
conditions tonight. Low confidence (20-40 percent chance) of rain
reaching KBNO-KBKE-KMYL before 03/00Z, high confidence (80-90
percent chance) of MVFR conditions and mountain obscurations with
rain developing across the north (KBKE-KMYL) between 03/00Z and
03/06Z. Chance of showers (40-60 percent) impacting Treasure Valley
terminals between 03/00Z-03/06Z. Widespread MVFR/IFR in rain and low
clouds over the KBKE and KMYL areas after 03/06Z with areas of MVFR
to the south. Mountains obscured. Areas of LLWS developing after
03/02Z. Surface winds: SW-NW 10-20 kt with gusts to 30 kt, except
becoming SE 5-15 kt in the Treasure Valley after 03/06z. 10k ft MSL
winds: W-SW 20-30 kt increasing to 35-50 kt after 06z.

KBOI...VFR. W to NW winds 10 kt or less becoming SE after 03/06z.
Low confidence (30-40 percent chance) of rain reaching the terminal
between 03/00-03/06Z, high confidence (85-90 percent chance) of rain
and low VFR ceilings after 03/06Z.

&&

.HYDROLOGY...Still looking for an unusually wet period
tonight and Monday over the area, particularly across the West-
central and Boise Mountains where rainfall totals of 1 to 2 inches
with locally higher amounts area anticipated by Monday evening.
Near-surface soil is slightly drier than is normal for this time of
year, so there is some room for at least initial rainfall to be
absorbed by the soil. Mainstem rivers currently are not forecast to
reach flood stage but some smaller tributary creeks and streams
could reach or exceed bankfull resulting in minor flooding. Mud and
rock slides could be an issue in steep terrain as well. Use caution
if traveling in the mountains on Monday and Tuesday, and avoid
recreation near rivers and creeks in mountainous and steep terrain.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Flood Watch from midnight MDT tonight through Monday evening
     IDZ011-013.
OR...None.

&&

$$

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