490
FXUS65 KBOI 021605
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1005 AM MDT Sun Jun 2 2024

.DISCUSSION...A strong moisture surge, which is forecasted to
qualify as a moderate atmospheric river, will spread
precipitation into our area this evening. Precipitation chances
over terrain and just upstream of terrain are near 100% until
monday evening when the moisture moves past our area. Storm
total rainfall forecasts are currently 1" in mountain valleys
and 2" along ridges. A few summits could even see up to 3" of
rain. Rainfall in terrain could be locally much higher as models
Monday afternoon show banded terrain driven storms. This raises
the confidence for flash flood potential as convection is
supposed to kick off after the bulk of the steady rain has
already fallen and near- surface soils are likely to be
saturated. While the latest model consensus generally shows a
decrease in the average QPF, some models show an increase. Our
forecast so far has matches WPC guidance which has been on the
higher end of the model range. Based on the strength of the
moisture surge (~500 kg/m/s), the potential for convection in
the afternoon, and the ENS indicating that models are still
calling for record setting rain: current QPFs are likely fine
until the afternoon forecast package.

This afternoon is still looking breezy, with gusts of 25-30 mph
in the Magic Valley and in SE Oregon. Winds will be even gustier
Monday afternoon, with gusts to 35-40 mph in the same area as a
cold front approaches. Temps this afternoon will be a little
colder than yesterday with the increasing cloud cover, and will
be colder still on Monday with the rain. No major updates to
the previous forecast until the afternoon package.

&&

.AVIATION...VFR today 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 (30-
50 percent) impacting Treasure Valley terminals between 03/00Z-
03/06Z. Areas of LLWS developing after 03/02Z. Surface winds: SW-NW
5-15 kt increasing to 10-20 kt with gusts to 30 kt after 02/18Z. 10k
ft MSL winds: W-SW 15-30 kt.

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

&&

.HYDROLOGY...Current 24 hour precipitation forecasts on Monday show
over 2 inches of rainfall on ridgetops and over 1 inch of
rainfall in valleys in the Central ID and Wallowa Mountains,
with nearly half an inch of rain in the Treasure Valley. 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. However, the long period of steady
rainfall on top of a decent snowpack could still lead to rapid
river rises and instances of minor flooding in streams and
creeks. For instance, Deadwood Summit SNOTEL`s latest reading
shows a snow depth of 35 inches at about 7000 feet with a snow
water equivalent of about 13.8 inches of water. With snow levels
likely around 10,000 feet for this storm, most precipitation
will fall as rain in this twenty four hour period. This along
with warm temperatures this last week will aid in snow melt.
Latest NOHRSC estimates show over 3 inches of water equivalent
will melt from mountain snowpack.

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. There is a
Hydrologic Outlook in effect providing details 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.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Tuesday night...Today looks to be
the last mild weather day for the short term period, with
slightly above normal temperatures and clear skies in the
morning, and widespread rain in Eastern Oregon and in the West
Central Mountains-Weiser River Valley by late tonight. A strong
plume of moisture, known as an atmospheric river, will begin to move
into the Pacific Northwest by late this evening, bringing
record rainfall and the potential for flash flooding due to
significant rain on snow melting. Zonal flow aloft will allow
for significant moisture transport inland, leading to a
prolonged stratiform rainfall event over Monday-early Tuesday.
Current model forecasts show high confidence in a significant
and abnormal rainfall event for our region, with precipitable
water values near 1 inch (99th percentile of values for this
time of year). The Ensemble Forecast Index, or a measure of how
abnormal the weather is for a time of year, shows Monday
reaching the maximum QPF (record rainfall) over the Central ID
Mountains which would mean we could see record breaking rainfall
over much of that region. The WPC has put much of the Central
ID Mountains in a slight risk of excessive rainfall, where the
chance of flash flooding is around 15%.

There is a low chance of convection on Monday night after the
initial wave of precipitation and a weak cold front passes, but
instability will be weak so confidence in thunderstorms is low.
Any slow moving thunderstorms would greatly increase the flash
flooding threat, especially in the mountains and near burn
scars, so this potential will have to be monitored closely.

Some clearing on Tuesday morning could allow for fog and low
stratus clouds to form in valleys, especially in areas with
poor drainage where standing water will contribute to fog
development. As the low pressure system with corresponding
atmospheric river moves east, westerly winds will pick up on
Monday afternoon. Winds will be strongest over southeastern
Oregon, with gusts as high as 40-50 mph over ridgetops.
Temperatures will increase as clearing continues on Tuesday,
with a ridge beginning to develop into the region, beginning a
sharp warming trend for the rest of the week.

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 percent
chance) of high temperatures exceeding 90F in the lower valleys
(including the Treasure Valley) and a low probability (less than 30
percent chance) of reaching 100F in the lower valleys (which would
be near record territory). Models also remain consistent in showing
enough moisture available for a slight chance (less than 30 percent)
of showers and afternoon/evening thunderstorms each day.

&&

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

&&

$$

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DISCUSSION...JM
AVIATION.....TL
HYDROLOGY....JM
SHORT TERM...SA
LONG TERM....BW