691
FXUS65 KBOI 031612
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1012 AM MDT Mon Jun 3 2024

.DISCUSSION...QPFs this morning have been lowered by around
0.05-0.1 inches as mesoscale models are showing the steady rain
ending an hour or two earlier than anticipated. However, the
chance for localized heavy rain will persist through the
afternoon as forecast sounding show 200-300 J/kg of CAPE. This
should be enough instability for terrain induced heavy showers
to develop. While there is a slight chance for lightning in
these showers, the CAPE is concentrated in the lower atmosphere,
so storm depth may not be most favorable for lightning. We`re
keeping the Flood Watch out this morning as the earlier end of
steady rain means more time for convective showers this
afternoon. WPC is predicting a 15% chance of flash flooding in
the Central Mtns due to excessive rainfall today. Highest
confidence in flash flooding will be under or near heavy showers
later today. If we fail to see convection produce heavy
showers, the watch will most likely be cancelled early, even
though rivers may still continue to rise after the event. Winds
will still be gusty this afternoon (up to 35-45 mph gusts) in
SE Oregon and far SW Idaho. Confidence in fog for sheltered
mountain valleys Tuesday morning has increased, with near
surface RHs being near/at 100% and clearing skies overnight.

&&

.AVIATION...At 03/16Z mountains obscured with areas of MVFR and
local IFR east of KMYL-KMUO in clouds and rain.  Rain showers and
local mountain obscuration west of KMYL-KMUO, improving from west to
east. A 20 percent chance of thunderstorms in Idaho 03/18Z-03/24Z.
Areas LLWS through 03/18Z. Surface winds: SW-NW 10-20 kt. Gusts to
25-35 kt 03/18Z-04/00Z. 10k ft MSL winds: W 30-50 kt.

KBOI...VFR with scattered rain showers, ending west to east by
03/21Z.  Decreasing clouds after 04/00Z.  Local IFR in fog 04/10Z-
04/14Z. Southeast winds around 10kt shifting to northwest 12-22kt
around 03/18Z, decreasing to 10kt 04/01Z.

&&

.HYDROLOGY...Current hydrologic forecasts indicate that, of the main
stem rivers, only the Middle Fork of the Payette in Crouch will
see a rise to action stage from this rainfall. However,
localized flooding from heavier convective showers are very
difficult to model with great precision, so there could be
impacts like rock slides in steep and mountainous terrain near
the heaviest rainfall totals. Some smaller tributary creeks and
streams could reach or exceed bankfull resulting in minor
flooding. Stages on a few gages have begun to show indications
of rising water, but it could take many hours for the peak of
the water to make its way through the system. Use caution if
traveling in the mountains on Monday and Tuesday.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Wednesday night...The atmospheric
river has moved into the region tonight, bringing widespread
rainfall to the region. As of 2:30 am MDT, the McCall airport
has seen almost a third of an inch of rain, with hi-res models
indicating the stratiform rainfall to continue through about 8
am. Current forecasts show around 2 inches of rain falling over
ridgetops and 1 inch of rain falling in central ID mountain
valleys, with locally higher amounts depicted on a few
ridgetops west of the Long Valley. The Snake River Valley can
still expect to see about half of an inch of rain, while areas
in the rain shadow (eastern side) of the Owyhee Mountains/Blue
Mountains will likely see much less precipitation from the
stratiform event. A cold frontal passage later this morning
will bring a push of convective showers, with a potential for
training thunderstorms, mainly over high terrain in the West
Central Mountains. However, higher wind speeds aloft (30-40 kt
at 700 mb) look to slightly limit the potential for training
shower and thunderstorms. This combination of heavy rain during
the night with the potential for heavy rain from convective
showers in addition to the snowmelt could lead to rapid stream
and creek rises and minor flooding. Therefore, the flood watch
will remain in effect through Monday evening.

The cold frontal passage later this morning will also bring
breezy winds across the region, with gusts near ridgetops
reaching 40-50 mph, and sustained westerly winds near 20 mph
everywhere except the Snake River Valley. This will help with
some initial drying, although patchy valley fog and low stratus
has a potential (~40% chance) to form in sheltered high terrain
valleys that see a combination of overnight clearing with high
rainfall totals from Monday. Another push of moisture across the
Northern Panhandle will allow for some slight precipitation
chances over the northern portions of the West Central Mountains
on Tuesday evening. However, confidence is low due to weak
upper level dynamics. Temperatures will remain near normal as
zonal flow persists through Tuesday. However, much hotter
temperatures are expected to begin on Wednesday with the advent
of an upper level ridge building into the region.

LONG TERM...Thursday through Monday...No significant changes in the
latest deterministic models and ensembles regarding the dominance of
an upper ridge across the region for the remainder of the week and
into the weekend. The summer-like pattern results in temperatures 15-
20 degrees above normal with highs in the 90s across the lower
valleys each day. There remains uncertainty in the amount of
moisture available to support convection. Probabilities of
afternoon/evening showers and thunderstorms remain low (15-30
percent chance). Models somewhat agree that a Pacific trough moves
through early next week for 5-10 degrees of cooling and drier/stable
conditions behind the exiting trough on Monday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Flood Watch through this evening IDZ011-013.
OR...None.

&&

$$

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