354
FXUS65 KBYZ 221715
AFDBYZ

Area Forecast Discussion...Updated
National Weather Service Billings MT
1115 AM MDT Wed May 22 2024

.UPDATE...

The latest river level forecasts have the Little Bighorn River at
Hardin exceeding minor flood stage. A Flood Watch has been issued.
River levels are forecast to crest late Thursday into early Friday
around 9ft which is 1 foot above minor flood stage. Heavy
precipitation of greater than 1 inch is expected in this area
starting tonight and lasting through late Thursday. This will
combine with precipitation anomalies of 150-300% to create
favorable flood conditions. Local residents should monitor river
levels and be prepared for possible flood warnings. Torgerson

&&

.DISCUSSION...
Today through Thursday night...

Deterministic storm tracks have come into good enough agreement to
have high confidence in the system bringing widespread moderate to
locally heavy rainfall and heavy mountain snow from Wednesday
night through Thursday night. Still a big spread on how much
precipitation will fall in a given area due to smaller differences
in storm track and speed, but even the precipitation reasonable floor
is a significant number. Here are the current NBM values for the 25th/75th/Mean
and deterministic forecast values.

Location        Lowest (25%) Highest (75%)  Mean     NBM Forecast
Big Timber      0.69 in      1.45 in        1.10 in    1.13 in
Billings        0.62 in      1.63 in        1.16 in    1.34 in
Hardin          0.61 in      1.71 in        1.22 in    1.42 in
Sheridan        0.40 in      1.39 in        0.96 in    1.17 in
Miles City      0.30 in      1.18 in        0.81 in    0.86 in
Broadus         0.23 in      0.98 in        0.65 in    1.16 in

The forecast values for areas east of Hardin are higher than the
mean at this time due to uncertainty on whether the upper low will
cut across far SE MT or stay south over NE WY. The Canadian/GFS
solutions are further south while the EC cuts the corner. With the
GFS/Canadian being ~2/3 of the NBM deterministic, the forecast for
these areas is higher than the mean, and in some areas higher than
the 75th percentile (in other words, on the high side of available
guidance).

Snowfall with this system continues to trend higher over area
mountains as the storm track has become better defined in a
favorable position to generate precipitation in our area. 700 mb
temperatures drop to around -8C Thursday as the core of the upper
low moves through the area, dropping snow levels down to 5500 feet
or so. Dynamic cooling and snowfall rates could push that down to
4000 feet which would bring at least a slight chance for
accumulating snow into the higher hills across the area. Current
forecast shows 10 to 18 inches being common snow totals across the
mountains, with western foothills above 5000 feet (Nye, Red
Lodge, Wilsall) in the Trace to 5 inch range. The higher hills
over northern Stillwater and Sweet Grass counties are also showing
2 to 5 inches with the aforementioned stronger dynamic cooling
where upslope winds are enhanced. The heaviest
snowfall/precipitation with this system will start in the west
overnight tonight, central and west tomorrow morning, central and
east tomorrow afternoo/evening before the system shifts out of the
area late Thursday night.

Winter Highlites...Upgraded the western mountains to a Winter
Storm Warning starting 6pm this evening through Thursday evening.
Posted a Winter Storm Watch for the Bighorn and Pryor mountains
starting 6am Thursday through 6am Friday. Will likely need
additional Winter Weather Advisories for western foothills and
valley locations. Will let the next shift take another look at the
dynamic cooling influence on the snow levels to make that call.
Overall the advisory areas will be mainly for snow on trees/power
lines and poor visibility as the roads should stay wet or at worst
slushy during this event.

Hydrology...Latest stream forecasts not showing a threat for
flooding at this time. Cold temperatures aloft should keep the
snowpack locked in and the rainfall, while intense for a period of
time Thursday, does not look heavy enough to cause any significant
flooding issues. That said you can expect ponding of water on
roads and in low lying areas, along with rises on small streams
and creeks. Those with interests near smaller waterways should
move livestock and equipment away from them, as well as out of low
lying areas that may hold water as a precaution before
precipitation kicks in tonight. Chambers


Friday through Wednesday...

As the previous system moves out of the region, and shortwave
ridging moves in, a slight chance (25-35% over the mountains,
10-20% over the plains) for precipitation will linger into Friday
afternoon. Then, Friday evening into early Saturday morning look
to remain fairly dry.

Energy from the next wave moving into the region is expected to
increase the chance of precip for Saturday afternoon through late
Sunday. Currently, for locations from Miles City west, there is a
50-60% chance (60-70% over the mountains) of at least 0.25" of
precip over the weekend. Mountain snow totals have trended down
as the snow level is expected to only briefly drop to 7,000 ft
overnight, but otherwise remain above 8,000 ft during the daytime.
Currently, there is only around a 20% chance of at least 6" of
snow over the mountains.

For Monday into midweek, an upper level ridge is expected to build
in, bringing mostly dry conditions and a warming trend. Beyond
that, the pattern becomes more uncertain, with some models holding
on to the ridging and others beginning to move another trough
into the region by late Wednesday. Additionally, the models that
do bring in a trough vary in strength and placement at this time.

High temperatures will be in the mid 60s to low 70s Friday
through Saturday, before cooling slightly into the low to mid 60s
Sunday. Then, as ridging builds in, highs increase into the 70s
Monday and upper 70s to low 80s for Tuesday and Wednesday. Archer


&&

.AVIATION...

Showers and isolated thunderstorms will begin to increase late
this morning, spreading from west to east through the day and into
Thursday morning. Mountains obscurations will also increase. Under
pockets of heavy rain, conditions will periodically decrease to
MVFR/IFR. Archer

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Thu     Fri     Sat     Sun     Mon     Tue
-------------------------------------------------------
BIL 065 041/049 035/065 042/070 045/063 045/073 048/080
    2/R 7+/T    42/R    04/T    66/T    21/U    01/U
LVM 059 036/042 033/063 039/064 039/060 039/070 044/079
    7/T 9+/T    22/T    17/T    65/R    11/U    01/U
HDN 068 041/051 035/067 039/072 044/063 043/073 045/081
    2/R 5+/T    61/U    02/T    66/T    31/U    00/U
MLS 069 043/054 036/064 041/072 046/064 044/073 047/080
    2/R 39/T    62/R    11/B    55/T    31/U    00/U
4BQ 069 042/057 037/064 040/070 044/061 045/071 046/079
    1/B 18/T    81/U    01/B    45/T    41/U    00/U
BHK 069 041/056 034/062 039/071 043/063 043/070 045/078
    1/B 28/R    82/R    01/B    35/T    32/R    10/U
SHR 065 038/051 031/062 036/068 041/060 039/070 043/078
    1/E 39/T    71/U    01/B    56/T    31/U    00/U

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning in effect from 6 PM this evening to
      midnight MDT Thursday night FOR ZONES 67-68.
     Winter Storm Watch in effect from Thursday morning through
      late Thursday night FOR ZONE 171.
WY...Winter Storm Watch in effect from Thursday morning through
      late Thursday night FOR ZONE 198.

&&

$$
weather.gov/billings