Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 250228
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
828 PM MDT Mon Apr 24 2017

.UPDATE...
Latest upper air analysis shows 100+ kt H3 jet nosing from the
great basin toward the central Rockies, with increasingly
divergent flow extending thru WY into our cwa. The combination of
increasing synoptic scale ascent and deepening upslope winds in a
moist and unstable airmass has allowed for plenty of showers
across our western cwa this evening. There has been some lightning
but that is diminishing since sunset. Cold advection associated
with Canadian surface high to our northeast has dropped temps to
the 40s...with low-mid 30s noted in NE MT already.

Upper and surface lows will become stacked later tonight in
northeast WY, allowing for significant precipitation along the
easterly upslope side of the Bighorns, extending across our far
southeast. Trailing convergent axis in central MT will keep pcpn
going in our west tonight into early tomorrow as well. Our
northeast parts from Miles City to Baker will remain dry with this
system.

Have made no changes to the inherited winter highlights, which
include a Winter Storm Warning for the Beartooth/Absarokas and
advisories for the Sheridan Foothills and across our far
southeast zones. Low level cooling will be a result of advection
and dynamics, and latest RUC wet bulb zero height forecasts
suggest a change to snow overnight, perhaps not til after 09z for
the lower elevations of Sheridan County. So, these areas will see
a good dose of rain before snow becomes a concern. Further west,
the Beartooth/Red Lodge foothills will also see a change to wet
snow late tonight or early Tuesday, but moisture will begin to
lessen by then. So although some wet accumulation is possible in
the Paradise valley and at Nye and Red Lodge, believe these areas
will see most of the precipitation in the form of rain. Melville
on the east side of the Crazy Mtns should also see some wet
accumulation. Accumulations of 3-6 inches still seems a good call
for our far southeast, especially over the hills and near the WY
border. Will need to keep an eye on Sheridan County per the
upslope. If Sheridan/Dayton/Big Horn change over earlier than
expected, higher snow amounts are possible. Also, the Bighorns
will see heavy snowfall and this looks to be a high end advisory
with up to a foot of snow above 6kft. Travel on I-90 thru Sheridan
and on Highway 212 thru southeast MT will be messy tomorrow
morning.

JKL

&&

.SHORT TERM...valid for Tue and Wed...

A few showers were moving E through the forecast area early this
afternoon. Abundant cloud cover was keeping thunder at bay.

A Pacific jet, seen on water vapor, was diving SE into the four-
corners region and will bring upper divergence to the region
through Tuesday as it carves out a deep trough. An upper low will
form over NE WY by 12Z Tue. and will push E into the Dakotas Tue.
evening. Associated surface low will deepen over E WY tonight.
Models agreed that a bullseye of QPF would develop N of this low
and affect areas from the Sheridan Foothills N and NE into SE MT
tonight through Tue. morning. Strong ENE winds will wrap around
this low into the forecast area. Backdoor cold front will push W
into the area bringing negative 850 mb temperatures as far W as
Yellowstone and Musselshell County Tue. morning. QPF amounts will
be greater than a half inch in some areas around the low.
Soundings showed the precipitation changing to snow around the low
by 12Z Tuesday. Snow ratios combined with expected QPF will allow
for several inches of snow. The snow, although wet, will combine
with the winds to support Advisory conditions. Therefore will have
an Advisory for snow tonight through Tuesday in SE MT and the
Sheridan Foothills.

Time-height cross sections continued to support high snowfall
amounts in the Beartooths/Absarokas and NE Bighorns into Tuesday.
The Bighorns may see Warning criteria snow, but will leave the
Advisory in place for now. Otherwise, rain will mix with snow
across the area overnight through Tue. morning. Some rain/snow mix
will linger in the SE Tue. afternoon. Snow amounts looked too
light to support an Advisory over remaining areas, but this will
need to be watched in subsequent shifts. Highs will reach only the
40s on Tuesday. Did mention some thunder over western zones Tue.
afternoon and evening with LI`s below zero degrees C.

Precipitation winds down Tue. night into early Wednesday under WNW
flow/weak ridging. Another Pacific jet dives SE into the western
U.S. Wed. afternoon through the end of the period, lowering
heights and bringing more showers. Wednesday will be a bit warmer
than Tuesday with highs in the 50s. Arthur

.LONG TERM...valid for Thu...Fri...Sat...Sun...Mon...

Extended period Thursday and Friday is cool with upslope flow
with low heights aloft but fairly disorganized structure to the
upper trough over the region. This will yield a couple of days
with modest precipitation and persistent cold temperatures with
accumulating mountain and likely foothill snowfall. But
accumulation rates will be slow and daytime melting could occur
below 6000 feet so impacts may be limited from a winter
perspective. The amount of precipitation will cumulatively cause
sloppy conditions for non paved roads and will continue to
increase the amount of moisture in the ground and in the snowpack
so streams and rivers will remain at elevated levels for this time
of year.

Upslope flow breaks on Saturday and a transition to an unsettled
northwesterly flow sets up for the remainder of the extended. This
will allow for a minor warmup but with still below normal
temperatures and more shower chances but amounts will not be as
great as the early week or late week events. borsum

&&

.AVIATION...

Increasing upslope flow this evening and overnight will cause
widespread low clouds and a mix of rain and snow. Mountain and
foothill obscurations will become widespread and all terminal
sites will encounter IFR conditions or worse tonight in low cigs
and reduced vis. AAG/borsum


&&

.PRELIMINARY POINT TEMP/POPS...

    Tue     Wed     Thu     Fri     Sat     Sun     Mon
-----------------------------------------------------------
BIL 037/047 035/056 039/049 036/046 034/052 036/058 040/058
    85/O    32/T    45/W    75/R    32/O    11/B    33/R
LVM 036/050 032/054 035/047 030/046 029/051 030/056 034/055
    87/T    46/T    75/W    55/O    42/O    22/R    33/R
HDN 037/049 034/058 039/050 037/048 035/054 035/060 039/060
    85/O    21/E    45/W    56/R    32/R    11/B    33/R
MLS 033/046 031/055 036/053 036/050 035/055 036/060 041/060
    11/E    11/B    23/W    33/R    21/B    11/U    23/R
4BQ 033/044 027/053 032/048 034/046 032/050 033/057 037/059
    77/O    11/B    24/W    44/O    33/O    21/B    33/R
BHK 027/043 024/051 028/051 030/050 031/053 031/057 035/057
    01/E    11/B    12/W    12/O    11/B    11/B    24/R
SHR 034/043 030/052 035/045 034/042 030/046 030/053 036/054
    ++/O    22/T    45/W    66/O    43/O    11/B    32/R

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory in effect until 6 PM MDT Tuesday FOR
      ZONES 36>38-58.
     Winter Storm Warning in effect until 6 PM MDT Tuesday FOR
      ZONE 67.
WY...Winter Weather Advisory in effect until 6 PM MDT Tuesday FOR
      ZONES 98-99.

&&

$$
weather.gov/billings


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