Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 210308

Area Forecast Discussion...Updated
National Weather Service Billings MT
908 PM MDT Thu Apr 20 2017


Regional radar shows increasing showers to the west this evening
as the next weather system pushes into the region. Precipitation
chances increase over the mountains and across western areas
overnight eventually spreading to central areas by daybreak
Friday as isentropic ascent and QG forcing increases over the
region. Have scaled back pops across central and western foothill
locations initially this evening as models are in agreement in a
slightly later onset. Also, adjusted winds to current trends.
Otherwise, the rest of the forecast remains on track with mountain
snow and low elevation rain expected for much of the area through
Friday. The Winter Weather Advisory for the Bighorn Mountains
still looks to be in good shape. The previous forecast discussion
and an updated aviation discussion are below. STP


.SHORT TERM...valid for Fri and Sat...

Main short term concern continues to be precipitation and high
elevation snow from late tonight through Friday night. 12z models,
in particular the GFS/NAM, threw a curve ball by severely
dropping lower elevation precipitation totals, or shifting the
location of the heaviest precipitation further south. This seems
to be a result of the interaction of the pair of H5-7 circulations
that develop along the divide, one over northern Wyoming and the
other near Great Falls. The drier models develop stronger mid-
level convergence between these systems and thus descent that
works against the low level isentropic ascent underneath. The
ECMWF/UKMET/Canadian have remained consistent with keeping this
mid level forcing separate enough or weak enough to prevent the
destructive lift interference, and the NAM for its part mainly
shifted the location of the heavier precipitation southward a bit.
The overall result of this change is less confidence in
categorical pops and location of precipitation amounts. Lowered
pops along and north of I-90 to Likely category as a first step
and also tweaked precipitation amounts down a tenth of an inch or
so including the Billings area. Don`t want to pounce on one run to
much since overall models have been pretty consistent the past
few days, and the Euro/Canadian remain consistent.

Snow accumulations in the southern mountains really remain
unchanged even with the 12z run adjustments as the highest
precipitation chances/amounts will be in those areas regardless.
Still looking at 8 to 12 inches of heavy wet snow above 7000 feet
with locally heavier amounts possible. The going advisory for the
Bighorn mountains is in good shape and as snow gets going later
tonight mid shift can assess whether an upgrade will be needed.
For the lower elevations will see a mix of rain and snow for areas
west of Billings late tonight into tomorrow morning. Could be a
slushy 1 to 2 inch accumulation of snow on grassy surfaces over
the higher hills and foothills. Lower elevation roads should remin
wet to locally slushy overnight, with wet roads during the day

This system is not going to leave quickly and may linger well
into Friday night across south-central zones. A few of the models
have developed more precipitation over SE MT than was previously
forecast. Can`t rule out some light snow accumulation Friday
night along the hills and foothills from Sheridan county northeast
into western Powder River county either. Will keep tabs on that
potential over the next few model runs.

Ridge aloft brings sunny and dry conditions to the area for
Saturday afternoon with temperatures back in the 60s. Chambers

.LONG TERM...valid for Sun...Mon...Tue...Wed...Thu...

A progressive and unsettled pattern will prevail over the area
through the extended period. The GFS/GEFS showed increasing model
spread with the pattern as the week progresses in time.

The flow will be relatively zonal until Wednesday, with a good
deal of energy moving through the flow. A cold front pushes into
MT on Sunday and moves ESE through the region Sunday night. Fairly
strong Q-vector convergence accompanies the system Sun. night with
at least a half inch of precipitable water. Lifted indices fall
below zero degrees C Sun. afternoon. Showers and possible
thunderstorms will spread E into the Billings area on Sunday, with
a good chance for showers across the entire area Sun. night.
Sunday will be warm ahead of the front, with highs well into the
60s. Monday will be cooler behind the front with continued chances
of showers. Cooler air continues to move into the area Mon. night
with chances for rain and snow showers. Scattered showers will
continue on Tuesday.

A trough will dig over the region on Wednesday, as a strong
Pacific jet dives SE into the Western U.S.. Models had a surface
low over the Intermountain West, extending N into MT. If this
system holds together, it will bring a good chance of rain to the
region. The system remains in the area through Thursday with
continued chances for rain. Arthur



Showers over the western areas and mountains will push slowly
eastward late tonight reaching KBIL toward 12Z-15Z. VFR will
transition to primarily MVFR due to ceilings late tonight. Rain
will continue to spread E through much of the area Friday. Expect
MVFR to prevail with local IFR conditions in heavier precipitation.
Obscurations will become widespread over the mountains tonight
and continue on Friday along with gusty NW surface winds. BT



    Fri     Sat     Sun     Mon     Tue     Wed     Thu
BIL 040/049 037/060 040/067 043/056 038/055 040/057 040/057
    67/R    63/O    12/T    43/W    33/W    34/R    33/R
LVM 034/047 030/059 039/062 037/054 034/054 035/055 034/054
    86/O    51/U    23/T    53/W    45/W    44/R    43/R
HDN 039/053 038/060 037/070 042/058 038/057 038/059 039/058
    37/R    63/O    01/B    43/W    23/W    34/R    34/R
MLS 039/061 041/058 041/068 044/056 038/054 039/058 040/058
    13/R    22/R    11/B    34/W    23/W    23/R    33/R
4BQ 038/058 037/059 040/069 043/057 037/053 037/057 038/056
    14/R    42/R    00/B    34/W    34/W    34/R    34/R
BHK 033/059 035/058 038/065 040/052 032/050 033/054 035/055
    01/B    11/B    11/B    35/W    23/W    24/R    33/R
SHR 037/048 035/057 036/066 040/054 036/053 036/054 037/053
    68/O    73/O    01/B    33/W    34/W    34/R    33/R


WY...Winter Weather Advisory in effect until 6 AM MDT Saturday FOR
      ZONE 98.


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