Area Forecast Discussion
Issued by NWS Billings, MT

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000
FXUS65 KBYZ 212133
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
333 PM MDT Sun May 21 2017

.SHORT TERM...valid for Mon and Tue...

Northerly flow over Montana with embedded waves in the flow
generating showers. Instability was relatively weak with cape
values 200-400j/kg, with the highest cape over the east. Shear was
weak as well some straight line shear was present over the east.
Expect showers with weak thundestorms to be a little more
organized this afternoon and early this evening and contain some
gusty winds and possibly pea size hail. The best chance of
stronger cells will be over southeast Montana.

Instability wanes this evening, but ascent should continue as a
stronger shortwave moves into southeast Montana early Monday
morning as a back door cold front at 850mb drives through. Will
thus keep the chance of showers going through the night with an
uptick in coverage Monday morning as the stronger wave moves in.
Models have come in line with timing of the wave and it looks to
be Monday morning with the strongest ascent driving south Monday
afternoon. The GFS was more aggressive in generating showers along
the east slopes as low level winds turn northerly. Not a true
upslope event here and believe the GFS is over done in its QPF
generation, but did raise PoPs to likely with strong QC forcing
over the south and along the east slopes. Freezing level drops to
around 9 thousand feet late tonight and would expect a few inches
of snow accumulation in the higher peaks late tonight and Monday
morning.

Tapered PoPs from north to south as forcing moves away and drier
air moves in. Mid level eights rise Tuesday and Tuesday night as
ridging builds in from the west. Monday will be cooler behind the
frontal boundary. Tuesday will see warmer temperatures and should
get back to slightly above normal as 850mb temperatures rise to
around 18c. Locations over the far east will be a little cooler on
Tuesday with highs in the mid 60s there. TWH

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

Models are in fairly good agreement Wednesday through Friday but
diverge somewhat by next weekend. For Wednesday, an upper low will
be situated just north of the Montana/Canadian border with an
associated trough across the Pacific northwest and northern
Rockies. The low will remain fairly stationary or move just
slightly east Thursday into Friday. This will result in a cool and
unsettled cyclonic flow aloft across our forecast area. We will
continue with chances for showers and isolated thunderstorms each
day. A cold front will sweep across the forecast area Wednesday,
which will be the warmest day in the extended. Temperatures ahead
of the front will warm well into the 70s to around 80 degrees.
Readings behind the front will drop a good 10-15 degrees for
Thursday and Friday with upper 50s to lower 60s common. The GFS
keeps the upper low and cyclonic flow across the area Saturday
into Sunday, but the ECMWF tries to shift it east. In any case,
next weekend has the potential to be unsettled and possibly cool
as well. Hooley

&&

.AVIATION...

Expect some scattered showers and thunderstorms for the remainder
of the afternoon and into early evening, mainly along and east of
a Roundup- KBIL-KSHR line. Some of these will produce brief heavy
rain and pea size hail. Shower chances will increase across the
entire forecast area after 06z. Expect some MVFR conditions in and
near any shower/thunderstorm activity. The mountains will be
obscured at times. Otherwise, VFR conditions will prevail through
tonight across the lower elevations. Hooley

&&

.PRELIMINARY POINT TEMP/POPS...

    Mon     Tue     Wed     Thu     Fri     Sat     Sun
-----------------------------------------------------------
BIL 048/066 044/073 052/079 046/065 044/062 043/065 045/068
    36/T    10/U    02/T    23/T    32/T    23/T    33/T
LVM 042/064 038/075 048/073 039/060 037/060 037/063 038/065
    36/T    10/U    02/T    24/T    44/T    43/T    32/T
HDN 047/067 042/073 047/081 045/067 044/064 042/068 043/069
    46/T    10/U    02/T    23/T    32/T    23/T    32/T
MLS 046/065 042/071 049/079 047/067 044/065 044/067 045/070
    55/T    20/U    02/T    22/T    22/T    23/T    33/T
4BQ 045/062 040/068 045/079 045/067 043/064 042/066 043/068
    56/T    20/U    02/T    22/T    22/T    23/T    32/T
BHK 043/061 037/066 041/073 043/065 041/063 040/064 040/067
    56/T    20/U    02/T    41/B    22/T    22/T    22/T
SHR 044/060 036/067 043/079 042/063 039/060 039/062 040/064
    46/T    20/U    02/T    24/T    43/T    33/T    32/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...None.
WY...None.

&&

$$
weather.gov/billings



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