Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 190847

Area Forecast Discussion
National Weather Service Billings MT
247 AM MDT Tue Sep 19 2017

.SHORT TERM...valid for Tdy and Wed...

Satellite imagery shows an energetic trof over the Pac NW and
northern Rockies, with diffluent flow and cold cloud tops
streaming into our cwa from southeast ID and western WY. Pacific
surface front has made it through two thirds of our cwa, and a
line of showers exists just behind this boundary and east of
Billings as of 230 am. Surface analysis shows an area of pressure
falls centered near West Yellowstone, a result of the strong jet
nosing across the northern Great Basin. There is another weaker
jet lifting thru central MT. This coupled jet, along with
diminished post-frontal downsloping, will enhance the pcpn across
our west early this morning, and have adjusted pops accordingly.
Overall the area of showers will track west to east through the
morning and early afternoon, with synoptic descent and increased
downsloping later today. However, cyclonic flow and cold pool
aloft (near -24C at 500mb) will allow for diurnally driven showers
this afternoon. There is some modest instability that extends to
colder than -30C, so would expect a little lightning and perhaps
graupel with any heavier showers. Finally, snow levels near 8000
feet right now over the Beartooth Absarokas should fall to
6500-7000 feet this morning, so expect some snow showers to get to
Cooke City. Beartooth Pass will receive more snow showers and
wind today, of course. Temps will be quite a bit cooler than
Monday with highs in the 50s to near 60F, warmest in our east.

Shower activity will diminish this evening as shortwave exits to
the northeast. Next shortwave dropping through the Pac NW will
bring the next period of orographic precipitation to mainly our
southwest mountains Wednesday and Wednesday night. 700mb temps
expected to rise to near 0C Wednesday before falling again
Wednesday night, so expect some fluctuation in the snow levels.
That being said, the high mountains favored by southwest flow
aloft (i.e. above Cooke City) will see snow accumulations late
Wednesday through Wednesday night, and beyond. Will continue to
highlight the mountain snowfall and wintry conditions for outdoor
recreationists and hunters in our messaging. Shower chances will
begin to spread across our western lower elevations by Wednesday
night. Temps on Wednesday will jump back up to the 60s, and maybe
near 70F across our east, before next dose of cold advection
Wednesday night.

Other issue for Wednesday is wind with strong mixed southwest
winds by afternoon, especially along our foothills including
Livingston to Big Timber. Could actually see some gap flow early
in the day before the airmass destabilizes again. Gusts to 50 mph
look like a good bet, and could see near 60F in the favored spots.
Winds further east like at Billings will also be gusty, in the
30-40 mph range.


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

Did not make much of a change to the going extended forecast.
There was a lot of model spread in the GEFS regarding the
Thursday-Friday system, and the deterministic models disagreed on
forecast details like the timing of the system, and QPF amounts
and placement through the period. So, just nudged forecast
parameters toward the latest blended guidance.

Strong jets will move NE through the forecast area Thu. and Thu.
night, bringing periods of jet divergence to the region. Upper
trough positively tilted over the Pacific NW Thu. morning,
gradually works it`s way into the forecast area through Sat. with
upper lows developing in the trough at times. Moisture will be
over the W and central zones Thu. for chances of showers and
mountain snow showers. The moisture overspreads almost the entire
area Thu. night for a likelihood of rain and snow showers W of
KBIL and rain showers elsewhere. There was upslope flow into the
foothills, but temperatures will be dropping through the 30s,
which may prohibit much snow from accumulating. Chances for
precipitation continue Fri. through Sat., but decrease with time
as the jet energy moves E of the area. Temperatures will be well-
below normal with highs in the 50s to mid 60s on Thu. and in the
40s and 50s on Fri. and Saturday.

The upper trough remains over the region on Sunday into Monday
with decreasing chances for showers. Timing on the trough`s
departure differed between the GFS and ECMWF, so went with climo
PoPs for Tuesday. There will be a slow warming trend Sun. through
Tuesday. Arthur



Conditions will be mainly VFR with localized MVFR in showers
today. There may also be an isolated afternoon thunderstorm. IFR
ceilings are expected in KSHR this morning. Otherwise, gusty NW
surface winds will affect KMLS and KSHR today. Expect widespread
mountain obscurations. Chances for showers will decrease this
evening and end overnight. Mountain obscurations will decrease
through the night. Arthur



    Tdy Wed     Thu     Fri     Sat     Sun     Mon
BIL 057 041/065 044/055 039/046 037/047 036/048 037/053
    7/T 21/N    23/W    75/W    43/W    33/W    21/B
LVM 052 038/057 037/050 034/043 030/044 030/047 030/051
    6/T 43/W    55/W    66/W    33/W    22/W    21/B
HDN 058 038/068 043/057 039/051 037/050 037/051 036/055
    7/T 20/N    23/W    75/W    43/W    33/W    22/W
MLS 061 041/068 044/060 041/053 038/052 037/053 036/056
    5/W 20/N    11/E    54/W    32/W    33/W    11/B
4BQ 057 038/069 044/061 040/053 038/049 037/048 035/052
    6/T 40/U    12/W    34/W    34/W    43/W    11/B
BHK 059 039/068 044/064 041/055 038/053 038/051 036/054
    3/W 20/N    01/B    34/W    34/W    32/W    11/B
SHR 055 034/067 041/056 035/048 033/047 033/045 030/049
    7/T 40/U    23/W    54/W    44/W    44/W    32/W




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