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000
FXUS65 KBYZ 230933
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
333 AM MDT Mon Apr 23 2018

.SHORT TERM...valid for Tdy and Tue...

A typically difficult rain vs snow spring forecast today. Weather
system on track to move over our CWA today with QPF guidance
suggesting widespread 0.50-1.00 liquid precipitation. Big
question remains how much of this precipitation will fall as snow
and then actually accumulate given the warmer ground conditions
and the late April sun angle. Bufkit soundings of GFS show warmer
low to mid level temperatures than the NAM and HRRR. Wet Bulb
temps are rather high to get much snow over our central zones.
That said...it would only take the GFS solution being off by 1
degree Celsius or less at 850mb to change rain to snow at
Billings. I suspect we will see at least a mix of rain and snow at
many locations by mid morning, but by then the sun angle will
limit accumulation on grassy surfaces, and all but prevent it on
road surfaces.

The foothills and mountains are another story. Deep upslope flow
will combine with strong jet induced processes to cool the lower
levels this morning and should lead to a complete change over to
snow. So inherited winter weather advisories in western foothills
looks good, and considering some of the steep lapse rates I would
not be surprised to see some higher foothill locations over-
achieve with 6 to 8 inches. Roads though should just be
slushy/sloppy by afternoon.

As the system shifts east it will affect the Pryors and BigHorns
region with northerly upslope and high precip. Several inches of
wet snow is therefore possible in these areas. So will issue a
winter weather advisory for the Big Horns from 9 AM to Midnight.

The other problem area is the higher hills in Powder River and
Carter Counties. The bulk of the forcing and precipitation to
affect this area is likely to occur this evening into the
overnight hours. Dynamic cooling after dark may be enough to
accumulate snowfall on roads. Models suggest road temps going just
below freezing in the Ekalaka vicinity around 10 PM. We believe
it best to let the day shift see how things are unfolding and how
well the models are handling the details rather than handcuff
them. So will hold off on any highlights in the southeast for now.

System exits the area by Tuesday for a dry and seasonally cool
day.

Hydrology...with widespread precipitation today we should see
some rises on area waterways. At this time flooding is not
expected but will monitor the hydrographs closely, especially in
foothill areas where snow melt is ongoing. BT


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

Ridge of high pressure will be established over the region
Wednesday afternoon but a quick moving clipper system will move in
from the north in the evening into Thursday morning, briefly
weakening the ridge. Although not a very strong system, we could
squeeze out some rain from this, mostly west of Miles City and
Broadus, so kept a slight chance of rain in the forecast for this
period. Expect temperatures in the upper 60s and low 70s
Wednesday to cool into the mid 60s Thursday.

The ridge of high pressure quickly rebuilds into the middle part
of the weekend as a large upper level low over Western US builds
the ridge over Montana. Model differences quickly emerge through
the weekend. The EC keeps this system a closed low and moves it
slowly eastwards through the weekend while the GFS opens up the
wave and moves it quickly through Montana on Saturday. Regardless
of the solution, warmer temperatures will round out the work week
and into the start of the weekend with a chance for showers, and
possibly some thunderstorms, returning to the forecast through
the weekend. Dobbs
&&

.AVIATION...

Widespread rain with some pockets of snow (especially along the
mountain foothills) will overspread the region along with
widespread mountain obscurations. Expect IFR conditions through
most of the afternoon with lingering pockets of LIFR conditions
in the heavier showers and low cigs. KSHR/KMLS will see these
conditions develop through late morning as the system moves east.
Improving conditions can be expected to move in gradually from the
west beginning this evening around 00z.
&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Tue     Wed     Thu     Fri     Sat     Sun
-------------------------------------------------------
BIL 041 029/057 036/070 035/066 039/077 044/078 044/071
    +/O 30/U    00/U    20/U    00/U    01/U    22/W
LVM 040 025/056 031/066 032/066 035/075 036/072 037/065
    +/O 30/B    01/U    21/U    01/U    12/W    22/W
HDN 042 030/057 031/071 033/067 036/078 042/079 043/073
    +/O 50/U    00/U    21/U    00/U    01/U    22/W
MLS 043 031/057 035/069 035/064 036/074 044/076 045/072
    +/R 60/U    00/U    10/U    00/U    00/U    11/B
4BQ 047 032/052 032/067 035/061 034/072 042/075 043/070
    +/O 81/B    00/U    21/U    00/U    01/U    12/W
BHK 046 030/053 032/066 037/060 037/070 043/072 044/069
    9/R 80/U    00/U    10/U    00/U    00/U    01/B
SHR 042 028/053 027/070 034/065 034/076 040/078 042/070
    +/O 71/B    00/U    31/U    00/U    01/U    22/W

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory in effect until noon MDT today FOR
      ZONES 40-41-56-64>66.
WY...Winter Weather Advisory in effect from 9 AM this morning to
      midnight MDT tonight FOR ZONE 98.

&&

$$
weather.gov/billings