Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 230258
AFDBYZ

Area Forecast Discussion...Updated
National Weather Service Billings MT
858 PM MDT Sun Apr 22 2018

.UPDATE...

Current forecast looks to be in good shape so no major updates
are planned this evening. IR satellite showing enhancing cloud
tops across southwest Montana this evening as a result of a
developing storm system that will affect our forecast area tonight
through the day Monday. Radar is filling in across our western
counties and light rain has begun the past half hour in
Livingston. Look for the precipitation to increase in coverage and
spread east during the night. The rain will change to snow very
late from about Big Timber westward and areas south along the
Beartooth/Absaroka Foothills where a snow advisory is in effect
from late tonight through Monday morning. Those in the snow
advisory areas may want to plan a little extra time for the
morning drive tomorrow. Hooley

&&

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

No big changes in the short term forecast. Models in good
agreement with precipitation getting going over western zones
early this evening as strong jet divergence develops overhead.
Can`t rule out an isolated thunderstorm early this evening western
mountains/foothills, but CAPE not supportive of a mention in
zones at this point so will leave out for now and monitor radar
trends. Precipitation will spread from west to east tonight into
tomorrow morning as the upper dynamics press across the area. The
upper divergence will deepen low pressure below 800mb which in
turn will increase low level east to northeast winds tonight. This
upslope flow will get quite deep and combine with conditional
instability to bring a very efficient precipitation producing set
up through at least tomorrow afternoon. QPF amounts from all the
models look impressive with 0.75 to 1.25 inch amounts looking
widespread from tonight through tomorrow. Latest ensemble mean for
Billings showing 0.90 inches of precipitation. With high
confidence in precipitation have categorical precip chances for
most locations by tomorrow morning when strongest forcing aloft
moves into the area.

Big question then is how much of this precipitation will fall as
snow and then actually accumulate given the warmer ground
conditions and the late april sun angle. Ran snow ratios based on
surface temperatures and came up with advisory type accumulations
for the western foothills from late tonight through about noon,
and will issue an advisory with this package for those locations.

Elsewhere expect that locations below 4500 feet will likely see an
inch or less accumulation on unpaved surfaces as the dynamic
cooling arrives during the day tomorrow over central and eastern
zones. The exception may be the higher hills and foothills from
southern Big Horn and Sheridan counties east into Powder River and
Carter counties. Snow rates may be heavy enough to produce
advisory accumulations in these elevated locations despite the
daytime, and if snow continues into Monday night that could also allow
for better accumulations. Would not expect roads to be a problem
in general during the daylight hours, but Monday night could be a
different story. Still a bit of time to see how this system plays
out before going with a highlite for these southern zones and
will let mid-shift interrogate another model run before pulling
the trigger.

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

Hydrology...with precipitation expected to be widespread tonight
into Monday night expect to see some rises on area waterways. At
this time flooding is not expected but will monitor the
hydrographs closely, especially in foothills areas where snow melt
is ongoing. Chambers

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

Wednesday starts out sunny with downslope winds, but a clipper
system will move through late in the day into the Wednesday night.
Cant rule out a stray shower as winds turn northerly and cooler
air drops in, but this looks like a mainly dry frontal push.
Amplified ridge aloft pushes into the area late Thursday into
Friday for dry and warmer conditions. Next storm system begins to
take shape just west of the area on Saturday. Right now Saturday
looks dry, but timing differences could bring showers in a bit
earlier so will have to keep an eye on that. This system could get
interesting as it looks to tap some gulf moisture and slide in
negatively tilted from Wyoming, with the ingredients for a multi-
day widespread precipitation producing system. Could be some
thunderstorm potential mixed in too. Still a long way out though
but bears monitoring this week. Chambers

&&

.AVIATION...

A storm system will impact the area tonight through Monday night.
Expect VFR conditions to deteriorate from west to east tonight as
precipitation develops. MVFR to IFR conditions are anticipated
tonight west of a Forsyth to Sheridan line, with all areas seeing
MVFR to IFR at times on Monday. Mountain obscuration will become
predominant by late this evening over the Beartooth/Absarok and
Crazy mountains, and by sunrise over the Bighorns. Precipitation
will start as rain but mix with or change over to snow in most
areas by Monday morning. Chambers

&&

.PRELIMINARY POINT TEMP/POPS...

    Mon     Tue     Wed     Thu     Fri     Sat     Sun
-----------------------------------------------------------
BIL 038/043 030/057 037/070 038/063 042/073 042/072 041/070
    99/O    20/U    00/U    21/U    00/U    01/U    22/W
LVM 034/040 025/057 031/065 033/065 036/070 037/069 037/063
    ++/O    20/B    01/U    21/U    01/U    12/W    22/W
HDN 038/044 031/058 033/070 036/061 040/072 041/072 040/071
    89/O    31/U    00/U    21/U    00/U    01/U    22/W
MLS 040/045 031/056 035/067 037/062 040/071 041/070 042/069
    69/R    30/U    00/U    10/U    00/U    00/U    12/W
4BQ 040/047 032/052 033/066 037/059 039/069 039/069 040/069
    49/R    71/B    00/U    11/U    00/U    00/U    22/W
BHK 039/044 030/053 032/066 036/056 037/067 039/066 040/064
    29/R    60/U    00/U    00/U    00/U    00/U    12/W
SHR 039/042 028/054 027/066 035/061 036/070 039/070 040/070
    69/R    62/O    00/U    21/U    00/U    11/U    23/W

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory in effect from midnight tonight to
      noon MDT Monday FOR ZONES 40-41-56-64>66.
WY...None.

&&

$$
weather.gov/billings


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