Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 232042
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
242 PM MDT Sun Apr 23 2017

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

One last afternoon of mild weather conditions before a wet and
chilly week kicks in. A very dynamic and unsettled weather pattern
will take over tonight through Tuesday. Concerns are heavy
mountain snow, and some mixed precipitation or snow at lower
elevations by Tuesday morning. This all leads to some increasing
hydro related concerns as well.

For this afternoon and tonight...it is currently pretty quiet
across our CWA as initial piece of energy ahead of upper trough to
the west tracks over central Montana. Much broader energy will
sweep across our CWA from SW to NE tonight spreading showers and
thunderstorms over the area. Deep convection will be somewhat
muted by cloud cover and limited CAPE, but a few thundershowers/storms
still a good bet based on soundings. This activity will be
exiting to the east by morning. A few inches of snow is possible
overnight in the mountains from convective snow showers.

Monday into Tuesday...much stronger short wave energy moves
across the central Rockies and carves out a broad trough across
the entire region. This energy has a good deal of Pacific moisture
associated with it as it takes aim at the SW mountains. Cross
sections show strong ageostrophic flow aiding height falls. This
really kicks in by Monday afternoon and continues through Tuesday
in the high country. Therefore, we have issued a Winter Storm
Watch for the Absaroka/Beartooth Mountains during this time. West
facing slopes will be hit hard Monday, transitioning to the north
facing slopes Monday night into Tuesday.

In the meantime, the lower elevations will see scattered showers
and possibly some thunder spread across the west and central zones
Monday. Monday night a back door cold front is progged to slip in
from the east. This will create upslope over the southwestern
foothills and an overrunning pattern. The 850 mb progs have temps
dropping below zero even at Billings, so we may see showers turn
to snow for the Tuesday morning commute. However, mainly just
grassy or natural terrain areas would likely see some
accumulations. But there may be a tight band of intense wet snow
that develops over Yellowstone, Big Horn and Stillwater counties,
and this will need to be monitored going forward for possible
advisories early Tuesday. The bulk of forcing ends by Tuesday
evening with precipitation winding down.

As for temps, we expect Monday to be relatively seasonal, but
Tuesday will be rather chilly and brisk with highs only in the
40s.

The entire week continues to look unsettled, so we will continue
to update the Hazardous Weather Outlook. Some model scenarios
produce 1 to 2 inches of precipitation for some locations in the
foothills and nearby areas. We will need to watch closely to see
how the land handles all the additional precipitation. For now, we
will stick with mentioning some ponding of water in low lying
areas and some rising water in area streams. Muddy rural roads
will probably become more problematic as well. BT

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

Active and generally cool weather anticipated through the latter
half of the week.

Next in a series of Pacific shortwaves will bring a good chance of
showers Wednesday afternoon and night. Flow over the western
CONUS will amplify by Thursday/Friday, resulting in a developing
cut off low over the great basin and central Rockies. This gives
us the opportunity for a combination of dynamic/upslope
precipitation, with great spread still exhibited by the models.
Latest 12z runs suggest heaviest precipitation along strong
baroclinicity to our south in WY. This seems most plausible at
this time given the strongly amplified upper jet, but could still
give us a somewhat significant period of precipitation especially
in our southern upslope areas, in the form of rain and snow. Low
level temps shown by operational models and most ensembles are
suggestive of some wet snow for our foothills, possibly including
Red Lodge and Sheridan if not places further north, and this will
need to be watched. For now, have adjusted pops to show highest
chances in our south. Have also lowered temps for Friday. After a
couple days with highs in the upper 40s and 50s Wednesday and
Thursday, it looks as if much of our cwa will struggle to rise
above the 40s Friday.

Fate of upper low as it cuts off to our south is still very much
uncertain, as the EC actually lifts it north to the Dakotas and
allows for wrap-around precipitation in our east thru Sunday. A
drier GFS solution keeps ascent to our south by Saturday. For the
forecast, will stick with a blend and show lower pops with the
onset of a warming trend by Saturday. We may see the effects of
next Pacific shortwave by Sunday.

JKL
&&

.AVIATION...

A Pacific disturbance and cold front will bring a good chance of
showers from west to east late this afternoon and tonight. Expect
local MVFR with these showers, and occasional mountain
obscurations in rain and snow. An isolated thunderstorm is also
possible. More showers can be expected on Monday, especially west
of KBIL-KSHR, with local MVFR and mountain obscurations. JKL

&&

.PRELIMINARY POINT TEMP/POPS...

    Mon     Tue     Wed     Thu     Fri     Sat     Sun
-----------------------------------------------------------
BIL 042/058 038/046 036/054 037/051 035/048 034/055 037/059
    45/T    88/O    34/W    54/W    45/R    32/W    11/B
LVM 036/055 034/050 033/053 032/050 030/048 029/053 032/056
    66/T    76/T    46/T    45/W    66/O    42/W    22/W
HDN 042/060 039/048 034/057 038/053 036/049 034/057 035/061
    44/T    87/O    23/W    54/W    44/R    32/W    11/B
MLS 041/058 035/048 032/055 036/051 035/052 035/056 037/061
    41/E    23/O    12/W    32/W    22/R    21/B    10/B
4BQ 042/059 036/048 028/052 032/049 034/047 033/052 035/058
    42/W    65/O    12/W    23/W    24/R    32/W    11/B
BHK 035/054 028/046 025/050 028/050 031/050 031/054 032/057
    51/E    22/S    11/B    12/W    12/O    21/B    10/B
SHR 039/056 037/045 032/053 035/049 034/043 032/049 033/054
    36/T    87/O    23/W    45/W    66/O    52/W    11/B

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch in effect from Monday afternoon through
      Tuesday afternoon FOR ZONE 67.
WY...None.

&&

$$
weather.gov/billings



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