Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 191511

Area Forecast Discussion
National Weather Service Bismarck ND
1011 AM CDT Mon Mar 19 2018

Issued at 1006 AM CDT Mon Mar 19 2018

Cancelled the Winter Weather Advisory for the western third of the
state with this update. Advisory continues for the south central
and upper James River Valley through noon. Radar and surface
observation trends have shown a decrease in snowfall coverage from
west to east this morning. Observations across the west and
central have also shown areas of lowered visibility even where
snow was light, so patchy fog was added in this update through the
overnight hours.

UPDATE Issued at 634 AM CDT Mon Mar 19 2018

Bowman radar shows snow across eastern Montana and southwest North
Dakota so will keep teh advisory going across teh far west for
now. Snow continues central and east. Current headlines will be
left in place.


.SHORT TERM...(Today and tonight)
Issued at 446 AM CDT Mon Mar 19 2018

Snow will end from west to east today, but will likely continue in
the James River valley until late afternoon. A slippery commute is
expected this morning in many areas, and that`s the main message
we are conveying initially.

As of 09 UTC, differential cyclonic vorticity advection is ongoing
across much of western and central ND downstream of one of several
shortwave troughs associated with a parent vorticity maximum which
is centered in southeastern Alberta per GOES-16 moisture channel
imagery and model-derived 500 mb analyses. This large-scale ascent
is almost entirely responsible for the ongoing snowfall now that
earlier frontogenesis has weakened considerably and the steep mid-
level lapse rates on the order of 7 C/km that were sampled by the
00 UTC Bismarck RAOB have slowly been reduced, too. Radar trends
largely reflect this scenario, as well, with less banded structure
since 06 UTC, though cellular pockets of heavier reflectivity have
persisted from Bismarck southeast toward LaMoure, which is likely
reflective of lingering instability aloft. Regardless, the 00 UTC
global model suite and high-resolution and rapid-refresh guidance
through the 07 UTC RAP and HRRR cycles all support continued light
snow over much of central ND through the morning, and across the
James River valley through the day. This is supported by forcing
associated with the large-scale differential vorticity advection,
which is well-reflected in 500-300 mb Q-Vector convergence in the
global model simulations that gradually shifts eastward today. We
relied on the multi-model consensus to build the PoP forecast for
today, with rapid reductions in snow chances over western ND this
morning, while maintaining 100 percent PoPs central until forcing
wanes after 18 UTC. There`s good model agreement in this scenario,
but the 00 UTC 3-km NAM in particular exemplifies our expectations
in its simulated radar output. Note that the leading mid-level
shortwave that will drive the lingering light snow today is being
forecast to slow its pace with eastward extent, which will result
in snow lingering in the James River valley through the day.

We have maintained the Winter Weather Advisory in most areas with
this forecast release since light snow is still ongoing. However,
we did decide to cancel the advisory across Mountrail, Ward, and
McHenry Counties, where drier northerly low- and mid-level flow
persisted much of the night north of a subtle cool frontal zone,
and which effectively impeded northward progression of snowfall.
While very recent radar trends still support light snow in those
counties the next several hours, event-total amounts will not be
high enough to warrant an advisory. Otherwise, we do expect that
we will be able to begin cancelling the advisory for some of the
counties in southwestern ND in the next several hours concurrent
with the eastward progression of the back edge of steadier snow,
but observational trends through 09 UTC supported maintaining it
for a bit longer with ASOS/AWOS visibilities still in the 1-3SM
range in southwestern ND at this hour. Regardless of where and
when steady snow is still occurring at daybreak, the commute is
expected to be characterized by snow-covered and slippery roads in
many areas. However, as precipitation intensity diminishes, lower
atmospheric temperatures warm slightly, and sub-surface fluxes are
able to assist with surface warming, the METRo model suggests that
roadways will likely warm enough for melting to occur as early as
late morning. That also will support allowing the advisories to
end no later than that time frame.

Finally, it appears that another quick-moving shortwave trough is
set to move into western ND tonight on the southern flank of an
eastward-advancing upper-level low that will be crossing southern
Canada. Model simulations depict a relatively sharp kink in mid-
level height fields with this wave, but differ slightly with its
timing and suggest the deep-layer ascent with it may be somewhat
meager. Even so, there`s enough simulated forcing and QPF signal
in guidance for us to carry 40 to 60 percent PoPs for light snow
in western ND late tonight. Only minor accumulations are
expected, though, given the expected forcing.

.LONG TERM...(Tuesday through Sunday)
Issued at 446 AM CDT Mon Mar 19 2018

An active weather pattern is expected during the long term, with
the main feature of interest the pronounced upper-level troughing
that is expected to become established over the Pacific northwest
by this weekend. Downstream of the developing trough, confidence
is high in broad ridging developing across the Northern Plains in
the midweek time frame, which will herald a warm-up. The 00 UTC
GFS and ECMWF support 850 mb temperatures warming to the 0 to +4 C
range by Thursday, with forecast surface temperatures in the lower
to middle 40s F south and west of the Missouri River beginning on
Wednesday. Given recent snowfall and above-freezing daytime highs,
we will likely need to consider fog potential in later forecasts.

By Thursday night and Friday, a negatively-tilted shortwave trough
is still forecast to cross the region. Global models have been and
continue to support a strong mass response to this wave, as shown
in the strong U-component wind anomalies in the NAEFS output that
are indicative of strong 850-mb pre-wave southeasterly winds from
40 to 50 kt that have been consistently advertised over the last
several days. The resultant theta-e advection and potential for
strong ascent on isentropic surfaces and potential frontogenesis
enhancements in this regime could indeed drive heavy precipitation
as the GFS and ECMWF QPF output supports. Precipitation-type could
be a forecast challenge given the potential presence of a strong
thermal gradient across the region, but accumulating, and possibly
heavy, wet snow could occur across parts of western and central
ND. Confidence in the details of this event is slightly greater
than average for this time range, but 00 UTC GFS ensemble members
suggest caution is still advised. For example, at Bismarck, the
21-member GFS ensemble suggests a range of QPF from 0.09" to 1.18"
in that period.

Thereafter, maintenance of the larger trough to our northwest for
the upcoming weekend suggests active weather will continue with
additional precipitation opportunities Saturday and Sunday.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 634 AM CDT Mon Mar 19 2018

Hazards to aviation include widespread IFR conditions in snow and
stratus. While the snow will gradually end from west to east the
IFR conditions in stratus will continue beyond 00Z.


Winter Weather Advisory until 1 PM CDT /noon MDT/ this afternoon
for NDZ019>023-025-034>037-042-045-046.



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