Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 231244

Area Forecast Discussion
National Weather Service Bismarck ND
644 AM CST Thu Feb 23 2017

Issued at 644 AM CST Thu Feb 23 2017

Radar trends and web camera images have shown an uptick in the
coverage of light snowfall from northeast Montana southeast into
north central ND since 11 UTC. This update adjusted PoPs through
mid morning to reflect those trends. Weak frontogenesis shown in
recent RAP runs in the 925-850 mb layer along a Glasgow to Minot
line might be aiding the light snow production somewhat. If that
is the case, the light snow may begin diminishing concurrent with
that signature after 15 UTC, though weakly cyclonic flow will be
in place through the day, justifying at least a slight chance of
light snow over western ND. Any accumulations will be minor.


.SHORT TERM...(Today and tonight)
Issued at 331 AM CST Thu Feb 23 2017

Cool, but near-normal temperatures will rule today.

As of 09 UTC, a weak high pressure ridge extends from Williston to
Bismarck. Weakly cyclonic flow is present aloft under influence of
the northern-stream jet. Moisture channel imagery suggests that an
intense mid- and upper-level jet streak is crossing the southwest
United States early this morning, with a maturing shortwave trough
(and mid-level low) just west of Salt Lake City. The 00 UTC models
unanimously agree that wave will eject into the middle Missouri
River Valley by Friday morning, keeping its associated surface low
and swath of cold-sector heavy snow well south of the state.

Closer to home, an area of light snow driven by a weak shortwave
trough marked by a weak and subtle lowering in the 500 mb height
fields in RAP 00-hour analysis output across northwest and north
central ND has weakened since 06 UTC. That was expected with the
weakening of that wave as it shifted eastward into more confluent
flow aloft east of Highway 83. Fog has been at a premium thus far
tonight despite light winds and a nearly-saturated boundary layer
(thanks in part to the wet snow that fell less than 24 hours ago).
We will nonetheless maintain a mention of patchy fog through this
morning across much of western and central ND as the consensus of
high-resolution and rapid-refresh guidance suggests some fog will
be maintained or further expand before 15 UTC. However, forecast
soundings suggest weak near-surface cold air advection is enough
to prevent the surface layer from decoupling, which in turn leaves
the lower atmosphere susceptible to turbulent mixing and favors
deposition of moisture as frost instead of fog (as we`ve observed
here in Bismarck). That`s also borne out in modified Richardson
number output. Thus, even though we have chosen to keep patchy fog
in the forecast in many areas, we don`t expect it to be widespread
or particularly long-lived in any one location at this time.

For today, we relied on the 00 UTC multi-model consensus to build
the forecast given good agreement in a relatively benign pattern.
We are calling for highs mainly in the lower 30s F, save for along
the Canadian border where forecast highs are from 25 to 30 F. Out
of respect to weak upstream radar echoes in northeast MT, broadly
cyclonic flow aloft, and simulated light QPF from recent HRRR/RAP
iterations and even some of the 00 UTC global models (especially
the ECMWF), we are carrying a slight chance of light snow across
western ND for most of the day.

Tonight, surface ridging will remain in control. The degree to
which low stratus is maintained is uncertain, with the 00 UTC GFS
and its associated MOS output more optimistic on clearing for a
wider berth of the area than most other guidance. That could
influence eventual temperature trends. Finally, with weak flow the
potential for fog may exist again, but forecast sounding profiles
once more suggest the lower atmosphere may be a bit too turbulent
to support its development. We therefore refrained from adding it
to the forecast at this juncture.

.LONG TERM...(Friday through Wednesday)
Issued at 331 AM CST Thu Feb 23 2017

Seasonably cold temperatures will continue through the long term
along with a few relatively low-end precipitation chances.

The 00 UTC GFS and ECMWF and their ensembles support maintenance
of a mean western United States trough through early next week,
with a potential eastward shift in the zone of lower heights by
next Thursday. That multi-model consensus favors northwest flow
aloft over North Dakota, perhaps interrupted only briefly in the
period centered on Tuesday when a stronger shortwave trough may
eject from the Rockies into the upper Mississippi River Valley.
This regime will favor climatologically-low 500 mb heights over
our area (near the 10th percentile of values for this time of the
year per the 00 UTC NAEFS) and a corresponding high probability of
below-normal temperatures, further supported by recent iterations
of the CIPS analog guidance. Temperatures at 850 mb will average
near -10 C through the period, supporting highs mainly from the
upper 20s to lower 30s F on most days and across most of western
and central North Dakota.

Precipitation-wise, weak shortwave troughs embedded in northwest
flow aloft will offer up a few chances of light snow. While the
predictability of waves in this regime is generally modest at
best, the last several model cycles have favored a stronger 500 mb
shortwave passage Saturday afternoon and evening. That could end
up being the focus of a high PoP/low QPF scenario since guidance
is also calling for steep low- and mid-level lapse rates, though
any snowfall accumulations would be minor. Otherwise, the stronger
wave ejecting out of the western trough near next Tuesday might
be another opportunity for more organized snowfall, but there`s
been wide variability in guidance from model to model and run to
run in that time period, and taken at face value the 00 UTC GFS
and ECMWF both suggest little in the way of impacts during that


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 644 AM CST Thu Feb 23 2017

Periods of MFVR/IFR ceilings and patchy fog will continue across the
region today with abundant low level moisture. Forecast models are
hinting at the stratus breaking up across the north this evening
with this trend possibly continuing south overnight.




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