Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 112134
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
334 PM CST Sun Feb 11 2018

.SHORT TERM...(This evening through Monday)
Issued at 334 PM CST Sun Feb 11 2018

Very cold air will return tonight along with dangerously low wind
chills, which will necessitate a Wind Chill Advisory across all of
western and central ND tonight through Monday morning.

Broad, but persistent surface pressure rises of 2 to 3 mb/3 hours
are ongoing across the region this afternoon in response to Arctic
high pressure that`s building southeast through central Alberta as
of 21 UTC. The 12 UTC multi-model consensus strongly concurs that
the ~1045 mb surface ridge axis will quickly build southeastward
into northern ND by 12 UTC. The 12 UTC NAEFS best characterizes
the strength of this Arctic high by signaling sea level pressures
in the 99th percentile for this time of the year over most of the
state by early Monday morning. Upstream temperatures beneath the
ridge axis in Alberta this morning were as cold as -25 F, and its
position across northwest and north central ND by Monday morning
will herald a scenario of very efficient radiational cooling with
a clear sky, relatively light winds, and strong ridge-related
subsidence in the Arctic air mass characterized by temperatures at
850 mb around -20 C. As a result, we leaned toward the cooler edge
of temperature guidance in many areas tonight and are calling for
overnight lows into the -25 F range in favored cool areas across
northwest and north central ND. There`s a bit more uncertainty in
the magnitude of cooling further south, where the air mass will be
a bit warmer and where high-level cloud cover may increase by the
morning downstream of a 500 mb trough digging into the Rockies. We
thus stayed the course closer to the consensus of the 12 UTC MOS
output for lows along and south of Interstate 94 tonight, which is
often a high-quality guidance selection near ridge axes to begin
with.

Although surface winds will relax steadily overnight as air mass
density changes subside with the arrival of the surface ridge, we
will maintain enough air flow to generate wind chills that exceed
our local -25 F criteria for a Wind Chill Advisory given the low
ambient air temperatures. We thus issued a Wind Chill advisory for
the entire area, beginning in the north at 00 UTC and across the
remainder of western and central ND at 04 UTC. The advisory is in
effect until 18 UTC. Note that given actual air temperatures near
-25 F in parts of northwest and north central ND tonight, marginal
Wind Chill Warning criteria of -40 F may be met at times, but we
refrained from issuing a warning at this time given the low wind
speeds and relatively short temporal duration of those marginal
conditions.

Otherwise, a few flurries have been occurring in association with
stratocumulus this afternoon, but those should diminish by 00 UTC
as the shallow boundary layer with its small layer of instability
shrinks. Finally, we have maintained a small chance of light snow
on Monday along the ND/SD border from Bowman and Hettinger toward
Fort Yates in respect to weak deformation that may develop along
that axis downstream of the aforementioned shortwave trough that
will be digging into the Great Basin in split flow aloft. Having
said that, most of the forcing will likely be relegated to the SD
side of the state line.

.LONG TERM...(Monday night through Sunday)
Issued at 334 PM CST Sun Feb 11 2018

An active pattern is expected to be in place during the long term
forecast period with mean cyclonic flow aloft continuing. However,
it appears that this will mainly be manifest as a roller-coaster
of temperatures rather than significant precipitation events as
the associated mean troughing over the western U.S. is expected
to maintain a positive tilt with only slow retrogression while a
subtropical ridge makes attempts at building northward into the
southeastern U.S. at times. On the sub-hemispheric scale, global
models suggest several shortwave troughs will cross our area in
the long term, with ensemble-driven confidence highest in a mid-
week wave passage. GFS and ECMWF simulations and their related
ensemble output have suggested warm air advection preceding that
shortwave trough may begin sooner, on Tuesday, than was believed
at this time yesterday, but the greatest pre-frontal mixing and
thus warmest day of the week will likely hold until Wednesday,
when the multi-model consensus still has highs peaking in the 30s
area-wide, with a few 40 F readings in southern ND. That same set
of model guidance still calls for an Arctic cold frontal passage
centered on Wednesday night, which may generate a widespread area
of light snowfall.

Predictability of smaller-wavelength waves in the flow drops off
significantly after Thursday. Yesterday`s GFS ensemble and the
deterministic ECMWF suggested colder air would likely remain in
place through next weekend, but today`s same guidance suggests
additional waves could yield additional warming and cooling in
short time frames between Friday and Sunday. The multi-model
consensus that drove our forecast naturally follows suit, but
specific outcomes for any given day are a low-confidence item.
For example, for next Saturday, the 21 members composing the 12
UTC GFS ensemble at Bismarck show a wide spread of temperatures
at 850 mb, ranging from +8 C to -19 C.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 334 PM CST Sun Feb 11 2018

Patches of MVFR ceilings linger across western and central ND this
afternoon, but will end by evening, with VFR conditions prevailing
across the area overnight into Monday morning.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory from 10 PM CST /9 PM MST/ this evening to
noon CST /11 AM MST/ Monday for NDZ017>023-025-031>037-040>048-
050-051.

Wind Chill Advisory from 6 PM this evening to noon CST Monday
for NDZ001>005-009>013.

&&

$$

SHORT TERM...CJS
LONG TERM...CJS
AVIATION...JJS/CJS



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