Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 220550
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1250 AM CDT Thu Oct 22 2020

.UPDATE...
Issued at 1244 AM CDT Thu Oct 22 2020

Precipitation banding well evident on regional radar at 0545Z near
and south of the southern North Dakota border. Expecting the brunt
of this band to stay about that far south as it progresses east
through the morning. We bumped up snow-liquid ratios slightly
across the southern border to account for the likely moderate
snowfall rates. No other changes needed with this update.

UPDATE Issued at 1033 PM CDT Wed Oct 21 2020

Current radar imagery shows a narrow west-to-east band of
reflectivity intensifying across southern Grant, Sioux, southern
Emmons, and McIntosh Counties. While there may still be some drier
air underneath these echoes, analysis suggests it is not as dry as
what was sampled by the 00Z Bismarck sounding farther to the
north, and some snow is likely reaching the ground. Therefore, the
start time of the Winter Storm Warning was moved up from 1 AM CDT
to now. The Winter Weather Advisory for Grant County has also
been moved up to start now. All other headlines remain the same.

UPDATE Issued at 927 PM CDT Wed Oct 21 2020

The two biggest concerns for this forecast update are a low to
mid level layer of dry air entrenched over central North Dakota
and an eastward-moving intense band of heavy snow currently over
northeast Montana.

The 00Z observed sounding from Bismarck shows a layer of very dry
air from 850 to 700 mb. This dry layer is being well simulated by
the 18Z NAM and GFS, but poorly by the most recent iterations of
the RAP and HRRR. While total saturation of the column may not be
completely necessary for snow to commence at any given location,
the observed sounding suggests it will take longer for snow to
reach the surface than what is being depicted by model
reflectivity and QPF fields. NAM and GFS 850 mb RH fields were
heavily relied on to delay the north and eastward progression of
the start time of snow over central North Dakota, which is the
most notable change to the forecast for this update. There was
also a slight reduction in total snow amounts along the Missouri
River Valley from Bismarck to Lake Sakakawea, but only by a few
tenths of an inch at most.

A strong mid level shortwave is producing an intense band of snow
over northeast Montana, with estimated snow rates as high as 2
inches per hour. Deterministic guidance with different model cores
and physics packages strongly suggest that this wave will
deamplify as it approaches the North Dakota border, with rapidly
dissolving frontogenesis expected by 06Z. However, the NAM and to
some extent the GFS continue to show strong lift within the
dendritic-growth zone over Williams County later tonight
accompanied by pockets of negative EPV, suggesting heavier snow
may be possible. Feel that the best course of action at this time
is to not add Williams County to the advisory and continue to
closely monitor observed trends for the next 1-3 hours.

Aside from these discussed challenges, the rest of the forecast
appears to be trending well. Snow is falling along and southwest
of line from Beach to Hettinger, but has not quite reached
Dickinson as of this writing. A chance of freezing drizzle mixing
in with the snow along the Highway 12 corridor continues for a few
more hours. But most, if not all impacts will be caused by snow.

UPDATE Issued at 605 PM CDT Wed Oct 21 2020

The biggest change for this update was to introduce chances for
freezing drizzle mixing with snow along the Highway 12 corridor
through the evening, along with areas of fog. Surface observations
at Hettinger and Baker, MT have routinely been reporting
visibility at or below 2 miles this afternoon with -SN and BR. Area
webcams suggest that the visibility restrictions are more due to
the fog/mist/drizzle than the falling snow. The presence of liquid
hydrometeors is also supported by the HRRR and RAP, whose
soundings at Bowman show a saturated layer from 0.5 to 1.5 km AGL
with temperatures greater than -10 C and strong upward vertical
motion. The start time for the Winter Weather Advisory in these
areas is 6 PM MDT, a little less than hour ahead of this update.
Despite the ongoing snowfall and freezing drizzle potential, we
are still okay with this start time given ground temperatures are
likely still above freezing, and fog being the more dominant
visibility restriction.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 1243 PM CDT Wed Oct 21 2020

A winter storm will impact the northern Plains tonight through
Thursday evening. The southward trend in model runs continues
today with the greatest snowfall potential across south central
and southeast ND. As confidence grows in amounts and location, we
have decided to issue a Winter Weather Advisory for the I-94
corridor and southwest ND and a Winter Storm Warning for locations
south of I-94 across south central ND.

Ahead of the upcoming system, broad but weak low-level warm air
advection is producing some light snow across southwest ND. This
area of snow should produce an inch or less.

Tonight, a trough is expected to deepen over the Rockies and
progress east towards the northern Plains. As this trough enters
the northern Plains mid to upper-level ascent will increase. A
700mb frontogenesis band will strengthen with the arrival of
the upper-level jet streak. The 12 UTC model suite is consistent
in keeping the best frontogenetical forcing near the southern
North Dakota border through Thursday morning. This is well
supported by ensemble guidance as the 12Z EC/GFS/CMC mean
ensemble probabilities keep the highest QPF axis along the
southern border. The southern tier of counties (mainly east of
the Missouri River) look likely to see the greatest snowfall
totals with probability of exceeding 6 inches in the 40 to 70
percent range (highest probs across the southern portions of the
counties). Guidance has continued to push the brunt of the forcing
(and QPF) over to the South Dakota side. North of I-94 to Highway
2 between one and four inches of snow may fall. North of Highway 2
an inch or less is expected.

.LONG TERM...(Thursday night through Wednesday)
Issued at 1243 PM CDT Wed Oct 21 2020

Potentially record breaking cold temperatures are forecast this
weekend into early next week. Another system may bring
accumulating snow this weekend as well.

Snow tapers off Thursday evening as high pressure settles in
behind the trough. A seasonably cold 850mb air mass is expected to
accompany the cold surface high with NAEFS guidance showing both
850mb temperatures and MSLP near climatological minimum and
maximum respectively Friday into early next week. Forecast lows
drop into the teens and single digits with daily highs
consistently below freezing. Some locations across southwest ND
may even see below zero temperatures early next week.

Another round of snow will be possible Saturday as the next trough
crosses the region. The latest global model runs show a very
similar setup to tonight/tomorrow`s system coming through
Saturday night into Sunday. While differences exist between
deterministic models in north-south placement, ensemble
probabilities increased with NBM producing higher end PoPs across
the south during this period.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1244 AM CDT Thu Oct 22 2020

Snow accompanied by MVFR to IFR ceilings and visibility will
spread across western and central North Dakota through the
morning, lasting through this afternoon. KMOT is expected to
remain outside of the heaviest snow, but will still see MVFR
ceilings at times. Winds gradually turn from east to north through
the TAF period, increasing to near 20 kts in areas this
afternoon.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 7 PM CDT /6 PM MDT/ this evening
for NDZ019-020-034>037.

Winter Weather Advisory until 7 PM CDT /6 PM MDT/ this evening
for NDZ017-018-031>033-040>044.

Winter Storm Warning until 7 PM CDT /6 PM MDT/ this evening for
NDZ045>048-050-051.

&&

$$

UPDATE...AE
SHORT TERM...AJ
LONG TERM...AJ
AVIATION...AE



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