Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 211145
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
645 AM CDT Wed Oct 21 2020

.UPDATE...
Issued at 636 AM CDT Wed Oct 21 2020

For this update we blended in recent HRRR/RAP QPF for today`s snow
in the southwest, bringing totals up to 0.5" in areas.
Additionally we increased sky cover through the day to account for
persistent stratus and increasing mid to high level clouds. No
major changes needed to the upcoming winter storm messaging, will
let upcoming updates utilize 12Z data for headline decisions.

&&

.SHORT TERM...(Today through Thursday)
Issued at 400 AM CDT Wed Oct 21 2020

A winter storm is expected to develop through areas of the
Northern Plains, beginning tonight and lasting through Thursday
evening. The trend in model runs over the past 24 hours has been
to shift the greatest snowfall potential south, with potential
winter storm criteria snowfall kept mostly south of the I-94
corridor. Given the moderate shifting in both magnitude and
location of greatest forecast snow, the Winter Storm Watch was
left in place with this shift.

Ahead of the upcoming system, broad but weak low level warm air
advection may produce some light snow in the south and west.
Otherwise seasonably cold temperatures remain the consistent story
with highs today in the 30s.

Tonight, a trough is expected to deepen over the Rockies and
progress east towards the Northern Plains. Under coupled jet
streaks and strong Q-vector convergence, mid to upper level ascent
will increase after 00Z tonight. The big question for this event
will be the track of a 700mb frontogenesis band, expected to
strengthen with the arrival of upper level divergence. Recent
dprog/dt trends, including most 00Z guidance, has best
frontogenetical forcing near the southern North Dakota border
through Thursday morning. This is well supported by ensemble
guidance where 00Z EC/GFS mean ensemble probabilities keep the
highest QPF axis along the southern border. The southern tier of
counties look likely to see the greatest snowfall totals with
probability of exceeding 6 inches in the 50 to 70 percent range,
though this is a step down from previous probabilities as a lot of
guidance has pushed the brunt of the forcing into the South Dakota
side. A gradient in probabilities lies along the I-94 corridor,
where there is room for mesoscale banding to develop slightly
further north than the current consensus. Interestingly this is
supported by some high- resolution HREF members which place
greater snowfall totals near I-94 as upper level support
increases after 12Z Thursday. However since almost all guidance
shows frontogenesis fields maximized still to the south through
the event, this northward high-end potential should be on the
lower side of the probabilistic window. In the produced
probability output, this places the I-94 corridor within a 25 to
45 percent chance of exceeding 6 inches with a 70 to 85 percent
chance of exceeding 2 inches. One positive side to the southward
trend in the low is that freezing rain is much less likely than
previously thought.

.LONG TERM...(Thursday night through Tuesday)
Issued at 400 AM 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. There are still details to work out
but there will likely be several opportunities for cold daily
records to break in this period.

Another round of snow will be possible Saturday as the next trough
crosses the region. 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 12Z TAFS through 12Z Thursday morning)
Issued at 636 AM CDT Wed Oct 21 2020

Persistent IFR/MVFR stratus continues across northern and central
North Dakota. Models are struggling with detecting this which
makes timing out improvement to VFR challenging. High pressure
building in may work to erode stratus gradually today, but likely
not until after 18Z. A low pressure system will bring in
widespread chances for snow beginning between 00Z and 06Z and
lasting through Thursday. Areas in southern North Dakota may
receive heavy snow with IFR conditions, impacting KDIK-KBIS-KJMS.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late tonight through late Thursday night
for NDZ019>023-025-034>037-042-045>048-050-051.

Winter Storm Watch from this evening through Thursday evening
for NDZ018-031>033-040-041-043-044.

&&

$$

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


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