Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 101826

Area Forecast Discussion
National Weather Service Bismarck ND
1226 PM CST Sat Dec 10 2016

Issued at 1225 PM CST Sat Dec 10 2016

Some areas in the north central will have borderline wind chill
advisory criteria, thus will allow the wind chill advisory to
expire. Latest iterations of the short term high res models indicate
some minor adjustments in hourly temperatures and max temps. Did
increase chances for light snow...with latest models indicating some
increasing frontogenesis this afternoon roughly from Williston to
Minot/Bismarck to Jamestown/Oakes.

UPDATE Issued at 1034 AM CST Sat Dec 10 2016

The Wind Chill Advisory is still in effect for all of western and
central areas until noon CST.

Slight changes were made to POPs to reflect the latest radar
trends. Light radar returns in the west will continue to fill-in,
as snow showers push eastward into the state from eastern Montana.
The forecast is on track for accumulations between 1 and 3 inches
for most of western and central areas. Some central areas may
receive more than 3 inches.

UPDATE Issued at 651 AM CST Sat Dec 10 2016

Overall, the going forecast is on track and all we did with this
update was blend observational trends and recent rapid-refresh
guidance into hourly forecast fields through 18 UTC. That did mean
we increased PoPs into the likely range over far southwestern ND
this morning, where light snow is falling already at Hettinger. We
also extended the patchy fog mention in south central ND through
15 UTC.


.SHORT TERM...(Today and tonight)
Issued at 401 AM CST Sat Dec 10 2016

The focus of the short term forecast is on tonight, when 2 to 4
inches of snow is expected across most of west and central ND.

Early this morning, areas of stratus continue, primarily downwind
of Lake Sakakawea toward the Bismarck/Mandan area. Recent trends
in web camera images and observations from the Mandan AWOS reveal
potential for patchy fog in this area of central ND through early
this morning. We therefore carried patchy fog in the forecast
from Garrison to Hazen and Bismarck and Mandan through 13 UTC,
when increasing mid-level clouds will yield subtle warming of the
boundary layer as already observed upstream at Dickinson. That in
turn should begin to reduce near-surface saturation. Otherwise,
temperatures in areas without clouds are in the -20 to -25 F range
over a wide expanse of northwestern and central ND as of 09 UTC.
Suffice to say that we are continuing the wind chill advisory as
planned through 18 UTC. In fact, we may end up needing to extend
it into the afternoon over parts of north central ND if the
diurnal temperature curve is stunted as clouds increase through
the day.

Mid-level warm air advection today will manifest itself as a band
of forcing on isentropic surfaces centered near 285 K, which will
support some fine and light snowfall. Flow on those 285 K surfaces
is not particularly strong and large-scale ascent will be lacking,
both of which call into question the probability of light snow
during the day. However, forecast soundings also show a deep layer
within the favored dendritic growth zone, and even the most recent
RAP and HRRR iterations favor light snowfall by afternoon, so we
have some 40-60 percent probabilities of snow expanding from
southwest ND early this morning into central ND by early afternoon
in the forecast. Any accumulations today should be negligible.

Tonight, a sharp but quick-moving 500 mb shortwave trough that is
seen on early-morning water vapor images near Seattle will rapidly
cross the region. The 00 UTC GFS, NAM, and ECMWF all call for a
period of strong 500-300 mb Q-vector convergence tonight, and all
of those solutions concurrently show a widespread QPF signature of
0.10 to 0.20 inches across western and central ND. There is also a
period of modest ascent on isentropic surfaces that peaks in the
03 to 06 UTC time period, also supporting widespread snowfall. The
cold air mass supports higher-than-climatology ratios of snow to
liquid, and individual methods like Cobb, Roebber, and the maximum
temperature aloft equation all suggest ratios near 20 to 1. Thus,
a widespread area of 2 to 4 inches of snowfall is forecast across
most of the area tonight. This will be a fluffy, low-density snow,
but winds are forecast to be relatively light, minimizing impacts.
Given that and a mean forecast snowfall of 3 inches, we have held
off on issuing any headlines with this forecast release. Note too
that mid-level frontogenesis is forecast to be weak at best and is
not simulated to be well-linked to the synoptic-scale ascent, so
the potential for banded snowfall appears low. That being said, if
dendritic growth is efficient then locally higher snowfall totals
could occur.

.LONG TERM...(Sunday through Friday)
Issued at 401 AM CST Sat Dec 10 2016

Frigid temperatures are here to stay through the long term thanks
to reinforcing surges of Arctic air Monday and Tuesday night.

The 00 UTC global suites were in good agreement in advertising the
persistence of a negative PNA regime with a mean trough at 500 mb
over the western United States through at least the next 10 days.
When combined with mean ridging over Alaska, this will provide an
open door to Arctic air to continue pouring into the area, and it
will be maintained locally given our deep snow field. The GFS and
ECMWF simulations from 00 UTC also simulated a deep (though rather
progressive) 500 mb low dropping from northern Canada at 12 UTC on
Monday to northern Ontario by midweek. Guidance suggests a short
wave trough orbiting the low will swing a cold front across the
area early Monday. Gusty post-frontal winds and a period of light
snow may both accompany the front, though taken at face value the
00 UTC model simulations call for peak boundary layer winds only
near 20 kt behind the front Monday. We nonetheless did rely on the
00 UTC MOS consensus for wind speeds in that forecast period since
it typically verifies better with post-frontal winds. Another
frontal passage is forecast to occur late Tuesday.

Forecast highs per the 00 UTC multi-model consensus are within a
few degrees F either side of zero through the week, with lows of
-5 to -15 F. Wind chills will likely be deep into advisory range
most of the period. However, after Monday, the forecast is mainly
dry until slight chance snowfall probabilities show up late in the
week. It`s then that some GFS and ECMWF ensemble members suggest
a strong shortwave trough may eject from the mean western states
trough at a far enough north latitude to bring snow to the area.
Confidence in that is low, though.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1225 PM CST Sat Dec 10 2016

Low pressure will move across the northern and central plains this
afternoon and tonight. This will bring a wide swath of mvfr-ifr
ceilings and visibilities west to east across the forecast area in
snow. Ceilings will improve to VFR from west to east after 12 UTC
Sunday. A southeast surface flow 10 to 20 mph today will diminish
tonight and shift north to northwest 5 to 10 mph Sunday morning.




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