Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 201018

Area Forecast Discussion
National Weather Service Bismarck ND
518 AM CDT Tue Mar 20 2018

Issued at 518 AM CDT Tue Mar 20 2018

Quick update to increase PoPs into the likely category early this
morning in northwest ND, where surface observations have revealed
rather expansive 3-5SM visibilities in light snow since 09 UTC.
The 06 UTC 3-km NAM best-represents this more stratiform region of
precipitation, and suggests it will quickly diminish by 13 UTC as
the primary region of forcing focuses further south, as previously
discussed below.


.SHORT TERM...(Today and tonight)
Issued at 433 AM CDT Tue Mar 20 2018

The main focus of this forecast release is on light snow potential
from western into south central ND through this morning.

Synoptically, broad cyclonic flow is present early this morning as
mean 500-mb troughing extends from the north central U.S. into the
Ohio River valley. Embedded within this regime is a closed low in
south central Canada, and there`s a rather short-wavelength, but
sharp shortwave trough on its southwest side that`s moving through
eastern MT early this morning. The latter wave has been very well-
anticipated the last few days, but early-morning radar and GOES-16
imagery suggests its core of mid-level positive vorticity may be a
bit further south than most guidance suggests. That includes the
rapid-refresh suites through the 07 UTC RAP and HRRR. Steep mid-
level lapse rates are present in advance of the wave, with values
from 700-500 mb averaging near 7.5 C/km. While deep-layer ascent
as evaluated diagnostically via 500-300 mb Q-Vector fields leaves
quite a bit to be desired, the presence of those steep lapse rates
and the convective nature to upstream radar echoes in eastern MT
favors light snow spreading into western ND through daybreak, and
into south central ND from 12 to 18 UTC. Most convection-allowing
model guidance, including the 00 UTC NCEP WRF ARW and NMM cores
and overnight HRRR simulations, have strongly favored a narrow
corridor of higher QPF in support of localized snow accumulations
of 1 to perhaps as much as 3 inches from McKenzie to Dunn Counties
with this activity. However, to this point upstream radar trends
have not been supportive of this evolution, and the subtle south-
ward displacement of the vorticity maximum aloft compared to what
model guidance expects adds a bit more uncertainty to a meso-beta
scale forecast such as that one. As a result, we`ve continued to
use rather broad strokes with our PoP and snowfall forecast over
western ND this morning, with PoPs maximizing in the 60 to 70
percent range and snowfall accumulations of an inch or less. We
will continue to monitor observational trends in case a narrower
region of semi-enhanced snowfall becomes apparent later on. Note
that we did carry low-end PoPs as far east as Bismarck/Mandan as
the wave shifts southeast by late morning, as well.

Otherwise, we maintained patchy fog in the forecast across all of
western and central ND this morning, though confidence that more
than modest visibility reductions will occur is low based on the
ASOS/AWOS trends thus far, and model-derived soundings that show
modest low-level mixing continuing underneath the stratus deck in
place across most of the area. The exception to the rule thus far
has been in Slope and Bowman Counties, where GOES-16 imagery and
web cameras have supported locally dense fog where local clearing
had occurred earlier.

Later today, we relied on the 00 UTC multi-model consensus with a
bit of extra nod to MOS-based inputs to call for highs in the low
to mid 30s F. Given highs exceeding freezing today in western ND,
which will likely lead to some melting of Sunday night`s snowfall,
and forecast soundings which show a low-level inversion forming in
support of minimal low-level turbulence, we added patchy fog into
the forecast once again tonight and Wednesday morning in western
ND. That move is also supported by the 00 UTC MET MOS and 3-km NAM
guidance. We refrained from adding fog across central ND at this
point since forecast soundings provide a less clear picture as to
whether or not the near-ground layer will be too turbulent for it
to form or not.

.LONG TERM...(Wednesday through Monday)
Issued at 433 AM CDT Tue Mar 20 2018

A relative warming trend is still expected Wednesday and Thursday
as mid-level heights and temperatures rebound as ridging builds
downstream of a deep upper-level low expected to become centered
off the British Columbia coastline in that time frame. The 00 UTC
multi-model consensus calls for highs mainly in the 35 to 45 F
range during this time frame, warmest southwest. Potential low
clouds and fog could pose some challenge to our temperatures as
recent snow melts, and as low-level southerly flow becomes well-
established by Thursday. The 00 UTC NAM was particularly robust
with moisture advection by late Wednesday night and suggests we
will need to be mindful of the low, but non-zero potential for
stratus becoming deep enough to yield light freezing drizzle at
some point by Thursday morning. That simulation was more robust
than other 00 UTC guidance with that signature, but it will be
something to monitor in succeeding model cycles.

Otherwise, the 00 UTC guidance continued calling for a shortwave
trough ejecting out of the developing western trough by Friday.
This lead wave has been forecast by most models to remain open at
500 mb (except for the 06 UTC NAM that just arrived), but there`s
long been a signal in guidance for strong theta-e advection with
screaming southeasterly 850 mb winds of 50+ kt in advance of it,
and precipitable water values in the 97.5th or greater percentile
per recent NAEFS cycles. There will be a tight thermal gradient in
play with this scenario, and precipitation-type issues will be a
challenge given above-freezing mid-level temperatures expected
over at least southwestern ND. The 00 UTC ECMWF was drier overall
with the Thursday night through Friday night timeframe, and that
does reduce confidence in the event somewhat. However, the 00 UTC
GFS ensemble members show distinct clustering in their QPF fields
at several locations, with values in the 0.30 to 0.60 inch range
at Bismarck, and slightly higher values at Minot and Jamestown.
Again, precipitation type will be a challenge, particularly the
further south and west one goes, and average, but notable spread
remains among the full set of ensemble members, but there remains
enough of a signal to suggest we will need to watch this period
for potential headline-worthy snowfall over parts of the area.

For this weekend and into early next week, there`s significant
disparity in the last few global model iterations with regard to
how the western trough will translate eastward. Thus, confidence
in forecast details drops significantly, but the main theme will
continue to be additional chances for rain or snow, especially
from Saturday night through Monday.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 433 AM CDT Tue Mar 20 2018

MVFR and IFR stratus will persist through the 06 UTC TAF cycle for
most of western and central ND. Light snow will likely impact
parts of western ND this morning, including the KISN and KDIK
terminals. Otherwise, patchy fog could also produce localized
visibility reductions through the morning, as well.




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