National Weather Service Raw Text Product

512 
FXUS63 KBIS 191407
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
907 AM CDT Mon Oct 19 2020

.UPDATE...
Issued at 904 AM CDT Mon Oct 19 2020

Snow continues to gradually taper off from northwest to southeast.
Only changes needed for mid-morning update were to delay the end
of the snow a bit on the western edge as it has been a bit
reluctant to dissipate. Have also increased cloud cover over the
area as the little bit of clearing that has developed quickly
becomes clouded up again as stratus over northern locations
continues to drop south. Some holes in the clouds are showing up
over southern Manitoba, but remain pessimistic about how much
could clear over our area given the trends over the past few days. 

UPDATE Issued at 645 AM CDT Mon Oct 19 2020

We increased PoPs to 100 percent early this morning southwest and
south central where light snow is ongoing as of this update, and 
gradually shifted the higher PoPs southeast and ended them by mid 
morning. Otherwise, we just blended observational trends into the 
rest of the hourly forecast fields through the morning.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 506 AM CDT Mon Oct 19 2020

Widespread snow tonight and Tuesday with potential travel impacts
due to snow covered and slippery roads is the short term emphasis.

Early this morning, light snow is falling in southwestern and far
south central ND, mainly along and south of the I-94 corridor, in
association with a weak shortwave trough. This snow is peripheral
to a surface ridge axis extending from northern MT into northern
ND, generally north of a midlevel baroclinic zone, and is being
primarily driven by synoptic-scale ascent including forcing in the
left exit region of a 100+ kt 300 mb jet streak centered across
southeastern MT at 09 UTC. Radar trends and model guidance suggest
this light snow will gradually shift east-southeast along the ND-
SD border before diminishing this morning.

Satellite trends reveal expansive and persistent cloud cover over
the region this morning, with pervasive stratus extending upstream
well into Canada. Guidance including the 00 UTC global suite and
more recent rapid-refresh guidance is very poorly-resolving most
of these clouds, especially near the surface ridge axis. Based on
observational trends, we took a pessimistic approach to forecast
sky cover today with mostly cloudy conditions advertised area-wide
through the day. Forecast highs are based on the NBM given well-
clustered guidance, and are in the 30s F. We maintained a chance
of light snow southwest through the day in closer proximity to the
midlevel baroclinic zone extending from eastern MT into SD.

For tonight into Tuesday, the 00 UTC global ensembles remained
very consistent in calling for passage of a shortwave trough and
related batch of strong QG-forcing aided by both low- and midlevel
warm air advection and DCVA aloft. Guidance continued to suggest
little in the way of frontogenetical forcing, with synoptic-scale
forcing dominating precipitation production. This tends to favor
a widespread snow event, and ensemble QPF plumes are relatively
well-clustered in the 0.20 to 0.40 inch range, albeit with a few
slightly higher and lower members included. Forecast soundings
show a deep saturated column favorable for riming/aggregation of
snowflakes supporting snow to liquid ratios less than 10:1, and
applying that to the multi-model consensus QPF yields event total
snowfall tonight through Tuesday of 1 to 2 inches for much of the
west and central, with up to 3 inches in parts of the south
central and James River valley where snowfall may linger longest
on Tuesday. The probability of exceeding advisory criteria across
central ND is only around 50%, however, we are emphasizing travel 
impacts given this will both be 1) one the first widespread snow 
events of the season, and 2) occurring before and during the 
Tuesday morning commute, with roads likely to be sub-freezing and 
supportive of snow cover and slippery conditions. Finally, we did 
add a slight chance of light freezing rain to southwestern ND late
tonight, too, when warm air advection may yield a relatively deep
warm layer of +1 to +2 C aloft, favoring the possibility of full 
hydrometeor melting and refreezing at the surface. The 00 UTC HREF
mean ice accumulation (FRAM) output supported that, as well.

Finally, in the wake of the shortwave trough and related surface
low, a period of gusty winds is possible Tuesday afternoon in the
west and south central. The 00 UTC GFS and NAM soundings show mean
boundary layer winds nearing 25 kt for a time. However, given the
high-density snow and afternoon temperatures expected to exceed 
the freezing mark in those areas, the potential for blowing or 
drifting snow appears very low if not non-existent.

.LONG TERM...(Tuesday night through Sunday)
Issued at 506 AM CDT Mon Oct 19 2020

Cold and winter-like weather will continue through the long term,
with increasing confidence in potential heavy snow Wednesday night
and Thursday over portions of the region.

Global ensemble members are highly confident and very clustered in
their simulation of a strong shortwave trough passage Wednesday
night and Thursday. The small subset of members that had a less-
amplified wave were primarily composed of the 12 UTC Canadian
ensemble system, and the 00 UTC membership of that ensemble has a
more amplified wave in line with the remainder of global guidance.
NAEFS guidance highlights the anomalous and intense upper-level
wind maxima associated with this wave, supporting strong dynamic 
ascent, and the 00 UTC deterministic GFS and ECMWF show potential
for low- and midlevel frontogenetical forcing as well, albeit 
potentially offset from the strong synoptic-scale ascent somewhat.
Regardless, this supports the ensemble QPF signatures with both 
the 00 UTC ECMWF and GEFS members and recent NBM cycles favoring 
an ~50% probability of event-total QPF of 0.50 inches or more with
this system, roughly corresponding to the potential for 6+ inches
of snowfall. Individual ensemble members continue with geographic
spread in the location of their QPF maxima, and so plumes for any
given point continue to have large spreads in potential outcomes,
though the ensemble means continue to favor the southern two 
thirds of ND for the heaviest QPF. Given the consistent signal and
increasing probabilities of QPF sufficient for heavy snowfall, we
have begun to message that possibility in our products, with 
potential for winter storm criteria being met. However, it is 
worth noting that moisture anomalies are lacking with this system,
and it does appear quite progressive in most guidance, both of 
which could tend to limit overall intensity of snowfall. Moreover,
while we are increasingly confident in the occurrence of heavy 
snowfall over some part of the region, exact amounts and the 
location of heaviest precipitation remain uncertain.

In the wake of that system, NAEFS guidance calls for anomalously
high surface pressure in the 90th to 97th percentile over the 
region by next weekend, supporting the very cold temperature 
guidance in the NBM. Forecast highs are only in the 20s F Friday 
through Sunday. An active pattern also remains in place, but the 
spread in guidance for any potential additional snow later next
weekend is large and thus confidence in that is low.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 645 AM CDT Mon Oct 19 2020

MVFR and IFR conditions in light snow southwest and south central
early this morning will improve by about 15 UTC as light snow
ends. Otherwise, mainly VFR ceilings are expected today although
pockets of MVFR ceilings are possible. Widespread snow with MVFR
and IFR conditions is expected across western and central ND
tonight, beginning in the west between 00 and 06 UTC and moving
into central ND between 06 and 12 UTC.


&&

.BIS WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...JJS
SHORT TERM...CJS
LONG TERM...CJS
AVIATION...CJS