Shields, North Dakota Weather Forecast Discussion

068
FXUS63 KBIS 131440
AFDBIS
Area Forecast Discussion
National Weather Service Bismarck ND
940 AM CDT Sat Sep 13 2025
.KEY MESSAGES...
- Several rounds of scattered showers and thunderstorms through
  the middle of next week.
- Medium to high probabilities for at least an inch of rain
  across the western half of the state through this weekend.
- Isolated severe thunderstorms possible through this evening,
  and again Sunday afternoon and evening.
- Temperatures favored to remain near to above average through
  the middle of next week.
&&
.UPDATE...
Issued at 940 AM CDT Sat Sep 13 2025
A large area of showers with a few thunderstorms exists across
south central North Dakota into the James River Valley, with
additional areas of lightning moving in from northern South
Dakota. Across the remainder of the forecast area, there are a
few scattered showers and thunderstorms, as more widespread
convection has moved into the southern Canadian Prairies.
Similar to the past few days, isolated storms have been
developing unpredictably, with no real rhyme or reason with
where and when they`ve been forming. Still thinking that
overall, precipitation will be less widespread later today, but
for now will keep carrying broad medium to high POPs through the
afternoon.
UPDATE
Issued at 654 AM CDT Sat Sep 13 2025
At 630 AM CDT, the most concentrated area of convection was training
over northwest parts of the state. Strong updrafts have been
embedded at times, most notably around a half hour ago near New
Town. MRMS estimates show a corridor of 2-4 inches of rain in just
the last 3 hours across central Mountrail County, including the town
of Stanley. Farther south, a larger area of rain was building into
southern North Dakota. These are partially the remnants of a cluster
of overnight severe storms that moved from west central to north
central South Dakota, and there are still a few stronger updrafts
embedded with this complex near Mobridge. We expect rain to fill in
across central North Dakota through mid morning, but conditions have
proven to be rapidly evolving over the past couple days.
&&
.DISCUSSION...
Issued at 449 AM CDT Sat Sep 13 2025
An elongated upper level low is spinning within a longwave trough
over the western CONUS early this morning. A northern circulation is
forecast to cutoff from the base of the trough later today, with
that area of cyclonic vorticity wrapping into its own closed low as
it ejects poleward off the Southern Rockies through the Central and
Northern Plains late tonight through Sunday night. This setup
combined with highly anomalous atmospheric moisture parameters will
continue to bring several rounds of showers with isolated to
scattered thunderstorms across western and central North Dakota
through the weekend. The time periods with the greatest expected
coverage/highest probabilities of showers are 1) this morning and
afternoon as a 700 mb low over northern Wyoming supplements DCVA and
2) Sunday afternoon and evening with the arrival of the mid/upper
low originating from the base of the longwave trough. Tonight looks
to be a drier period, but we are still carrying a 20 to 30 percent
chance of rain at most locations through the night given the pattern
and availability of moisture. Details on timing, placement, and
intensity of showers and storms will continue to have very low
predictability.
A risk for isolated severe storms remains in the forecast through
early this evening, and returns for Sunday afternoon and evening.
There is little to no projected change in the overall CAPE/shear
parameter space through Sunday evening, with MUCAPE consistently
around 500-1500 J/kg and effective bulk shear around 25-35 kts. The
higher ends of these ranges are favored to be more over central
North Dakota, and lower to the west. Hail larger than quarters and
wind gusts higher than 60 mph seem less likely to occur this
weekend, but this pattern has proven to be full of surprises thus
far. Surface-based convection also seems unlikely. Interestingly,
the HRRR and RAP erode CIN in southwest North Dakota this afternoon,
but their forecast soundings show a suspiciously well-mixed, very
shallow boundary layer underneath a deep layer of high RH.
Nevertheless, the 00Z HREF did paint its most concentrated UH tracks
over southwest and south central North Dakota, but with a temporal
peak of late morning/early afternoon. Another potential hazard to
keep in mind during the afternoon hours today and Sunday is a non-
zero potential for non-supercell tornadoes/landspouts, as several
ingredients that favor their development are forecast to be present.
The greatest impact from showers and storms through the weekend is
now likely to be locally excessive rainfall. There is still a 50 to
70 percent chance for at least one inch of rain along and west of
Highway 83 through Sunday night, and this does not take into account
what has already fallen. However, there will be a high spatial
variability to final rainfall totals given the convective
influences. The context of this is exemplified by the HREF localized
probability-matched mean QPF, whose 24-hour totals range from as low
as a few hundredths of an inch to as high as 5 inches. We have
already seen this in observed data as well, with pockets of 2-6
inches estimated by MRMS scattered across a large area with an
estimate of less than a tenth of an inch. Any location that sees
training convection or multiple rounds of heavy rain through the
weekend has a risk for localized flooding.
High temperatures this weekend are likely to be around 70 to 80. It
will also remain unusually humid for mid-September with dewpoints in
the 60s for much of the area. The excessive moisture and rainfall
could allow fog to develop pretty much anywhere tonight into Sunday
morning.
There is still timing uncertainty on the departure of the deeper
mid/upper lower that originates from the base of the longwave
trough, with ensemble clusters ranging from as early as Monday
morning to as late as Monday evening. Until the system fully moves
into Canada, chances for showers will remain in the forecast, but
the probability for thunderstorms looks much lower on Monday.
However, a trailing shortwave that is forecast to dig into the
Pacific Northwest on Sunday could just as soon return chances for
rain by late Tuesday. This shortwave looks to be much weaker, and it
could end up stalling or meandering around the Northern Plains to
Western Great Lakes. Ensembles do seem consistent on a broad
troughing pattern over the region for the middle of the week, with
temperatures cooling closer to average values of highs in the lower
to mid 70s and lows in the 40s. All ensemble clusters then show at
least a brief period of ridging toward the end of the week, albeit
with considerable timing and amplitude differences.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 654 AM CDT Sat Sep 13 2025
Periods of showers and scattered thunderstorms are expected through
this evening. The heaviest showers are likely to produce periods
of MVFR to IFR visibility reductions. A few stronger
thunderstorms could also produce hail and erratic, strong wind
gusts. Chances for showers and storms are lower late tonight.
MVFR to IFR ceilings are possible across much of western and central
North Dakota this morning, with some LIFR possible along the
Canadian border. MVFR ceilings could focus more over western North
Dakota later this morning into the early afternoon. Southwest and
south central North Dakota are more likely to have prevailing VFR
ceilings this afternoon, but northern parts of the state could see
MVFR ceilings prevailing through tonight. There is also a potential
for fog to develop across all of western and central North Dakota
late tonight.
Outside of thunderstorms, winds are expected to become easterly
around 10 kts this afternoon, perhaps a little stronger near
KJMS.
&&
.BIS WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...Jones
DISCUSSION...Hollan
AVIATION...Hollan

NWS BIS Office Area Forecast Discussion