Grand Rapids, North Dakota Weather Forecast Discussion

908
FXUS63 KBIS 120945
AFDBIS
Area Forecast Discussion
National Weather Service Bismarck ND
445 AM CDT Fri Sep 12 2025
.KEY MESSAGES...
- Several rounds of scattered showers and thunderstorms through
  the middle of next week.
- Medium probabilities for at least an inch of rain across the
  western half of the state through this weekend.
- Isolated severe thunderstorms possible late this afternoon
  through Sunday.
- Temperatures favored to remain near to above average through
  the middle of next week.
&&
.DISCUSSION...
Issued at 444 AM CDT Fri Sep 12 2025
A positively tilted longwave trough axis extends from central
Saskatchewan to the Sierra Nevada early this morning, with a lee
surface trough from southwest North Dakota to the Nebraska
panhandle. Shortwave energy ejecting from the base of the longwave
trough into the Northern Plains has been responsible for clusters of
showers and thunderstorms since last evening. At 4 AM CDT,
convection was much calmer across western and central North Dakota
than earlier in the night. A few showers were bubbling in the
northern James River Valley along outflow from a departing MCS. A
larger area of mostly light rain was focused from around Dickinson
to Hettinger, with a few embedded updrafts recently showing signs of
intensification. There were also isolated to scattered showers with
infrequent lightning lifting northeast through the southern James
River Valley. In other news, patchy dense fog has developed across
northwest North Dakota. Surface observational trends and webcams
suggest that visibility around one quarter mile or less is highly
terrain-dependent. Will therefore continue to handle with a Special
Weather Statement unless or until conditions change.
Low chances for showers and thunderstorms remain in the forecast
this morning and afternoon, mainly across the southern half of the
state, but think that drier conditions will be more common today as
flow aloft briefly becomes anticyclonic. Surface low pressure
broadening over western and central South Dakota will help maintain
the humid air mass in place, with widespread dewpoints in the 60s
and possibly reaching above 70. Most of western and central North
Dakota should be a few degrees cooler today, but still above average
with highs around 80 to 85.
From tonight through this weekend, the longwave trough axis will
pivot to a negative tilt, with the base of the subsequent shorter
wavelength trough moving poleward across the region late Sunday into
Monday. This will bring waves of strong synoptic forcing across the
western half of the state, resulting in several rounds of showers
and scattered thunderstorms. The timing of highest confidence in
showers and storms is during the day Saturday when a 700 mb low is
forecast to close underneath the left exit region of an upper jet,
and deterministic models show a repeat of this setup Sunday evening.
NBM probabilities for at least an inch of rain through Monday have
risen to 50 to 70 percent across the western half of the state, and
the axis of highest probabilities has shifted slightly eastward to
between Highways 85 and 83. The potential for high rain amounts is
well supported by ensemble climatological tools showing near record
values for precipitable water and low level specific humidity, as
well as a shift of tails in the EFI QPF output. We do expect that
final rainfall amounts will be highly variable given convective
influences.
Enough CAPE and shear are forecast to be present to support an
isolated risk for severe storms at times from late this afternoon
through the weekend. Overall confidence in the convective aspect of
the forecast remains low however, with CAMs continuing to show a
wide variety of outcomes with placement, timing, and intensity of
convection. In addition, most, if not all convection is likely to be
elevated. There does appear to a low-probability scenario for a
supercell or two south and west of the Missouri River late this
afternoon and evening, which temporally corresponds to the highest
forecast CAPE (as much as 2000 J/kg per HREF mean) as well as a
projected temporary enhancement in deep layer shear (as high as 40
kts). The arrival of stronger forcing later tonight into Saturday
should weaken atmospheric instability, but there is still potential
for a few embedded storms to become strong to severe at times, as is
being advertised by CAMs` UH tracks and highlighted by SPC`s
convective outlooks through Sunday. Central North Dakota has a
relatively higher risk (albeit still low) for a severe storm over
the weekend than the west.
The favored departure time of the negatively-tilted trough has been
pushed back to Monday evening/night, shifting the most likely period
of drier weather to that time frame. By late Tuesday, another upper
low is forecast to meander into the region. This low could stall and
bring several more rounds of showers and isolated thunderstorms to
the region mid to late next week, but discernible differences in the
synoptic pattern begin to emerge in cluster analysis by Thursday.
Cooler daytime temperatures are expected on Saturday and Sunday.
Forecast highs each day are generally in the 70s, but any locations
where it rains most of the day could be stuck in the 60s. The NBM
temperature distribution shows a subtle warmup for Monday and
Tuesday before a more distinct cool down to near average
temperatures by the end of the week.
&&
.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 133 AM CDT Fri Sep 12 2025
VFR conditions are expected to prevail at all terminals initially in
this forecast cycle. However, scattered showers and thunderstorms
will be making their way into southwest North Dakota early this
morning. Added a VCTS mention at KDIK for this potential. Otherwise,
lowering cigs and the development of fog is expected at KXWA/KMOT
closer to the 12Z time frame this morning. MVFR/IFR cigs and IFR
vsbys will be possible at these two terminals just prior to and
after sunrise. These conditions should gradually improve mid/late
morning. Southeast winds will persist through the overnight hours
before turning to the east to northeast mid/late morning with the
exception of KJMS where a east-southeast component to the wind will
persist longer through the day.
&&
.BIS WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Hollan
AVIATION...WFO ABR

NWS BIS Office Area Forecast Discussion