149
FXUS63 KBIS 181930
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
230 PM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Isolated showers and a rumble of thunder or two possible
  across the southwest this evening. Then low to medium chances
  (20 to 40 percent) for isolated to scattered showers and
  thunderstorms on Wednesday. A few storms may become strong to
  severe in south central North Dakota during the afternoon and
  evening.

- Near to slightly above average temperatures favored mid week
  through the weekend.

&&

.DISCUSSION...
Issued at 230 PM CDT Tue Aug 18 2026

Conditions through the later half of the workweek will be
defined by the redeveloping upper level ridge over the western
and central CONUS. For today, cyclonic flow is found across the
northern Plains as a stacked upper level low begins to move over
Hudson Bay. Seasonably cool highs in the lower 70s north to
lower 80s southwest can thus be anticipated across the forecast
area this afternoon. Mainly dry conditions are expected today,
though a handful of short term CAMs continue to advertise low
(<20 percent) potential for some showers across portions of the
far southwest later this afternoon into this evening as a
shallow shortwave perturbation ejects off the Hudson bay low.
Interrogation of BUFKIT model soundings during this period does
reveal at least moderate instability (1000 - 1250 J/KG of model
MUCAPE) and decent bulk shear values (30 to 40 knots) developing
over the southwest late this afternoon, which could allow some
isolated rumbles of thunder. The elevated nature of these
storms will likely keep these storms sub severe in nature,
though very dry near surface conditions, modest DCAPE values
approaching 1000 J/KG, and sufficient 1-3km shear values around
35 knots may allow for gusty winds when and where any storms do
develop. The overall potential for severe weather remains very
low.

Late tonight into Wednesday, a redeveloping upper level ridge
over the western CONUS will subject the northern Plains to
northwesterly flow. With the passage of another, slightly more
robust, shortwave sliding along this northwesterly flow, chances
for showers and potential some thunderstorms increase (10 to 20
percent) from west to east through Wednesday morning and
afternoon. With surface flow turning south to southeasterly
allowing for greater moisture flux into the southcentral and
south east Wednesday afternoon during peak heating, an uptick in
shower and storm coverage can be anticipated. The overall
convective environment appears to be fairly modest, with MUCAPE
values approaching 1000 J/KG and fairly poor bulk shear values
initially around 25 knots or less. Through the late afternoon
and evening, increasing bulk shear values (up to around 30 to 40
knots across the south central into the south east) will brief
be of a stronger to marginally severe storm or two. The main
hazards associated with any severe storm would be large hail up
to the size of quarters, and damaging wind up to 60 MPH. SPC has
a small portion of south central North Dakota under a level 1
of 5 risk for severe weather on Wednesday. Diminishing MUCAPE
and increasing CIN through the mid to late evening is then
expected to limit the storm potential through the remainder of
the night. Otherwise for Wednesday, the building upper level
ridge moving into our west will allow for the greater
penetration of the thermal axis into the northern Plains,
promoting near normal high temperatures in the lower to upper
80s.

Looking ahead into the remainder of the week, near to slightly
above normal and mainly dry conditions are anticipated as the
upper level ridge becomes more situated over the Great Plains.
Some cooler highs in the upper 70s are possible across the north
Friday afternoon as a weak impulse slides down the eastern
extent the ridge, but otherwise highs in the 80s are expected
through at least Saturday. The ensemble becomes increasingly
discordant late Saturday into Sunday as it attempts to resolve
the evolution of a more robust pacific trough / low cutting
across the southern Canadian Prairies. A slight majority (55
percent) of ensemble members favor a rapid flattening of the
ridge late Saturday into Sunday, allowing for low to medium
chances (20 to 40 percent) for precpitation and somewhat cooler
highs in the upper 70s to mid 80s on Sunday. A minority cluster
(45 percent) of members instead favor a slower evolution of this
pattern, allowing for a brief amplification of the ridge over
the northern Plains heading into Sunday, before rapidly
flattening late Sunday into early Monday. This pattern would
delay the onset of precpitation until later in the day Sunday,
and would allow highs mainly in the 80s to persist through the
weekend. In either scenario, the ensemble broadly shows a return
to warmer and drier weather heading into the early half of next
week as a redeveloping ridge pattern takes hold of the central
CONUS.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 230 PM CDT Tue Aug 18 2026

VFR ceilings and visibility is generally expected at all
terminals throughout the 18Z TAF period. Chances for isolated
showers and a thunderstorm or two increase across the southwest
this evening, then across the west and portions of central
North Dakota through Wednesday morning. Overall coverage of
either is expected to be limited, and thus confidence is too low
to include mentions at any given terminal at this time.
Otherwise, light and variable winds this afternoon and overnight
will reorganize out of the south to southwest through Wednesday
morning.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Adam
AVIATION...Adam