483
FXUS63 KBIS 200230
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
930 PM CDT Tue Aug 19 2025

.KEY MESSAGES...

- Hot, humid, and breezy weather is expected Wednesday, with
  moderate to high heat impacts.

- Isolated severe thunderstorms are possible Wednesday evening
  and Wednesday night, with low to medium chances of showers and
  thunderstorms Wednesday evening through Thursday.

- Much cooler temperatures, lower humidity, and mainly dry
  weather is expected Friday through Tuesday.

&&

.UPDATE...
Issued at 925 PM CDT Tue Aug 19 2025

A mild quiet evening continues across western and central North
Dakota. No major changes were needed for this update.

UPDATE
Issued at 554 PM CDT Tue Aug 19 2025

No major changes were needed for this update as the weather
remain quiet and warm across western and central North Dakota.
Just blended in the latest observations of the sensible weather
fields to the going forecast.

&&

.DISCUSSION...
Issued at 240 PM CDT Tue Aug 19 2025

The forecast is highlighted by heat Wednesday, followed by low
to medium chances of thunderstorms Wednesday night through
Thursday evening. We are then highly confident in much cooler,
and lower-humidity weather from Friday through this weekend.

An upper-level ridge is building into the region this afternoon,
with the attendant rising heights aloft minimizing diurnal cloud
development and mitigating any chance of convection. Low- and
midlevel warm air advection will increase tonight as a lee-side
trough develops with increasingly southwesterly/cyclonic flow
aloft in and downstream of the Northern Rockies. Steepening
midlevel lapse rates with the advection of an associated
elevated mixed layer into the Northern Plains, and a low-level
jet around 40 kt centered around 850 mb, both imply elevated
convective potential in and of themselves late tonight and
Wednesday morning, mainly in north central ND near the terminus
of the low-level jet. However, very warm temperatures aloft and
somewhat shallow low-level moisture will result in substantial
CIN for elevated parcels, reducing those odds considerably. CAMs
are consistent signaling the aforementioned capping will keep
deep moist convection at bay. As a result, we have a dry
forecast in place, though we fully expect these processes to
manifest as altocumulus decks, and it could support low-end PoPs
with later forecast updates.

Surface dewpoints in the 60s F and the low- and midlevel warm
air advection will ensure low temperatures tonight are in the
60s F, which is about 10 degrees above normal for late August.
Given the moist near-surface layer, we added patchy fog into the
forecast late tonight and Wednesday morning in east central ND,
from Rolla/Rugby/Bottineau south toward Harvey, Carrington,
Jamestown, LaMoure, and Ellendale. Further west, high dewpoints
will also persist, but the increasing southerly low-level flow
may inhibit fog formation, instead suggesting possible low
cloud (stratus) formation from about 09 to 15 UTC.

On Wednesday, the surface to 700 mb thermal ridge axis will be
centered along the ND/MT border in vicinity of a surface trough
axis and related deep boundary layer mixing. Forecast highs are
around 100 F in western ND in that zone, and are in the mid 90s
F over central ND where higher surface dewpoints in the 60s F
will persist through the day on gusty south winds. The result
will be a hot and breezy day with heat impacts. The apparent
temperature (heat index) values are forecast around 100 F in
parts of western ND where we have a Heat Advisory in place. We
considered expansion of that advisory further south and east,
all the way to the Highway 83 corridor, but forecast heat index
values are a few degrees shy of the 100 F criteria for that
headline. Even so, Wet Bulb Glob Temperature (WBGT) forecasts,
which are indicative of heat impacts to outdoor activities, are
high across all of western and central ND, so we are stressing
the need for precautions with outdoor activities on Wednesday.
If later forecasts either increase forecast high temperatures a
bit more, or if dewpoints end up slightly higher owing to the
effects of evapotranspiration, as has been the case recently,
the Heat Advisory may need to be expanded.

Very warm midlevel temperatures with 700 mb temperatures as high
as +15 C are expected Wednesday afternoon and evening, which is
forecast to contribute to large MLCIN reflective of significant
capping in the moist sector ahead of the surface trough. Still,
the very hot, deeply-mixed air mass near the trough may locally
minimize capping, including where that air mass intersects with
the richer moisture in northwest/north central ND. This is all
consistent with a conditional severe thunderstorm risk in parts
of western and central ND, highest near the Canadian border, as
the CAPE-shear setting is favorable for supercells if capping
were to be overcome. In respect to the MLCAPE on the order of
2000-3000 J/kg and effective-layer shear around 30 kt, but only
low odds of capping being overcome, we are messaging potential
hazards as ping pong ball size hail and damaging winds up to 60
mph Wednesday evening through Wednesday night. Gradual height
falls aloft and a cold frontal passage on Wednesday night will
shift the low thunderstorm chance and an attendant strong-severe
storm risk eastward, though forecast soundings suggest
significant capping may persist overnight.

On Thursday, the cold frontal zone will gradually continue its
eastward movement, reaching the eastern Dakotas by evening.
Large-scale ascent and winds aloft will both increase as a broad
upper-level trough crosses the region. However, timing of the
frontal passage with respect to the diurnal heating cycle, and
lingering midlevel capping, both lower confidence in there being
strong-severe storms in our forecast area. This is also
reflected in machine learning probabilities that are instead
focused east of the local area, moreso in the Red River Valley.
Nonetheless, we do have a chance of showers and thunderstorms in
the forecast, gradually diminishing by Thursday evening.

Beginning Friday and continuing through the weekend, we expect
highs only in the mid 60s to to lower 70s F, with lows mainly
in the 40s F. This will also be accompanied by much lower
humidity (dewpoints in the 40s F) as deep-layer northwesterly
flow develops as an upper-level low moves across central and
southern Canada. Spread in ensemble guidance/NBM members is very
low in this timeframe, leading to high confidence in forecast
temperatures and dewpoints. We are expecting mainly dry weather,
though the cyclonic flow could result in a low shower chance in
later forecasts. We also expect northwest winds to be in the
breezy to windy categories, mainly Friday and Saturday, though
the chance of reaching advisory criteria is low per the most
recent ensemble guidance.

Heights aloft will gradually rise next week as broad upper-level
ridging takes shape in the western U.S., but dry mean northwest
flow is expected to prevail in the Northern Plains, with highs
locally remaining in the 70s F early next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 554 PM CDT Tue Aug 19 2025

VFR conditions are generally expected through the period. KJMS
could see a bit of patchy fog overnight but confidence is too
low at the moment to add mention in the forecast. Winds will
mainly be out of the south, becoming breezy at times, especially
west and north.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from 1 PM CDT /noon MDT/ to 7 PM CDT /6 PM MDT/
Wednesday for NDZ001-002-009-010-017-018-031-032.

&&

$$

UPDATE...ZH
DISCUSSION...CJS
AVIATION...ZH