891
FXUS63 KBIS 261420
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
920 AM CDT Sat Jul 26 2025

.KEY MESSAGES...

- Hot and humid today, with afternoon heat indices around 100
  in south central North Dakota and the James River Valley.

- Isolated to scattered severe thunderstorms are once again
  possible across western and central North Dakota late this
  afternoon through tonight. Expected hazards include hail up
  to the size of golf balls and wind gusts up to 70 mph.

- Periodic showers and thunderstorms are expected to continue
  Sunday through the first half of next week. Isolated severe
  thunderstorms are possible over this time period.

- Temperatures expected to fall below average by the middle of
  next week.

&&

.UPDATE...
Issued at 909 AM CDT Sat Jul 26 2025

A weak pressure pattern with light surface winds was situated
over western and central ND this morning. A few showers and
thunderstorms were located along the ND/SD border, and over
northwest ND. This activity will continue to propagate east
through the morning. Better CAPE is situated over the JRV, but
CIN is also plentiful here. There is less of a cap along the
International Border, shear is weak but should be increasing
through the morning. Both these areas of convection may linger
through the morning, but are expected to remain sub-severe. We
made some minor adjustments to pops to account for both of
these areas.

UPDATE
Issued at 644 AM CDT Sat Jul 26 2025

Widely scattered showers, and more recently a narrow line of
thunderstorms in the southern James River Valley, are being
maintained by warm air advection associated with the low level
jet across southern North Dakota early this morning. Expect this
activity to diminish as the LLJ wanes through the morning.
Meanwhile, a separate area of weaker warm air advection and low
level frontogenesis has brought an isolated thunderstorm into
northwest North Dakota. This activity may linger as it moves
into north central North Dakota later this morning, but
lightning will become less likely as it enters a more stable air
mass there.

&&

.DISCUSSION...
Issued at 503 AM CDT Sat Jul 26 2025

Southwest flow aloft will persist over the Northern Plains today as
a surface deformation zone emerges from southwest to northeast North
Dakota. A deepening surface low over western South Dakota will
promote a reinforcement of rich boundary layer moisture, with
dewpoints expected to rise into the mid 60s to mid 70s this
afternoon, highest in the James River Valley. It will be another hot
day, with highs mostly in the upper 80s to mid 90s. This pushes heat
indices to around 100 across south central North Dakota and the
James River Valley and forecast maximum wet-bulb globe temperatures
into the extreme category. A Heat Advisory remains in effect for
these areas from noon to 8 PM CDT.

Extreme CAPE as high as 4000 to 5000 J/kg is forecast to build
across the southern half of the state this afternoon, and stronger
flow aloft will promote strengthening shear on the order of 40-50
kts in the 0-6 km layer. However, mid level capping and weak forcing
are likely to preclude convective initiation through this
afternoon. The few CAMs that do initialize convection prior to
mid evening generally show only one to two discrete storms from
southwest to south central North Dakota. Should this outcome
occur, very large hail would be the primary hazard. A
conditional tornado threat could also emerge with discrete
storms on account of the increasing low level jet amplifying
hodograph concavity. But this would be limited in both space
(south central North Dakota) and time (early evening).

By 03Z, a mid level shortwave is forecast to eject off the Black
Hills into southwest/south central North Dakota. CAMs are in much
better agreement that this forcing will initiate more widespread
convection from northwest South Dakota into southern North Dakota
late this evening, spreading to the north and east through the
night. Storm mode is difficult to discern with a lack of a well-
defined surface boundary, but a parallel orientation of the MLCAPE
gradient and deep layer shear/wind vectors would likely tilt more
towards a linear mode vs. discrete. The strongest shear is forecast
to be displaced slightly north of the strongest buoyancy, which caps
our forecast most likely high end wind gust at 70 mph.

Flow aloft begins to turn more zonal on Sunday. The strongest
forcing is forecast to depart to the east by peak heating, but some
CAMs do show isolated late afternoon/evening convection across
various parts of the state. However, another EML, the lack of a
strong forcing mechanism, and rising mid level height tendencies
are more likely to suppress convection on Sunday. Should an
updraft become sustained above the LFC, there would be
sufficient CAPE and shear to promote a severe storm. But
convection from tonight could also greatly influence the
evolution of the mesoscale environment on Sunday. All this is
to say we likely will not have high forecast confidence in
convection on Sunday until the morning of. Otherwise, Sunday is
looking to be just a few degrees cooler but still humid.
Forecast highs range from the lower and mid 80s north to lower
90s south. Heat indices may approach 100 in the southern James
River Valley late Sunday afternoon.

A gradual transition from zonal to northwest flow aloft is favored
by ensembles from early to mid next week. The initial zonal flow
regime keeps medium chances for showers and storms in the forecast
across most of the state late Monday afternoon through Monday night.
A high shear/low CAPE environment could promote a few strong to
severe storms, and the SPC has introduced a Marginal Risk across
southwest and far south central parts of the state. The transition
to northwest flow driven by a deepening Hudson Bay low and
amplifying Rockies ridge should allow a cooler and drier air mass
under Canadian high pressure to glide into the Upper Midwest. This
pattern limits daily shower and storm chances to southwest parts of
the state by the middle of the week, and brings a reprieve from the
hot and humid conditions. Forecast highs from Tuesday through Friday
are mainly in the 70s, with lower dewpoints in the 50s to lower
60s.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 644 AM CDT Sat Jul 26 2025

Isolated showers and thunderstorms are possible this morning. KXWA
will see a nearby storm through 13Z. Otherwise, impacts to TAF sites
are unlikely this morning. Widespread showers and thunderstorms
are expected to develop across southern North Dakota this
evening and move northeast through the night. Erratic and strong
wind gusts, hail, and brief periods of IFR/LIFR visibility
restrictions from heavy rain are possible with the strongest
storms. Outside of thunderstorms, VFR conditions and light winds
are expected through the forecast period.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT /7 PM MDT/ this evening for
NDZ023-025-034>037-042-045>048-050-051.

&&

$$

UPDATE...TWH
DISCUSSION...Hollan
AVIATION...Hollan