995
FXUS63 KBIS 180637
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
137 AM CDT Mon Aug 18 2025

.KEY MESSAGES...

- Isolated thunderstorms are possible across western North
  Dakota overnight.

- Dense fog advisory for the north central, central, and
  portions of southeast North Dakota.

- Isolated thunderstorms possible on Monday. A strong to severe
  storm is possible across south central North Dakota, with the
  main hazards being hail to the size of ping pong ball and
  wind gusts to 60 mph.

- Warmer and drier weather expected through mid week. Some
  moderate heat impacts are possible.

&&

.UPDATE...
Issued at 126 AM CDT Mon Aug 18 2025

Fog continues to advect westward into south central North
Dakota. A Fog Advisory was issued where the dense fog is, it is
in effect until 9am CT. The next shortwave in Montana has fired
storms that are now currently on the North Dakota border. They
will move into the state in this hour, but should remain sub-
severe.

UPDATE
Issued at 948 PM CDT Sun Aug 17 2025

In the last hour and a half, a cluster of strong to severe
thunderstorms has been impacting western Divide County, now sneaking
into far northern Williams County. It appears these storms are
at the nose of weak warm air advection with ample CAPE
(2000-2500 J/kg) but weaker effective shear (around 25 kts). Was
anticipating a weakening trend now that the sun has set, but
the exact opposite was occurring at the time of this writing.

The other concern for the near-term forecast will be the potential
for fog development. Low stratus is steadily expanding westward
through north central North Dakota and the James River Valley.
Expecting a hybrid radiation/advection fog setup, with ceilings
lowering to the surface and fog formation along the peripheries of
the low clouds.

UPDATE
Issued at 738 PM CDT Sun Aug 17 2025

Monitoring two separate areas of convection early this evening.
One is a batch of mostly showers with infrequent lightning
along the James River Valley. This activity is tied to a slow
moving 700 mb trough that is forecast to drift into the Red
River Valley later this evening. It is located along the eastern
edge of a pool of 2000 J/kg, but both SBCIN and MLCIN are
present, and effective bulk shear is analyzed to be no greater
than 25 kts. However, radar estimates that 3 to 5 inches of rain
has fallen in northeast Stutsman County in the past couple of
hours as an area of heavier rain persisted in that location.

The second area is a line of isolated showers and thunderstorms
that is slowly drifting into northwest North Dakota. A couple
storms became strong at times prior to entering the state, but
intensity has quickly waned on this side of the border. Current
trends suggest that the chance for a stronger storm in western
North Dakota remains very low.

&&

.DISCUSSION...
Issued at 352 PM CDT Sun Aug 17 2025

Upper level wave will push eastward and out of the state
tonight. Weak instability close to this wave could bring some
shower and thunderstorm activity across the east through this
evening. Being displaced from the wave combined with a lack of
shear, severe weather is not expected. Some CAMs have isolated
thunderstorm activity in the west overnight tonight along an
axis of instability. The lift for this looks limited, thus just
put in slight chances. If a storm can develop, modest shear
could bring a strong storm although the chance for severe
weather looks low. Winds will become calm tonight and with
lingering high dewpoints will bring RH values near 100%. This
could bring patchy to areas of fog to much of the CWA tonight
into Monday morning. Lows tonight will generally be in the mid
50s to mid 60s. The axis of instability moves over central ND on
Monday. A weak warm front also looks to set up Monday warming
temperatures back into the 80s for most areas with light winds
also expected. Either weak disturbances in the westerly flow
aloft or this frontal boundary at the surface could bring
isolated thunderstorms in the axis of high cape. Either one of
these disturbances would provide weak lift, thus the confidence
in thunderstorm formation is low. The potential is there for an
isolated severe storm if one can develop. SPC currently has a
Marginal Risk (level 1 of 5) in south central portions. CAPE is
high yet shear is modest. Storm mode would be multi cluster to
perhaps supercell. Thus the main hazards would be ping pong ball
size hail and 60 mph winds. Monday night, thunderstorm activity
will diminish and another night of light winds and high RH
values could return patchy to areas of fog. Given the similar
setup to tonight have added fog mention in for Monday night into
Tuesday morning as well.

A broad ridge aloft then looks to settle in over the state
Tuesday. This will bring further warming temperatures in the 80s
to perhaps 90s in the west. Heat Risk could even reach the
moderate level in some western areas. Even with some afternoon
instability, ridge aloft should cap any chances for
thunderstorms Tuesday. After a mild and dry night Tuesday night,
further warming and generally dry conditions will be expected on
Wednesday. Widespread mid 80s to mid 90s are forecast, with
perhaps some upper 90s in the west. This could bring more
widespread moderate to perhaps isolated major Heat Risk across
the CWA. Breezy southerly winds may also be found, as well as
some lower RH in the west. Fuels are likely too green for any
fire danger across the area. Surface low and perhaps a cold
front could return shower and thunderstorm chances Wednesday
night into Thursday morning. Thunderstorm activity could then
linger into Thursday with some cooler temperatures possible in
the north behind the mentioned front. Depending on timing of
this front there could be at least some isolated severe weather
chances as well.

Late next week into next weekend looks mainly dry and cool as
northwest flow is forecast to remain across the area. Cooler
highs in the 70s with lows in the 40s and 50s are forecast.
Perhaps some breezy northwest winds may also be found in this
pattern next weekend. These temps and winds are dependent on
how this northwest flow pattern sets up, although NBM
temperatures spreads are currently small indicating higher
confidence in this pattern change.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 126 AM CDT Mon Aug 18 2025

VFR with PROB30 for thunderstorms in the west, to IFR for dense
fog in the central and east. Gusty winds may occur with the
storms for the next few hours, otherwise FEW clouds. For the
fog, this will continue through the morning. Expect CIGs of
200-400ft all night. Fog should clear up after 14z. KBIS was
left out of the fog as of now, but may need to be added in the
next few hours if the fog continues to move east, or form over
BIS.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for NDZ002>005-
010>013-021>023-025-036-037-047-048-050-051.

&&

$$

UPDATE...Smith
DISCUSSION...Anglin
AVIATION...Smith