216
FXUS63 KBIS 270241
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
941 PM CDT Fri Jul 26 2024

.KEY MESSAGES...

- Strong thunderstorms are possible from Lake Oahe to the James
  River Valley late this evening through tonight.

- Daily low to medium chances (20 to 40 percent) for
  thunderstorms remain in the forecast through the middle of
  next week. Some storms could be strong to severe over central
  North Dakota Saturday and Sunday.

- Near to above normal temperatures will continue, with highs
  mostly in the mid 80s to mid 90s through next week.

&&

.UPDATE...
Issued at 940 PM CDT Fri Jul 26 2024

At 930 PM CDT, line of showers was located from the western
Standing Rock Reservation to the Turtle Mountains. Orientation
of this line has mostly been parallel to upstream dry slotting
associated with a stacked low over northern Saskatchewan, and is
more meridional than model guidance projected. Lightning
activity has slightly increased along the southern flank of the
line as it is approaching greater elevated buoyancy, and some
isolated convection has been trying to develop downstream from
near Steele to Cooperstown. High-resolution guidance initiated
since after 18Z has been more inconsistent on convective
evolution, displaying a wide variety of scenarios. Any updrafts
that are able to sustain to the east of the main line of showers
will have the best chances of becoming strong, but MUCAPE is
forecast to decrease over time.

UPDATE
Issued at 724 PM CDT Fri Jul 26 2024

For unclear reasons, thicker smoke at the surface never materialized
this afternoon in western North Dakota. With all stations reporting
10 mile visibility and no evidence of near-surface smoke upstream in
surface observations, the mention of smoke has been removed from the
forecast. Periods of thicker smoke aloft will remain possible for
the foreseeable future, though.

Scattered high-based showers are located from southwest to north
central North Dakota early this evening. Very little lightning has
been observed over the past few hours, which confirms the
latest SPC mesoanalysis of MUCAPE no greater than 100 J/kg
where showers are present.

Later on this evening, recent CAMs have consolidated on a
solution that develops a line of storms along a low level baroclinic
zone from the Standing Rock Reservation to the James River Valley
around 10 to 11 PM CDT. Given the time of initiation, this
convection would most likely be elevated but could have access to
MUCAPE as high as 2000 J/kg and effective bulk shear near 30 kts.
While shear vectors are oriented parallel to the initiating
boundary, the overall weak background forcing could dictate more of
a mixed multicell/cluster mode as is being depicted by CAM
reflectivity output. We are therefore expecting the potential for a
few strong to marginally severe storms as a reasonable high-end
outcome. The area of convection should slowly drift eastward over
time, but individual storm motion parallel to the boundary
could promote training in an environment with 1.5 to 1.75 inch
precipitable water and warm layer cloud depths around 3 to 4 km.
This could lead to a localized risk of excessive rainfall.

&&

.DISCUSSION...
Issued at 235 PM CDT Fri Jul 26 2024

Painting the synoptic picture, western and central North Dakota
currently sits under the influence of southwest flow aloft as an
upper level trough is starting to nudge into the region. A stacked
low is located over northern Saskatchewan with a cold front
extending south into central and southwest North Dakota. The stacked
low and front will slowly move off east through the weekend, but
western and central North Dakota will remain in persistent
zonal/southwest flow for several days as broad western US troughing
settles in.

For the rest of today, we will see plenty of mid to high clouds
along the cold front as it moves east. Forecast soundings suggest
plenty of dry air near the surface so cloud bases are fairly high.
Thus, while we continue to see radar echoes all along this boundary,
it is likely that only a little bit of light rain is making it down
to the surface. Later this afternoon and into the evening,
instability will increase along and ahead of the cold front which
will also lead to an increase in shower and thunderstorm coverage
and intensity. That being said, it`s a little bit unclear if showers
and storms will be along or behind the front. The best chance for a
strong storm or two will generally be early this evening in the
vicinity of the James River Valley. MLCAPE along the front is
forecast to be in the 1000 to 1500 J/kg range with deep layer shear
in the 30 to 40 knot range. If storms were able to fire in the warm
sector, they would have quite a bit more instability to work with,
but it seems likely that substantial capping will keep this scenario
from happening. Either way, the parameter space will be sufficient
for some organized or semi-organized convection. The most likely
scenario is a strong storm or two over the James River Valley this
evening with some gusty winds to 50 mph and small hail. Storms will
generally move out in the mid to late evening hours with some
showers lingering through the overnight.

For Saturday, we will stay in southwest flow aloft but most of the
larger scale forcing will have moved off to the east. However, a
diffuse stationary boundary extending north northeast from a weak
surface low over northwest South Dakota and several outflow
boundaries from tonight`s convection will be lingering around,
likely leading to enough convergence to trigger some storms by the
late afternoon and evening. Models are currently having a hard time
deciding on how far west the instability axis will extend which will
have a big impact on the coverage of strong to severe storms on
Saturday. The most likely scenario is that we will see some isolated
to scattered severe storms across the central and into portions
of the Devils Lake Basin and James River Valley. The
environment here will be characterized by moderate instability
(1500 to 2500 J/kg MLCAPE) and deep layer shear around 35 to 45
knots. Thus, any storms that can get going here will have the
potential to become severe. Hail to the size of golf balls and
winds to 60 mph will be the main threats. The tornado threat
appears to be fairly low, but when there are plenty of
boundaries floating around, we cannot completely rule it out.
The main limiting factor will be the uncertainty regarding storm
coverage given generally weak forcing, but another fly in the
ointment (per some forecast soundings) could be some dry air
entrainment into updrafts. Storms will once again weaken in the
late evening hours, potentially giving way to some more showers
overnight.

The setup on Sunday doesn`t look all too different from Saturday as
far as parameter space goes, but maybe a bit further south and east.
However, we will have a stronger shortwave moving into the southwest
around 00z Monday, which may end up leading to some sort of
cluster/MCS moving across the state through the night. It`s a bit
early to talk specifics, but these storms could also be strong to
severe with a better wind threat potentially depending on how the
boundary layer evolves.

We will then continue to have daily low to medium chances (20 to 40
percent) for showers and storms Monday through Wednesday. CSU
Machine Learning guidance is also suggesting that low severe weather
chances may hang around Monday and Tuesday which seems reasonable
given the parameter space many of the deterministic models are
suggesting.

Regarding the heat, it will be another hot one on Saturday with
highs expected to be in the mid 80s to mid 90s. However, the area
with the highest dewpoints (east) will be cooler than today so for
now, there are no plans for further Heat Advisories. We will cool
down slightly into the 80s on Sunday before the next wave of heat
and humidity moves in Monday and through the week. The hottest
day of the period will be on Tuesday when highs will generally
be in the lower 90s to lower triple digits. Thus, more heat
headlines could be needed eventually for Tuesday. We don`t cool
down much through the rest of the week with plenty of highs in
the 80s and 90s expected through Friday.

Finally, the HRRR-Smoke model continues to be aggressive in
bringing in some thicker near surface smoke to the area late
this afternoon and through the night. While smoke aloft is
likely to hang around for at least several more days, it looks
like we may get a break from near surface smoke after tonight.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 940 PM CDT Fri Jul 26 2024

VFR conditions are mostly expected to prevail through the forecast
period. Scattered showers will remain possible across southwest and
central North Dakota this evening. Later this evening into tonight,
thunderstorms may develop from far south central into eastern North
Dakota, possibly impacting KBIS and KJMS. Any storm would be
capable of producing MVFR to IFR visibility from heavy rain,
erratic wind gusts, and small hail. Saturday morning should be
dry, but showers and storms could develop once again across
western North Dakota Saturday afternoon. Smoke is no longer
expected to reduce surface visibility, but thicker smoke aloft
will remain possible for the foreseeable future.

&&

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

&&

$$

UPDATE...Hollan
DISCUSSION...ZH
AVIATION...Hollan