521
FXUS63 KBIS 170237
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
937 PM CDT Mon Sep 16 2024

.KEY MESSAGES...

- Severe threat is becoming confined to far north central North
  Dakota into northeast North Dakota. Key threats are damaging
  wind gusts, large hail, and very heavy rain.

- An active pattern continues through the week, with strong to
  severe storms again possible Tuesday night.

- Temperatures will trend cooler through the week, after
  Tuesday, with forecast highs in the 60s to lower 70s by next
  weekend.

&&

.UPDATE...
Issued at 929 PM CDT Mon Sep 16 2024

For late evening update have trimmed McLean and Bottineau
Counties out of the watch. Have also trimmed down thunderstorm
chances over areas either not affected or in the path of the
ongoing convection. Only exception is a few showers/storms
lifting through eastern Montana which may enter western North
Dakota later tonight per latest CAMs, but chances for this
appear low. For additional info on ongoing storms in north
central North Dakota, please refer to last mesoscale discussion.


UPDATE
Issued at 757 PM CDT Mon Sep 16 2024

** Mesoscale Discussion **

The severe-thunderstorm risk will be greatest over north central
and northeastern ND the remainder of the evening, with a lower
risk of severe storms as far south as the Highway 200 corridor.

As of mid-evening a cluster of strong-severe storms has evolved
east-southeast of Minot, north of a surface frontal zone/outflow
boundary. These storms are elevated per trends in low-level CIN,
but low-level moisture has been gradually increasing (surface
dewpoints rising into the mid 60s F) downstream of the storms
in McHenry, Pierce, and Rolette Counties. This is resulting in a
deeper moisture profile that is sufficient for increasing MUCAPE
on the order of 2000 j/kg for the elevated storms, and objective
analysis suggests 30-35 kt of effective-layer shear ahead of the
ongoing storms, favoring a continued large hail and transient
damaging wind risk with storms as they evolve generally eastward
the next few hours. An increasing low-level jet combined with
the eastward advance of an upper-level vorticity max will both
likely result in development of additional storms across north
central and northeastern ND the next few hours. However, simple
extrapolation of ongoing storms would have them exiting the
Rolette and Pierce County areas by around 11 pm CDT. Upscale
growth and storm interactions with the elevated storms will
likely limit peak hailstone sizes, though the deep-layer shear
and CAPE combination still supports quarter to golf ball size
hail potential with the most-intense storms.

Further south, congested cumulus in the Bismarck/Mandan area
reflected what was only a weakly-capped boundary layer, but it
appears background ascent has been insufficient for sustained
convective initiation even as this agitated cumulus moved north
closer to the surface front in eastern McLean, Sheridan, and
Wells Counties. The boundary layer will gain appreciable CIN on
the side of the frontal zone receptive to boundary-layer based
storms with sunset, so the conditional odds of severe storms is
low and diminishing quickly south of the Highway 200 corridor.
This will also result in an end to the earlier-mentioned tornado
risk, which was predicated on a surface-based storm developing
in close proximity to the aforementioned surface boundary.

CJS

UPDATE
Issued at 713 PM CDT Mon Sep 16 2024

Have issued a severe thunderstorm watch for most of north
central North Dakota until midnight tonight. Otherwise,
remainder of forecast remains in tact. Please see mesoscale
discussion in previous update for details.

UPDATE
Issued at 516 PM CDT Mon Sep 16 2024

** Mesoscale Discussion **

As of late afternoon, a quasi-stationary/remnant outflow
boundary extends from southern Mercer county northeast toward
Turtle Lake-Mercer in McLean County, to just north of Harvey, in
southern Pierce County. Surface winds have veered slightly south
of that boundary (e.g., at Bismarck and Linton), suggesting that
low-level confluence is increasing in its vicinity. One-minute
satellite imagery reveals shallow cumulus along that boundary,
and earlier some orphan anvils focused in Mercer and Oliver
Counties, but those have become less-focused in the last 30
minutes. Overlap of mid 60s F surface dewpoints and mid 80s F
temperatures is greatest from northern Burleigh into Sheridan
and Wells Counties, along and just south of that boundary, and
reflects the relatively-greatest (albeit still only medium)
probability for surface-based thunderstorms to develop before 00
UTC. RAP-based objective analysis has ~2000 J/kg of MLCAPE in
that corridor and 30-40 kt of effective layer shear, favoring a
mix of multicellular to transient supercellular storms if
sustained initiation occurs. However, as mentioned, there are
only medium probabilities of that sustained initiation occurring,
which will be driven by whether or not sufficient ascent related
to the main vorticity maxima aloft further northwest in the
Minot vicinity overlaps with the buoyant sector on the warm side
of the frontal zone. Hodographs are not very lengthy aloft, and
deep-layer shear vectors oriented with a large parallel
component to the surface boundary suggest that if surface-based
storms develop they will likely grow upscale into messy modes
after 1 to 2 hours. As a result, potential hazards remain large
hail up to golf ball size (potentially largest in the first 1 to
2 hours of any surface-based development, before smaller sizes
become favored with upscale growth), damaging wind gusts up to
60 mph, and a potential tornado owing to sufficient low-level
SRH and ambient vorticity along with large low-level CAPE along
the boundary. However, this is all conditional on sustained
surface-based initiation occurring.

Across north central ND to the north of the boundary, where
surface temperatures are in the 60s to lower 70s F, storms are
expected to be elevated in nature. Sufficient bouyancy (MUCAPE
on the order of 1000 J/kg) exists for strong-severe storms, but
effective-layer shear is weaker, on the order of 25 kt, which
may marginalize the hazard magnitude in north central ND north
of the boundary (including in places like Minot and Rolla). The
forcing for ascent will be greater in those area, closer to the
vorticity maxima aloft, so storm coverage will be greater to
the north of the boundary, in north central ND, though.

CJS

&&

.DISCUSSION...
Issued at 230 PM CDT Mon Sep 16 2024

Currently, large cutoff upper level low pressure centered over
central California, with a broad southwest flow aloft pattern
over the Dakotas. Well defined embedded wave continues to lift
northeast across the region (now over mainly northeastern ND),
along with all associated showers and thunderstorms north of
I-94 and mainly across northern areas of the state.

We remain fairly unstable this afternoon, with Mixed Layer CAPE
near 2500 and MUCAPE up to 3000, especially for those areas
that cleared this afternoon across much of the south. 0-6km wind
shear is modest, around 30-40 knots. CAMs have been consistent
(as the midnight shift last night pointed out) of most if not
all convective activity across north central into the Turtle
Mountains later today into the evening, along with some members
showing signs of activity in the James River Valley. This is
where the Slight Risk is maintained by SPC. Still, with the
amount of unstable air, the amount of moisture, and the
potential forcing with any subtle mid level wave lifting
through, will keep at least a slight chance of thunderstorms all
areas through this evening.

S/WV ridge momentarily builds over the Northern Plains later
tonight through most of Tuesday, as the aforementioned upper low
starts to meander east-northeast into the Intermountain West.
This will result in a warm and dry day for tomorrow, along with
ample sunshine.

Later on Tuesday and into Tuesday night, a warm frontal boundary
will be lifting north across the region, as the upper low swings
northeast into the Northern High Plains of eastern Wyoming and
Montana. This will result in another round of showers and
thunderstorms (some strong to severe) across the region during
this timeframe.

We will remain in an active upper level pattern through the work
week, with multiple periods of showers and potential
thunderstorms. There is also a very favorable probability for
temperatures trending cooler, with highs in the 60s to lower 70s
by next weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 713 PM CDT Mon Sep 16 2024

Scattered to numerous thunderstorms this evening over north
central North Dakota, including the KMOT terminal. Isolated
thunderstorms possible over remaining areas, but have refrained
from mentioning in individual TAFs given low confidence in
timing or development. Later tonight low status is expected to
move into north central and parts of northwest North Dakota,
leading to areas of IFR ceilings through late Wednesday morning.
By Wednesday afternoon, VFR conditions are expected across the
area.

&&

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

&&

$$

UPDATE...CJS/JJS
DISCUSSION...NH
AVIATION...NH