151
FXUS63 KBIS 162216
AFDBIS

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

.KEY MESSAGES...

- Thunderstorm chances continue through tonight, with an
  isolated to scattered severe thunderstorms possible, along
  with 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 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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1244 PM CDT Mon Sep 16 2024

Chances for showers and thunderstorms will continue across
central North Dakota through much of the day while slowly
drifting north and eastward. Overall, VFR conditions are likely
to prevail, though some lower ceilings cannot be ruled out in
and around areas of showers and storms. Easterly winds around 5
to 10 kts will turn to the southeast and increase to 10 to 15
kts this afternoon, with some higher gusts possible, before
diminishing by early evening. Low IFR stratus will be possible
at KMOT later tonight, and possibly at KXWA. Low level wind
shear is also forecast for late tonight for KJMS.

&&

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

&&

$$

UPDATE...CJS
DISCUSSION...NH
AVIATION...NH