469
FXUS63 KBIS 160132
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
832 PM CDT Sat Jun 15 2024

.KEY MESSAGES...

- Scattered severe thunderstorms are possible across western and
  central North Dakota tonight through early Sunday morning.
  Expected hazards include damaging winds up to 80 mph and hail
  up to golf ball size, with a tornado or two possible.

- Isolated to scattered severe thunderstorms are possible across
  south central and southeast North Dakota Monday through Monday
  night.

- Warm and humid today, followed by below normal temperatures
  through the first half of next week. Windy in the northwest on
  Sunday.

&&

.UPDATE...
Issued at 829 PM CDT Sat Jun 15 2024

Quick update to issue a Tornado Watch for all of western and
most of central North Dakota through 2 AM CDT / 1 AM MDT.

UPDATE Issued at 631 PM CDT Sat Jun 15 2024

Minimal updates were needed for the gridded forecast early this
evening. We did slightly adjust precipitation chances to better
reflect the latest trends and observations and blended in the
latest observations to the forecast. For more details regarding
the latest on the severe threat, please see the mesoscale
discussions above.

&&

.MESOSCALE...
Issued at 735 PM CDT Sat Jun 15 2024

Convection continues to struggle across western and central North
Dakota this evening. At 730 PM CDT, the only storms were widely
scattered along a line from near Dickinson to Hettinger. One of the
storms between Dickinson and New England briefly became severe, but
has since weakened. We now think the threat ceiling for this part of
the state is much lower than previously anticipated due to
increasing CIN and a lack of stronger mid to upper level forcing. It
is noteworthy though that the recent intensification corresponded
with the arrival of a patch of high clouds, indicating a relative
bump in deeper layer ascent. Other patches of of high clouds over
eastern Montana may eventually catch up to the ongoing convection,
but it is also possible this convection may not survive that long.
Confidence in the longevity of severe storms over southwest into
south central North Dakota this evening is low.

Expectations for a potentially significant wind threat later this
evening from northwest to north central North Dakota remain on
track. The upstream complex is showing signs of congealing from near
Glasgow to Jordan, MT, with a projected time of arrival in the
northwest between 9 and 10 PM CDT. An emerging trend in recent high-
resolution guidance is for convective development along an eastward
progressing cold front from southwest into south central North
Dakota later overnight, as far south as the South Dakota border.
CAMs that contain this convection show strong UH tracks in an
environment with forecast MUCAPE around 2000-3000 J/kg, but there is
uncertainty on the strength of shear, which is expected to be higher
trailing the front.

Issued at 600 PM CDT Sat Jun 15 2024

The lack of sustained strong to severe convection thus far can
likely be attributed to poor mid to upper level forcing. Recent
attempts at convective initiation are noted on satellite in an area
of more focused low level warm air advection over far southwest
North Dakota and far northwest South Dakota. Rapid-refresh guidance
gives this development the best chance of surviving residual
capping. Effective bulk shear has increased to 40-45 kts as far east
as the Morton/Grant/Sioux County line, so any sustained updrafts
could quickly become severe.

Through 8 PM CDT, the threat for severe weather in northwest North
Dakota is very low. However, this will quickly change as the
developing line of storms in central Montana approaches. CAMs have
been very consistent in advertising an intensifying bowing/linear
complex moving into the northwest later this evening. SPC
mesoanalysis is already showing 40-50 kts of 0-3 km shear and 45-55
kts of effective bulk shear in place, and these figures could
increase further with winds expected to increase in both the mid and
low levels. A thermodynamic gradient parallel to the expected storm
motion has also setup over northwest North Dakota, with DCAPE over
1000 J/kg on the buoyant side. The ongoing central Montana
convection will soon be moving into a more unstable environment,
which should give us a better idea of severe expectations across the
northwest and north central later this evening into tonight.

Issued at 423 PM CDT Sat Jun 15 2024

At 4 PM CDT, a diffuse surface trough was analyzed from the
southwest corner of the state to near the Turtle Mountains. Attempts
at sustained convective initiation are evident along this boundary,
with the most successful attempt as of yet occurring over
Mercer County as of this writing. Another more prolonged attempt
at sustained convection is noted in Bowman/Slope Counties where
agitated cumulus have been percolating for the last hour. The
storms developing near Beulah/Hazen would seemingly have better
odds of sustainment given its location on the nose of southerly
winds advecting in mid 60s dewpoints. But approaching high
clouds in the southwest could also be a trigger for sustained
convection there. The Mercer County development is in an
environment with higher CAPE (near 3000 J/kg SBCAPE) but lower
shear (30-35 kts effective), and this is being borne out in
radar observations with rapidly intensifying updrafts with no
tilt. Farther west, there is relatively lower CAPE (around 2000
J/kg SBCAPE) but stronger shear (40-45 kts effective).

&&

.DISCUSSION...
Issued at 238 PM CDT Sat Jun 15 2024

Upstream troughing digging into the Pacific Northwest places the
Northern Plains under southwest flow aloft this afternoon. At the
surface, a lee trough is developing just west of the ND/MT border.
The air mass downstream of the lee trough is capped, but low to mid
60s dewpoints coupled with strong diurnal heating has already
resulted in 2000 J/kg SBCAPE. A gradual lowering of mid level
heights combined with low level convergence could provide enough
forcing for convective initiation to overcome the capping in western
North Dakota late this afternoon, and there are already signs for a
sustained storm on satellite imagery along the MT/ND border near
Trotters. Bulk shear through all layers AGL is forecast to increase
this afternoon and especially this evening. Any storms that develop
along the lee trough later this afternoon are favored to remain
discrete, with long hodographs supporting very large hail up to
tennis ball size.

With a weakening of the capping inversion and an increase in the low
level jet with poleward moisture transport early this evening,
convection should become more widespread across western North
Dakota. The enhanced southerly return flow could introduce a brief
tornado threat with any discrete storm, with 0-500 m SRH increasing
to 100-200 m^2/s^2 and 0-1 km bulk shear increasing to 15-25 kts.
This may be especially true for the northwest due to a deepening
northeast Wyoming low augmenting the surface wind field to a
predominant easterly direction. Later in the evening and into the
early overnight hours, several CAMs bring severe storms that
originate in central Montana into northwest and north central North
Dakota as an intense bowing complex. Simultaneously, existing
convection could grow upscale and merge into a forward-propagating
linear complex. The kinematic environment over northwest and north
central North Dakota looks favorable for a higher-end damaging wind
threat, with 0-3 km shear exceeding 40 kts and near surface winds
directly opposing storm motion. This is where the highest potential
for significant severe winds of 75 to 80 mph exists.

Later tonight, following the convection, a deepening low over
southern Saskatchewan will drive a cold front across the state. This
will result in a cooler, drier, and windier Sunday. The strongest
winds are expected in the northwest, where afternoon gusts could
approach 45 mph. Outside of the southern James River Valley which
could approach 80 degrees, high temperatures will primarily be in
the upper 60s to mid 70s on Sunday.

Multiple rounds of showers and thunderstorms are expected over the
north central CONUS Sunday night through Monday night as a longer
wave trough translates across the Northern Rockies. For Sunday night
into Monday morning, there are medium chances (40 to 70 percent) for
showers and storms across the southern half of the state, including
a lower probability risk for elevated strong to marginally severe
storms posing a large hail threat along the South Dakota border and
east of the Missouri River. This will be conditional on the
northward advancement of elevated buoyancy into a a highly sheared
environment. For Monday afternoon through Monday night, there are
high chances (70 to 90 percent) for showers, with thunderstorm
chances ranging from low northwest to high southeast. All convection
over this time frame looks to be elevated, but MUCAPE around 1000-
2000 J/kg paired with effective bulk shear of at least 50 kts could
support a severe threat with any storm whose elevated inflow layer
contains no CIN. Buoyancy prognostics limit this threat spatially to
south central and eastern North Dakota.

Showers remain in the forecast for Tuesday as the base of the
shortwave trough swings through, though thunderstorm chances will be
much lower. A southwest flow aloft regime is favored to continue
through the week, with low confidence in the timing of shower and
storm chances. Temperatures are favored to remain below through at
least the middle of next week before warming back closer to normal
by next weekend. The coolest time period is expected to be Tuesday
through Tuesday night, with highs mainly in the 60s and lows mainly
in the 40s.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 631 PM CDT Sat Jun 15 2024

A few isolated storms have developed over the past couple of
hours across southwest North Dakota. The expectation is that
these storms will increase in coverage as they move east
northeast across the state. A few of these storms could become
strong to severe. Hail up to the size of golf balls and winds up
to 70 mph will be possible under the strongest storms. These
storms may impact KDIK and KBIS and then potentially impact KJMS
after midnight. Another wave of storms should enter the
northwest around the 02z to 03z time frame. This second round of
storms could be a bit more significant with hail up to the size
of golf balls and isolated gusts as high as 80 mph. A tornado
or two is also possible. These storms should track from
northwest North Dakota and into the north central part of the
state in the late evening and into the overnight hours
(potentially impacting KXWA and KMOT). MVFR to IFR conditions
are likely under the heaviest thunderstorms. Winds will also
become gusty and erratic under or in the vicinity of any
thunderstorms.

&&

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

&&

$$

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