205
FXUS63 KBIS 300144
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
844 PM CDT Wed May 29 2024

.KEY MESSAGES...

- Severe thunderstorms remain possible across western North
  Dakota through this evening, with the threat diminishing
  during the overnight hours. Hail up to the size of ping pong
  balls and 70 mph winds are the primary threats.

- There is an 80 to 90 percent chance of a wetting rain (a tenth
  of an inch or more) for much of western through central ND,
  including the James River Valley, through Thursday morning.

- Temperatures will cool slightly on Thursday, then go on a
  warming trend to near or slightly above average through the
  weekend.

- Shower and thunderstorm chances may linger across the north
  and east for Thursday. They may then be isolated to scattered
  on Friday. Periodic shower and thunderstorm chances will then
  return Sunday through the first half of next week.

&&

.UPDATE...
Issued at 843 PM CDT Wed May 29 2024

The Wind Advisory was allowed to expire on time at 8 PM CDT.

The threat for severe storms appears to be winding down. At 830 PM
CDT, the strongest storms were located over central Williams County.
Radar analysis suggests these storms may have briefly approached
severe limits at times, but have been very pulse-like in
nature. This fits with recent SPC mesoanalysis that shows
sufficient shear for sustained updrafts located west of these
storms. Further south, convection is struggling to sustain aside
from a stronger storm entering southeast Adams County. Despite
the lack of shear, there is still enough DCAPE to maintain the
Severe Thunderstorm Watch. But it is looking much more likely
that the severe threat has concluded for the evening.

UPDATE
Issued at 540 PM CDT Wed May 29 2024

** MESOSCALE DISCUSSION **

Strong-severe storm risk is increasing in far western ND and is
expected to peak in the 23 to 02 UTC (6 pm to 9 pm CDT) window.

As of 22 UTC, low-level convergence is increasing along the
ND/MT border region, with veering winds upstream in MT resulting
in a more well-defined surface wind shift. Satellite images show
deepening cumulus along/ahead of the wind shift north all the
way to the Canadian border, and objective analysis data reveals
that MLCIN has diminished as far north as McKenzie County. The
residual capping along the wind shift in Williams and Divide
Counties may also diminish sufficiently for sustained convective
initiation in those areas in the next 1-2 hours. MLCAPE on the
order of 1500-2000 J/kg will promote intense updrafts, but their
individual longevity will be limited by weak effective-layer
shear on the order of 20-30 kt. We therefore continue to expect
multicellular to transient supercell structures in this event,
with peak hailstone sizes from quarter to ping pong ball size.
Large temperature-dewpoint spreads and the high-based nature of
storms presents an outflow wind risk, as well, especially in
southwestern ND where DCAPE exceeds 1000 J/kg. However, 0-3-km
bulk shear has diminished to less than 20 kt, which will likely
limit a more organized damaging wind risk, instead favoring a
more transient wind threat of 60-70 mph with any clustered or
collapsing storms. Mean winds and right-moving supercell motion
vectors (which would apply to any transient supercells) favor a
general east-northeast motion around 20 kt this evening. Based
on these speeds and extrapolation along with recent Warn-on-
Forecast (WoFS) guidance, the convection would reach a line from
roughly Watford City to Dickinson and Hettinger in the 00 to 01
UTC time frame. The southwestern ND area has the relatively
highest, albeit still modest deep-layer shear, largest bouyancy,
and weakest capping, so the severe-storm risk will be greatest
in that area. Further north, in Williams and Divide Counties and
vicinity, the environment is less favorable for organized severe
storms due to deep-layer shear being weakest there. Moreover,
further east in central ND, residual capping is also expected to
marginalize the severe storm risk later this evening as
convection reaches that area in a few hours.

CJS

&&

.DISCUSSION...
Issued at 320 PM CDT Wed May 29 2024

A Severe Thunderstorm Watch is now in effect across western
North Dakota until 10 PM CDT. A slight risk of severe weather
remains through tonight, more so later this afternoon through
this weekend. The overall threats has not change much since this
morning. Thermal ridge will bring ample amounts of mildly
capped CAPE. The stronger shear however still looks to remain
behind the cold front and not necessarily in the warm sector.
This effective shear of around 30 knots may not be enough to
sustain stronger super cells. This combined with higher LCLs and
hodographs still indicating the better helicity is in the 0 to
3 KM level than the 0 to 1 km level has lead to lower confidence
in an isolated tornado threat. Thus will stick to the large
hail and strong winds. Hail up to ping pong ball size still
seems the more likely size, although an isolated golf ball size
is possible. High DCAPE and adequate 0 to 3 KM shear also
indicate a decent wind threat today with gusts up to 70 MPH.
Timing wise, look for storms to start developing in the late
afternoon especially across the southwest. Then look for upscale
growth through the evening that pushes eastward. The shear
vector to the disturbance today will promote multicell and
clusters. Perhaps a discrete cell is possible initially,
although the approaching cold front will make this threat short
lived. By early to mid evening storms become more clusters to
lines of storms and slowly progress eastward into the overnight
hours. The cold front progressing eastward may start to tamper
with the instability perhaps lowering the hail threat, although
the wind threat will remain into central portions. So overall
between now and around midnight local time in western and
central portions be sure to monitor for the latest severe
weather watches and warnings. Overnight this looks to transition
into a broad line of showers and thunderstorms. Some stronger
storms may be embedded at times through tonight. Otherwise the
strong southeast winds will become a breezy westerly wind behind
the front tonight, with breezy southerly winds lingering in the
east ahead of the front. There remains a Wind Advisory into
early evening for these strong southeasterly winds. Lows tonight
will be mild and generally in the upper 40s to upper 50s.

Cold front will continue its eastward push through Thursday with
a broad trough lingering in the north. Shower and thunderstorms
will be found along this front in the east, with some isolated
to scattered coverage possible in the north. Temperatures will
slightly cool behind this front, with breezy westerly winds also
found. A broad trough pattern then looks to linger on Friday.
This could bring about near normal temperatures and some
isolated to scattered showers and thunderstorms. The threat for
severe weather looks low at this point. A dry and warming
pattern then looks to start the weekend on Saturday. Sunday
continues to be warmer. Another passing wave could bring a few
showers and thunderstorms. An active yet mild pattern could
then be found to start next week. The chances for severe weather
at this point are low, although the CSU-MLP does have some low
probabilities, more so on Sunday. Something to monitor through
the weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 634 PM CDT Wed May 29 2024

Strong to severe thunderstorms are possible across western North
Dakota this evening. The strongest storms will be capable of large
hail and damaging winds up to 60 kts. If a storm moves over a
terminal, a period of IFR to LIFR visibility with erratic winds can
be expected. The showers and storms are forecast to spread into
central North Dakota later tonight, and eastern North Dakota early
Thursday morning, but the potential for severe storms is far lower
there. A period of MVFR ceilings are possible at KJMS Thursday
morning. Outside of storms, strong southeast winds this evening will
turn westerly later tonight into Thursday. Low level wind shear will
develop from KBIS to KJMS later tonight.

&&

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

&&

$$

UPDATE...Hollan/CJS
DISCUSSION...Anglin
AVIATION...Hollan