076
FXUS63 KBIS 251131
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
631 AM CDT Tue Jun 25 2024

.KEY MESSAGES...

- A few isolated strong to severe thunderstorms are possible
  from south central North Dakota to the James River Valley this
  afternoon and evening. Potential hazards include hail up to
  the size of quarters and damaging winds as high as 60 mph.

- High chances for showers with medium chances for thunderstorms
  Thursday afternoon through Thursday night. Isolated to
  scattered severe storms are possible over this time period.

- Below normal temperatures Friday through Sunday, with highs
  mostly in the mid 60s to mid 70s.

&&

.UPDATE...
Issued at 630 AM CDT Tue Jun 25 2024

Since 530 AM CDT, a rogue shower or two developed over southern
Logan County and passed over Fredonia and Kulm into northwest Dickey
County. Reflectivity has approached 50 dbZ at times, but no
lightning has been observed. Additional stray showers cannot be
ruled out across the far south this morning, but most areas will
remain dry.

&&

.DISCUSSION...
Issued at 415 AM CDT Tue Jun 25 2024

Quasi-zonal flow aloft lies over the region early this morning, with
surface ridging building from southern Alberta into eastern Montana
and the western Dakotas. A weak mid level shortwave crossing the
Northern High Plains with downstream theta-e advection over a zonal
thermal gradient has generated widely scattered showers along the
ND/SD border early this morning, with a couple thunderstorms just
across the border on the South Dakota side. With SPC mesoanalysis
still showing the northern gradient of MUCAPE extending into far
southern North Dakota, we will keep the low chance of a thunderstorm
in the forecast through early morning, but it is more likely that
there will be no lightning on this side of the border. Any lingering
showers should exit our forecast area to the south and east no later
than an hour or two after sunrise.

Quiet weather with a mostly sunny sky is expected this morning, with
flow aloft beginning to transition to northwest as a Hudson Bay
trough begins to dig into the Great Lakes. A trailing shortwave
ejecting off the southern Canadian Rockies early this morning is
forecast to reach northern parts of the state this afternoon. The
northwest and north central will be one of the two focal areas for
potential shower and thunderstorm development this afternoon and
evening. The other focal area is from south central North Dakota
into the James River Valley where forcing for ascent is much weaker,
but greater boundary layer instability is forecast underneath a
cyclonic northwest flow pattern. Multiple CAMs have simulated
convective initiation in this part of the state later this
afternoon, and even a few have produced UH tracks. But with only
around 500 J/kg CAPE to work with and model soundings suggesting low
EL heights, think it will be difficult for any updraft to tap into
the 50-60 kts of deep layer shear that is forecast. Effective shear
could still be as high as 30 kts though, which could support a few
stronger to borderline severe storms, should convection initiate.
The SPC has introduced at Marginal Risk (level 1 out of 5) for
severe storms this afternoon and evening from Lake Oahe to the
southern James River Valley. It should be noted that even though
there is no forecast risk of severe weather across the north,
boundary layer thermodynamics are only slightly less favorable there
compared to the south, with similar shear and stronger forcing. That
is to say, a few stronger storms cannot be ruled out with the
arrival of the shortwave in northern North Dakota later this
afternoon into the evening, and this could translate southeast
through the James River Valley into the early overnight hours.

As alluded to above, it will be around 5-10 degrees warmer across
the south than the north today, with forecast highs in the mid 70s
to mid 80s. It will also be breezy across the north, with northwest
winds increasing to around 20 mph this afternoon and gusting as high
as 30 mph. Later tonight into Wednesday, high pressure sliding down
from Alberta/Saskatchewan will become the dominant synoptic feature
for the Northern Plains, with the northwest flow aloft transitioning
from cyclonic to anticyclonic. This will bring a period of much
quieter weather to the region tonight through the day Wednesday,
which may allow overnight temperatures to fall into the upper or
perhaps mid 40s in the favored low-lying areas from Beulah/Hazen to
Hebron/Glen Ullin to Mott and Hettinger. Highs on Wednesday are
forecast to range from around 70 northeast to 80 southwest.

A well-advertised Pacific Northwest trough is forecast to move
ashore Wednesday night, amplifying an upper ridge over the Northern
High Plains. This should allow the nocturnal low level jet to
strengthen, which could result in scattered shower development
across western and south central North Dakota Wednesday night into
Thursday morning. MUCAPE is forecast to be very low to nonexistent
(only a few hundred J/kg at most) over this time period, but
steepening mid level lapse rates could promote a weaker thunderstorm
or two. During the day Thursday, the Pacific Northwest trough will
begin crossing the Northern Rockies, forcing the downstream upper
ridge axis through the Dakotas. This setup should result in low
level southerly flow advecting 60s surface dewpoints into the region
while strengthening CIN under an EML. A lee trough is also forecast
to develop near the MT/ND border Thursday afternoon. Lowering
heights in response to the approaching mid/upper trough should help
erode CIN along the lee trough, which seems like the most likely
area for surface-based (or mixed layer) convective initiation late
Thursday afternoon. There is still ensemble spread in the placement
of the lee trough and tangential instability axis, which can be seen
in ensemble cluster analyses that show two lower-membership clusters
that contain nearly all GEFS members but very few members from other
ensembles that have everything shifted farther east into western
North Dakota. However, all clusters show an environment favorable
for severe convection, with mean CAPE around 1000-2000 J/kg and deep
layer shear around 30-40 kts, and deterministic models suggest
values of both could end up higher. Deep layer shear and mean wind
vectors orthogonal to the lee trough imply a discrete, supercellular
mode could be favored initially, with upscale growth likely
following outflow interactions and the arrival of stronger forcing.
Even if surface-based convection fails to initiate or sustain,
widespread showers and storms are still likely later Thursday
evening through Thursday night with the arrival of strong DCVA
attendant to the mid/upper trough. All hazards could be in play for
discrete storms, including a tornado or two, though any tornado risk
would likely be much smaller in space and time compared to the
overall severe threat, and it could also be limited by both residual
MLCIN and a shortage of streamwise vorticity per forecast
hodographs. A transition to a linear mode would greatly favor a
damaging wind threat over other hazards. The risk for localized
flash flooding appears lower given the synoptic setup, but if any
training convection develops, anomalously high precipitable water
with warm cloud depths exceeding 3000 m could promote a heavier rain
concern.

The passage of the trough on Friday will drag much cooler air into
the region, with below normal temperatures expected through the
weekend. The NBM is focused on Saturday being the coolest day with
highs only in the mid 60s to lower 70s, and Friday could be just as
cool in the northwest. Lower chances for showers and thunderstorms
remain in the forecast on Friday, but the risk for severe weather
should be very low to nonexistent. The synoptic pattern for
Wednesday through Friday could repeat itself almost verbatim on
Saturday through Monday, but confidence in any potential for severe
storms on Sunday is lower given the forecast time range, and
ensembles do not favor below normal temperatures following the
passage of this trough.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 630 AM CDT Tue Jun 25 2024

VFR conditions are likely through the forecast period. However,
there is some potential for MVFR ceilings to move into northern
and eastern North Dakota late tonight. Introduced a scattered
2,500 ft mention to KXWA, KMOT, KJMS for this TAF issuance. A
few showers and thunderstorms could develop from northwest to
central North Dakota later this afternoon and evening, but the
probability of impacts to aviation at any given terminal is low.
Light winds early this morning will become northwest around
12-17 kts this afternoon, strongest across the north where gusts
to around 25 kts are possible.

&&

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

&&

$$

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