159
FXUS63 KBIS 240822
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
322 AM CDT Mon Jun 24 2024

.KEY MESSAGES...

- A severe thunderstorm or two will continue to impact north
  central parts of the state early this morning, with hail up to
  golf ball size and damaging winds up to 70 mph possible.

- Isolated severe storms are possible across northwest and
  central North Dakota this morning, with the threat ending from
  west to east through the afternoon. Expected hazards include
  hail up to golf ball size and damaging winds up to 70 mph,
  with the higher hail risk in the afternoon in the James River
  Valley.

- Warm and humid across the eastern half of the state today,
  then slightly cooler and mostly dry Tuesday and Wednesday.

- Medium to high chances for showers and thunderstorms Thursday
  afternoon through Thursday night.

&&

.DISCUSSION...
Issued at 321 AM CDT Mon Jun 24 2024

Shortwave energy riding over the top of a mid to upper level ridge
axis has sparked areas of showers and thunderstorms from eastern
Montana into western and central North Dakota early this morning,
with forcing being aided by a low level jet with strong moisture
transport. The strongest convection has been focused along the
Canadian border in Burke, Renville, and Bottineau Counties. Two long-
lived supercells have now weakened and transitioned to more of a
compact linear mode, but another supercell upstream has entered
northeast Burke County as of 3 AM CDT. Stronger storms were also
previously located along the Missouri River in northeast Montana,
with hail up to ping pong ball size at Wolf Point earlier in the
night. That convection also weakened as it moved into northwest
North Dakota, but small hail and gusty winds remain possible.
Finally, scattered showers with occasional lightning have blossomed
within a low level warm air advection zone across southwest North
Dakota in a much weaker effective shear environment. There is medium
to high forecast confidence that showers and storms will persist
moving east across the state this morning into the afternoon, but
low confidence on intensity. Through this morning, the strongest
shear remains farther north.

Convective evolution later this afternoon is far more uncertain,
with CAMs already having struggled with overnight activity. The air
mass downstream of an eastward advancing cold front/dryline will
become very unstable this afternoon, with surface dewpoints
increasing to the 70s and CAPE approaching 4000-5000 J/kg in a
moderately sheared environment. Strong capping above the boundary
layer and the unknown convective evolution this morning contribute
to the uncertainty of convection this afternoon, and the best
forcing through all atmospheric layers is not lined up spatially. On
one end of the forecast spectrum, a few severe storms could develop
this afternoon east of Highway 83, which could be new convective
initiation or re-intensification of lingering convection from
earlier in the day. This outcome has been offered in consecutive
runs of the NAMnest, for example. On the other end of the forecast
spectrum, morning convection could exit the forecast area by the
afternoon, with no additional development for the rest of the day.

The aforementioned cold front/dryline is forecast to cross the state
from west to east through the day. The eastern half of the state
will have a warm and muggy afternoon, with highs in the mid 80s to
lower 90s and afternoon heat indices pushing the mid 90s in the
southern James River Valley. It will be only slightly cooler but far
less humid to the west. Later tonight, a mid level shortwave riding
over a low level baroclinic zone could bring some scattered showers
and thunderstorms along the South Dakota border. Any storms should
remain well below severe limits due to negligible bouyancy.

A transition to cyclonic northwest flow is expected on Tuesday, with
surface ridging building from the southern Canadian Rockies into the
northern High Plains. A few afternoon and evening showers are
possible along the Canadian border, with breezy conditions. On
Wednesday, northwest flow aloft transitions to anticyclonic with the
broad surface high becoming more focused over the region. This
should result in a very quiet weather day. Highs on Tuesday and
Wednesday will generally range from the 70s north to 80s south.

There continues to be strong model consensus for an upper level low
landing on the Pacific Northwest Wednesday night, crossing the
Northern Rockies on Thursday and reaching the Northern Plains on
Friday. NBM PoPs begin to increase Wednesday night as a preceding
lee trough intercepts the nocturnal low level jet. Then
deterministic global models continue to indicate the potential for
widespread showers and storms later Thursday through Friday, with
NBM PoPs as high as 90 percent. CSU machine learning severe
probabilities have increased over the past 24 hours, and a quick
look at the CAPE/shear parameter space in deterministic guidance
suggests all severe hazards could be in play. Heavier rainfall could
also be a concern with this system, with NAEFS/ECMWF showing 90th-
95th climatological percentile values of precipitable water. Warm
and/or humid conditions are possible on Thursday, but below normal
temperatures are favored on Friday and Saturday following the
passage of the trough, with the NBM now advertising highs as cool as
the mid and upper 60s.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 102 AM CDT Mon Jun 24 2024

Showers and thunderstorms are possible across western and central
North Dakota overnight into Monday morning before shifting off to
the east. A few storms could be strong to severe, with the highest
chance of a severe storm north of KMOT. VFR conditions are otherwise
expected through the forecast period. Southeast winds overnight will
turn to the northwest on Monday, increasing to 10-15 kts with a few
20-25 kt gusts during the afternoon.

&&

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

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan