025
FXUS63 KBIS 231145
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
645 AM CDT Sun Jun 23 2024

.KEY MESSAGES...

- Isolated severe thunderstorms are possible along and north of
  Interstate 94 late this evening through Monday morning.
  Expected hazards include hail up to quarter size and damaging
  winds up to 70 mph.

- Conditional risk for isolated severe storms across central and
  eastern North Dakota Monday afternoon and early evening. The
  chance of a storm developing is less than 20 percent, but any
  storm that does form would be capable of hail up to golf ball
  size and damaging winds up to 70 mph.

- Warmer today and Monday, with highs mostly in the 80s to lower
  90s. Humid central and east on Monday.

- Drier Tuesday and Wednesday, followed by increasing shower and
  storm chances through the rest of the week.

&&

.UPDATE...
Issued at 644 AM CDT Sun Jun 23 2024

Surface observations and webcam and satellite imagery suggest fog
across western and central North Dakota early this morning is very
patchy and shallow, favoring low-lying areas near rivers, creeks,
and streams. Glen Ullin continues to report visibility less than one
quarter mile, and an NDAWN webcam on the east side of Lake Oahe near
the mouth of the Cannonball River shows dense fog. Otherwise, there
is no evidence of more widespread denser fog. The fog should quickly
lift by around 8 to 9 AM CDT.

&&

.DISCUSSION...
Issued at 444 AM CDT Sun Jun 23 2024

The Northern Plains sit under northwest flow aloft early this
morning with an embedded trough/low spinning over southern Manitoba
and an upstream ridge extending north from Yellowstone National Park
through western Saskatchewan. The surface wind and pressure fields
are unremarkable, with general broad, weak ridging over the region.
This has allowed patchy fog to develop throughout western and
central North Dakota. The severity of the fog appears to be very hit
and miss, with only Glen Ullin consistently reporting visibility
less than one quarter mile.

The upstream ridge is forecast to shift eastward and develop
positive tilt over the course of the day, allowing for an increasing
southeast return flow over eastern Montana and the western Dakotas
along the eastern periphery of a strengthening thermal ridge. The
deep layer subsidence and capping inversion above the boundary layer
will keep dry weather and a mostly sunny sky in the forecast through
the afternoon, with highs reaching the mid and upper 70s northeast
to near 90 southwest.

With the ridge axis shifting off to the east this evening, flow
aloft will begin turn southwesterly with a strengthening low-level
jet delivering strong low level moisture transport through western
North Dakota. A developing surface low in response to lowering
heights over southwest Saskatchewan with an attendant frontal
boundary extending south along the ND/MT border is likely to provide
enough forcing to initiate convection either over northwest North
Dakota late this evening or over northeast Montana earlier in the
evening before moving into the northwest. All CAMs simulate at least
a few storms with varying degrees of intensity and placement,
tracking east/southeast into central North Dakota later tonight into
Monday morning, possibly reaching as far south as the I-94 corridor
in south central North Dakota later tonight and perhaps even as far
south as the SD border in the southern James River Valley early
Monday morning.

The strength of convection moving from west to east across the state
late this evening through Monday morning will be highly dependent on
the degree of convective inhibition and strength of effective bulk
shear within the storm inflow layer rooted above the stable boundary
layer. Elevated buoyancy should not be a limiting factor for storm
intensity, with HREF mean MUCAPE around 1500-2000 J/kg. HREF mean
soundings in northwest North Dakota do generally project around 30-
40 kts of effective bulk shear. With most CAMs showing at least one
appreciable UH track (greater than 75 m^2/s^2) and HREF neighborhood
probabilities for the parenthesized threshold reaching 30 percent,
we will continue to message isolated severe storms along and north
of Interstate 94 late this evening through Monday morning. While the
CAPE/shear parameter space is generally unsupportive of hail or
winds beyond baseline severe thresholds, the potential for high
cloud bases with DCAPE around 1000/1500 J/kg and onion bulb-shaped
thermodynamic profiles in the lower levels on model soundings pushes
our wind gust forecast to 70 mph. These storms are not expected to
be as prolific of rain producers as we have seen more recently, but
a couple CAMs do show potential for training convection that could
lead to localized rain amounts as high as 2 inches.

On Monday, the surface low moving from southern Saskatchewan into
central Manitoba should deepen, with troughing extended south
through the central Dakotas. The air mass along and downstream of
the trough is projected to be very moist, with surface dewpoints
rising well into the 70s across the eastern half of the state
through Monday afternoon. A trailing dryline feature is forecast to
work its way through western and central North Dakota Monday morning
and afternoon, though it is interesting that the strong
isodrosothermal gradient notably lags behind the surface wind shift.
Monday looks to be the warmest day of the week for most areas,
with highs in the lower 80s northwest to lower 90s south.

There is currently little to no model support for any convection
developing in our forecast area Monday afternoon due to the strength
of a low to mid level capping inversion and lack of any
distinguishable mid to upper level forcing. However, the presence of
low level convergence within a volatile air mass with extreme
CAPE as high as 4000-5000 J/kg along with the trailing density
gradient which could theoretically generate horizontal
convective rolls pushing against the top of the boundary layer
gives us pause in ignoring the very low probability but high
impact outcome of the LFC being breached. We will continue to
message a highly conditional, very low probability risk for
isolated severe storms capable of golf ball size hail and
damaging winds up to 70 mph. If MLCIN turns out weaker than what
is forecast, the threat ceiling for hail could rise. But again,
it should be emphasized that the most likely outcome (by far)
is that there will be no severe storms in western and central
North Dakota, including the James River Valley, Monday afternoon
and evening.

A weak mid level impulse traversing a latitudinal low level
baroclinic zone along the ND/SD border could generate some showers
and sub-severe thunderstorms across southern North Dakota Monday
night. Otherwise, the Tuesday through Wednesday timeframe looks to
be drier. Northwest cyclonic flow could generate some afternoon
showers across the north on Tuesday, but there does not appear to be
enough instability for any thunderstorms. Surface high pressure is
then forecast to build down from Canada Tuesday night into Wednesday
as the northwest flow aloft turns anticyclonic. Tuesday and
Wednesday will be slightly cooler, especially to the north and east
where 70s are more likely for highs. Southwest and parts of south
central North Dakota should remain in the 80s.

There is 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 indicate the potential for widespread showers and storms
later Thursday through Friday. Details on any severe potential
remain far from certain at this time range, but the mean of all
global ensembles does bring 1000-1500 J/kg CAPE and 35 kts of
surface to 500 mb shear into the state Thursday evening, and CSU
machine learning severe guidance also paints low probabilities over
the region. 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 slightly below normal temperatures of
highs in the 70s are now favored on Friday and Saturday following
the passage of the trough.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 644 AM CDT Sun Jun 23 2024

Patchy fog will continue across western and central North Dakota
early this morning. Visibility will be reduced to MVFR levels at
times at KBIS, with other major terminals unlikely to see impacts.
VFR conditions are otherwise expected through the day. Chances for
showers and thunderstorms will increase from northwest to southeast
later this evening through tonight. A few stronger storms are
possible with erratic gusty winds and heavy rain. Winds will become
southeasterly around 10-15 kts from west to east this afternoon
through the evening. A period of low level wind shear is likely at
KMOT late tonight.

&&

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

&&

$$

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