610
FXUS63 KBIS 230921
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
421 AM CDT Thu May 23 2024

.KEY MESSAGES...

- Scattered severe thunderstorms (risk level 2 out of 5) across
  southern North Dakota this afternoon and evening. Expected
  hazards are large hail up to 2 inches in diameter and damaging
  winds up to 70 mph, with a tornado or two possible.

- Periods of scattered to widespread showers are expected today
  through Friday, especially tonight through Friday morning when
  some snow could briefly mix with rain.

- Well below normal temperatures for most areas today through
  Friday night, with widespread overnight lows in the 30s.

- Low to medium chances (20 to 40 percent) for showers and
  thunderstorms over the holiday weekend, with temperatures
  warming back closer to normal.

&&

.DISCUSSION...
Issued at 421 AM CDT Thu May 23 2024

A closed upper low at the base of a positively tilted trough sits
over southern Idaho early this morning. Shortwave energy eminating
from the trough combined with low level warm air advection has
generated and maintained scattered showers across northern and
central North Dakota. This activity is expected to move northeast
through the morning, with the northern extent stalling near the
Canadian border. Later this morning and into the afternoon, the
upper low/trough will kick into Wyoming with rapid lee cyclogenesis
forecast near the Black Hills. This should lead to an increase in
shower activity (and eventually thunderstorms), especially over
western North Dakota. The warm front attendant to the surface low is
forecast to advance northward into far south central and southeast
North Dakota this afternoon, bringing afternoon temperatures there
into the 60s and 70s. Highs will otherwise be limited to the 50s
today, and perhaps only upper 40s in parts of the northwest.

The warm frontal boundary and adjacent warm sector will be the focal
point for scattered severe thunderstorms this afternoon and evening.
Latest HREF guidance brings mean SBCAPE around 1000-2000 J/kg from
the Standing Rock Reservation to the southern James River Valley by
mid afternoon. Some capping is noted in model soundings, and only a
slight cooling of the mid levels is forecast. Due to the strong
forcing at all levels of the atmosphere, the capping would not
prevent convective initiation, but it could act to slightly dampen
the threat ceiling. That threat ceiling could reach into significant
severe territory, with effective bulk shear of at least 50 kts and
very long hodographs with low level curvature. Confidence in the
development of strong to severe convection is high, with all CAMs
showing at least a few UH tracks of 100 m^2/s^2 or higher. Forecast
confidence in storm mode is lower though, with shear and anvil level
wind vectors pointing ENE and 850-300 mb mean wind vectors pointing
NNE over a W-E warm front and a N-S dryline/surface trough. Thinking
conceptually and pairing that with CAM reflectivity output, there
would seem to be a slight favoring of an initial discrete mode
followed by a possible evolution into linear/bowing segments. With
higher deep layer RH acting to reduce dry-air entrainment of
updrafts, a shallow depth of the LFC-FZL layer, and ELs extending to
the tropopause, discrete storms are likely to produce up to 2-inch
diameter hail given the CAPE/shear parameter space. Forecast DCAPE
less than 1000 J/kg and the lack of a more focused linear mode point
toward damaging winds being a lesser concern, but the 0-3 km shear
forecast of around 35-50 kts has prompted us to increase our
forecast wind threat to 70 mph. Finally, there is a small window
both spatially and temporally for a tornado risk. The area of
concern is most likely to be confined to along and just south of the
warm front, with the highest HREF probabilities of STP exceeding 1
focused over Logan and McIntosh Counties. 0-500 m SRH increasing to
100 m^2/s^2, 0-1 km shear increasing to 10-20 kts, low LCLs, and a
large streamwise vorticity component in modeled hodograph analysis
are all contributing factors to the tornado risk. Aside from what
has been discussed, there is also a lower severe storm risk north of
the warm front from around the southwest corner of the state to the
northern James River Valley. This would mainly be elevated
convection with large hail as a primary hazard. The expected timing
window for the overall severe threat is from around 3 PM to 11 PM
CDT. The highest severe probabilities (risk level 2 out of 5) are
located along and south of a line from near Mott to Bismarck to near
Carrington.

The surface low is forecast to deepen and lift east-northeast into
the southern Red River Valley through the night. After evening
convection pushes east of the forecast area, the highest
precipitation chances will focus on the wrap-around deformation
band, which is forecast to be located from north central to southwest
North Dakota around midnight, shifting to the Turtle Mountains/
Devils Lake Basin to near Bismarck around 7 AM CDT Friday morning.
Forecast temperatures under the axis of highest precipitation rates
have risen by a degree or two, which has lowered the probability of
snow mixing in. We are still carrying a brief period of a rain/snow
mix during the late night and Friday morning hours, per NBM
precipitation type probabilities. The highest chance of snow mixing
in is now located in the Turtle Mountains.

Temperatures will be unseasonably cold for most areas tonight
through Friday night. Overnight lows will mainly be in the 30s both
of these nights, and frost could be a concern behind the departing
low Friday night. Highs on Friday will be highly dependent on
lingering showers and residual cloud cover. The Turtle Mountains
will likely be the coldest part of our forecast area, where highs
could actually struggle to climb out of the lower 40s. In contrast,
the southwest corner of the state could see highs in the lower 60s,
which is still around 10 degrees below normal. A lower amplitude
shortwave trailing the departing low could bring some showers and
perhaps a few thunderstorms into western North Dakota Friday
afternoon and evening. While forecast CAPE is only a few hundred
J/kg, effective bulk shear around 35-45 kts could support a stronger
storm or two.

Warmer temperatures are expected over the holiday weekend, but it
will remain cooler than normal with highs mainly in the 60s and lows
mostly in the 40s. North Dakota is now forecast to lie in more of a
split flow regime, with the strongest forcing to our north and
south. There could still be enough forcing and instability for some
showers and thunderstorms at times, but aside from Sunday afternoon
and evening when the left exit region of an upper jet is forecast
over the area, NBM PoPs have lowered to around 10 to 20 percent at
most. A more appreciable warm up is finally favored by the middle of
next week, with ensemble mean height fields showing ridging over the
Northern Plains. This would coincide with a couple days break from
shower and thunderstorm chances until later in the week when
ensembles hint at a transition to southwest flow aloft.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 421 AM CDT Thu May 23 2024

There are multiple aviation hazards this forecast period as a strong
low pressure develops southwest of North Dakota. Scattered showers
will be possible at all terminals this morning, but more so at
KXWA, KMOT, and KBIS, and surrounding areas. Showers will then
become more widespread this afternoon and evening, along with
thunderstorms across the south. Some storms could be severe with
large hail and damaging winds as high as 55-60 kts. Significant
visibility restrictions can be expected with any thunderstorm,
and prevailing MVFR visibility is otherwise possible under
showers. Ceilings are forecast to lower to MVFR and eventually
IFR levels from north to south late tonight through Thursday
afternoon. Northeast winds will increase to 15-20 kts on
Thursday, with gusts to 25-30 kts.

&&

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

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan