060
FXUS63 KBIS 192040
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
340 PM CDT Sun May 19 2024

.KEY MESSAGES...

- A few strong to severe storms will be possible across southern
  portions of western and central North Dakota late this
  afternoon and into the evening.

- The strongest storms are likely to be across the far south
  central, mainly over Sioux and Emmons counties. Expected
  hazards include large hail to ping pong ball size and damaging
  wind gusts to 70 mph, with a tornado or two possible.

- Expect below normal temperatures with daily chances for rain
  this coming week through the upcoming holiday weekend.

&&

.DISCUSSION...
Issued at 340 PM CDT Sun May 19 2024

Western and central North Dakota currently sits under southwest
flow aloft as a broad western US trough approaches. A surface
low is currently located over eastern Wyoming and should move
into western South Dakota by late afternoon and early evening.
Precipitation chances will increase during this time frame from
south to north along an inverted surface trough to the north of
the low and an east to west oriented warm front near or just
south of the South Dakota border. The highest rain chances will
be across the south central and into portions of the James River
Valley (60 to 90 percent) with low to medium chances further
north and west (30 to 60 percent).

As far as the severe weather potential goes, it does seem
reasonable that we could see a fairly brief window for a strong
to severe storm or two across southern portions of western and
central North Dakota. However, the best chance for a severe
storm or two is likely to be over the far south central, mainly
over Sioux and Emmons counties. Confidence has decreased a bit
regarding the severe weather potential as clouds have been
stubborn to clear out in the most favored area for initiation
per the latest CAMs. Thus far, the clouds have kept any
appreciable instability south of the area with MUCAPE values
only approaching 500 J/kg or so south of the I-94 corridor.
MLCAPE values only max out in the 100 to 250 J/kg range across
the south. That being said, RAP forecast soundings and SPC
mesoanalysis trends suggest that we could see a small area of
MLCAPE across portions of the far southwest and south central by
22z to 01z. Given the abundant cloud cover, this could be
overdone a bit. One aspect that will not be lacking is shear
with widespread effective shear forecast to be in the 50 to 65
knot range. This enhanced shear can also be gleaned from the
widespread billow clouds on visible satellite imagery across
south central North Dakota and north central South Dakota. Thus,
if the warm front can lift north far enough and clear the far
south central out it may not take a ton of instability for a
strong to severe storm to pop. A quick glance at the latest CAMs
suggests rapid convective initiation sometime in the 23z to 00z
hour. The strongest updraft helicity tracks are generally
confined to around the North Dakota/South Dakota border.
Instability should wane rather quickly after 01z and
thunderstorms will likely be in the weakening phase as they move
into the James River Valley. A few storms could remain strong
until around 03z before they move off into the Grand Forks
forecast area. In a worst case scenario, storms across the far
south central could see a brief window to produce damaging winds
to 70 mph, hail to the size of ping pong balls, and a
conditional risk for a brief tornado or two. Further north and
west, isolated to scattered showers with a few embedded rumbles
of thunder will be possible.

Some showers may linger across the north late tonight into
Monday morning as the shortwave moves off to the northeast. We
will stay in southwest flow aloft through at least Tuesday as
waves continue to rotate around the slow moving broad trough.
The flow then flattens out and becomes nearly zonal midweek
before the next potent trough approaches. Ensembles and global
models continue to struggle with the evolution of the late week
pattern regarding timing and track of this system. Either way,
rain chances appear medium to high (40 to 70 percent) for most
of western and central North Dakota Thursday into Friday. What
is up in the air still is where the heaviest precipitation will
fall and thunderstorm potential. Thus far, NBM is painting a
picture of low to medium chances for thunderstorms (20 to 40
percent) during this same time frame. CIPS analogs still suggest
some chances for severe weather towards the end of the week and
into the holiday weekend but CSU Machine Learning guidance does
not.

Lows tonight may become fairly chilly across the southwest
dipping into the mid to upper 30s. For now, frost is not
expected but it will be something to watch. Lows will be in the
40s elsewhere. Highs will be in the 60s Monday through Wednesday
and then we could see some 50s across the north on Thursday.
Friday could be the coolest day of the week with NBM forecast
highs in the lower 50s to lower 60s but the spread is rather
large, likely due to model/ensemble uncertainty regarding the
pattern. A warmup will then be possible over the holiday
weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1227 PM CDT Sun May 19 2024

VFR conditions will prevail through the first part of the period
at all sites. However, chances for showers and storms will
increase late this afternoon and into the evening hours,
especially across the south central and into the James River
Valley. This activity also may usher in some MVFR or low VFR
ceilings to areas across the central and east, including KBIS,
KJMS, and KMOT. Some guidance is a bit more bullish on potential
IFR ceilings over the south central and James River Valley by
late tonight or early Monday morning, but this is a low
confidence solution at the moment. Thus, we will leave it out
of any site specific forecast for now. A few strong to severe
storms will be possible across southern North Dakota. The
strongest storms are likely to be mainly along the South Dakota
border but KBIS could see a storm in the vicinity and KJMS has a
better chance of a storm actually moving overhead. Further
north, a few scattered showers will be possible. Winds may
become gusty and erratic in and around thunderstorms.

&&

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

&&

$$

DISCUSSION...ZH
AVIATION...ZH