068
FXUS63 KBIS 240008
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
708 PM CDT Thu May 23 2024

.KEY MESSAGES...

- Scattered severe thunderstorms (risk level 2 out of 5) across
  southern North Dakota this 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 this
  evening through Friday, and some snow could briefly mix with
  rain tonight through Friday morning.

- Well below normal temperatures for most areas tonight 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.

&&

.UPDATE...
Issued at 708 PM CDT Thu May 23 2024

We continue to monitor two general areas for strong to severe
thunderstorms - one in the James River Valley, and one in
southwest North Dakota. There is one main thunderstorm line
extending across much of Logan and McIntosh Counties, with
uncertainty on the upstream severe potential due to cooler
surface temperatures and an overturned airmass. Upstream in the
southwest is one main area of convection in Adams Counties and
south into Perkins County, SD. This is the convection that WoFS
had been advertising throughout the past few hours, with
potential for these storms to remain strong to severe into Grant
and Sioux Counties. Instability drops off quickly to the north,
so we are expecting the severe potential to stay south of the
Bismarck/Mandan area and I-94 corridor, although scattered
thunderstorms are still possible in this area. Our general
expectation is for the severe weather risk area-wide to diminish
by around 9 PM CDT from both airmass overturning and diurnal
destabilization with the onset of sunset.

UPDATE
Issued at 514 PM CDT Thu May 23 2024

We have expanded the Severe Thunderstorm Watch to include
Hettinger, Adams, Grant, and Sioux Counties. Per the Mesoscale
Discussion below and current radar trends, we are expecting
continued development in southwest North Dakota that will move
into the watch area. The most robust development at the moment
is in south central North Dakota in Emmons County, with a
cluster of thunderstorms ongoing in LaMoure and Dickey
Counties.

UPDATE
Issued at 440 PM CDT Thu May 23 2024

** MESOSCALE DISCUSSION **

Severe-storm risk will continue over the Dickey/LaMoure/McIntosh
County vicinity through early evening, with an increasing
strong-severe storm risk westward along and north of the ND/SD
border to the Emmons, Sioux, Grant, and Adams/Hettinger County
areas through the next several hours.

As of 2130 UTC, a mix of supercells and increasingly clustered/
messy convective modes is ongoing near and north of the warm
frontal zone in the southern James River valley. Strong deep-
layer shear, sufficient low-level hodograph curvature to favor
right-moving supercells (over left-movers) north of the warm
front, and modest bouyancy favor a continued severe risk with
this activity. The dominant threat will be large hail, though
peak sizes may be limited by storm interactions as mode becomes
more messy by 2230 UTC. Recent increases in low-level SRH (with
0-1-km SRH over 200 m2/s2 on the KBIS VWP, which is also north
of the warm frontal zone) support low-level mesocyclones and as
a result a tornado or two remain possible in far south central
ND and the southern James River Valley of ND. Overall though the
tendency for upscale-growing modes and modest thermodynamics may
temper the threat ceiling. Extrapolation takes this activity out
of our area and into southeastern ND by 6 pm CDT, but recent WoFS
cycles have presented moderate chances of additional development
as far west as Emmons County in the next 60 minutes, which
would prolong some risk (mainly in the form of large hail) in
this general area.

Meanwhile, further west into southwest and far south central ND
to the west of the Missouri River, an axis of somewhat higher
surface dewpoints in the lower 50s F extends from Mobridge to
Hettinger, north of the surface warm front. Weak to moderate
bouyancy and strong deep-layer shear is present in this area,
and satellite trends presenting increasing ascent approaching
from the southwest and deepening convective towers suggests the
probability of strong-severe storm development is increasing as
far west as Adams/Hettinger Counties the next few hours. WoFS
cycles also support this notion. Large hail would also be a risk
with any supercell(s) that could evolve in this corridor, and
would likely be the main risk given a potentially more elevated
nature to these storms further north of the warm front.

CJS

UPDATE
Issued at 329 PM CDT Thu May 23 2024

Quick update for the issuance of a Severe Thunderstorm Watch for
Emmons, Logan, McIntosh, LaMoure, and Dickey Counties until 11
PM CT. Still expecting potential development in this area within
the next hour or two.

&&

.DISCUSSION...
Issued at 120 PM CDT Thu May 23 2024

Latest surface analysis shows a frontal boundary draped over
southern North Dakota, with surface low pressure over west
central SD. North of the front east to northeast winds remain
over much of western and central ND. Temperatures north of the
front were generally in the mid and upper 40s north, to the mid
50s central. As you get into southern ND, generally along the
I-94 corridor and south, temperatures increase into the 60s and
although moisture isn`t great, as get near the aforementioned
boundary, we see surface dewpoints into the middle 50s. A strong
shortwave trough from southeast Montana into north central WY,
with another wave situated over the High Plains of eastern WY.
Subtle cloud top cooling/glaciation over west central SD and
Moderate cu development over far southwest ND into northwest SD
may be the indication of the forcing that will be approaching
this afternoon, which if you extrapolate would put possible
development over south central ND around 20-22 UTC.

This current area of south central ND southward into north
central SD is currently characterized with ML CAPE of 1000-1500
J/KG advecting north into southern ND, along with effective
shear of 45 to 60 knots currently situated over southern ND.
This puts southwest and south central ND in a favorable position
for strong to severe thunderstorms from mid-late afternoon and
into this evening. the latest HREF Upward helicity Neighborhood
Proximity values are very similar to the 00Z run this morning in
the location centered around Emmons, Logan, McIntosh and Dickey
counties and if anything a little stronger and extended both
west and east along the aforementioned surface boundary over
southern ND. SREF sig Severe potential is also very near this
position and with a bit higher probabilities than earlier runs.
Compared to this morning we don`t think hazards have changed. We
will still message 2 inch hail, 70 mph winds and a tornado or
two possible. The tornado threat looks to be favored over the
southern JRV, and back possibly to the Missouri River. The hail
threat would cover most of southwest and south central ND along
and south of the I-94 corridor. The wind threat also looks to be
favored along and south of the SD border, but as the overnight
shift mentioned, the 0-3KM shear vectors are very strong over
northwest and north central SD, so any convection that would
tend to bow out would have an increased wind potential if it
develops over, or tracks into North Dakota. For all of these
threats, the area extent is pretty limited, with the I-94
corridor a cutoff for the northern extent. As for the timing.
Some of the CAMS were indicating convection developing earlier
over southeast ND and far southwest ND, but in general, where we
anticipate strong to severe convection this afternoon, the
timing seems to be still pointing to around the 20-22 UTC
timeframe, through around 02-03 UTC.

Although the southwest and especially the south central are
favored for severe storms this afternoon/evening, all of western
and central can expect scattered to showers and thunderstorms
through evening and lingering into the overnight hours.

Late tonight and early Friday the focus shifts to the north and west
with the potential for frost. As the system begins to exit, current
forecast trends clear out far western ND after around 09 UTC. The
northwest looks to be the most likely area to see some frost. Will
monitor but the areal extent and duration is quite small. We have
areas of frost over portions of Divide and Williams counties and
gardeners may want to take preventative measures. At this time
tough will hold off on a frost advisory.

Depending on the amount of dynamic cooling with the system, we
could still see a mix of rain/snow over north central ND as the
system lifts into Southern Canada. It`s possible the higher
elevations in the Turtle Mountains could see some light
accumulations, otherwise expect mainly a mix or a brief
changeover to light snow before ending.

There will also be some strong winds on the back side of this
system. It`s possible we may need some wind highlights Friday
morning, or even possibly during the overnight hours in the
southwest. With the severe threat today and quite a bit of
uncertainty in the strength of the winds overnight will hold off
on wind highlights at this time.

Friday will be quite cool, especially north central into
northeast ND where highs will struggle to get out of the 40s.
Most areas will see highs in the 50s.

Frost potential will be a concern late Friday and into Saturday
morning and a frost advisory may be needed for a good portion
of the forecast area. A weak pressure pattern is situated over
the area bringing light winds. This would favor frost if we have
little sky cover. This seems to be the case, but one potential
failure mode would be stratus/fog developing. There does appear
to be a signal for fog in the latest bufkit soundings. Will
continue to monitor.

A warming trend is then expected through the remainder of the
Holiday Weekend and into early next week. There will be some
hit and miss chances for showers and thunderstorms through the
weekend, but no big washouts at this time. The greatest risk for
showers and/or thunderstorms looks to be in the far south
central and into the James River Valley on Saturday
afternoon/evening. Uncertainty increases in the Sunday/Monday
timeframe but there will be a progressive wave or two brining a
chance of threat for showers and thunderstorms. At this time,
the threat for severe storms looks minimal through the Holiday
weekend and through the first half next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 708 PM CDT Thu May 23 2024

Scattered showers with areas of MVFR-IFR ceilings remain across
northwest and north central ND this evening. Thunderstorms over
southern ND (mainly along and south of the I-94 corridor) 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/IFR visibility is
otherwise possible under showers. Ceilings will continue to
lower to MVFR/IFR levels from north this evening. Improvements
in cigs/vsbys are expected to move into the far northwest,
including KXWA, later tonight, with most of the are remaining
MVFR to IFR or lower into Friday morning. Northeast winds will
remain steady at 15-20 kts through the night, with gusts to
25-30 kts. Conditions will improve late in the TAF period.

&&

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

&&

$$

UPDATE...CJS/Jones
DISCUSSION...TWH
AVIATION...TWH/Jones