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774
FXUS63 KBIS 221406
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
Issued by National Weather Service Hastings NE
906 AM CDT Wed May 22 2024

.KEY MESSAGES...

- Areas of frost across the western half of the state early this
  morning.

- Medium to high chances for rain (50-60 percent north to 80-90
  percent south) Thursday through Friday. Some snow could mix
  with rain in parts of western North Dakota late Thursday night
  into Friday morning.

- Isolated severe thunderstorms are possible across south
  central and southeast North Dakota Thursday afternoon and
  evening.

- Unseasonably cold for most areas Thursday through Friday
  night.

- Slightly warmer over the holiday weekend, with medium chances
  for rain Sunday through Memorial Day.

&&

.UPDATE...
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Issued at 904 AM CDT Wed May 22 2024

Temperatures have risen into the 40s this morning, reducing the
risk for frost development. Therefore, the Frost Advisory has
been allowed to expire.

UPDATE
Issued at 641 AM CDT Wed May 22 2024

The early morning forecast remains on track. Latest CAM and NBM
guidance has spatially increased probabilities for thunderstorms
across much of western and central North Dakota this afternoon and
evening, and this is supported by buoyancy/instability analysis on
model soundings. The forecast has been updated to reflect this.
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&&

.DISCUSSION...
Issued at 457 AM CDT Wed May 22 2024

A surface ridge is analyzed over western North Dakota early this
morning, under northwest flow aloft. The cyclonic flow has
maintained some scattered mid level clouds moving into western North
Dakota with an occasional light shower. But any low lying areas not
directly under clouds have consistently observed temperatures in the
30s. The Frost Advisory remains in great shape. Frost should quickly
melt an hour or two after sunrise.

The surface ridge and upstream mid to upper level ridge are forecast
to glide eastward across the region today. Highs this afternoon
should once again mainly be in the 60s. Later in the afternoon and
evening, increasing low to mid level frontogenesis coupled with mid
level isentropic ascent and low level warm air advection could
generate scattered showers and perhaps a weak thunderstorm or two
over parts of western and central North Dakota. The highest
probabilities for rain through tonight have shifted northward with
this forecast package, but most of western and central North Dakota
carries at least a 20 percent chance of a shower over this time
period. The northward shift of both clouds and rain have increased
forecast lows tonight into the upper 30s across the far north.
Therefore, frost no longer appears to be a concern for tonight.

A potent shortwave presently digging into the Pacific Northwest will
pass over the Northern Rockies tonight and begin ejecting into the
Northern Plains on Thursday, with lee cyclogenesis forecast near the
Black Hills. There still remains some uncertainty on the evolution
of this system, both between the GEFS and other global ensembles and
now also between CAMs and all global ensembles. First, the mean
position of the surface low in the 00Z HREF is close to Mobridge,
SD, while in global ensembles it is farther south, closer to Pierre.
The differences between the GEFS and other global ensembles are more
timing and amplitude based, with the GEFS holding on to its quicker
progression. There are two ensemble clusters in the 00Z iteration
that show the quicker solution, with a combined membership of only
24 percent. Furthermore, those two clusters contain 43 out of 50
GEFS members and only two total members from the other two
ensembles. The NAM and CAMs are also aligned more with the two
majority clusters` timing progression. So while the GEFS cannot be
completely discarded, it was not strongly considered in the forecast
process. That leaves us with a southern vs. northern outcome, which
is highly important for the prospects of severe weather Thursday
afternoon and evening.

The warm sector is progged to be characterized by MLCAPE approaching
1000-2000 J/kg, effective bulk shear around 50 kts, very long
hodographs with some low to mid level curvature, dewpoints in the
50s, and a breaching of convective temperatures under falling
heights and DCVA/WAA. Therefore, it can be surmised that convection
will initiate either along the warm front, in the warm sector, or
both Thursday afternoon. The warm front could be as far north as the
I-94 corridor at initiation time, but it could also be as far south
as northern South Dakota. All hazards could be in play in what
appears to be a mixed storm mode scenario. The tornado risk would be
more spatially and temporally confined than hail and wind, but RAP
STP values have been as high as 3 near the triple point, and HRRR
hodographs show strong streamwise vorticity. There could also be an
elevated severe hail threat that extends north of the warm front
from left-moving supercells, which are evident as negative UH tracks
in some CAMs. If the severe threat expands northward into North
Dakota, it will most likely be confined to areas along and south of
the I-94 corridor and along and east of the Missouri River. But
again, there is a very plausible outcome of the environment
supportive of severe storms remaining in South Dakota.

The Storm Prediction Center has placed the aforementioned parts of
the state under a Marginal Risk (level 1 out of 5) for severe storms
on Thursday. Given the uncertainty on storm mode but high level of
shear, we are messaging hail up to ping pong ball size and 60 mph
winds. Because of the spatial and temporal limitations and overall
uncertainty on the northward extent of the warm sector, we are not
messaging a tornado threat at this time.

Widespread rain in the deformation wrap-around region of the low
will fall across the Northern Plains Thursday evening into Friday.
Storm total rain amounts could be highly variable given the expected
convection on Thursday, but the NBM is advertising a 30 to 70
percent chance of exceeding half an inch, highest east and lowest
northwest. There is growing concern that dynamic cooling could allow
snow to mix with rain Thursday night and Friday morning. This
potential is now well supported by both global ensembles and CAMs.
In fact, it is not inconceivable that there could be accumulating
snow on grassy surfaces. Details of this aspect of the forecast will
need to be fine-tuned with future updates.

Aside from the warm sector on Thursday, where highs could approach
or exceed 70, well below normal temperatures are expected from
Thursday through Friday night. Both nights will have lows in the
30s, and with the low pressure system having departed by Friday
night, at least areas of frost are anticipated. During the day
Friday, temperatures will have a hard time making it out of the 40s
wherever rain and clouds are more prevalent. A final note about this
system is that it will be windy, especially in the southwest. But it
is not expected to be a high-end wind maker for North Dakota, and it
appears at this time that any advisory for winds would likely be
limited to the southwest.

Temperatures will warm up for the holiday weekend, but highs are
favored to remain below normal, mainly in the 60s. There is still
potential for a shortwave passage Sunday into Memorial Day with
medium chances for rain, but this feature is now less pronounced and
has shifted south in global ensemble mean height fields.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 641 AM CDT Wed May 22 2024

VFR conditions are likely to prevail through the forecast period.
Chances for scattered showers and possibly a few thunderstorms will
increase across the western half of the state later this afternoon
and focus more across the north later tonight. The highest
probabilities for both showers and a thunderstorm are at KXWA, and
the lowest are at KJMS. If a shower or storm moves over a terminal,
a brief period of MVFR/IFR visibility restrictions is possible.
Later tonight, MVFR/IFR ceilings are forecast to move down from
Canada. These may reach KXWA and KMOT by 12Z, but think shortly
thereafter is more likely. Light and variable winds are expected
through this evening, except at KJMS where northwest winds will
sustain around 10 kts, with some 20 kt gusts possible this
afternoon. Winds will then turn to the east-northeast late tonight
and begin increasing to 10-15 kts.

&&

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

$$

UPDATE...Wekesser/Hollan
DISCUSSION...Hollan
AVIATION...Hollan