Severe Thunderstorm Warning 8, NWS Bismarck, ND

AREA FORECAST DISCUSSION
National Weather Service Bismarck ND
345 PM CDT Tue May 9 2023

.SHORT TERM...(This Evening through Wednesday Night)
Issued at 345 PM CDT Tue May 9 2023

The forecast through Wednesday night will be characterized by warm
temperatures and isolated to scattered showers and thunderstorms.

As of mid afternoon, scattered convection is developing in several
areas as convective temperatures near 70 F have been reached. Weak
to moderate bouyancy has developed owing to steep midlevel lapse
rates and increasing low-level moisture, with MLCAPE on the order
of 500-1000 J/kg. Recent VWP hodographs from KBIS reveal effective
layer bulk wind differences on the order of 45 kt in southern ND,
decreasing to 30 kt in the north per KMBX VWP data. The scattered
storms in south central ND developed on the leading edge of the
greatest low-level moisture return characterized by dewpoints in
the lower to mid 50s F, and are also in the setting with the most
favorable wind profiles for marginally-severe storms through the
early evening. The main risk will likely be marginally-severe hail,
with some risk of a transient supercell in south central ND given
the magnitude of deep-layer shear and broad cyclonic curvature in
VWP-based hodographs, but the overall risk will be limited by the
lack of greater bouyancy. Elsewhere, convection is also developing
near a surface wind shift in western ND and far eastern MT, which
may continue into the evening as well. MLCAPE is weaker in western
ND, but there is also a risk of a strong storm with that activity
too, perhaps moreso as if shifts eastward into somewhat greater
MLCAPE this evening.

Overnight, we have maintained a broad area with a 20-30 percent
chance of showers and thunderstorms given weak signals in CAMs for
continued activity and forecast soundings which suggest sufficient
elevated bouyancy for a continued risk of convection along a mid-
level warm frontal zone. However, our confidence in the timing
and placement of any activity is low with only a broad and weak
low-level jet on the order of 25 kt forecast. We did add patchy
fog to the forecast late tonight and Wednesday morning in parts of
the west and south central, too, with weak flow and increasing
low-level moisture.

On Wednesday, shortwave ridging aloft and broad warm air advection
will allows high to reach well into the 70s F. A weak surface wind
shift will be in place from southwestern into central ND, and
that may serve as an impetus for convective initiation by late day
amid moderate bouyancy. However, residual capping shown on model-
forecast soundings and weak synoptic-scale forcing raises
uncertainty in whether storms will develop prior to Wednesday
night. Deep-layer shear may be somewhat weak, but in some models
is sufficient for a marginal severe storm risk.

.LONG TERM...(Wednesday Night through Tuesday)
Issued at 345 PM CDT Tue May 9 2023

Highlights in the long term forecast include high confidence for
a widespread rain event Thursday through Friday, and a drier and
warm pattern late this weekend into early next week.

A strong, negatively-tilted upper-level trough is forecast to move
northeastward into Colorado on Thursday, resulting in the formation
of a middle- and upper-level low that will slowly move into the
Northern Plains Friday and Saturday. Low- and midlevel warm air
advection beneath increasing forcing for ascent in the cyclonic
flow aloft will set the stage for increasing precipitation from
south to north Thursday, and we are calling for widespread 70-90%
chances of rainfall Thursday night and Friday as the middle- and
upper-level low approaches the region. Global ensembles favor the
highest odds of significant rainfall being centered in southwestern
and south central ND, with the probability of 1 inch or more of
event-total rainfall ranging from 80% along the ND/SD border, to
70% along the I-94 corridor from Beach to Bismarck/Mandan and
Jamestown. Those probabilities of at least an inch of rainfall
decrease to the north, but remain notable in the 30% range along
the Canadian border.

Two forecast considerations that offer some measure of uncertainty
are 1) The location of the northern gradient in heaviest rainfall,
and 2) The potential for thunderstorms in the south, especially on
Thursday. Where the northern gradient in rainfall is concerned, we
do note that global ensemble clusters offer two camps, one which has
the axis of 1" or greater of QPF confined closer to the ND/SD
border, and one which allows that amount of QPF north all the way to
the Highway 2 corridor. This represents the spread in scenarios
with regard to how far north the upper-level low comes by Friday,
and is also reflective of potential for one or more dry slots to
move north through the area as the low approaches. Nonetheless,
the theme that we want to convey is high probabilities of rainfall
in this period, but with odds favoring the greatest totals in
southwestern and south central ND. Indeed, the probability for 2
inches or more of rainfall is in the 40-50% range in southwestern
ND. Where thunderstorm risk on Thursday is concerned, the degree
to which the boundary layer is able to destabilize is uncertain
with cloud cover and precipitation, though a low risk of strong to
severe storms could develop in areas where appreciable surface-
based CAPE is able to take shape. Machine learning guidance based
on the GEFS suggests this is most likely in SD compared to ND,
though. Regardless, there will be steep-enough midlevel lapse
rates for a chance of embedded thunderstorms in many areas
Thursday through Friday.

The middle- and upper-level low is forecast to gradually weaken this
weekend, but depending on how quickly it does and where its located,
precipitation could linger through at least Saturday. The consensus
of global ensembles is for ridging aloft to build into the area by
Sunday and Monday, though, offering a warming trend, with spread in
NBM members small and supportive of highs in the 70s F Sunday and
especially Monday. There's then some possibility of a weak cool
frontal passage from the north around Tuesday, which results in a
bit lower confidence in forecast highs by that time period. The
overall theme from CIPS analogs is for a much drier weather pattern
in the 6-10 day forecast period, though.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
Issued at 345 PM CDT Tue May 9 2023

Mainly VFR conditions are expected through the 18 UTC TAF cycle,
but isolated to scattered showers and thunderstorms could produce
localized MVFR conditions. Light winds and increasing dewpoints
may also produce stratus and/or fog late tonight through Wednesday
morning, which was included in the KMOT TAF where confidence is
highest. Elsewhere, the probability of MVFR conditions late
tonight and Wednesday is only around 30 percent, which was too low
to include in the 18 UTC TAFs.

&&

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

SHORT TERM...CJS
LONG TERM...CJS
AVIATION...CJS

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