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129
FXUS63 KBIS 280921
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
421 AM CDT Fri Jun 28 2024

.KEY MESSAGES...

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- Showers and thunderstorms are possible this afternoon and
  evening, with 60-80 percent chances in northern North Dakota
  and 20-30 percent chances in the south.

- Strong storms with small hail and gusty winds are possible
  this afternoon and evening, but the severe storm risk is low.

- It will be breezy to windy today, especially in southwestern
  North Dakota where gusts of 35 to 45 are expected, highest in
  Bowman and Slope Counties.

- Severe thunderstorm potential (a Level 2 of 5 Risk) in the
  form of large hail and damaging winds returns Sunday night.
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&&

.DISCUSSION...
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Issued at 421 AM CDT Fri Jun 28 2024

Early this morning a weak cold frontal boundary is slowly moving
eastward across the area, and is located along the Highway 83
corridor as of 08 UTC. Last evening`s convection has moved well
east of the frontal zone, into the far eastern Dakotas/Minnesota
as of this hour. The air mass in the wake of that QLCS is much
too convectively-processed and stabilized for any renewed storm
development with the eastward-moving front early this morning.
However, the moist and stable boundary layer is contributing to
significant stratus over central ND, and areas of fog in north
central ND where light upslope/easterly winds into the terrain
of the coteau region occurred prior to the frontal passage. We
expect the stratus and fog to gradually lift as the front moves
eastward, but it may linger into the early part of the day in
parts of central and especially north central ND.

The main feature of interest today and tonight is an upper-level
low initially over southwestern Canada early this morning, which
is forecast to move east-southeast, reaching western and central
ND by late afternoon and evening. Upstream radar imagery reveals
an area of showers already associated with the cyclonic flow in
the vicinity of this upper-level low, and guidance is consistent
in showing showers and some thunderstorms moving into the area
and/or further developing with diurnal destabilization this
afternoon. Precipitation chances this afternoon and evening
range from 60 to 80 percent in northwest and north central ND,
where synoptic-scale ascent will be strongest near and north of
the upper-level wave, to 20 to 30 percent along the ND/SD border.

Boundary layer destabilization with residual low-level moisture
characterized by dewpoints in the 45 to 55 F range and cooling
temperatures aloft will contribute to around 500 J/kg of MLCAPE.
This is sufficient for scattered thunderstorms this afternoon
and evening, but the bouyancy profile is rather "skinny" owing
to modest midlevel lapse rates and a lack of greater low-level
moisture. This may tend to limit updraft accelerations, and
marginalizes the severe-storm risk despite effective-layer shear
around 40 kt. As a result of the expected thermodynamic
setting, and both GEFS-based machine-learning and HREF-driven
calibrated severe storm guidance having probabilities less than
5 percent in our area, we have chosen to focus our messaging on
the potential for strong storms with gusty winds and small hail
rather than explicitly mentioning severe storms in our outlooks.
However, the reasonable worst-case scenario for this afternoon
and early evening would be for a marginal severe storm with
large hail to the size of quarters and/or damaging wind gusts up
to 60 mph, mainly in central ND where the CAPE-shear setting
will be most favorable. Confidence in the coverage of that risk
wasn`t high enough to explicitly message at this time, though
mesoscale trends will be monitored through the day in case a
more well-defined area of concern becomes apparent.

Otherwise, a stronger cold front is expected to move across
western ND this afternoon and central ND this evening, in
tandem with the upper-level trough. The 00 UTC guidance
forecasts a few-hour period of strong low- and midlevel cold
air advection with the front centered on southwestern ND in the
late afternoon and early evening. This includes significant
"thermal packing" at the 850 mb level with the frontal passage,
which often signals a period of strong winds when that cold air
advection and its associated momentum transfer occur in an
already-well-mixed boundary layer after peak heating of the
day. Given this trend, along with a signal in ECWMF ensembles
(EFI values of 0.8), and forecast soundings from various
guidance suggesting mixed-layer winds of 40-45 kt, we have
issued a Wind Advisory for Bowman and Slope Counties this
afternoon and evening. We gave consideration to including
additional counties north and east of there, but momentum
transfer is more uncertain with latitude given the midlevel
ascent associated with the upper-level low. Moreover, with
eastward extent the timing of the frontal passage will be late
enough in the day that the duration of strong winds is more
uncertain. Nonetheless, it will still be windy across all of
southwestern ND this afternoon and evening, and there is a low
to medium probability that the Wind Advisory could need to be
expanded a bit with later updates.

Highs today will range from the mid 60s in northwestern ND,
where precipitation and the cold front will arrive first, to
mainly the 70s elsewhere. Lows tonight will be cool, in the
40s F in most areas, as high pressure originating over Canada
begins building into the area. That will set the stage for
below-normal temperatures on Saturday when highs are forecast
to only be in the 60s F, albeit with dry weather, a mostly sunny
sky, and lighter winds as ridging dominates at the surface and
aloft.

Southeast return-flow will develop on Sunday as middle- and
upper-level ridging shifts eastward ahead of a trough that will
move into the northern Rockies. This will establish a warming
trend, and low-level moisture advection as a lee-side surface
low develops over MT/WY. The timing of an upstream impulse in
the increasing southwesterly flow suggests convection will be
favored to develop over MT in the afternoon and evening, and
may then move across western and central ND Sunday night. We
anticipate a severe-storm risk in the form of large hail and
damaging winds will accompany this activity given the expected
CAPE-shear setting, and as a strong low- level jet develops
overnight. However, forecast soundings do suggest strong capping
associated with a warm elevated mixed layer may be a challenge
to the overall event. Machine-learning guidance and the SPC
outlook that contains up to a Level 2 of 5 Risk in parts of
western ND nonetheless support messaging of a severe storm risk
Sunday night in western and central ND.

A severe-storm risk may exist Monday as well, depending on
the timing of a surface frontal zone and the effects of any
overnight and early-day convection. However, strong southwest
flow aloft ahead of an advancing upper-level trough and rich
low-level moisture would yield potential severe storms if the
frontal passage is slow enough and sufficient heating occurs.

Global ensemble members in the NBM are closely clustered with
the large-scale pattern through Monday, heralding a confident
general forecast. Beyond Monday, approximately 70% of ensemble
members suggest broad troughing will persist across the area
for much of next week, which would support cooler temperatures
and 20 to 40 percent chances of showers and thunderstorms.
However, spread in guidance increases, especially by the 4th
of July, when there`s about a 30% chance of a bit warmer and
drier pattern than the ensemble mean and NBM call for.
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&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
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Issued at 421 AM CDT Fri Jun 28 2024

MVFR to LIFR ceilings and patchy fog are ongoing in central and
especially north central ND as of 09 UTC. They will persist
until a cold front passes, but improving conditions are
forecast between 12 and 14 UTC behind that front. Mainly VFR
conditions are expected during the day today, but isolated to
scattered showers and thunderstorms are forecast, mainly after
18 UTC and mainly in northern ND. MVFR ceilings may also return
to northern ND after 00 UTC (a 40-60 percent chance).
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&&

.BIS WATCHES/WARNINGS/ADVISORIES...
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Wind Advisory from 2 PM this afternoon to 9 PM MDT this
evening for NDZ040-043.
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&&

$$

DISCUSSION...CJS
AVIATION...CJS