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308
FXUS63 KBIS 261357
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
857 AM CDT Wed Jun 26 2024

.KEY MESSAGES...

- Quiet weather today, with highs ranging from the upper 60s in
  the Turtle Mountains to lower 80s in far southwest North
  Dakota.

- Scattered severe thunderstorms (risk level 2 out of 5) are
  likely across the western half of the state late Thursday
  afternoon into Thursday night. Expected hazards include very
  large hail up to 2 inches in diameter, damaging winds as high
  as 70 mph, and a few tornadoes.

- Below normal temperatures Friday through Sunday, with highs
  only in the mid 60s to lower 70s on Saturday.

&&

.UPDATE...
-- Changed Discussion --
Issued at 857 AM CDT Wed Jun 26 2024

Broken low level stratus has expanded across the majority of areas
north and east of the Missouri River. These low clouds are
still expected to dissipate later this morning through the early
afternoon. Thus, the forecast still remains on track.

UPDATE
Issued at 649 AM CDT Wed Jun 26 2024

The forecast for today remains on track. Broken low stratus
continues to expand southward across northern and eastern North
Dakota early this morning. The clouds should stall around mid
morning and gradually dissipate through early afternoon.
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 444 AM CDT Wed Jun 26 2024

The Northern Plains sit under northwest flow aloft early this
morning, with a downstream shortwave approaching Lake Superior and
upstream ridging over the Northern Rockies. At the surface, high
pressure is centered over southern Saskatchewan. The upper level
ridge and surface high will shift eastward today, bringing a quiet
weather day to western and central North Dakota with no rain and
light winds. Broken low stratus moving down from Canada early this
morning will likely reach as far south as the Highway 52 corridor by
late morning, then become more scattered and gradually dissolve
through the afternoon. High temperatures today are expected to range
from the upper 60s northeast to lower 80s southwest.

A highly amplified trough with a closed upper low at its base will
make landfall on the Pacific Northwest coast later today. The
continued eastward progression of downstream features will increase
surface southeasterly and low level southerly flow over the western
Dakotas and eastern Montana this evening through tonight. A
respondent increase in low level warm air advection and moisture
transport could allow showers and a few thunderstorms to develop
across western and south central North Dakota later tonight into
early Thursday morning. The current suite of CAMs contains a wide
disparity in coverage, placement, and start time of convection, but
a mid level shortwave preceding the stronger upstream trough should
eventually provide extra forcing support for showers and a few
storms along and east of Highway 83 through Thursday morning. Severe
weather is highly unlikely through Thursday morning, with HREF mean
MUCAPE remaining under 1000 J/kg and the strongest instability
staying farther west. There could still be a stronger storm or two
though late tonight through Thursday morning, with effective bulk
shear possibly increasing to around 30 kts.

The highly amplified upstream trough is forecast to cross the
Northern Rockies Thursday afternoon, with lee cyclogenesis in
southeast Montana and an inverted surface trough extending north
into southern Saskatchewan. To the east of the developing surface
low, south to southeast BL winds will advect 60s surface dewpoints
into western North Dakota underneath an EML. There are several
potential scenarios for convective initiation and evolution Thursday
afternoon and evening. First, there is the unknown on what role
morning convection could play in the evolution of the mesoscale
environment Thursday afternoon. Latest guidance does suggest
weakening of the capping inversion over western North Dakota late
Thursday afternoon, which could be driven by both an increase in
diurnal heating and lowering mid level heights. While the best mid
to upper level forcing will lag behind in Montana during the late
afternoon, increasing low level warm air and theta-e advection may
provide enough lift for air parcels to exceed the LFC. This initial
late afternoon development would be favored to occur along and west
of Highway 85. Residual capping could dampen the threat ceiling to
some extent. However, MLCAPE around 2000-3000 J/kg paired with 40-50
kts of 0-6 km shear is more than sufficient for severe weather. The
initial storm mode would almost certainly be discrete, favoring very
large hail as the primary hazard. But there could also be a tornado
risk with discrete storms through early evening, with 0-1 km shear
forecast around 10-20 kts, low LCL heights, and at least some
potential for streamwise vorticity ingestion per forecast hodograph
analysis.

Regardless of the coverage of discrete supercells across western
North Dakota during the late afternoon and early evening on
Wednesday, numerous thunderstorms are likely to develop across
eastern Montana and enter western North Dakota sometime during the
evening, likely intercepting any ongoing convection as the storms
progress eastward through a favorable severe environment into
central North Dakota during the late evening or early overnight
hours. Storm mode during this part of the event would likely quickly
evolve into a combination of multicell and linear, transitioning the
primary severe hazard to damaging winds. Once storms pass the
Highway 83 corridor, there is a clear signal in CAMs for a drop off
in storm intensity, although some deterministic models maintain
sufficient, albeit weaker CAPE and shear into the eastern half of
the state late Thursday night.

Given the potential for a higher-end CAPE/shear parameter space with
both discrete and linear storm modes, our most likely threat ceiling
forecast calls for hail up to 2 inches in diameter, damaging winds
up to 70 mph, and a few tornadoes. But there are still potential
failure modes that could make this event not as severe or not as
widespread.

On Friday, the base of the trough is forecast to pass through the
state, with initial dry slotting (except far north) followed by
strong cyclonic vorticity and wrap-around deformation. This keeps
shower and thunderstorm chances in the forecast through Friday
night, ranging from 20 percent south to 70 percent north. Buoyancy
could be very low to nonexistent on Friday, but it would only take a
few hundred J/kg in a highly sheared environment for storms to cause
trouble.

With the arrival of the trough, a period of unseasonably cool
weather will begin on Friday, with highs across the north only
forecast in the mid to upper 60s. Then behind the departing trough
on Saturday, the NBM continues to advertise only around 65 to 70 for
highs across the entire region, with lows Friday and Saturday nights
mostly in the 40s to lower 50s. A gradual warm up back to 70s and
80s for highs is then favored for Sunday into next week. There is
strong model consensus given the forecast time range for another
potent trough moving from the Pacific Northwest to the Northern
Plains late Sunday through Monday, and CSU machine learning guidance
is already hinting there could be a severe storm potential with this
system.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 649 AM CDT Wed Jun 26 2024

MVFR to perhaps IFR ceilings are expected across northern and
eastern North Dakota this morning, including at KXWA, KMOT, and
KJMS. The categorical restrictions may not be prevailing
through the whole morning though. Ceilings should improve to VFR
by early afternoon, and VFR conditions are otherwise expected
through the forecast period. Light and variable winds trough
the afternoon will become southeast around 10 kts this evening
through tonight. Chances for showers and perhaps a few
thunderstorms will begin to increase across western and south
central North Dakota overnight into Thursday morning. Forecast
confidence on the timing, coverage, and placement of development
remains low, but probabilities are high enough to include a
VCSH mention at KBIS later tonight.

&&

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

&&

$$

UPDATE...Telken
DISCUSSION...Hollan
AVIATION...Hollan