084
FXUS63 KBIS 271235
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
735 AM CDT Thu Jun 27 2024

.KEY MESSAGES...

- Numerous severe thunderstorms (risk level 3 out of 5) are
  expected across the western half of the state between 4 PM CDT
  this afternoon and 2 AM CDT late tonight.

- Initial storms developing in western North Dakota during the
  late afternoon and early evening hours will be capable of
  producing very large hail up to tennis ball size, damaging
  winds up to 70 mph, and a few tornadoes.

- A line of severe storms could then move through western and
  into central North Dakota during the evening and early
  overnight hours. Should this line develop, it will be capable
  of producing significant damaging winds as high as 80 mph.

- The threat for severe weather greatly diminishes along the
  Highway 281 corridor, but isolated severe storms (risk level 1
  out of 5) are still possible there later tonight.

&&

.DISCUSSION...
Issued at 733 AM CDT Thu Jun 27 2024

A severe thunderstorm outbreak appears likely across the western
half of North Dakota late this afternoon through the evening and
into the overnight hours. While there is a risk for a few tornadoes,
very large hail (up to tennis ball size) and significant damaging
winds (as high as 80 mph) are the primary hazard concerns.

Early this morning, increasing low level warm air advection and
moisture transport underneath an upstream transient ridge have
generated showers and a few thunderstorms over the Standing Rock
Reservation to Lake Oahe in northern South Dakota. The showers and
storms are forecast to expand and spread north and east through the
morning and early afternoon. Severe weather is not anticipated with
this activity, though latest rapid refresh guidance does show
potential for up to 500 J/kg MUCAPE and 30 kts effective bulk shear.

A potent trough over the Pacific Northwest early this morning will
cross the Northern Rockies by this evening, inducing lee
cyclogenesis over southeast Montana this afternoon. Persistent
southeast BL flow will advect 60s surface dewpoints into the western
Dakotas while reinforcing a capping inversion this morning and
afternoon. Closer to the lee trough in far western North Dakota,
less cloud cover should allow for greater BL destabilization by late
afternoon, and lowering mid level heights in advance of the
approaching trough should also contribute to the weakening of CIN.

All CAMs are simulating convective initiation along and west of
Highway 85 between 4 PM and 6 PM CDT. By this time of day, consensus
guidance shows MLCAPE as high as 3000 J/kg and 0-6 km shear around
50 kts. With an initial discrete mode favored, this environment
lends itself to intense supercells capable of very large hail. There
is also a risk for a few tornadoes, but that risk carries less
certainty and will likely be for a much smaller geographic area. The
biggest question on the tornado risk is whether the surface wind
field will be backed enough (i.e., have enough of an easterly
component) to generate the streamwise vorticity necessary for
tornadogenesis, which model hodographs currently show is iffy. As of
this writing, HREF and RAP analyses project the most likely area for
tornadogenesis from around Watford City to Dickinson, but this
aspect of the forecast could change later today.

Back in eastern Montana, there is higher confidence in convective
initiation on a more widespread basis by the early evening as the
approaching trough drives a cold front into the region. Given the
stronger forcing along with shear vectors turning more parallel to
the surface boundaries, these storms could quickly grow upscale into
linear segments with embedded mesocyclones and race eastward across
the western half of the state through the evening. The highly
unstable air mass and strong shear increasing with depth AGL could
set the stage for a damaging wind event. Forecast values of 3000
J/kg MUCAPE, 30-40 kts 0-3 km shear, 50 kts 0-6 km shear, and DCAPE
around 1000 J/kg are all supportive of damaging winds as high as 80
mph.

This discussion up to this point has focused on the most likely
outcome. However, despite all the requisite ingredients for severe
convection, there are several potential failure modes that could
change the outcome of the event, for either better or worse. If CIN
is stronger than forecast over western North Dakota, the initial
threat of discrete supercells could fail to materialize. If the
orientation of the low level wind field is changed, the tornado risk
could either increase or be nullified. There may also now be some
questions on the evolution of the convective event as a whole, with
the 06Z HRRR now simulating primarily a discrete mode only for the
entire event, which would greatly reduce the higher-end damaging
wind potential. But then the 07Z HRRR followed up with the strongest
linear signatures seen in model data yet, further exemplifying the
lingering forecast uncertainty. Considering all of this, the
evolution of the mesoscale environment today will be of the utmost
importance, even moreso than usual, in determining the outcome and
evolution of this event. But it should be emphasized once more that
all ingredients will be in place for a more notable severe event.

The threat for severe storms may continue into the eastern half of
the state later tonight, but a less favorable downstream environment
should lower both the coverage and threat ceiling. The threat for
severe storms will end from west to east late this evening through
tonight, but a few trailing sub-severe storms will remain possible.
Given the progressive nature of the system, the threat for localized
flash flooding is low, but not zero. Finally, this conceptually
seems like a favorable set up for a wake low following the severe
line of storms, but that phenomenon is highly unpredictable.

Other forecast highlights beyond the short term period include the
following:

- Shower and thunderstorm chances, ranging from 20 percent
  south to 80 percent far north, return Friday afternoon and
  evening as the trough axis reaches the Northern High Plains. A
  few stronger storms cannot be ruled out, but the strongest
  shear may not overlap the greatest buoyancy of less than 1000
  J/kg CAPE.

- Unseasonably cool temperatures Friday night through Saturday
  night, with lows mostly in the 40s and highs on Saturday only
  in the 60s. A warm up back to near normal temperatures is then
  favored for Sunday into the work week.

- Another upper level trough could bring scattered to widespread
  showers and thunderstorms to the region late Sunday through
  Monday. Deterministic and CSU machine learning guidance
  indicate at least some potential for severe weather.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 733 AM CDT Thu Jun 27 2024

MVFR ceilings are expected to develop from south to north later
this morning, mainly through central North Dakota, and persist
through much of the forecast period, possibly lowering to IFR
from KMOT to KJMS later this afternoon. Scattered showers with
embedded thunderstorms are also forecast to spread across south
central and eventually into eastern North Dakota this morning
through mid afternoon. Southeast winds will increase to 15-20 kts
this afternoon across all of western and central North Dakota.
Numerous severe thunderstorms are expected late this afternoon
through tonight, beginning in western North Dakota then moving
through central North Dakota. The strongest storms could produce
gusts up to 70 kts, but 50-60 kt gusts will be more common with
the main line of storms. Isolated to scattered storms that
develop ahead of the line in western North Dakota during the
late afternoon and early evening could produce hail up to tennis
ball size and a few tornadoes. Periods of at least IFR
visibility restrictions can be expected with any storm.

&&

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

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan