256
FXUS63 KFGF 270906
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
406 AM CDT Thu Jun 27 2024

.KEY MESSAGES...

- There is a level 1 out of 5 risk for severe thunderstorms across a
  portion of eastern North Dakota this evening.

- There is a level 1 out of 5 risk for severe thunderstorms across
  the region Friday afternoon and evening.

- There is a low chance for severe thunderstorms again Monday
  across the region.

&&

.DISCUSSION...
Issued at 406 AM CDT Thu Jun 27 2024

...Synopsis...

Ridging is currently in place, with a mid/upper low deepening
in the Pacific NW. Pattern across the Northern Plains remains
progressive however, and ensemble trends continue to show strong
consensus on this general zonal/quasi-zonal flow to continue
into next week. The variances will be on the timing, track,
strength of each passing mid level trough and associated surface
low/front evolution that causes variances in timing, location,
and magnitude of any severe impacts or heavy rain. Temperatures
vary with passing shortwave ridges/troughs with a tendency for
below average temperatures at time (notably this weekend behind
the trough on Friday. Each passing trough carries at least some
potential for severe thunderstorms, with details tied to the
mentioned uncertainties.


...Severe threat this evening...

SW flow ahead of the wave in the Pacific NW is building into the
Northern Rockies and several surface troughs/warm fronts to the west
eventually will shift east towards eastern ND as a LLJ develops this
evening. Current consensus shows a modest elevated instability axis
(1000-2000 J/KG) developing into our west around 00Z but quickly
diminishes this through 03Z due to the shifting surface
pattern. The window for organized/severe convection seems to be
tied to upstream activity arriving as the LLJ strengthens in the
warm sector and instability remains high enough. The WAA/LLJ
should support increasing coverage of showers/elevated
convection, but if profiles are as narrow/weak as shown by
consensus/mean HREF when upstream convection arrives then severe
convection is much less likely. Slightly higher elevated CAPE
lingering longer in our west or south (as a subset of guidance
shows) would still support a lingering severe threat into parts
of eastern ND, but activity should be very elevated. Primary
threats remain hail, and wind where momentum transfer of heavy
rain/elevated downdrafts can mix down higher gusts in the higher
low level shear environment.

...Severe threat Friday afternoon/evening...

Deformation zone is favored by latest trends to shift north through
Friday morning towards the US/Canada border, lingering into the
afternoon. A mid level dry slot is shown to allowing for some
clearing across the southern Red River Valley into west central
MN. This could aid in destabilization and there is a signal for
SB CAPE to increase above 1000 J/KG in some of these areas. This
will tend to be in a post frontal shear environment (backing
low levels) but deep layer shear remains high (top heavy
shear profiles). Still, due to the strong effective shear we
could see supercell development carrying and larger hail
threats. The two main scenarios based on current trends include
elevated clustering or isolated supercells with larger hail to
ping pong ball size (maybe larger), but less of a tornado
threat. A less likely scenario (shown by one cluster of
ensembles) brings better instability farther north where low
level veering near the surface front and slower moving frontal
zone could allow for a conditional tornado threat.

...Severe threat Monday....

Another mid/upper wave follows a very similar track and evolution
as the system Friday, with a period of widespread showers or
embedded thunderstorms. There is a signal for a deeper warm sector
building into our region within most clusters, and higher probs for
SB CAPE to exceed 1000 J/KG during the peak heating/diurnally
favored period of the afternoon/evening. CSU GEFS based machine
learning (15-30% severe probs) are supported by 100% chance for
greater than 55 NBM CWASP and 50-60% probability for NBM CWASP
values exceeding 65. That aligns with a threat for isolated to
scattered severe convection. This is still farther out, so
details/impacts are still inherently uncertain.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1135 PM CDT Wed Jun 26 2024

Mid level stratus persists in the vicinity of KDVL, but is
expected to gradually clear by around 10Z. VFR conditions then
prevail through the remainder of the morning hours, with
increasing cloud cover during the afternoon. Showers and
scattered thunderstorms are expected to work into the area
through the evening hours, then persist overnight. Went with the
mention of VCSH for now until confidence increases regarding
onset timing of better rain and thunderstorm coverage. Winds
shift to the southeast by mid morning, then increase heading
into the afternoon with gusts approaching 25 knots.

&&

.FGF WATCHES/WARNINGS/ADVISORIES...
ND...None.
MN...None.

&&

$$

DISCUSSION...DJR
AVIATION...Lynch