897
FXUS63 KFGF 271457
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
957 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.

&&

.UPDATE...
Issued at 957 AM CDT Thu Jun 27 2024

Patchy dense ground fog has dissipated.

Radar/satellite imagery indicates showers/embedded thunder
continues to slowly propagate east/northeast, now located across
south central North Dakota. This activity will be the main
concern for today, and is being forced by elevated (700mb) warm
air advection - with guidance also indicating weak MUCAPE near
this level. Guidance indicates this forcing/instability will
continue to slowly propagate northeast, across the valley by
6pm. The main uncertainty at this point is coverage of showers
through today. This evening and overnight, attention turns to
more widespread showers/thunder and the marginal severe risk
(will continue to assess through the day).

UPDATE
Issued at 650 AM CDT Thu Jun 27 2024

Patchy dense ground fog has developed over parts of west
central MN, with highly variable impacts. So far webcams and obs
have been trending towards this improving quickly this morning.

Showers and isolated/embedded thunderstorms are southwest of our
CWA and spreading northeast (handled well by current HREF) and
this is accounted for in the current forecast. This initial area
of showers should spread into our area later this
morning/towards midday and then transition north.

&&

.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 /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 650 AM CDT Thu Jun 27 2024

VFR is currently prevailing in eastern ND and most of northwest
MN. Patchy ground fog in west central MN is resulting in
variable flight conditions near KBJI, which could lift/clear
out with daytime mixing quickly this next hour or two.

A period of showers will arrive from the southwest midday in
eastern ND and this progress northeast with lower chances for
thunderstorms near the TAF sites this afternoon. Widespread
showers and thunderstorms develop this evening and overspread
the region along with MVFR and IFR ceilings. A few stronger or
severe storms will be possible in ND (lower coverage and harder
to anticipate impacts at this range).

&

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

&&

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

&&

$$

UPDATE...TG/DJR
DISCUSSION...DJR
AVIATION...DJR