636
FXUS63 KFGF 161818
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
118 PM CDT Mon Sep 16 2024

.KEY MESSAGES...

- There is a level 2 out of 5 risk for severe storms this
  evening. Damaging winds and hail will be the primary threats,
  with an isolated tornado possible.

- There are multiple chances for rain and thunderstorms through
  this weekend.

&&

.DISCUSSION...
Issued at 1245 PM CDT Mon Sep 16 2024

...Synopsis...

18z surface analysis indicates two pressure troughs across the
region, with the main density gradient along outflow from this
mornings thunderstorms. Thunderstorms are attempting to fire
along a weak surface theta-e gradient in the James River Valley.
These parcels are likely surface based, but as time goes on and
storms track northeastward, they will likely quickly become
elevated should they survive long enough. Severe chances are
likely to develop later this afternoon into the early evening
hours. The environment is favorable for primarily hail, damaging
winds, and heavy rainfall. There is a low chance for an isolated
tornado for surface-based supercells south of the front, but
that probability heavily hinges on supercells arising.

The pattern through the week will feature southwest flow, with
multiple impulses of shortwaves. This will bring the potential
for continued rainfall and thunderstorms through this weekend.
Severe storms are possible but mesoscale features would need to
be resolved to feel confident enough to up messaging.

...SEVERE STORMS TODAY...

The main surface features to watch through the day today will
be two theta-e boundaries, one that appears closer to the
international border and the other from a remnant outflow
boundary/reinvigorated by differential diabatic heating along
the central Red River Valley. Guidance continues to be very
strong in its assertion that this outflow will begin to act as a
warm front and surge northward through the day today. While this
is appearing to begin to occur now, there is high uncertainty in
how far north this mesoscale feature will get. Synoptic forcing
will be more than enough in northeast North Dakota given a solid
CVA plume pushing into eastern ND and ageostrophic divergence at
the apex of the ridge. For this reason, there is high confidence
in thunderstorms arising north of US Highway 2. The probability decreases
as you get further and further south away from synoptic
forcing. It does appear based on initiating convection upstream
that mesoscale forcing alone may be enough, so it does appear
surface based convection will arise this afternoon.

As storms generate north of the boundary, these are likely to
be very elevated. Shear is more than enough for organization
(generally effective bulk shear of 20-30 knots). The bulk of the
shear is within the 1-3km layer, so 0-3km bulk is 30 knots. The
main issue right now is how the convective mode evolves this
evening. The two main scenarios that are possible are
multicellular/transient supercells. This scenario would feature
more potential for larger hail to golf ball size hail and heavy
rainfall. The other scenario would be a more organized linear
structure that is aligned south to north, but this is a much
lower probability scenario. If this arises, 0-3km shear will be
more than enough for damaging winds to make it to the surface
despite the elevated nature.

Further south, more surface-based thunderstorms are possible,
but this will be further away from synoptic forcing. As such,
there is high confidence that convection down here will be
much more sparse as reflected by CAMs. The downside is that
curvature within the lowest 3kms appears sufficient for
potential surface-based supercells. Within the lowest 1km, there
is solid streamwise vorticity to facilitate a tornado threat
this afternoon if supercells can remain discrete.

Thunderstorms will eventually transition to a heavy rain threat
if linear structures perpendicular to 0-3km shear do not arise.
Training thunderstorms would become fairly likely, especially if
the frontal boundary can align much better with shear vectors
(more southwest to northeast oriented front). Flash flooding is
not likely to be a widespread threat but may arise in isolated
locations.


&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1201 PM CDT Mon Sep 16 2024

TSRA is the biggest impact for the TAF period, although location
is very uncertain. The greatest chances are in the vicinity of
GFK and DVL. Storms will be capable of 50 knot wind gusts and
hail. Thunderstorms will exit the area by 10z for most, with the
potential lingering at BJI through 16z.

&&

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

&&

$$

DISCUSSION...Perroux
AVIATION...Perroux