-- Discussion containing changed information from previous version are highlighted. --
172
FXUS63 KFGF 171135
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
635 AM CDT Mon Jun 17 2024

.KEY MESSAGES...

-Training showers and thunderstorms will bring the threat for
 flash flooding across the region through Monday and Tuesday.
 The Flood Watch has been expanded to include the majority of
 eastern North Dakota and all of northwest Minnesota.

-Embedded severe thunderstorms are likely to arise through
 Tuesday with the threat for golf ball size hail and a lower
 risk for 60 mph winds.

&&

.DISCUSSION...
Issued at 1237 AM CDT Mon Jun 17 2024

...Synopsis...

00z upper air analysis indicates an 850mb warm front that has
become the focus for strong isentropic ascent across South
Dakota and southern Minnesota. This warm front will lift further
north with its strong warm air advection through the day today.
At the surface, a very strong theta-e gradient will set up
across the central plains and keep temperatures today very cool
in the 50s. Efficient moisture transport associated with this
warm front will create widespread showers and thunderstorms this
afternoon and evening, lasting through a majority of the night
into Tuesday morning.

Thunderstorms should exit the area by late Tuesday afternoon at
the latest, with additional showers lingering overnight Tuesday.
There are signals for another stout wave to bring efficient
rainfall to the area, although instability with this wave is
much less so there is lower confidence in potential flooding.

...FLASH FLOOD THREAT...

There is a strong signal for widespread heavy rainfall beginning
Monday afternoon and lasting through the overnight hours. The
main question at this point is how much of the area sees
flooding and where the strongest impacts will be felt.
Confidence has diminished in flooding potential across the
southern zones due to the transient nature of the 850 warm
front. Of greater concern for flooding based on CAMs is
redevelopment west of the Red River Valley. For this reason, the
Flood Watch has been expanded to encompass the majority of the
area.

A very strong low level jet will generate continuous
convection, feeding persistent instability and moisture into the
zone of forcing. This is what we know 100% will occur overnight
tonight. What we don`t know is the evolution of surface features
that will provide for more focused ascent and create a zone of
potential flash flooding. CAMs have been really struggling to
resolve certain things, but one of the most concerning is a few
CAMs bringing a stationary front across eastern North Dakota.
This is a very low confidence scenario at this point, but one
that may arise nonetheless. At this point in time there are two
potential outcomes through Tuesday:

SCENARIO 1: The stationary front is fairly weak or nonexistent
and there is no zone of focused ascent. This scenario would have
limited flash flooding potential as the main forcing zone (the
elevated warm front) will be fairly transient. Heavy rainfall is
still likely to occur.

SCENARIO 2: The stationary front is strong enough to induce
convection along a line. In this scenario, where convection
fires WILL have flash flooding. The zone will be fairly tight
thanks to strong instability. This is the higher impact
scenario. Where this zone of ascent sets up though we do not
know at this time. It could set up anywhere as far west as the
Sheyenne River Valley and Devils Lake Basin to as far east as
northwest Minnesota.

Given the convective nature of this and the multiple moving
parts, it is very likely portions of the Flood Watch will need
to be removed periodically throughout the day and overnight
tonight, particularly as confidence increases in the greatest
area of convection. This is a very low confidence scenario with
the potential for higher impacts so take the entire situation
and forecast as fluid at this point. Despite a downgrade from
moderate to slight in the latest Excessive Rainfall Outlook, it
is important to note that this is due to uncertainty. The
potential severity of flash flooding has not changed. Prepare
for at least heavy rainfall with the potential for flash
flooding.

...SEVERE WEATHER THREAT...

Through the day today, strong differential diabatic heating will
likely strengthen a surface theta-e warm front across southern
South Dakota and southern Minnesota. This will keep the bulk of
surface instability away from our area, meaning the bulk of
convection that fires this afternoon will be elevated. Strong
southwesterly flow within the 850mb layer will advect a
persistent elevated mixed layer to our area, giving strong
sustained convection with strong instability. This means the
primary threat will be hail. The main uncertainty at this point
in time is how large potential hail will be and will depend on
anything becoming a supercell. Despite strong bulk shear from
the surface up, it is more likely that thunderstorms will only
see shear from 850 and up, which is substantially less.
Effective-layer bulk shear north of the warm front is on the
order of 20-30 knots, so there is a greater probability for
multi-cellular clusters. If parcels can tap into a bit more
shear closer to the surface, the potential for elevated
supercells is there. Without supercells, the greatest hail size
we will most likely see is 1.25, but with supercells it would be
possible to push past 1.5.

As the surface warm front pushes into our area after midnight,
convection in the vicinity of this warm front may pose the risk
for severe wind gusts to 60 mph, although confidence is low due
to timing with a potential diurnal inversion near the surface.
Redevelopment is likely Tuesday afternoon. Flow parallel to the
initiating boundary will create more linear structures,
creating a primarily damaging wind driven impact. Severe
convection should only last about 1-3 hours before exiting into
northeast Minnesota.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
-- Changed Discussion --
Issued at 635 AM CDT Mon Jun 17 2024

VFR conditions prevail at all TAF sites this morning, with
showers starting to work into the southern portions of the
forecast area. Shower activity is expected to remain mostly
south of KFAR through much of the day, with only perhaps a stray
shower or storm moving into the vicinity through late this
afternoon. Widespread showers and storms are possible starting
this evening, however, with strong to severe thunderstorms as
well as very heavy rain expected. Impact potential will be
greatest between 05Z and 12Z in most areas, with KBJI taking a
bit longer for storms to work east of the area. MVFR to IFR CIGs
and visibility is the expectation during showers and
thunderstorms.
-- End Changed Discussion --

&&

.FGF WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
ND...Flood Watch from 1 PM CDT this afternoon through Wednesday
     morning for NDZ007-008-014>016-024-026>030-038-039-049-
     052>054.
MN...Flood Watch from 1 PM CDT this afternoon through Wednesday
     morning for MNZ001>009-013>017-022>024-027>032-040.
-- End Changed Discussion --

&&

$$

DISCUSSION...Perroux
AVIATION...Lynch