556
FXUS63 KFGF 172321
AFDFGF

Area Forecast Discussion
National Weather Service Grand Forks ND
621 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- There is a level 1 out of 5 risk for severe storms Wednesday
  afternoon and evening, including the potential for supercells.

&&

.UPDATE...
Issued at 621 PM CDT Tue Sep 17 2024

Forecast remains on track tonight with some elevated warm air
advection forced showers along an advancing theta-e gradient
moving south to north through eastern North Dakota after
midnight through sunrise. Could see some gusty winds potentially
nearing 40 mph but otherwise limited impacts expected. Further
north of under current cloud cover winds remain light(er) but
still likely not calm enough for fog formation on a widespread
scale tonight in the far north valley.

&&

.DISCUSSION...
Issued at 202 PM CDT Tue Sep 17 2024

...Synopsis...

Large upper cyclone is readily viewable on water vapor satellite
imagery this afternoon centered over the Great Basin, with the
Northern Plains residing under southwest flow aloft just downstream
of the cyclone`s exit region. At the surface, low pressure is
deepening in the northern High Plains around eastern MT, with a
stationary front extending eastward through ND into far
northwest MN. South of this front, summer-like temperatures
well into the 80s are occurring, with some locations still
liable to touch 90 in the southern Red River Valley this
afternoon. North of the front, cooler temperatures in the 70s
and low stratus clouds will set the stage for another night of
fog potential in northeast ND as upslope easterly flow allows
the stratus cloud deck to scrape against the ground.

Tonight into Wednesday morning, the upper cyclone rapidly moves
into the northern High Plains in a negatively tilted fashion,
with rapid deepening of the surface cyclone also anticipated.
This will provide a branch of forcing and moisture within its
moist conveyor wrapping into the low from the south and east
into ND and western MN. This will bring around a 40% chance for
showers and thunderstorms sweeping across our area tonight into
Wednesday morning. Thunderstorms are not anticipated to be
severe tonight due to lack of better shear and overall wind
energy associated with this branch of forcing.

Getting into Wednesday, as the low rapidly develops, wind energy
markedly increases over ND into western MN. This coupled with
sufficient instability and moisture will drive the chance for
severe storms Wednesday afternoon into evening. More on this
below.

By Thursday, the low starts to trek northeastward bringing with
it a moisture scouring cold front, gusty winds, and beginnings
of an eventual cooling trend. Depending on how fast the front
moves into MN, there could be a small area that holds potential
of seeing stronger storms Thursday afternoon near central MN.

This weekend into next week, most ensemble guidance depicts
broad troughing into the Plains and Upper Midwest. This will
continue chance for rain and possibly thunderstorms along with
trending temperatures closer to average this time of year, which
will feel more fall-like rather than the summer-like conditions
we`ve been experiencing as of late.

...Severe storm potential Wednesday...

Setting the stage for severe potential Wednesday will be
continued poleward advection of moisture within eastern ND and
western MN ahead of a dryline stretching into eastern ND. This
along with daytime heating under marginally steep lapse rates
aloft will allow for a sector of instability with MLCAPE values
progged in the 1000-2000 J/kg. With such strong dynamics nearby,
the wind field over eastern ND into MN will be increasing,
contributing to 40+ kt of shear to 500 mb. More impressive
regarding the wind field is in the lower levels. Most guidance
depicts strengthening winds in the 0-3 km, with notable veering
with height contributing to increased helicity values. This is
providing high chance for 0-1 km storm relatively helicity
values exceeding 100 m2/s2 by Wednesday late afternoon into
evening within the relatively moist/unstable sector. Should
discrete storms develop in this regime, supercells will be
favored with all hazards possible, including the chance for
large hail and tornadoes. The potential for all hazards is
illustrated nicely by some recent ML guidance such as Nadocast
and CSU severe probabilities.

However, there are notable questions on the potential coverage
of robust thunderstorms. This is mainly driven by the
unfavorable timing and/or displacement of forcing for ascent
depicted by guidance. Most of the synoptic forcing for ascent
will tend to move northward into Canada. At the surface, there
isn`t much evidence to suggest a surging dryline to help lift,
with the cold front more likely to move through after sunset
when capping increases and surface-based parcels become
increasingly unlikely. At this time, the chance for widespread
severe storms is very low due to this, although mesoscale
forcing in a few areas coupled with broad weak ascent with the
nearby upper low may be enough to help a few robust
thunderstorms.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 621 PM CDT Tue Sep 17 2024

VFR conditions remain for all despite the below 3sm reports at
DVL, likely some form of wildlife impacting the sensor or some
other non meteorological phenomenon. Could see some LLWS near
2000 ft but guidance has varying opinions and thus will opt
until later for hopefully some better agreement before putting
it in the taf. Winds look to be quite gusty after sunrise
tomorrow with southerly 20kt gust near 30kt in the southern
valley with lower peak gusts elsewhere.

There is a chance for scattered showers and thunderstorms
between 06-15Z across the region, 15-18Z for Minnesota, as a
low pressure system approaches. However, confidence wasn`t high
enough in coverage from these to include TAFs for now.
Thunderstorms are not anticipated to be severe, although could
still hold gusty erratic winds over 30kt in addition to
lightning.

KTVF still holds an AMD NOT SKED due to AWOS outage at the
site.


&&

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

&&

$$

UPDATE...TT
DISCUSSION...CJ
AVIATION...TT