523
FXUS63 KDLH 190902
AFDDLH

Area Forecast Discussion
National Weather Service Duluth MN
402 AM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Showers and thunderstorms linger through the current overnight
  hours into mid-morning.

- This afternoon into the evening, a second round of
  thunderstorms are expected to initiate along and ahead of a
  cold front, with scattered severe storms (Level 2 out of 5) possible.
  Large hail, damaging winds, a couple tornadoes, and locally
  heavy rain may occur.

- Seasonable temperatures return through the weekend and into
  next week, with additional shower and storm chances (30-50%)
  on Saturday and periodic low-end rain chances (20-30%) next
  week.

&&

.DISCUSSION...
Issued at 401 AM CDT Thu Sep 19 2024

Current Conditions - This Morning:

Elevated (i.e. not surface-based) scattered showers and storms
have developed overnight in a warm air advection regime in
conjunction with forcing for ascent from a 40 kt low-level jet
over central and north-central MN. This activity is occurring
in the warm sector of a mature, vertically stacked low-pressure
system centered over far southeastern Saskatchewan with a cold
front draped south across the central Dakotas. The storms are in
an environment of effective bulk shear around 30 kt and some
MUCAPE around 1000-1500 J/kg, leading to a mixed mode of
pulse/clustered storms. While severe weather is not expected the
remainder of the current overnight hours, a few of these storms
could produce small hail (pea size to half-inch) and some
locally gusty thunderstorm winds to 30-40 mph.

While showers and storms should continue into the morning hours
as convection moves northeast through north-central and parts
of NE MN, the low-level jet will be weakening. Therefore the
converge and intensity of storms should reduce into mid to late
morning. Lingering low and high level cloud cover in the wake
of this morning precipitation may lead to some impacts on the
severe weather potential this afternoon and evening, primarily
when storms would initiate.

Severe Storms this Afternoon - Evening:

As the aforementioned vertically-stacked low continues trekking
northeast into southern/central Manitoba this afternoon and
evening. As it does, the cold front will advect east into
north-central MN by the afternoon and north-eastern MN by early
evening. While broken cloud cover will likely be ongoing
through much of the day, CAM guidance and model soundings depict
an atmospheric environment with steep low-mid level lapse rates
and strong moisture advection of PWATS of 1.5-1.7 inches near
the daily maximum values of sounding climatology for mid-
September should still be sufficient to produce 1500-3000 J/kg
of MLCAPE, albeit in a somewhat narrow corridor. Combine these
parameters with 30-40 kt of 0-6km bulk shear mainly
perpendicular to the cold front, and this sets the stage for
strong to severe storms to develop around the 3-5 PM timeframe
(as CAMs still suggest) as initial discrete/supercellular storm
modes before transitioning into several clusters/broken line of
storms during the evening. Given this expected time of storm
development and the cold front position at that time, the model
trends continue to point to storms developing east of Brainerd
to International Falls.

For specific severe storm hazards, the initial supercell mode
would briefly support hail up to around ping-pong size hail or
slightly larger during the late afternoon before growing upscale
into the broken line/clusters of storms into the evening, at
which point damaging wind gust threat will be increasing and the
large hail threat will be decreasing. There is also the
potential for a tornado or two during the late afternoon into
early/mid evening for storms that remain surface-based due to
0-1km storm relative helicity of 100 to locally 200 m2/s2. This
severe threat should diminish into the mid to late evening hours
as storms track east into the tip of the MN Arrowhead and
north-central WI with the loss of daytime heating and waning
instability. In addition to the severe threat, the high moisture
environment will also support the potential of locally heavy
rain due to quick downpours/high rain rates. While storms will
be progressive enough to limit training storms and or any more
widespread heavy rain threat, localized rainfall amounts of 1-2"
are possible, with the highest amounts possible in NW WI where a
WPC Marginal Excessive Rainfall Outlook (Level 1 out of 5) is
forecast. Rain should exit east of the Northland Thursday night.

Friday - Next Week:

Despite cyclonic flow behind the cold front on Friday, a drier
airmass in place should keep precipitation chances at bay for
the Northland. Ensemble guidance does show some good agreement
in a mid/upper-level trough skirting the International
Border/clipping the Northland on Saturday into Saturday night,
bringing a cold front eastward across the Northland with it.
Some shower/storm chances (30-50%) are forecast along and ahead
of the front. There is some uncertainty as to how much
instability/daytime heating will occur ahead of the front, but
Saturday afternoon and evening may need to be watched for a
low-end severe threat (5% severe chance). Temperatures on Friday
and Saturday will still be above-average for late September,
with widespread highs in the 70s to near 80 degrees.

A more seasonal airmass is forecast in the wake of Saturday`s
cold front, with widespread highs in the 60s and nightly lows
in the 40s on Sunday through at least the middle of next week.
During this timeframe, there will also be a somewhat active
upper-level pattern of a series of shortwave troughs that could
keep rain chances periodically in the Northland next week
(20-30% chances for now), though there will be dry time between
rounds. This pattern doesn`t look like a washout by any means,
but global ensembles quickly diverge with regards to what the
upper-level pattern will look like mid-late next week, so more
specific details are fleeting at the moment.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1254 AM CDT Thu Sep 19 2024

A band of scattered showers and thunderstorms are developing
from central/north-central Minnesota up into far NW Ontario as
a southerly low-level jet continues to strengthen tonight. This
jet will continue to support scattered shower and thunderstorm
potential through the current overnight hours, with activity
slowly moving northeast with time. No severe weather is expected
with this round and storms should weaken by mid-morning as the
low-level jet weakens and pushes northeast. Several terminals
will also experience LLWS through early this morning because of
the low-level jet. An additional round of scattered showers and
storms is expected to develop ahead of a cold front mid-
afternoon in northeast Minnesota and then move eastward into
which will move eastward through the Minnesota Arrowhead and
northwest Wisconsin this evening. Have included PROB30 mentions
for the current forecast thoughts on the best timing of these
storms. Some of the afternoon/evening storms could become strong
to severe, with hail and gusty thunderstorm winds, but have
left this mention out of the TAF. Expect MVFR to at times IFR
visibilities with any thunderstorms today. Otherwise, VFR
conditions are expected outside of showers and thunderstorms.

&&

.MARINE /FOR NEAR SHORE WATERS OF WESTERN LAKE SUPERIOR/...
Issued at 401 AM CDT Thu Sep 19 2024

Southeasterly winds along the South Shore are expected today
will become onshore for the Twin Ports and North Shore, with
some gusts to and become onshore for the Twin Ports and North
Shore again today, with some gusts of 10-15 knots possible for
the North Shore and 15-20 knots for the Twin Ports and portions
of the South Shore. Rain showers and thunderstorms are likely
today first during the late morning and early afternoon, and
then the better potential in the afternoon to evening. The
afternoon to evening storms could produce strong wind gusts and
large hail.

Breezy southwesterly winds develop for Friday behind the cold
frontal passage late Thursday, with gusts up to 15-20 knots
before turning light Friday night and backing to the southeast.

For the open water discussion, refer to the NWS Marquette Area
Forecast Discussion at weather.gov/mqt.

&&

.DLH WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.
MARINE...None.

&&

$$

DISCUSSION...Rothstein
AVIATION...Rothstein
MARINE...Rothstein