904
FXUS63 KMPX 172001
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
301 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Active weather pattern returns, with the chance for showers
  and thunderstorms Wednesday and Thursday.

- Slight risk (Level 2/5) for severe weather across eastern
  MN/western WI Thursday. Large hail and damaging winds are the
  main threats, however tornado potential does exist.

- Additional chance for widespread precipitation during the
  second half of the weekend, followed by significantly cooler
  air next week.

&&

.DISCUSSION...
Issued at 256 PM CDT Tue Sep 17 2024

Well defined upper-low over the Rockies is progged to lift northeast
and occlude over northern CONUS/southern Canada over the next few
days. The Upper Midwest is positioned in a region of broad southerly
flow to the northeast of this feature. 850 mb analysis reveals a 30-
40 kt LLJ stretching from northeast Nebraska to west central
Minnesota. Scattered morning showers that developed on the nose of
the LLJ in southwestern MN have all but dissipated, with just a few
lingering showers near the Iowa/Minnesota border at this hour. We
expect the remainder of the afternoon to remain dry for most of the
region, however we cannot rule out a few isolated showers or a storm
within the ongoing southerly flow regime. Coverage of any convection
should be very limited given subsidence in the mid-levels. Highs are
generally on track to reach the mid to upper 80s, although peak
heating has been slower across southwestern Minnesota where cloud
debris from the morning showers has been slow to clear. We`ve
included slight chance PoPs in western Minnesota overnight, as the
intensification of the nocturnal LLJ will likely produce isolated to
scattered convection. Our discussions on the ops floor have centered
around the eastern extent of PoP coverage tonight, given the poor
handling of showers along the eastward shifting LLJ the past few
evenings. Essentially, this means we wouldn`t be entirely surprised
to see some showers work east towards the Twin Cities
tonight/early Wednesday.

The aforementioned upper-low is forecast to become negatively tilted
as it occludes over the northern CONUS tomorrow. The approach of the
upper-low will swing an initial zone of ascent across the Upper
Midwest Wednesday, which will lead to the reintroduction of PoPs
across the area. Forecast soundings capture a thermal nose around 5k
feet, owing to the ongoing warm air advective regime. We`ll lean
against earlier solutions for convective initiation due to this
complication, however we do expect PoP coverage to increase later in
the day as the low-level jet intensifies. Our latest forecast
features 20-30% PoPs across western MN through 00z, followed by an
increase to 40-60% PoPs across south central MN through 06z. Showers
and thunderstorms will continue eastward through very early
Thursday. The SPC has included a Day 2 Marginal Risk across western
Minnesota, where initial storm development may pose a large
hail/damaging wind threat.

By Thursday morning the closed upper-level low is forecast cross the
international border north of the Dakotas and move over southern
Manitoba. The trailing surface cold front will be the focus point
for renewed convection and potential severe weather later Thursday
morning and into the afternoon. SPC has expanded the Day 3 Slight
Risk with the newest outlook, such that all of eastern Minnesota
and much of western Wisconsin are now highlighted under the area of
greatest severe weather potential. The environment is forecast to be
supportive of severe weather given steep mid-level lapse rates,
strong instability, and sufficient shear tied to the strong
vertically stacked system. There are questions in how far east the
cold front will travel prior to convective initiation, as well as
the state of the environment following Wednesday night`s round of
convection. Right now we are leaning on a solution that supports a
large hail threat around/shortly after initiation, followed by more
of a damaging wind concern as the convection blossoms into an
organized line of storms. Low-level curvature in the forecast
hodograph will support the potential for brief tornadoes. Storms
will end from west to east as the front moves through the area.

Dry weather returns for Friday and most of Saturday, though
scattered showers will be possible as a clipper-type wave advances
across the Dakotas Saturday morning. Better chances for widespread
rain will be tied to the advance of a surface low in the central
Great Plains on Sunday. The evolution of the northern wave
(mentioned above) and the southern surface low has been handled
differently across recent ensemble runs, such that confidence in
rainfall amounts remains in flux. Should wetter members come to
fruition, it`s plausible that some locations across the area may
observe 7-day rainfall totals upwards of 1.5-2 inches. On the flip
side, the drier nature of several ensemble members makes this a low
confidence forecast at this time. What is more certain is the
arrival of cooler September air for the second half of the weekend
and into early next week. The latest blended guidance advertises
highs in the upper 60s/low 70s and lows in lower 50s for much of
next week. Fall lovers do not get too excited yet -- while it`s a
little beyond the current forecast period, all signs point upward
for a warmer than normal end to the month.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1215 PM CDT Tue Sep 17 2024

It will be another 24-30 hours of southerly winds and VFR skies.
Like yesterday, not anticipating storms near the western IA/MN
border to survive the afternoon, with even RWF expected to
remain dry. The big question for tonight is do we see showers
develop again tonight like we have seen the last 2 nights.
Sunday night they were across eastern SoDak & Neb, last night it
was western IA into southwest MN. If we were to see it happen
again tonight, it would likely shift east to be more aligned
with the I-35 corridor. Confidence in them happening again is
too low to include in any of the TAFs at this point, but it is
something to keep an eye on.

KMSP...No weather is expected today. We don`t have any showers
in the TAF for Wednesday morning, but if we repeat what happened
with shower development the last 2 nights to the west, they
would likely form a bit farther east than they did the last 2
nights, meaning MSP would be in line to start Wednesday with
some shra/tsra activity. Nothing shows this happening, so we
don`t have anything in the TAF, but it is something we`ll have
to continuously reconsider through the night.

/OUTLOOK FOR KMSP/
THU...VFR. SHRA/TSRA likely. Wind SSW 15G25 kts.
FRI...VFR. Wind WNW 5-10kts.
SAT...VFR. Chc shra/tsra. Wind SE 5-10 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.

&&

$$

DISCUSSION...Strus
AVIATION...MPG