899
FXUS63 KMPX 191140
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
640 AM CDT Thu Sep 19 2024

.KEY MESSAGES...

- Potential for very large hail today with a Slight Risk (level
  2 of 5) this afternoon and evening.

- Rain is increasingly likely this weekend, but uncertainty
  remains high with the rainfall forecast.

- Our summer like warm with highs in the 80s continues through
  Saturday before cooler, more early fall like, air arrives
  Sunday with highs retreating back into the 60s and 70s.

&&

.DISCUSSION...
Issued at 258 AM CDT Thu Sep 19 2024

One look at any satellite image of North America and you will
easily be able to see the main low driving our weather. It is
currently spinning near the Montana/North Dakota/Saskatchewan
border area. This low will continue to occlude farther as it
slowly tracks across the Canadian Prairies. Closer to home, a
cold front will track across Minnesota today. This will help
provide the forcing needed for our storm chances later today.
Early this morning we have seen more or less isolated air mass
thunderstorms with little risk of anything strong. This will
continue to be isolated in coverage through the morning with
little severe chance thanks to weak instability at this time of
the morning.

This afternoon into evening will be the main event. Mid level
lapse rates will deepen through the day today setting up a
favorable thermal profile. Surface heating should overcome any
capping later today allowing for CI. Forecast CAPE values in the
CAMS is generally in the 2000 to 3000 J/kg range. With this
instability and the shear from veering winds we should have an
environment supportive of supercells. The question is where will
the forcing be best once the CI starts to occur? Current
guidance favors east central and south central Minnesota. This
is from the forecast position of the front once enough surface
heating has occurred to lead to CI. Generally along and to the
east of the I-35 corridor still seems like the best chance. With
ample CAPE and high values of SHIP, there are numerous large
hail analogs in forecast soundings. So hail will be the main
threat, but with supercells all severe threats remain possible.

Drier weather moves in on Friday with high pressure moving in
from the central plains behind the frontal passage. Despite a
cold frontal passage WAA should keep Friday temperatures well
above normal in the 80s. This WAA will also be in place
Saturday making it another above normal temperature day. The
frontal passage Saturday into Saturday night should finally cool
us down with northerly winds aloft and CAA. This should provide
for a strong enough cold front to give us a decent chance at
rain on Saturday. Looking towards ensemble guidance, most
members agree and have rain on Saturday. However the rainfall
amounts still have a significant amount of spread, so the
uncertainty in the forecast remains high. So while most will see
rain, it could be anything from a few hundredths to a good
soaking rain of 1-1.5 inches. This should start to get better
resolved in guidance once we get past our current system and the
wave driving this round moves into western North America.
Sunday into next week continues to have more chances for rain,
but there is much more spread within ensemble members tied to
multiple shortwaves transiting the north central CONUS. What
remains likely is cooler, but closer to normal temperatures next
week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 640 AM CDT Thu Sep 19 2024

Isolated thunderstorms will continue this morning mainly
impacting the Wisconsin terminals. Another round of storms are
expected this afternoon into evening. Best chances at MSP, MKT,
RNH, and EAU with a chance at STC. This afternoon into evening`s
round will be the main event with MVFR to IFR expected as the
storms move through. Should be more around an hour in impacts
with the timing of TEMPO and prevailing groups longer in the
12Z TAFs due to uncertainty. This will be revised in future
issuances of the TAFs as we get closer and confidence increases.
Outside of the storm chances just a shift from southerly winds
to westerly winds as a front moves through today.

KMSP... We should be done for the rest of the morning now at the
terminal for storms. The next round will be between 21Z and 00Z.
The actual time for storms should be more around an hour, but
uncertainty in timing has kept the window larger.

/OUTLOOK FOR KMSP/
FRI PM...VFR. Wind SE 5-10kts.
SAT...VFR. Chc SHRA/TSRA. Wind SE 5-10kts.
SUN...VFR. Chc SHRA/MVFR. Wind NE 5-10 kts.

&&

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

&&

$$

DISCUSSION...NDC
AVIATION...NDC