086
FXUS63 KMPX 191755
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1255 PM 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 /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1245 PM CDT Thu Sep 19 2024

|Aside from a few lingering showers near (and moving away from)
KEAU and a narrow swath of MVFR ceilings over central MN
impacting KSTC, all sites start out as VFR this cycle. Mainly
VFR conditions expected thru late afternoon, then rapidly-
developing convection is expected over eastern MN late this
afternoon, sliding eastward over western WI through this
evening. TSRA with visibilities down to around 1SM will impact
most eastern WFO MPX sites, including the potential for
strong/gusty winds so have also added that mention for the
afternoon-evening thunderstorms. Behind the line, quickly
clearing skies with shifting winds are expected late this
evening through overnight tonight. There may be sufficient
moisture and calm winds at KEAU to promote ground fog close to
daybreak.

KMSP...Best timing for TSRA into MSP looks to be in the 22z-00z
timeframe, but could last as long as 01z. Will monitor radar
trends when storms develop and adjust the timing as needed. Some
storms could be severe, with strong winds, large hail and a few
tornadoes.

/OUTLOOK FOR KMSP/
SAT...VFR. Chc SHRA/TSRA. Wind SE 5-10kts becoming W.
SUN...VFR. Slight chc SHRA/MVFR. Wind NW 5-10 kts.
MON...VFR. Slight chc SHRA/MVFR. Wind NE 5-10 kts.

&&

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

&&

$$

DISCUSSION...NDC
AVIATION...JPC