357
FXUS63 KMPX 242319
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
619 PM CDT Mon Jun 24 2024

.KEY MESSAGES...

- Enhanced Risk (Level 3/5) of severe thunderstorms across
  eastern Minnesota and western Wisconsin this evening. Severe
  thunderstorms will be capable of producing very large hail,
  damaging wind gusts, and a few tornadoes.

- Forecast confidence is increasing that the "cap" will weaken
  or break, allowing storm development this evening.
  Thunderstorms that do develop will quickly strength and become
  severe.

- Given the state of the ongoing flooding, recent excessive rainfall,
  and thus low flash flood guidance values, a Flood Watch is
  has been issued for potential flash flooding due to heavy rain
  across the Twin Cities metro and portions of S MN and W WI
  for this evening.

- Active pattern continues with another round of widespread
  rainfall Thursday afternoon through Friday night. Local river
  levels remain on the rise following the recent rainfall. River
  flooding remains a concern, as several sites have yet to crest
  in moderate or major flood stage.

&&

.MESOSCALE...
Issued at 558 PM CDT Mon Jun 24 2024

Things remain fairly quiet for now thanks to a substantial capping
inversion evident in a special 330 PM sounding launched at our
office earlier. This cap is the major source of uncertainty heading
into this evening, and the big question remains if ascent ahead of
an approaching 700 hPa trough will be enough to break the cap and
initiate severe storms over the next few hours. Satellite trends
indeed show thickening cloud fields over central MN near St. Cloud
indicative of such ascent, and many models do eventually produce
convection although where this convective initiation occurs varies
widely from model to model, anywhere from Mankato to Duluth.
However, the aforementioned satellite trends near St. Cloud suggest
this may be the primary area to focus on, and several WoFS members
are keying in on this area as well. If this does end up being the
location where convection develops, it could put areas in and near
the Twin Cities metro in the path of potentially significant severe
weather later this evening. Very strong instability and low- level
shear will be more than favorable significant wind damage and
tornadoes. But again, this is highly conditional on storms
developing in this area. Should storms fire further east into
western Wisconsin, as some models suggest, weaker capping would
favor more widespread storms but the environment wouldn`t be as
supportive of significant severe storms as it is further west. We
will keep a very close eye on trends over the next one to two hours
for a better idea on where/if significant severe weather can
develop.

&&

.DISCUSSION...
Issued at 344 PM CDT Mon Jun 24 2024

The stage is set for an active evening across southern
Minnesota and western Wisconsin. What we`ve been calling a "boom
or bust" set up the past few days is quickly becoming an
apparent "boom". Forecast confidence has increased and supports
convective initiation this evening across portions of southern
and central Minnesota. Current observations pin the warm front
across southern Minnesota and it`s expected to continue to push
north through late afternoon. Temperatures have warmed into the
mid to upper 80s with sfc dewpoints in the low to mid 70s over
SW MN. Overall forecast scenario remains valid with potential
for initial supercells to grow upscale into a bowing MCS and
track southeast into SE MN and W WI. 20Z SPC mesoanalysis
highlights extreme instability building in across eastern
Dakotas, SW Minnesota, and western Iowa. The instability
gradient is roughly from Alexandria to Rochester and will serve
as the "bumper" for any MCS that may develop later this
evening. ACCAS remain across portions of southern and central
Minnesota with a few light showers (or virga). Mid level lapse
rates of 8.5 C/km continue to spread east into Minnesota across
a moisture rich airmass that is resulting in the high-end CAPE
exceeding 5000+ J/kg. The 20Z day 1 SWO update echoed concerns
for potential significant damaging wind gusts (65+ kts) for the
eventual MCS. It also expresses the uncertainty for when storms
initiate. A special 21Z ROAB at KMPX will hopefully shed light
on the strength of the EML with 700 hPa temps 12-14C and if the
CIN has begun to weaken in addition to what instability has
built in. Unfortunately I do not have that data evaluable yet
but the latest ACARS at MSP show a very favorable shear profile
that`s supported in the KMPX VAD. Effective shear values of 50+
kts and SRH of 200+ m2/s2 will be enough to support initial
supercell storm mode. Low level shear will likely improve as the
LLJ strengthens this evening.

Several WOFS runs in a row have highlighted two separate areas of
initiation this evening. The first is south-central MN from 21-23z
and then second is north in central MN 23-01z. Other CAMs have
hinted at two different clusters and this scenario would support the
increased concern for heavy rain across portions of SE MN given the
high-end thermodynamic environment and abundant moisture available.
Initial storms will likely be supercellular in nature and be capable
of producing very large hail, damaging winds, and possibly a tornado
before growing upscale into a MCS with primary concern of damaging
winds downstream. If the split initiation scenario is correct, the
northern/later area will still have plenty of instability to work
with as the strengthening LLJ orientation is favorable to replenish
any moisture behind the first cluster of storms. Both clusters will
likely grow into MCS and turn SE-ward into the gradient with
additional convective development likely across portions of S MN
and W WI. Given the recent stretch of excessive rain and flooding,
low Flash Flood Guidance values, a flood watch was issued for SE MN
and W WI for the potential flash flooding this evening due to
repeated rounds of heavy rain/storms. Severe threat will decrease
after midnight as the complexes exit to our SE. Any lingering
convection should stay sub-severe but an isolated hail threat may
exist in additional the the heavy rainfall threat.

By Tuesday morning, we`ll dry out and have a few days to recover
from this round of storms. Some instability may linger and could
support a few showers in the afternoon. Highs remain in the low to
mid 80s. Wednesday and Thursday are cooler, in the upper 70s and low
80s, but less humidity. Attention turns to the next system progged
for Thursday night into Friday, with Thursday night/Friday morning
looking to be the most likely window for seeing more showers/storms.
The 12z guidance has plenty of variability/spread that makes sense
given we`re 4 days out. The heaviest rain threat appears to favor
Iowa at this time, but we`re still on tap for another round of
widespread rain, whether or not it`s heavy/excessive will be
resolved later on. A tropical airmass (and dewpoints in the 70s)
will stay just to our south, but 12z ECMWF highlights PWAT values
approaching 2.0" by 12z Friday. This will be capable of producing
heavy rain and potential severe given favorable upper diffluence
with a LLJ moving through, so we`ll need to remain vigilant as the
active pattern continues for the next week or so.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 558 PM CDT Mon Jun 24 2024

Large concerns still regarding the potential for severe storms
with destructive wind gusts this evening. Current thinking is
that discrete supercells initially form in central MN and track
east-southeast, quickly growing into a strong bowing convective
system. Terminals most likely to see storms are STC, MPX, RNH,
and EAU. Have delayed storm arrival time by about an hour at
most terminals, though. Have also kept southwesterly LLWS this
evening into tonight at STC, RNH, and EAU. Apart from the storm
potential, VFR conditions are expected the entire period.
Southeasterly winds will turn more southwesterly into tonight
before a cold front moves thru flipping them to west-
northwesterly. Current gusty winds will slow this evening to
near 5 knots and then increase to near 10 knots Tuesday
afternoon.

KMSP...Still thinking storms develop in central MN within the
next hour or two and track into the metro from 01-03Z. Kept with
the prevailing -TSRA from 01-04Z with a TEMPO of +TSRA and very
strong winds (hurricane force wind gusts possible) from 01-03Z.

/OUTLOOK FOR KMSP/
WED...VFR. Wind NW 5-10 kts.
THU...VFR. MVFR/-TSRA likely overnight. Wind S 5-10 kts.
FRI...VFR. MVFR/-TSRA likely morning. Wind S 10-15 kts.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...Flood Watch until 1 AM CDT Tuesday for Anoka-Carver-Chisago-
     Dakota-Freeborn-Goodhue-Hennepin-Isanti-Le Sueur-Ramsey-
     Rice-Scott-Sherburne-Steele-Waseca-Washington-Wright.
     Heat Advisory until 8 PM CDT this evening for Blue Earth-Brown-
     Chippewa-Faribault-Lac Qui Parle-Martin-Nicollet-Redwood-
     Renville-Watonwan-Yellow Medicine.
WI...Flood Watch until 1 AM CDT Tuesday for Dunn-Eau Claire-Pepin-
     Pierce-Polk-St. Croix.

&&

$$

MESOSCALE...DPH
DISCUSSION...BPH
AVIATION...CTG