617
FXUS63 KMPX 242045
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
345 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. &&

.UPDATE...
Issued at 1236 PM CDT Mon Jun 24 2024

Confidence is increasing on thunderstorm evolution this
afternoon and evening, such that a Flood Watch for the potential
for flash flooding as been issued for portions of eastern MN and
western WI.

17Z SPC mesoanalysis highlights a pronounced instability
gradient from Alexandria to Rochester. It is also along this
line where the H7 +12C isotherm is located, which is generally
the level that convection has trouble developing (capped).
Elevated ACCAS/light showers and a few thunderstorms are ongoing
across western MN, although ceilings are greater than 12kft. As
low pressure treks eastward across South Dakota this afternoon,
very rich Theta-e air will advect northward into western and
southern MN. Mid level lapse rates in excess of 8.5 C/km will
spread over this moisture-rich airmass and result in extreme
instability likely exceeding 5000 J/kg. In addition, the 850 mb
jet will strengthen to 40 kts and impinge on the surface front
near the Mississippi River. Despite marginal H7 temps, it is
believed this front and LLJ will initiate convection late this
afternoon across east central MN. CAMs have trended this way in
big fashion today. The initial cluster will form and propagate
southeast along the instability gradient into far western WI
and southeastern MN. Given extreme instability and favorable
shear, very large hail and destructive winds are possible. SPC
has upgraded this area to a level 3 Enhanced Risk. Following the
initial cluster, additional clusters may redevelop again
upstream with the LLJ orthogonal to the front. The LLJ will
replenish the atmosphere and maintain a healthy pool of
available instability. The renewed cluster(s) will also have the
potential for very large hail and destructive winds. Further,
potential training of these clusters appears likely and multiple
CAMs highlight an increasing likelihood of localized totals
exceeding 3 inches from the Twin Cities metro area, southeast
toward Eau Claire, Rochester, and La Crosse. Given the state of
the ongoing flooding, recent rainfall, and thus low flash flood
guidance values, a Flood Watch is necessary for these areas this
evening. Outflow from previous clusters could displace
subsequent clusters slightly farther southwest, so the watch was
given a wide berth to the southwest to account for this
potential. H7 temps may limit this to some degree, however.

&&

.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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1230 PM CDT Mon Jun 24 2024

Our main concern continues to revolve around the potential severe
storms with destructive wind gusts to form this evening. Forecast
confidence has increased to include in the 18Z TAFS. Thunderstorm
initiation is now expected after 21Z across central Minnesota before
congealing into a line of severe thunderstorms that will track
southeast. Potential for impacts include STC/MSP/RNH/EAU. Latest
TAFS use the 12z CAMs to support the earlier timing for TS
development. There`s still some uncertainty with where storms
develop, so kept the TS in a TEMPO for STC for now until there`s
better confidence on if/when/where the cap breaks. Storms remain
possible overnight for AXN/RWF/MKT, but chances at these terminals
are low enough to keep the TS mention out until later updates.

KMSP... Increased confidence that thunderstorms will develop over
central MN after 21Z and tracking across the metro this evening.
Opted for prevailing -TSRA between 22z-04z with a TEMPO of +TSRA and
strong wind gusts (hurricane force gusts possible) from 23z to 01z.
There is still variability in timing of TSRA but TSRA now appears
likely for MSP.

/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.

&&

$$

UPDATE...Borghoff
DISCUSSION...BPH
AVIATION...MPG