527
FXUS63 KMPX 201228
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
728 AM CDT Thu Jun 20 2024

.KEY MESSAGES...

- Two excessive rainfall events are expected late tonight
  through Friday morning, with another Friday night into
  Saturday morning. If the heavy rainfall footprints from these
  two events overlap, we could see a high impact excessive
  rainfall event Friday night.

- Though heavy rainfall will be the main threat, the second wave Friday
  night will have the potential for a few severe thunderstorms,
  with large hail and damaging wind gusts from bowing line
  segments driving that severe risk.

- Going into next week, the weather pattern will become more
  progressive, but not necessarily dry. This means fronts will
  come and go as opposed to lingering for days on end.

&&

.DISCUSSION...
Issued at 319 AM CDT Thu Jun 20 2024

Well, hopefully you all enjoyed our 24ish hours of dry weather
yesterday, with our next stretch of wet weather already getting
kicked off this morning across eastern SoDak into western MN. The
main concern over the next couple of days will come in the form of
two heavy rainfall events, late tonight into Friday morning, with a
second, stronger wave for Friday night into Saturday morning. Each
event on their own would likely not pose a significant excessive
rainfall risk, but the concern is the potential for some overlap in
the heavy rain footprints from the two events. If we are to see any
overlap in the heavy rain footprints with these two events, then
where that overlap occurs will have the potential for seeing a high
impact excessive rainfall event Friday night.

Starting off with the here an now, we`ve seen a saturated
environment develop within a zone of upper level diffluence with the
right entrance region of an upper jet up along the international
border. We actually have mostly clear skies from south central into
east central MN and western WI early this morning, which is
associated with a drier airmass in these areas. This dry air will
make it tough to get precip out of western MN through the day, so we
did decrease PoPs today as you head toward southeast MN. The heavier
precip will remain to our west through the day, with the LLJ
expected to remain focused across Nebraska into south central SD.

For tonight we`ll see a weak shortwave eject out of the four corners
region that will help to both strengthen and nudge the LLJ east
through the night. Current expectations are that we`ll see a cluster
of storms develop this afternoon at the nose of the LLJ over central
SoDak, with an elevated MCS riding the nose of the LLJ east through
the night across southern MN. The severe risk looks pretty low with
this round of precip as it`s elevated nature will limit the strong
wind potential, while limited instability keeps the hail threat in
check as well. What it does look to do is put down a swath of heavy
rain, likely a widespread 1-3", with amounts possibly pushing 5" if
we see any training/repeated rounds of thunderstorms. Where exactly
this swath of heavy rain falls is still uncertain. Within the 20.00
HREF you see this heavy rain getting dumped out as far south as
between Sioux Falls and Sioux City (FV3), to as far north as
Brookings, SD up toward Cambridge (NSSLwrf). The current forecast
from WPC favors the HREF mean, with the heavy rain signal for
tonight into Friday morning setting up from roughly Marshall over to
the Twin Cities.

How all of this activity impacts the environment for Friday in terms
of the placement of the surface warm front and the location of
thermal gradient up through h85 will determine where round 2 of
heavy rain goes. With that said, when we see extreme weather
patterns break, they tend to do so with an exclamation point. In the
winter, this takes the form of a prolonged arctic outbreak ending
with lows getting down to around -30F. In the summer, it`s a heat
wave breaking with a day or two with highs topping 100. Taking a
step back and looking at the synoptic setup for Friday night, it
certainly looks to have the potential to be that exclamation mark
for this particularly wet pattern we`ve been in since this past
weekend. Friday, a stronger shortwave will come out of Colorado,
this will increase the strength of the LLJ and associated
IVT/moisture transport. This with MUCAPE values in excess of 2000
j/kg in the warm sector feeding this LLJ with Pwats around 2" (climo
max for this point in June) and you have all the large scale markers
in place to support a high impact excessive rain event, it just
comes down to where the mesoscale details setup. If we get lucky,
the activity from tonight into Friday morning will help push round
two farther south, keeping the worst of it off of the rainfall
footprint from tonight. In a wost case scenario, we see an overlap
of the second rounds heavy rainfall with the first. If this occurs,
then the potential exists for a much more widespread and high impact
excessive rainfall event Friday night. The Moderate Risk from the
WPC day 2 excessive rain outlook highlights where this potential for
doubling up on heavy rainfall events is greatest across south
central MN, though WPC did mention that a further upgrade to a High
risk of excessive rainfall is possible if it does look we will
double up on heavy rain swaths tonight and Friday night. Though
uncertainty remains on exactly where each heavy rain event will
unfold, given the potential high end excessive rainfall impacts we
have over the next couple of days, it will be important to keep up
with any changes to the forecast for the next 48 hours.

Although we`ve kind of brushed over the severe risk, it can`t be
ignored with that second wave. We`ll see a very unstable environment
develop Friday afternoon along the Neb/SD border (around 3000 j/kg
of mlCAPE) ahead of the second shortwave coming at us for Friday
night. This airmass will be advected into southern MN through the
night with the LLJ and although we shouldn`t see that degree of
instability, 2000 j/kg of MUCAPE with 40 kts of 0-6km shear will
certainly support the potential for some stronger updrafts capable of
producing marginally severe hail, with the threat for bowing line
segments supporting a wind threat as well. However, the severe risk
looks to be more of a secondary threat Friday night when compared to
the excessive rain potential.

This weekend, we`ll see the eastern ridge finally break down, which
will allow the front coming through early in the day on Saturday to
push east through the Great Lakes through the weekend. However, we
will remain within the zone of active westerlies, which will
translate into a still active, though this time progressive weather
pattern. We`re starting to see some agreement on when the next wave
arrives, which is looking to be late in the day on Monday into
Monday night, with the next wave in the ensembles showing up at the
end of next week into the following weekend. Given the frequent
frontal passages, we won`t be able to build much heat this far
north, with it looking increasingly likely that the Twin Cities will
make it through June without seeing a high of 90 or warmer. If that
happens, it will be the first time since 2014 (another very wet
Spring/Summer) where we made it through June without seeing our
first high of 90 or greater for the year.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 700 AM CDT Thu Jun 20 2024

Rain over western MN this morning will encounter dry air over
eastern MN and western WI, so it is not expected to make much in
the way of eastward progress today. In fact, weather conditions
should be VFR through today. The one exception will be at EAU
and possibly RNH. For our WI terminals, there are MVFR cigs back
over central WI. East winds will likely bring MVFR cigs into
EAU for a bit this morning, with it less certain whether or not
that make it as far west as RNH. This afternoon, thunderstorms
will develop over SD, with a thunderstorm complex expected to
move across southern MN overnight. Leaned on the timing of the
HRRR to bring the TEMPO TS groups in the 12z TAF. Besides TS
potential, the MVFR/IFR cig potential will increase considerably
overnight as the warm front push north across Iowa.

KMSP...Can`t rule out occasional light rain today, but rain
chances this period will be highest with the complex of storms
expected to move across southern MN tonight. Besides TS chances,
we will likely see cigs drop below 2k feet overnight.

/OUTLOOK FOR KMSP/
FRI...Periods of -RA/MVFR, chc IFR/TSRA. Wind SE 10-15 kts.
SAT...TSRA/IFR likely early, bmcg MVFR in aftn. Wind SW 10-15
kts becoming NW.
SUN...VFR. Wind NW 10-20 kts.

&&

.HYDROLOGY...
Issued at 319 AM CDT Thu Jun 20 2024

Obviously, with the ongoing flooding, the last thing our rivers
need is a couple of more heavy rain events tonight and Friday
night. It should be stressed, that river forecasts from the
NCRFC only include 24 hours of QPF in the forecast, which means
the current river forecasts you see include very little of the
of the rain we are expecting through Saturday (the river updates
Thursday morning will start to include some of this additional
rain). As a result, expect more movement in the crest forecasts
until we get a better handle on how much and where
precipitation falls through Saturday morning. The main impacts
from this next round of rain is that first, we`ll see crests
occur later than currently forecast, with higher forecast levels
possible as well, which isn`t good news given all the points
we`re already forecasting to see moderate or major flooding.
Given the latest WPC QPF, the basins that have the greatest
concern for seeing crests higher than currently forecast are the
Redwood, Cottonwood, Minnesota, Crow, and Mississippi from St.
Paul on downstream.

&&

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

&&

$$

DISCUSSION...MPG
AVIATION...MPG
HYDROLOGY...MPG