521
FXUS63 KMPX 151154
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
654 AM CDT Sat Jun 15 2024

.KEY MESSAGES...

- Widespread showers and thunderstorms likely today with
  locally heavy rainfall of 2"+ possible.

- An intense heat wave will result in periods of hot and humid
  conditions and multiple rounds of thunderstorms, with the
  potential for severe weather and heavy rainfall.

&&

.DISCUSSION...
Issued at 339 AM CDT Sat Jun 15 2024

This is a very active forecast with three main stories to unfold:
multiple chances for severe weather coming up, anomalously high
rainfall totals leading to potential flooding, and our first
summer-like heat wave.

Severe Weather: There will be on and off chances for thunderstorm
development through Tuesday, with the potential for some of these
storms to be severe. With our setup today, we will see the current
line of storms over eastern Nebraska push eastwards into Iowa. A
broad shield of light precip will move through north of the
stronger, convective storms into the southern half of Minnesota. As
the rain gradually slides into Wisconsin, a couple hours of clearing
could be enough to foster scattered thunderstorms over southern and
east-central Minnesota in the afternoon/evening. The latest CAMS
favor storm development around 5-7pm. These would gradually weaken
and last through early tomorrow morning. Heading into tomorrow, a
frontal boundary will track across MN/WI during the evening/night
time frame. A strengthening LLJ, plenty of available moisture, and
an EML with steep lapse rates along the stationary front, will lead
to a conditional risk of strong to severe thunderstorm development.
The primary threats would be damaging wind and large hail. On Monday
and Tuesday, northwestern and central Minnesota will see additional
chances for t-storms. Strong instability aloft will again be
favorable for large hail in the strongest storms, and any organized
clusters could produce damaging wind gusts.

Rainfall Totals: Following collaboration with neighboring offices
and the WPC, we have highlighted a broad area of slight/marginal
risk for heavy rain leading to flooding concerns across portions of
Minnesota/Wisconsin for the next five days of the Excessive Rainfall
Outlook. Take your pick of any ensemble or deterministic model
output, and you`ll find PWAT values of 2+ inches as the LLJ
helps crank the Gulf moisture into the northern Great Lakes
region later today. This is expected to continue throughout much
of this week, with little time for relief from persistent
rains. The latest suite of ensemble runs are largely in
agreement of a maximum QPF trail stretching from eastern NE/SD
up through northwestern WI throughout the next 7 days. Values in
excess of 8-10" could be possible depending on the exact
location of the heaviest rainfall totals each day. To put that
into perspective, that would be nearly a 1/3 of our average
annual rainfall... in one week. Because of this, flooding will
be a big concern especially given the already wet/active spring
we`ve had.

Heat Wave: There is growing confidence in heat risk concerns heading
into next week as an upper-level ridge builds eastward, bringing
anomalously high h500 heights to the eastern half of the CONUS.
Surface temperatures on Sunday will creep into the lower 90s,
with dew point values in the low-mid 70s. This will result in
heat indices up to 95-100 degrees for much of the area. Given
the fact that it will be the first hot and humid day of the
year, we want the public to be aware and prepared. The Excessive
Heat Watch for the Twin Cities metro highlights the area that
will most susceptible to heat impacts due to the urban effects.
Little relief will follow with overnight lows in the low 70s,
and highs reaching the mid-upper 80s both Monday and Tuesday
most likely.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 654 AM CDT Sat Jun 15 2024

Showers continue to advance northward into Minnesota, but they
are fighting quite a bit of dry air below 10kft. As deeper
moisture arrives, showers will become more widespread and
heavier this afternoon and evening. Some thunderstorms will
develop toward the back end of the showers later today and
tonight, which could impact all TAF sites. Cigs will deteriorate
as the steadier rain moves in and could drop to IFR or MVFR
levels tonight. Skies will clear Sunday morning as hotter air
arrives.

KMSP...Spotty showers for the morning with steadier activity
moving in this afternoon and evening. TS possible tonight. VFR
for much of the day, but MVFR expected later today and tonight.
ESE winds could gust 20-25 kts today, then shift south Sunday.

/OUTLOOK FOR KMSP/
SUN...VFR. TSRA/MVFR likely late. Wind S 10-15 kts.
MON...VFR with MVFR/TSRA likely. Wind SE 10-15G20 kts.
TUE...VFR with MVFR/TSRA likely. Wind S 10-15G25 kts.

&&

.HYDROLOGY...
Issued at 315 PM CDT Fri Jun 14 2024

The extended period of heavy rainfall brings with it a potential
return to river flooding in the 5-10 day period, extending to the
end of the month on the mainstems. HEFS graphics show the potential
threat nicely, and also show the degree of uncertainty with large
ranges even within the blue "most likely" band by day 10.
Factors that will determine flood severity (minor vs. moderate or
worse) include how fast the rain falls...a number of 6-12 hour
periods with less than an inch adding up to 3-5 inches is better
than a couple of 2-3 inch events in a short period. We will also
need to be wary of a boundary setting up with training storms moving
from southwest to northeast over the same area.
In summary, this scenario bears watching closely over the next week,
with lots of uncertainty along the way.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...Excessive Heat Watch from Sunday afternoon through Sunday
     evening for Anoka-Carver-Dakota-Hennepin-Ramsey-Scott-
     Washington.
WI...None.

&&

$$

DISCUSSION...BED
AVIATION...Borghoff
HYDROLOGY...CCS