Flood Watch 6, NWS Twin Cities, MN

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
300 AM CDT Mon Aug 5 2024

.KEY MESSAGES...

- Widespread rain today with most seeing over an inch of rain.
  Locally heavy totals of over 2 inches possible, especially
  from eastern Minnesota into western Wisconsin.

- Thunderstorms more likely across southern Minnesota with a few
  stronger storms possible with a conditional severe threat.

- Cooler, highs in the 70s, and continued chances for rain for
  the rest of the week.

&&

.DISCUSSION...
Issued at 300 AM CDT Mon Aug 5 2024

Today and Tonight... Rain has already started across Minnesota
this morning and will continue in parts of the state throughout
the day extending into Wisconsin later this morning. The areas
with rain will vary over the day, but the forcing will be
broadly present across both Minnesota and Wisconsin today. The main
axis of rainfall should not be in the same area throughout the
day, so no one should be in a complete wash out. This is all
setup with a low moving across South Dakota into northern Iowa
with a warm/stationary front setup near the border with
Minnesota. This actually is similar to what we often see in a
major winter storm with a favorable area of synoptic lift in the
northeast quadrant of the low (where we will be much of today).
The winter storm similarities continue as a driver of the
higher QPF values will be area of frontogenesis today. This
ascent of warm moist air into this quadrant of the low over us
today will be the driving force for the rain. This has
precipitable water values over the 90th percentile. With all
this said, the next question would be are we going to see
flooding like we did back in June? Looking at soil moisture maps
soils are still wet, but not as saturated as they were back
during the June flooding. They are wet enough though, that a
heavy rain rate could lead to flooding. What could save us today
is that the cloudy and evaporatively cooled air could limit
surface heating such that convection has a hard time occurring.
Without a convective driver high rain rates would be harder to
achieve. Thermal profiles in model soundings show stout
inversions that will be hard to get past for the development of
deep convection. Still CAM solutions like the HREF PMM put a
widespread swatch of one inch of rain with regions of closer to
2 to 3 inches. One area where this does not hold as true is
farther south near the I90 corridor, as here there could be
less synoptically driven rain and therefore higher temperatures.
These higher temperatures will allow for more instability to
build and a better chance for thunderstorms. With ample shear in
model soundings it will not take much of an updraft for a
strong to severe storm to form. So we will have to keep an eye
on this area for CI, as if it can occur a storm could become
strong quickly. This will be a very conditional environment
however so it will be very boom or bust on the convective front.
The surface low will continue tracking across Iowa as we head
into tonight and as we move into the northwest quadrant of the
low rain chances will decrease.

Tuesday through Sunday... As Monday's low moves from the mid
Missouri Valley into the mid Mississippi valley our synoptic
forcing will come to an end and with it the rain. High pressure
will move into the Upper Midwest behind this system. So outside
of a chance for some lingering showers early Tuesday morning,
Tuesday will be a dry day with clearing skies. Tuesday through
the weekend will see highs in the 70s as the cooler airmass that
moved in with today's system remains over us. The next chance
for rain comes on Wednesday as a wave moves across the Upper
Midwest. Forcing favors northern Minnesota with the best chances
to our north for additional rain. We could see rain, but
chances get lower the farther south you go. Past Wednesday
guidance diverges. There is likely another wave moving through
late week, but even the deterministic models have a large spread
so taking an ensemble approach we see an even larger spread.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1258 AM CDT Mon Aug 5 2024

VFR conditions the rest of the night, but MVFR skies and visbys
will develop as rain moves across the region later today. Could
have some thunder at times, especially at KRWF, KMKT, and KMSP,
but for now only have a Prob30 at KMKT. Winds will be out of the
east. IFR conditions are possible this evening.

KMSP...VFR conditions the rest of the night and this morning. As
rain moves in, expect MVFR visbys and ceilings to develop this
afternoon, with IFR ceilings possible later this evening and
overnight. There is also a chance for thunder, but did not
include a PROB30 at this time.

/OUTLOOK FOR KMSP/
TUE...VFR. Wind NE 5-10 kts.
WED...VFR/-SHRA, chc -TSRA. Wind S 5-10 kts.
THU...VFR. Wind NW 5-15 kts.

&&

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

&&

DISCUSSION...NDC
AVIATION...JRB

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