590
FXUS63 KMPX 200557
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1257 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Multiple rounds of showers and thunderstorms are expected
beginning this evening into Monday morning and Tuesday through
Tuesday night.

- Increasing likelihood of strong to severe thunderstorms Tuesday PM
across southern Minnesota and western Wisconsin. SPC day 3 SWO
brings an Enhanced Risk (3 out of 5) into SE MN.

- Heavy rainfall is likely and totals may exceed 3 inches through
mid-week. This will likely lead to rises on rivers and their
tributaries into the holiday weekend.

&&

.DISCUSSION...
Issued at 312 PM CDT Sun May 19 2024

Radar returns from western to central MN have not been able to reach
the surface thru this afternoon due to very dry low-level air in
place (evidenced in MPX`s 12Z sounding). Returns near the MN/IA
border have had more success reaching the surface, as southerly flow
has begun to advect greater moisture into our region. Isolated to
scattered showers will become more common as we go thru this evening
in response to moisture advection and a strengthening LLJ moving in
from the west. Eventually, clusters of thunderstorms will develop
near midnight along the nose of the LLJ, likely over south-central
to southern MN, and move east-northeast thru WI into Monday morning.
We`re not expecting much in terms of severe weather (owing to the
minimal CAPE and marginal wind shear), but the 12Z HREF LPMM
forecasts most of southern MN and into western WI receiving at least
0.5-0.75" of QPF tonight thru Wednesday. There are also several
embedded streaks of QPF over 1.25" to upwards of potentially 2",
which seems possible if localized areas can receive repeated rounds
of storms. Rain should mostly exit to our east by mid-Monday
morning, while overcast low stratocumulus will be slower to break
up. But, some larger pockets of clearing should occur by the
afternoon allowing temperatures to warm into the low to mid 70s.

Most guidance favors keeping us dry through at least the first half
of Monday night. However, the HRRR, and to extent the GFS, ECMWF,
and GEM, do try to develop convection and QPF over southern MN
(mainly along I-90) where there is a reservoir of 1000-2000
J/kg MUCAPE. This precip would theoretically develop near a
stationary boundary and in response to an approaching upper-
level trough and jetstreak. While shear again looks marginal,
likely leading to a more multi-cell storm mode, a severe wind
gust or hail stone cannot be ruled out. This region has also
been outlined as a Marginal risk (1 out of 5) in SPC`s 1730Z Day
2 Convective Outlook. Tuesday basically looks like a lock for
widespread, accumulating rainfall from the aforementioned trough
developing a strong, synoptic cyclone as it crosses the
Rockies. NAEFS and EPS climatological guidance both show the
surface low becoming abnormally deep and having anomalously high
moisture transport as it travels into the Upper Mississippi
Valley into Tuesday evening. The first round of showers and
thunderstorms looks to occur during Tuesday morning/early
afternoon, moving northeast out of our CWA by mid-afternoon.
This precip should develop underneath divergent upper-level flow
and within the right-entrance region of a downstream jetstreak.
CAMs show a brief break in the precip occurring before another
round of showers and thunderstorms develops over IA (close to
the surface low and frontal boundaries) and moves northeast
Tuesday afternoon/evening. This round will eventually affect the
entire CWA by Tuesday evening leading to more accumulating
precipitation. Another focus with this system will be the
potential for severe weather later Tuesday. With such a strong
synoptic system forecast, the kinematics will be in place for
strong to severe thunderstorms within the impending buoyant warm
sector. Current consensus within the guidance places the warm
sector from southeastern MN into western WI. Indeed, forecast
severe variables and soundings show strong shear and veering
wind profiles, especially close to the warm front. Where the
uncertainty looms is how much daytime heating and thus
destabilization can occur? Mostly cloudy skies should be
overhead from the copious amounts of moisture and lift, so
breaks in the clouds will be necessary to increase CAPE and
improve low-level lapse rates. If some heating can occur, we
could have a potentially dangerous severe weather environment to
monitor. SPC has also recognized this possibility and issued an
Enhanced risk for much of the Midwest, including southeast MN
and southwest WI. A Slight risk extends farther north due to the
uncertainty of the northern extent of the warm sector.

Returning to storm total rainfall with this system through Wednesday
afternoon, the NBM has a mean of 1.25-2" of QPF for most of the MPX
CWA. Increasing to the 75th percentile of the ensemble results in
MPX`s area seeing at least 1.75" with localized amounts approaching
2.5". The 90th percentile is where things get really interesting
where area-wide amounts generally rise to well over 2.75", showing
the high precip potential of this system. Would not be surprised to
see localized streaks of even higher amounts, depending on where
rainbands set up. This will also have be monitored as flooding could
become an issue, especially for low-lying and urban areas.

Guidance shows the cyclone pushing off to our east on Wednesday,
cooling our highs into the 60s and causing precip to taper off.
However, temperatures should recover towards normal to end the week
as another shortwave crosses the Rockies by Friday. This shortwave
looks to bring another good chance of rain to our region during
Friday, though likely not as significant (in terms of amounts)
compared to Tuesday. Looking beyond the period, guidance hints a
couple more chances of precip through the end of May as a continued
train of shortwaves is favored. But, model differences grow during
this time leading to decreased forecast certainty.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1255 AM CDT Mon May 20 2024

A mix of -SHRA/-TSRA continue to make their way
northeastward early this morning. Prior to the "main event"
approaching all sites over the next few hours, an initial line
of convection extends just north of MKT east towards EAU. This
line should move through those sites over the next few hours
before the widespread rain reaches the area. Winds for starters
are light from the east with either a northerly/southerly
component dependent on terminal location with regard to the
initial line of rainfall. Prior to sunrise, winds will quickly
shift to the NW once a trough progresses through. At that time,
cigs will reduce to IFR for before mixing out before noon.
Confidence increasing on more storms possible for MSP southward
but will likely introduce next routine TAF.

KMSP...A line of -SHRA/-TSRA will reach the field ~0730z before the
primary band of precip arrives between 9-10z. Confidence remains
with cigs dropping to marginally IFR levels as the activity pushes
out and then improves to VFR early to mid afternoon.  Will need to
watch for redevelopment of -TSRA this afternoon and will likely be
introduced with the 12z TAFs as confidence increases. Winds becoming
west to northwest by early afternoon near 10 kts.

/OUTLOOK FOR KMSP/
TUE...MVFR or IFR. TSRA likely. Wind SE 10-15G25kts.
WED...MVFR bcmg VFR. Chc -SHRA. Wind W 15G25kts.
THU...VFR. Wind W 10 kts.

&&

.HYDROLOGY...
Issued at 655 AM CDT Sun May 19 2024

A heavy rain event remains on track for Tuesday, with a large
footprint of 1 to 3 inches of rain, with some locations pushing
4 inches. The greatest rainfall totals are expected across
southwest, west central, and central MN. Exactly where and how
much rainfall we see will determine where a threat for river
flooding will exist going into Memorial Day weekend, but given
current rainfall forecasts, flooding during the holiday weekend
may be possible in the Cottonwood, Redwood, Minnesota (upstream
of Mankato), and Crow river basins.

&&

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

&&

$$

DISCUSSION...CTG
AVIATION...Dunleavy
HYDROLOGY...MPG