216
FXUS63 KMPX 211213
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
713 AM CDT Fri Jun 21 2024

.KEY MESSAGES...

- Flood Watch for southern Minnesota (including the Twin Cities Metro)
  and western Wisconsin into Saturday. Multiple rounds of
  thunderstorms will produce widespread 2-4" with up to 5" of
  rainfall possible. If storms can train over the same area,
  significant flooding is possible.

- Additional rain and storm chances in the week ahead, though a pattern
  change will signal the end of the prolonged heavy rain events
  that have become commonplace as of late.

- River flood concerns will extend into next week, with several
  local sites forecast to reach major flood stage.

&&

.DISCUSSION...
Issued at 327 AM CDT Fri Jun 21 2024

Our late week soaker is underway across southwest Minnesota early
this morning. This is the first of two rounds of heavy rain that are
forecast to move through the region through Saturday morning, which
will work in tandem to amplify flooding concerns. Regional radar
shows an elongated area of rain and thunderstorms spanning from
southern Minnesota to central Nebraska. The heaviest rainfall is
occurring near the intersection of the South Dakota/Iowa/Minnesota
borders, where an MCS has developed on the northern edge of the low-
level jet. At first glance, much of the CAM guidance suggests the
heaviest precipitation will drift northeast towards the Twin Cities
(tied to a shortwave in the 700mb flow), though mesoanalysis would
say otherwise. Given radar trends, it appears the complex will
follow more of a west to east track this morning (feeding off of the
925mb moisture transport), which will put heaviest QPF to the
south of I-94. Flash flooding concerns will remain high as the
MCS gradually shifts east this morning, especially for locations
that fall under multiple hours of training thunderstorms. A
widespread 1-2" is expected across southern Minnesota through
midday, with locally higher amounts possible.

The morning round of heavy rain will exit into central Wisconsin
this afternoon, which will bring a bit of break in the highest PoPs
across much of the area. With that said, most of the CAMs continue
to develop scattered showers and storms across the region this
afternoon, likely due to the favorable moisture transport and
ongoing isentropic ascent. A deeper, more defined shortwave and
accompanied surface low is forecast to strengthen across the central
Plains today and will be a focus point for another round of heavy
rain heading into this evening. From a synoptic viewpoint, this 1-2
punch of heavy rain is no surprise given NAEFS PWATs well into the
99th percentile, low-level moisture transport supporting a Gulf-like
tropical airmass, and continued ascent along a warm front that is
forecast to stall across southern Minnesota. The "worst-case
scenario" has been discussed over the last few shifts, where
tonight`s convection lines up over the same locations that observe
the heaviest of this morning`s rainfall. Leaning on the 00z HREF
gives the idea that this scenario may not come to fruition, however
the HREF also heavily favors a more northeast trajectory in the
morning convection/MCS. As mentioned above, the current feeling is
that the morning round will take a more southerly path, so
unfortunately we cannot rule out the "double down" scenario at this
time. As far as round two goes, heavy rain is again forecast to
develop on the nose of the low-level jet in South Dakota/Nebraska
and will move east heading into this evening. This is where the
mesoscale details will become the key players for hydro and severe
weather concerns. SPC`s Day 1 convective outlook keeps a Slight Risk
for severe weather along and south of I-94 in Minnesota and western
Wisconsin. The risk for severe weather appears highly dependent on
two factors: 1. The northern extent of the warm front 2. How much
clearing/dry time occurs after the departure of this morning`s
precipitation. Guidance suggests that moderate instability will
develop south of the frontal boundary, across much of southern
Minnesota. Damaging wind will be the primary concern this
evening/overnight, however hail and a few tornadoes are possible.
The instability gradient itself will be a key to watch as the
afternoon rolls on, as it will likely provide and early insight
into where the heaviest rain will track tonight. Latest model
indications take the bulk of the activity to a more southern
solution, say along the MN/IA border, but enough variability
exists that we`ll have to continue to rely on a more blended
solution. Another 1-2" of rain will fall in the heaviest band
through early Saturday, with locally higher amounts once again
possible. Rainfall rates could approach 0.5-1"/hr. The concern
is that locations across southern Minnesota/western Wisconsin
observe the higher end amounts in both rounds, which could push
storm totals above 5". WPC has maintained a widespread Moderate
Risk for Excessive Rainfall to address the flash flooding
potential through early Saturday which coincides with our Flash
Flood Watch.

Drier weather will return for the second half of the weekend.
Temperatures will climb back into the mid to upper 80s Monday prior
to the arrival of the next cold front that aims to bring the chance
for rain and storms Monday into Tuesday. The upper-level pattern
will remain active heading into next week, however the recent trend
of widespread heavy rain producers will take a back seat for at
least a little while. Our forecast trends on the dry side Tuesday,
Wednesday and Thursday, followed by PoPs climbing late next week.
While we get a break from the off and on heavy rain events, the
concerns with river flooding will persist well into next week.
Multiple sites are forecast to reach major flood stage. A special
hydro section below this discussion contains more information.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 649 AM CDT Fri Jun 21 2024

This is a low confidence TAF set as models have really struggled
with handling this convective environment. At the very least,
tried to reduce the hours of prevailing TS mention we had going
given all of the uncertainties. We will see a broad shield of
rain move through western WI this morning, but confidence is low
with where/when additional rounds of storms will develop. With
the warm front expected to move up to about I-90 this
afternoon, current thoughts is that greatest chance for TS
development this afternoon will be down near that boundary,
with MKT being most at risk for see TS development. Precip
coverage will expand overnight as the LLJ intensifies in
response to a shortwave moving across Neb. There is a lot of
spread with where the cluster of storms that develops tonight
goes, but given what we just saw overnight, we definitely favor
the models that keep the heaviest precip tonight down along the
MN/IA border. Given the southern MN focus for forcing, did keep
AXN/STC dry after this morning during this TAF period. With this
warm front just making it past I-90, the MPX terminals (with
the possible exception of MKT) will remain north of the warm
front this period, which means persistent MVFR/IFR cigs can be
expected.

KMSP...Given the fact the warm front is expected to remain south
of MSP, it does look like the greatest risk for TS this
afternoon/evening will be south of MSP. Given all of the
uncertainties, pushed additional TS chances into prob30s.

/OUTLOOK FOR KMSP/
SAT...TSRA/IFR likely early, bmcg MVFR in aftn. Wind SW 5-10
kts becoming NW.
SUN...VFR. Wind NW 10-15 kts.
MON...VFR. Chc -TSRA/MVFR late. Wind SE 10-15 kts.

&&

.HYDROLOGY...
Issued at 335 PM CDT Thu Jun 20 2024

Main item to pass along is the latest river forecast that were
issued this morning include 48 hours of QPF. This means the
forecast hydrographs on the webpage capture most of they heavy
rain that is expected the next couple of days. As a result,
moderate or major flooding is expected at many of the sites
along the mainstem rivers.

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
MN...Flood Watch through Saturday afternoon for Anoka-Blue Earth-
     Carver-Dakota-Faribault-Freeborn-Goodhue-Hennepin-Le
     Sueur-McLeod-Nicollet-Ramsey-Rice-Scott-Sibley-Steele-
     Waseca-Washington-Wright.
     Flood Watch through Saturday morning for Brown-Martin-Redwood-
     Renville-Watonwan-Yellow Medicine.
WI...Flood Watch through Saturday afternoon for Dunn-Pepin-Pierce-
     St. Croix.

&&

$$

DISCUSSION...Strus
AVIATION...MPG
HYDROLOGY...JRB