Mesoscale Precipitation Discussion 0485
NWS Weather Prediction Center College Park MD
1053 PM EDT Fri Jun 21 2024

Areas affected...Southeast SDAK...Southern MN...Northern
IA...Western Wi...

Concerning...Heavy rainfall...Flash flooding likely

Valid 220255Z - 220845Z

SUMMARY...Considerable flash flooding event likely to broaden in
coverage (S MN/N IA) through overnight periods.  Embedded within
locally signficant, life-threatening flash flooding likely across
portions of southeast SDAK/northwest IA.  Additional 3-6" totals
are expected.

DISCUSSION...GOES-E 10.3um EIR already show a developing MCS with
broadening canopy over SE SDAK/SW MN/NW IA with tops cooling below
-65C indicative of the unstable air over 2000 J/kg MLCAPE.  The
synoptic environment continues to support further strengthening as
well as slow eastward advancement with impressive dual jet ascent
outflow channels to spur strengthening surface and low level wind
flow/flux into the developing MCS.  This is strengthening a
surface low in far northeast NEB along a stubborn west-east
stationary front that extends north of LRJ-CAV-OLZ to the WI/IL
border.  Surface to 850mb flow is nearly southerly, orthogonal to
the boundary with VWP 35-40kts of LLJ extending nearly the width
of Iowa.  This is transporting high theta-E air with surface Tds
in the low to upper 70s that remains in the 60s though
850mb...allowing for AoA 2" total PWats and solid flux to support
2"/hr rain rates, occasionally increasing to 2.5-3"/hr with storm
scale/isallobaric convergence centers over the coming hours.

Though there is a subtle dual jet feature in the upper-jet to
support the stronger divergence, the main steering flow becomes
fairly unidirectional through 700-300mb just a bit north of due
east to allow for a prolonged length of training convection. 
While the flow should allow for 25-30kts of eastward motions this
will allow for hours of training with 3-6" totals likely to occur
across S MN/Northern IA.  This area, especially over far SE SDAK,
and of the Iowa border is nearly fully saturated with NASA SPoRT
0-40cm saturation ratios of 80-90+%, so nearly all this will be
run-off or support rapid inundation, this will likely lead to a
broad area of considerable flash flooding with probable embedded
areas of locally significant, life threatening flash flooding
throughout the overnight period.  Areas in north-central Iowa and
southeast MN will see saturation values rise and coverage of flash
flooding incidents are likely to increase, but be a bit less in
coverage/magnitude that points further west.

Of note, but of lower certainty and toward the end of the valid
period (06-09z) The larger scale synoptic trough is exiting the
northern Central High Plains into W SDak, this will once again
bring mid to upper level height-falls and result in some backing
of the low level flow and eastward propagation of the surface low
and frontal zone.  While current convection is exhausting the bulk
of instability, backed flow as well as steepening lapse rates may
maintain instability to generate upstream thunderstorm development
across south-central to south-east SDAK, or more likely allow for
cells to persist/back-build in the vicinity of the surface low/MCV
increasing duration of moderate precipitation over these already
considerably flooded areas; so local trends will need to be
closely monitored for this potential uptick that may result in
further 1-3"+, but again, remains highly conditional/uncertain. 

Gallina

ATTN...WFO...ARX...DMX...FSD...MPX...

ATTN...RFC...MBRFC...NCRFC...NWC...

LAT...LON   44569479 44529242 44069068 42799066 42649262
            42269534 42709680 43089777 43629793 44279664