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AWUS01 KWNH 200824
FFGMPD
MNZ000-SDZ000-201430-

Mesoscale Precipitation Discussion 0638
NWS Weather Prediction Center College Park MD
424 AM EDT Sat Jul 20 2019

Areas affected...South Dakota

Concerning...Heavy rainfall...Flash flooding possible

Valid 200830Z - 201430Z

Summary...Thunderstorms developing across South Dakota will become
aligned west-east through the morning. Heavy rainfall rates of
more than 1"/hr are expected, and storms are likely to train north
of a front in Nebraska. Rainfall of 1-3", with locally higher
amounts, is forecast to occur on top of saturated soils and low
FFG. Flash flooding is possible.

Discussion...A surface boundary has dropped into Nebraska early
this morning, leaving E/NE winds across South Dakota. This
boundary also demarcates the exceptionally unstable air (RAP
MUCape > 5000 J/kg) from more modest instability (2000-3000 J/kg)
across the discussion area. Despite the somewhat more stable
surface air across SD, a mid-level baroclinic gradient exists
across the state which is functioning as a convergent boundary to
drive low-level ascent. This is occurring into a broad region of
mid-level frontogenesis, which is overlapped by the RRQ of an
intense 130kt jet streak shifting through Mantioba and Ontario
provinces of Canada, and aided by weak PVA ahead of a shortwave
noted on WV imagery. This deep layer ascent will continue to
produce thunderstorms in the favorable thermodynamic environment,
which should become even more favorable for heavy rainfall as
PWATs surge towards 2 inches, or 2.5 standard deviations above the
climatological mean, on backing low-level flow of 20-30 kts.

With the west-east aligned forcing through a deep portion of the
atmospheric column, thunderstorms should become linearly organized
across portions of southern and central South Dakota, reinforced
by any outflow boundaries, and move eastward along the robust
850-300mb thickness gradient. This setup will be favorable for
training of storms, and the 0-6km mean wind becomes parallel to
both the Corfidi vectors and the mid-level fgen gradient. With the
environment remaining favorable for heavy rainfall, and HREF
neighborhood probabilities for 1"/hr persisting for many hours at
40% or more, training of storms could produce rainfall in excess
of 3" in some locations. This will exceed local FFG, which in some
places is as low as 1" in 3 hours due to 14-day rainfall more than
300% of normal, and HREF exceedance probabilities for 3-hrs peak
above 50% through the morning. Flash flooding could be quick to
develop as heavy rain quickly turns to runoff on top of these
saturated soils

Although the most recent iterations of the high-res CAMs are in
good agreement that a west-east oriented swath of heavy rain will
occur, uncertainty abounds in the exact latitudinal placement of
this feature. Believe the most likely placement will end up along
and just north of I-90 which is roughly collocated with the +12C
700mb isotherm to act as a lid to convection, and this is
reflected by the latest HRRR, ARW2, and NamNest.

Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...ABR...FSD...UNR...

ATTN...RFC...MBRFC...

LAT...LON   45470064 45329927 45139807 44939729 44749688
            44459657 44269644 44039642 43759655 43559696
            43449755 43389831 43379909 43419991 43470064
            43550169 43610224 43680276 43840319 44190337
            44620327 45020293 45270238 45430153


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