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AWUS01 KWNH 210805
FFGMPD
MOZ000-IAZ000-KSZ000-NEZ000-211400-

Mesoscale Precipitation Discussion 0647
NWS Weather Prediction Center College Park MD
405 AM EDT Sun Jul 21 2019

Areas affected...Southern and Central Nebraska

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

Valid 210800Z - 211400Z

Summary...Thunderstorms expanding along a stationary boundary will
fill in across Nebraska through the morning. Slow storm motions in
an environment favorable for heavy rain will produce 2-3" of
rainfall, with local amounts up to 5". Flash flooding is possible.

Discussion...Regional radar mosaic this morning shows 2 mesoscale
convective systems (MCS) encroaching on the discussion area. The
first is associated with an MCV exiting KS into western NE, while
the second is a long lived trailing stratiform MCS dropping slowly
SW from northern MO. Between these two features, a convectively
enhanced stationary front is draped west to east across southern
NE, along which new convection is developing from both directions.

There is good model consensus that storms will unzip along the
length of this front from the west due to upper diffluence and
weak PVA, as well as from the east due to backbuilding of
convection along the outflow from the eastern MCS. A 25-35kt 850mb
LLJ measured by area VWPs is supporting persistent moist advection
and providing the impetus for destabilization as MUCape climbs to
2000-3000 J/kg along the boundary. The large scale ascent through
low-level convergence, isentropic upglide of the LLJ, and upper
diffluence will continue to support blossoming of thunderstorms
along the boundary as the environment continues to be resupplied
by moist unstable air from the south. The unimpeded LLJ into the
region suggests convection will be long lived, at least through
daybreak, before the typical early-day minimum develops towards
this end of this MPD valid period.

With PWATs near 2 inches and instability robust, rain rates have
been estimated by KUEX WSR-88D to exceed 2"/hr, and HREF
neighborhood probabilities maintain modest chances for these rates
to continue into the early morning. The boundary and best moisture
convergence appears to be a little south of the HREF members
forecast, and closer to recent HRRR runs. However, the exceedance
probabilities for 3-hr FFG and 6-hr/5yr ARI reach 30-40%,
suggesting that even though FFG across this area is relatively
high, it can be exceeded. This will be most likely where nearly
stationary storm motions occur to prolong the duration of heavy
rain, or in the vicinity of storm mergers which could at times
further enhance the rain rates. High-res CAMS are in very good
agreement that a stripe of 2-3" of rainfall will occur across this
area, with some pockets of 5" possible. This may lead to flash
flooding, especially if it occurs across areas where NWM soil
moistures are already nearly saturated over south-central NE.

Weiss

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

ATTN...WFO...EAX...GID...LBF...OAX...TOP...

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

LAT...LON   41919862 41809767 41649686 41469615 41219570
            40939533 40689516 40419512 40209518 40079533
            39999559 40019588 40069636 40119702 40179804
            40199861 40249929 40430007 40610057 40840083
            41100082 41590032 41849974 41909929


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