573
AWUS01 KWNH 170148
FFGMPD
MNZ000-IAZ000-NEZ000-SDZ000-170600-

Mesoscale Precipitation Discussion 0449
NWS Weather Prediction Center College Park MD
948 PM EDT Sun Jun 16 2024

Areas affected...eastern NE, southeast SD, far northwest IA, far
southwest MN

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

Valid 170147Z - 170600Z

Summary...Showers and thunderstorms will slowly increase in
coverage through the evening across Nebraska, and then expand into
South Dakota late. Rainfall rates will steadily intensify to
1-2"/hr, which through training could produce 1-3" of rain. Flash
flooding is possible.

Discussion...A cold front analyzed by WPC draped from
south-central MN southwest into southern NE is becoming a focus
for convective development as noted via cooling cloud tops on the
GOES-E IR imagery. Ascent along this front is being forced by
low-level convergence and isentropic ascent as the 850mb winds
from the south impinge into the front at 20-25kts. This ascent is
occurring into favorable thermodynamics as reflected by SPC RAP
analyzed MLCAPE as high as 4000 J/kg south of the front with a
very sharp instability gradient along it, overlapped with PWs
ranging from 1.1 inches north of the front to as high as 1.8
inches to the south. At the same time, a shortwave is ejecting out
of western NE, while a jet streak lifting northeast into Manitoba
and Ontario will more favorably place the diffluent RRQ over the
region to additionally enhance ascent.

The next few hours remain uncertain, but the past few runs of the
HRRR have become increasingly aggressive with development along
the cold front, which appears to be trending in the right
direction based on current satellite. With synoptic ascent
continuing to increase, and the LLJ surging to as high as 30-40
kts from the south, the overlap of ascent and thermodynamics
should become even more robust resulting in widespread development
across NE. Additionally, as the LLJ strengthens and veers subtly
more to the SW, this will cause Corfidi vectors to locally back to
cause some backbuilding along the front, resulting in slower cells
with longer duration training as the 0-6km mean winds track
parallel to the front. The HREF indicates rainfall rates have a
40-50% chance of exceeding 1"/hr, which through training should
produce stripes of 1-3" of rainfall, with locally higher amounts
possible, along the front.

Farther to the north, the intensifying LLJ will help draw the
higher moisture northward, and SREF 850-700mb moisture flux
anomalies are progged to reach as high as +4 sigma into NE by 06Z.
This will result in intense convergence of the moisture transport
vectors along the nose of this LLJ, resulting in a secondary axis
of elevated thunderstorm development. This axis is likely to
become the primary mechanism for heavy rainfall and flash flooding
overnight, but some development and training until 06Z could
result in isolated flash flooding along this boundary before that
time as well. In general, this will be a priming event across
SD/MN with more significant flash flood potential occurring
overnight.


Weiss

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

ATTN...WFO...ABR...DMX...FSD...GID...GLD...LBF...MPX...OAX...
UNR...

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

LAT...LON   44379629 44009498 43449406 42739383 42299464
            41929562 41289763 40569949 40200041 40210098
            40690115 41360055 41959983 42449944 42870018
            43230109 43860055 44229885