Areas affected…Northwest KS, Southern NE
Concerning…Heavy rainfall…Flash flooding likely
Valid 120200Z – 120800Z
Summary…Thunderstorms increasing in coverage will expand into
Nebraska and Kansas this evening. Rainfall rates may reach 3″/hr
at times, and despite increasingly fast storm motions, rainfall of
2-4″ is possible. This will likely be enough to produce flash
flooding.
Discussion…Rapidly expanding and cooling cloud tops on GOES-16
IR imagery near the CO/NE/KS line is indicative of a growing
cluster of thunderstorms. These thunderstorms are developing in
response to a surface low pressure analyzed near the CO/KS line,
which will likely move northeast along a warm front which is also
slowly pushing to the north. Additionally, a shortwave is noted in
WV imagery near the northeast tip of CO, which is in tandem with
robust diffluence within the RRQ of a jet streak to the north,
producing enhanced ascent into the region.
The thermodynamic environment is extremely favorable for
convection with heavy rain, and will remain so well into the
overnight. Surface dewpoints are as high as 78F south of the warm
front, with PWATs on the 00Z U/A soundings at LBF and DDC between
1.5-1.7 inches, near the all time highs for the date and more than
2.5 standard deviations above the climatological mean.
Additionally, MUCape is exceptionally high, analyzed by the 00Z
RAP to be in excess of 4000 J/kg across much of KS south of the
warm front, further fueling intense thunderstorm development as
the low and shortwave move eastward. Cyclonically veered 850mb LLJ
from the pooled instability will resupply favorable dynamics into
the discussion area, and as it lifts isentropically atop the warm
front will further focus and enhance rainfall across the area.
CAMs are in good agreement in a large swath of 2-3″ of rainfall,
with pockets of higher rain totals likely. In this region, FFG is
quite low, 1″/1hr and 1.5-2″/3hrs, due to pre-saturated soils from
7-day rainfall departures as much as 400-600%. This suggests that
any heavy rainfall will almost immediately turn to runoff and
flash flooding. These low FFGs and pre-saturated soils are
important to the flash flood risk tonight, as it is likely the
thunderstorms will organize into an MCS in 40 kts of bulk shear,
and will begin to forward propagate as cold pool dynamics become
favored due to somewhat drier air noted in forecast profiles.
Storm motions may exceed 30 kts to the east, so the best flash
flood risk will be near the western portions of the discussion
area and closer to the surface low where mergers/training is
possible, before forward propagation develops later into the
night. The chance for rain rates above 1″/hr will remain high to
the east, but temporal duration of rainfall will likely become
reduced, thus lowering the flash flood potential further east into
KS/NE.
Weiss
…Please see www.wpc.ncep.noaa.gov for graphic product…
ATTN…WFO…BOU…DDC…GID…GLD…ICT…LBF…PUB…TOP…
ATTN…RFC…ABRFC…MBRFC…
LAT…LON 41709939 41699884 41589845 41159786 40609744
40159743 39889769 39559787 39229848 39009938
38850010 38680079 38490126 38360160 38260200
38240233 38350258 38740257 39610240 41130255
41110256 41280232 41270214 41270166 41300110
41450046 41580002