SPC MD 1951



MD 1951 CONCERNING TORNADO WATCH 640… FOR NORTHWESTERN NEBRASKA…ADJACENT PORTIONS OF EASTERN WYOMING AND SOUTHWESTERN SOUTH DAKOTA

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Mesoscale Discussion 1951
NWS Storm Prediction Center Norman OK
0538 PM CDT Tue Sep 10 2019

Areas affected...Northwestern Nebraska...adjacent portions of
eastern Wyoming and southwestern South Dakota

Concerning...Tornado Watch 640...

Valid 102238Z - 110015Z

The severe weather threat for Tornado Watch 640 continues.

SUMMARY...A risk for tornadic supercells is expected to continue and
perhaps increase through 5-7 PM MDT across northwestern Nebraska,
near/southwest of Chadron, with additional strong to severe
thunderstorm development continuing near the state border vicinity,
into areas near and northeast of Valentine NE.  It is not clear that
a new severe weather watch will be needed northeast of WW 640 in the
near term, but trends are being monitored for this possibility.

DISCUSSION...The environment currently most conducive to
boundary-layer based thunderstorm development appears across parts
of the northwest Nebraska into southeastern Wyoming, where moist
southeasterly surface flow appears to be supporting mixed-layer CAPE
of 1500-3000 J/kg.  This is in the vicinity of the surface front,
which extends eastward along and just south of the South
Dakota/Nebraska border.

Convection across southeastern Wyoming has been discrete, including
at least one sustained tornadic supercell to the northwest through
north of Goshen WY, aided by strong deep-layer shear beneath 30-40
kt southwesterly 500 mb flow.  A weak mid-level perturbation is
forecast to emerge from the higher terrain and progress through
broader-scale anticyclonic mid-level flow across western Nebraska
and South Dakota by early this evening.  As it does, ongoing
discrete supercell development is expected to gradually shift
eastward into the northwest Nebraska Panhandle vicinity,
near/southwest through south of Chadron by 23-01Z.  Enlarging
low-level hodographs across this region through this period may
maintain or even support increasing tornadic potential, before
convection increasingly consolidates into a larger convective
cluster.

Considerable thunderstorm activity is already ongoing near/north of
the surface front downstream, into areas west through north of
Valentine NE, with an intensifying southerly low-level jet forecast
to focus strengthening warm advection across this region into early
evening.  Aided by large CAPE, in the presence of steep lapse rates,
and strong deep layer shear, activity will continue to pose a risk
for severe hail. 

Although strongest renewed convective development is expected to
remain focused across northwestern/north central Nebraska and
adjacent portions of southern South Dakota, some activity may at
least attempt to advect northeast of the watch area before gradually
weakening.

..Kerr.. 09/10/2019

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...FSD...ABR...LBF...UNR...CYS...

LAT...LON   43300408 43360214 43570077 43579927 42459980 42010199
            41830342 41910444 42330473 43300408 

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