Mesoscale Discussion 1099
NWS Storm Prediction Center Norman OK
0554 PM CDT Fri Jun 25 2021
Areas affected...northeast Colorado...northwest Kansas...and central
Concerning...Severe Thunderstorm Watch 316...
Valid 252254Z - 260000Z
The severe weather threat for Severe Thunderstorm Watch 316
SUMMARY...Thunderstorms continue across many portions of Severe
Thunderstorm Watch 316. The main threats this evening will be hail
and strong, gusty thunderstorm winds. The overall severe threat
should diminish later this evening with the loss of diurnal heating.
DISCUSSION...Several areas of thunderstorms are ongoing across the
Severe Thunderstorm Watch: one in northeast Colorado, one across
central Nebraska, and across northwest Kansas.
Across central Nebraska, discrete thunderstorms are ongoing along a
convergence zone associated with, and to the south of, a remnant MCV
over north-central Nebraska. Despite weak lapse rates, a warm, moist
boundary layer across the region has allowed surface-based CAPE to
increase to in excess of 2000 J/kg ahead of the ongoing
thunderstorms. Given the degree of instability, coupled with
deep-layer shear around 40 knots, thunderstorms may continue to pose
a severe threat through the evening. Hail and gusty thunderstorm
winds would be the main threats. The threat should lessen with time
this evening as diurnal heating and boundary-layer decoupling reduce surface-based CAPE and the weakening of the convergence zone limits
forcing for ascent.
Across northeast Colorado, several areas of linear thunderstorms
continue to move broadly east toward western Kansas. The strongest
such storms are currently moving northeast through Cheyenne County,
Colorado. Additional thunderstorms are moving southeast across Yuma
County, Colorado, and will enter northwest Kansas shortly.
Instability here is less than areas farther east given a drying of
the lower levels. Consequently, however, greater
temperature-dewpoint spreads should allow for at least some risk for
strong, gusty thunderstorm outflows. This threat should continue for
a few more hours before generally weakening later this evening with cooling/decoupling of the boundary layer.
Across northwest Kansas, thunderstorms have recently developed out
of the persistent cumulus field associated with a weak convergence
zone that is apparent on KGLD radar. Here, low-level moisture is
greater than areas farther east. The result is surface-based
instability between 1500 and 2000 J/kg. However, effective-layer
shear is less than across Nebraska given the lessening influence of
the aforementioned MCV. Current expectation is that these
thunderstorms should move very little this afternoon until being
impinged upon by the outflow associated with the thunderstorms
moving out of Colorado. Given the degree of instability, gusty
thunderstorm outflows and hail will be possible with the strongest thunderstorms. This threat should diminish with time this evening.
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