Valid 052000Z – 061200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MIDWEST…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN/CENTRAL HIGH PLAINS…
…SUMMARY…
Severe thunderstorm clusters will pose a threat mainly of damaging
winds today, across upper parts of the Midwest, Mississippi Valley
and Great Lakes. Other isolated severe storms with downburst winds
and hail are possible over a portion of the northern and central
High Plains.
…20Z Update…
The northern extent of the Enhanced and Slight Risks has been
trimmed based on the eastward progression of an MCS across northern
WI and Upper MI this afternoon. Scattered to numerous thunderstorms
are still expected to form along a southeastward-moving cold front
across central/southern MN into western WI over the next couple of
hours. Damaging winds appear to be the main threat with this
convection as a linear/cluster mode should dominate. But, some large
hail may occur initially given moderate to strong instability
(MLCAPE of 1500-2500 J/kg) and 35-40 kt of effective bulk shear.
This severe threat will likely spread into parts of IA and southern
WI through this evening.
Farther west, isolated convection is beginning to form along the
Bighorn Mountains in northern WY in association with modest
low-level upslope flow and a weak mid-level perturbation moving over
this region. A small cluster of storms may ultimately evolve out of
this initial development later this evening as it spreads
southeastward ahead of a surface cold front. Both isolated large
hail and severe wind gusts will be possible.
The potential for convection in between these two regimes across
parts of central/eastern NE still appears limited due to persistent
low-level cloudiness hampering diurnal heating and destabilization
to the south of the front. Have maintained low-end severe
probabilities across this region, but this threat appears quite
conditional/uncertain.
..Gleason.. 08/05/2019
.PREV DISCUSSION… /ISSUED 1117 AM CDT Mon Aug 05 2019/
…Upper Midwest and Great Lakes region…
An MCS continues eastward through northwestern WI this morning and
is associated with a progressive MCV that is embedded within the
base of an amplifying upper trough. The moist boundary layer
downstream continues to destabilize with 2000 J/kg MLCAPE expected
this afternoon. Destabilization in conjunction with increasing
mid-upper flow within the amplifying trough suggest intensification
of the current activity and/or redevelopment along the residual
outflow boundary should foster organized severe storms into the
afternoon from northern WI into upper MI. Farther south and west,
across southern WI, MN into northern IA surface dewpoints are near
70 F which will support 2500-3000 J/kg MLCAPE as diabatic heating
of the warm sector continues. Storms are expected to redevelop along
and south of the southeast-advancing cold front this afternoon.
Vertical wind profiles with 30-40 kt effective bulk shear will
support organized storm structures that will eventually evolve into
lines and bowing segments with damaging wind and hail into the
evening.
…Northern and central High Plains…
Severe thunderstorms appear most probable later this afternoon into
early tonight from northeastern WY across parts of southwestern SD
into northwestern NE. Activity is forecast to form over the terrain
of northern WY in a regime of upslope flow and diabatically
minimized MLCINH. Severe hail and wind gusts will be the main
concern, though uncertainty remains regarding storm longevity and
potential support from self-sustaining cold-pool processes.
Convection should follow a moisture/instability axis southeastward
at least into parts of the Black Hills/Sandhills area this evening,
with forced ascent along the cold pool.
$$