ACUS01 KWNS 191237
SPC AC 191236
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0736 AM CDT Fri Jul 19 2019
Valid 191300Z - 201200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS EAST-CENTRAL MN
AND NORTHERN WI...
A regional outbreak of severe thunderstorms with strong tornadoes
and potentially widespread significant wind damage may occur from
central Minnesota across northern Wisconsin during the mid-afternoon
...Upper Midwest/Great Lakes...
Upgrading to Moderate Risk this outlook for the potential threats of
multiple strong tornadoes and a derecho later this afternoon and
evening. CAPE/shear/SRH parameter space in this region is forecast
to be exceptionally rare for mid July amid a predominately zonal
flow regime across the northern states.
A convective cluster is ongoing across north-central SD aided by
warm/moist advection around 700 mb along a pronounced baroclinic
zone. 00Z CAM guidance had a decent handle on this activity earlier
this morning and are consistent with a decaying trend through late
morning as advection shifts east amid pronounced MLCIN as sampled by
the 12Z Aberdeen sounding. There is a low confidence scenario that a
strong enough cold pool is becoming established with this early-day
cluster such that total decay does not occur. If this occurs, robust
boundary-layer heating downstream may be sufficient for an
increasing threat during the afternoon of strong to severe wind
gusts. This could still yield a scattered to widespread damaging
wind scenario, albeit in an earlier time frame than progged by
The more probable scenario is for an extremely unstable air mass
(MLCAPE > 4000 J/kg) to become established to from southeast SD
through central/southern WI as the elevated mixed-layer overspreads
surface dew points rising into the mid to upper 70s. A surface
cyclone over central SD should track towards the Twin Cities through
early evening. Strong low-level convergence near/northeast of this
cyclone along the warm front should sustain surface-based storm
development across central into east-central MN. Once initiation
occurs, intense supercells will develop rapidly, with an attendant
threat for all severe hazards including strong tornadoes amid 50-60
kt effective shear and 0-3 km SRH > 300 m2/s2. The very warm/moist
thermodynamic profiles will also support generation of strong cold
pools which should eventually yield a bowing, forward-propagating
MCS across northern WI into parts of northern Lower and Upper MI.
Strength of the vertical shear suggests the potential for intense
bowing segments capable of yielding a derecho with significant
severe wind gusts.
...Lower Great Lakes...
An MCV progressing east over the Lower Great Lakes this afternoon
should provide enough lift for scattered thunderstorms development.
Air mass over the region will be moderately to strongly unstable but
modestly sheared. Expectation is for predominantly multicellular,
outflow-dominant storms capable of isolated damaging wind gusts.
Another round of 700-mb warm/moist advection will occur tonight
within a tight baroclinic zone well north of a surface front arcing
across NE into northeast WY. Large elevated buoyancy and strong
effective shear will support a risk for isolated large hail. The
risk for severe wind will likely be tempered by a more stable
sub-cloud layer in the second convective round.