Strong Tornadoes and Potentially Widespread Significant Wind Damage Possible
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 to evening.
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…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.
…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.
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 guidance. 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.
..Grams/Gleason.. 07/19/2019 – SPC