Weekend Severe Weather Chances
By Hunter Anderson
June 30, 2018 - 8:57 AM CST | 44 1 0
A couple rounds of strong thunderstorms have barely clipped (northwest) Wisconsin over the past couple days. Today and tomorrow most of the state is expected to join in on the stormy activity at least once. Quick notice: we're experimenting with verifying our forecast maps that will be shared with our followers from here on out. Ultimately we hope to create something more automated by the fall in regards to overlaying severe reports over our thunderstorm outlook graphics. Now let's get to those thunderstorm outlooks!
SATURDAY 6/30: SYNOPSIS...
A ridge over the eastern U.S. will remain in place through the period as a trough transitions across the northern plains. A belt of stronger mid-level flow will reside over the Dakotas and Minnesota. Embedded in this belt are multiple perturbations associated with either convective feedback or are synoptic driven. A cold front will extend from northern Minnesota through the central plains by this afternoon.
A boundary layer characterized by mid-upper 70s dew points and surface temperatures into the 90s will be in place ahead of the cold front by the afternoon. Steeper lapse rates aloft this humid environment will yield CAPE values upwards to 4500-5000j/kg. Larger scale forcing for ascent will arrive with one of the more vigorous waves late this afternoon and thunderstorms should fire from Iowa up through western Wisconsin by 5PM. Although deep layer shear will support organized updrafts, the 0-6km vectors are parallel to the source of forcing and thus will create linear storm modes or initial clusters that would transition to linear convective systems. Large hail will be the main threat initially with any robust updrafts then damaging winds will become the main hazard once the cold pool becomes established. Some uncertainty remains as to how far east activity will spread by midnight tonight as well as the severity so kept the general/isolated strong storm wording through eastern Wisconsin.
SUNDAY 7/01: SYNOPSIS...
The ridge out west will eventually flatten out and give way to more zonal flow across the northern Rockies and adjacent Pacific Northwest ahead of the next shortwave over the coast of British Columbia. Out east the aforementioned ridge will remain in place through the period as a negatively tilted shortwave propagates through the western Great Lakes. A strong impulse in the shortwave should initiate strong convection presuming favorable diurnal heating. In the mid levels, 40-50kts of 500mb flow looks likely across Minnesota, Wisconsin, Upper Michigan, and Ontario. At the surface a slow moving cold front will push east throughout the day and should settle from southern Ontario down through the Mid-Missouri valley by the late afternoon.
The timing of the differential cyclonic vorticity advection, large scale ascent, and cold front all seem to coordinate well with the diurnal heating cycle. However uncertainty remains with regards to early morning convective debris and how efficient the environment can rekindle itself by the time the aforementioned sources of lift move through. If enough recovery is realized, SBCAPE values upwards to 2000-2500 j/kg are possible, and deep layer shear vectors are oriented slightly more orthogonal to the forcing of the synoptic scale cold front. This could create a more scattered/discrete mode with transient supercell structures possible with the strongest updrafts. Any residual outflow boundaries could have an affect on near-storm environments as well across eastern Wisconsin. Hodographs are fairly straight, so damaging winds will be the main threat, but a tornado or 2 can't be ruled out at this point especially if a storm can utilize old outflow.
We're looking forward to seeing how well/poor these maps verify and will continue to learn from the forecasting perspective. I don't issue category 3 (red) outlooks often, but if the atmosphere can recover by late morning Sunday, eastern Wisconsin could surprise some people. I feel confident enough at this point to issue the "code red" but all joking aside, it's not the highest category we have with regards to severe weather outlooks. We have another category (magenta) that's used on days where we expect significant and widespread severe.