Anticipating & Communicating the Severe Threat Near La Crosse, WI: 6/2/2020 - Operations Proving Ground

On the afternoon of June 2nd a shortwave trough was traversing the Northern Plains (Figure 1a). This feature, along with an area of 850-700mb frontogenesis, aided in sustaining elevated convection that had been slowly moving west to east across Minnesota through the day. Meanwhile, at the surface a nearly stationary synoptic boundary extended across southern Minnesota and central Wisconsin. By 20Z, the environment along and south of the boundary was characterized by MLCAPE that increased from ~1000 to 3500 J/kg (Figure 1b), mid-level lapse rates of 8.0-8.5°C/km (Figure 1c), and effective bulk shear of 20 to 35 kts (Figure 1d). While shear was on the lower end of the parameter space most well-suited for severe convection, it was nevertheless sufficient for such a risk, especially since the highest values of shear were juxtaposed along the boundary where lift would be maximized. The potential for severe weather was outlined by SPC’s 2000Z Convective Outlook as an Enhanced Risk in an area extending from southern Minnesota/northern Iowa to Michigan based on the threat of severe hail and wind, potentially significant in some areas (Figure 2).

Throughout the morning, convective allowing model (CAM) guidance had generally suggested that storms would initiate in southwestern Minnesota between 21Z and 00Z (6/3), and then propagate eastward as an MCS thereafter. There were some signs in the CAMs by midday that convection might initiate further east into Wisconsin along the aforementioned surface boundary. Still, the best opportunity for convective initiation (CI) in that area was around or after 23Z.
Prior to 20Z, MLCIN was in excess of 100 J/kg along the extent of the boundary from central Wisconsin through southern Minnesota. The strong cap had suppressed any convective attempts to this point. However, visible satellite imagery between 20Z and 21Z portrayed an increasing cumulus field in that region (Figure 3), indicating a weakening cap and the increasing potential for thunderstorms to initiate. Closely monitoring the Day Cloud Phase RGB continually increased confidence in CI across southeast Minnesota (see FDTD GOES Applications Webinar), even though this was 2-3 hours earlier than anticipated based on previous CAM output. And by 21Z CI did in fact occur across southern Minnesota, including southeast Minnesota near La Crosse, WI. Noting the development of convection and the potential for severe hazards given the aforementioned environmental parameters, NWS La Crosse posted an IDSS graphic which outlined a localized area where an isolated severe threat would be enhanced over the next 1.5 hours (Figure 4). This message was relayed approximately 45 minutes before severe hail reports began being received. Having an understanding of the mesoscale parameters in which thunderstorms were developing allowed the La Crosse NWS office to quickly alter their messaging to highlight changes to the most likely scenario of outcomes regarding the near-term severe potential.

Figure 3: GOES-16 mesosector visible satellite imagery valid 20:00Z - 21:59Z 6/2/20 with surface observation and lightning overlay.

The event was only just getting started though. Convective coverage was poised to increase as a line of elevated thunderstorms approached the synoptic boundary from the northwest and moved toward a prime thermodynamic environment (Figure 5). Although these elevated thunderstorms had produced little in the way of severe weather to this point, that was likely to change as the storms gained access to surface-based effective inflow layer air along the surface boundary. At 2244Z, NWS La Crosse provided a mesoscale update via NWSChat outlining the risks given the environmental parameters. Not only did they continue to highlight the risk for significant hail given the high MLCAPE and steep mid-level lapse rates, they also noted the potential for severe wind gusts in association with the approaching line of storms as it started to encounter higher instability. In addition to the NWSChat update, NWS La Crosse again posted a short-term IDSS graphical update highlighting a localized area of about 8 counties where the severe threat was maximized in the next 1-2 hours (Figure 6).

Around 23Z damage reports from severe wind gusts began in association with the line of thunderstorms in western Wisconsin. Infrared satellite imagery indicated cooling cloud tops and the development of above-anvil cirrus plumes as the line of convection strengthened and transitioned from elevated to surface-based. Additional development occurred along the surface boundary just ahead of the incoming line of thunderstorms. Shortly thereafter the two areas of thunderstorms essentially merged and became a forward propagating MCS that produced numerous damaging wind reports across Wisconsin and Michigan. See the CIMSS Satellite Blog Post on this event for more satellite imagery and information.

Deep understanding of the mesoscale environment and the multiple features at play led to enhanced IDSS from NWS La Crosse via graphics posted to social media and updates on the severe threats on NWSChat. These services were especially important given the changes with respect to the time of impact. Furthermore, the updates yielded a continuous flow of information with regard to localized hazards and served to fill the watch-warning gap. This level of IDSS shows what can be achieved by continually assessing the mesoscale environment, anticipating the short-term convective trends and associated hazards, and translating that into precise, actionable information for our partners and the public.

Special thanks to Dan Baumgardt, SOO at NWS La Crosse, for his review, feedback, and insight on this analysis.