Lake Michigan Horizontal Shearing Instability Snow Band

 A lake effect snow band occurred along the long axis of Lake Michigan during 19 January 2024, and conditions were apparently favorable for the development of numerous mesovortices that would spin up and propagate toward the south shore with Indiana. Surface observations showed north-northwest winds across the western half of the lake gusting from 20-30 kt, while winds were nearly calm on the east side of the lake. A strong gradient in velocity was thus present near the middle of the lake in the east-west direction. Such a gradient likely aided in a horizontal shearing instability causing the lake effect snow band to roll up into fairly large meso-vortices. A feature associated with these vortices was the enhancement in radar reflectivity and thus snowfall rates along the northwest side of the area of rotation, and decrease in reflectivity/snowfall rates along the immediate southeast side of rotation.

KLOT radar reflectivity from the morning of 19 January 2024 showing the lake effect band moving from north to south across Lake Michigan and rolling up into individual vortices.

Mid-day two panel plot of reflectivity and storm-relative velocity observed by the KLOT radar. Images are annotated in order to show the rotation present within the lake effect snow band, both in the reflectivity plot on the left and storm-relative velocity plot on the right. Also note that reflectivity/snowfall rates are locally highest along the northwest side of the area of rotation.

Surface observations and composite reflectivity showing the lake effect snowband with mesoscale vortices, as well as the strong gradient in wind speed going from the west side of the lake to the east side.

HRRR model output showing strong gusty northwest winds over the west side of Lake Michigan and nearly calm winds on the east side which yielded an intense gradient in wind speed near the middle of the lake favorable for possible horizontal shearing instability.

A RAP model sounding valid at 1900 UTC 19 January 2024 near the location of the lake effect snow band. Steep low-level lapse rates and a saturated airmass favored convectively enhanced lake effect snow, with CAPE most certainly higher after the sounding is modified at the surface for the actual lake temperature and dewpoint.