. . The 1200 UTC sounding from the morning of 10 Dec indicates that a large portion of the local airmass was warmer than -15C. In fact, the dendrite growth level was centered around 850 mb (1.7 km MSL). We believe that this change came about as follows: 1) mechanical and buoyant mixing combined to transfer latent and sensible heat upward. This mixing created a relatively deep, moist boundary layer over the lake, and 2) vertical mixing was further augmented by the convection within the LES bands, deepened the warm, moist columns to heights well above 700mb. Thus, the airmass within the LES band over Sault Ste. Marie had warmed enough to make capped column and dendrite production likely. Also, the mean mixing ratio between the surface and 850 mb nearly doubled, increasing the potential amount of supercooled water available for riming. Over the surface of the lake, this mean mixing ratio was presumed to be considerably larger.
. . Crystals forming and riming at or above 1.7 km would require roughly 15 - 30 min to fall to a height of 0.5 km, the approximate height at which the winds became light and variable within the near-surface level region of the convergence zone. The winds above that level were northwesterly and fairly strong (winds from sounding -- not shown in figure). At 30 kt, falling particles would advect 8 - 16 nm over a 1.2 km depth. An average of infrared imagery for the period 1145 - 1745 UTC shows that the most persistent cold tops in the vicinity of Y62 to be roughly 22 km (~ 11 nm) west-northwest of the station. That is, the most active portion of the band was in an ideal position for depositing heavy precipitation on the city. There is a second persistent cold top region further west, but that snow would have been falling into the lake.