Other - Snow Squalls and Snow Squall Warnings

When we speak of a ‘flash freeze’, we are describing a situation in which wet pavement turns icy. At the initial time on the left, we have a snow squall – say along an arctic boundary - producing a brief period of heavy snow. The pavement may be above freezing, in which case, snow falls and initially melts on contact. With strong low-level cold advection in the immediate wake of the squall, and diabatic cooling associated with melting, water freezes on the road forming a dangerous layer of ice. This results in treacherous travel conditions in the period even after the lowest visibilities and heavy snow have past. I should also mention that sometimes even with cold pavement temperatures, that frictional warming beneath tires can cause brief melting which then refreezes on sub-freezing road surfaces. This frictional effect can be especially true in high traffic areas where braking occurs. If roads are untreated, the road hazard will be worse. While the presence of road chemicals can mitigate the potential for flash freeze occurrence, the forecaster generally doesn’t know which roadways are treated.