High pressure will be moving overhead tonight and with very dry air and light winds, the atmosphere will set up for excellent radiational cooling. Radiational cooling occurs when winds are light, skies are clear and dew points are low. Essentially in this atmospheric set-up, heat from the ground escapes easily into the upper atmosphere allowing for a net cooling at the surface. Computer model guidance is calling for lows in the upper 30s but with this pattern we’ll undercut the computers and call for mid-30 by morning. Lower 30s will be possible across the northern parishes of Acadiana and the National Weather Service just issued a freeze warning at press time (634pm) for Allen, Evangeline, St Landry and Avoyelles Parishes.
Even though our “measured” temperatures may not reach the freezing mark, we will likely see widespread frost by morning. On a smaller scale of radiational cooling, a micro-meteorological process of frost formation can occur given the right conditions and only when temperatures reach 32 degrees or colder. On frosty nights/mornings this will occur within an inch (or less) of a given surface. When winds are light and air is dry moisture from the ground evaporates which uses energy/heat. The energy/heat used in the process escapes into the atmosphere allowing for a net cooling at the surface. This is why we see frost. But we “officially” measure temperature 6ft above the ground so even though we may not have freezing temperatures measured, the frost process can occur within a much smaller scale, normally within a fraction of an inch from the ground or any exposed surface that is not heated such as car tops, house roofs etc. This can also happen on leaves of a plant, so that’s why it’s prudent to cover the plants and strop the process of radiative cooling when we have temperatures in the mid-upper 30s. In fact, given the right atmospheric conditions, frost can occur when “official” temperatures are in the lower 40s! After a frosty start tomorrow expect a nice moderating temperature trend into the weekend. Wednesday night/Thursday morning could be frosty again mainly in northern areas with the projected low close to 40…but that could change depending on our winds. Otherwise expect highs to get back into the 70s for Thursday afternoon and reach the upper 70s through the weekend. Overnight lows will moderate back into the upper 40s to lower 50s for the weekend.