The snow forecast last night for Lexington had meteorologist predicting anywhere between 1-3 inches to some meteorologistpredicting 4-8 inches. The differing was due to a complicated setup that lead to a difficult forecast. The West side of Lexington got less than half an inch while the East side of Lexington has reports of a little over an inch. We didn’t get as much snow as was expected because the surface temperature wasn’t able to drop below freezing long enough for the snow to accumulate. There was not cold air advection (the process in which the wind blows from a region of cold air to a region of warmer air) with this storm, if temperatures were going to dropbelow freezing it was going to be because of dynamics. More specifically a process we call dynamic cooling, which is a positive feedback mechanism. Dynamic cooling occurs from rising air and heavy precipitation, which works to cool the air from the top down. As air rises it cools, which lowers the freezing level and causes snow to form aloft. That snow falls into warmer air below and melts. Melting is a cooling process because it cools the air around it by absorbing heat. When there is heavy precipitation more melting occurs aloft, which results in more cooling and causes the freezing layer to continue to drop and can reach the surface.
There are many factors that go into forecasting a nighttime low. Cold air advection (CAA), cloud cover, and dew point are a few. With this system CAA wasn’t present and there was heavy cloud cover associated with the rain, which traps outgoing radiation and keeps the surface warmer. For the low to dip below freezingdynamic cooling was going to be the driving mechanism. Temperature cannot fall below the dewpoint, when the temperature equals the dewpoint the atmosphere is saturated. For most of the night the dewpoint was above freezing, it wasn’t until the dewpoint fell under 32 degrees that the temperature was able to fall under freezing. The air temperature did not fall under freezing until 5:05am when it was measured at 31.9 at the Fayette co Mesonet site. At this time the soil temperature was 32.9 degrees and the wind was from the SW at 3mph. This tells me CAA was not the reason for the temperature drop because wind from the south is warm or warm air advection (WAA). Dynamic cooling dropped the temperature of the column of air below freezing, but was unable to cool the surface enough for snow to accumulate.
As we can see from the Kentucky Mesonet, the low stayed above freezing or right below it for most of the state.
Dynamic cooling started cooling the atmosphere below freezing around 10pm and it can be seen on radar.
Radar was showing snow but people would look out there window and see all rain and wonder where the snow was. Radar is not able to see at the surface, the farther away from the radar site a specific location is, the higher up in the atmosphere the radar is sampling. For example in Maysville the radar is showing snow, but at the time there were reports of all rain. The NWS radar in Wilmington samples the atmosphere in Maysville ¾ of a mile above the surface. There was snow aloft overMaysville, but it was melting back to rain before it reached the surface. Lexington Airport received 1.05 inches of rain from the system yesterday. If it would have been all snow using the 10:1 ration we would have gotten 10.5 inches of snow from this event but that was not the case we only ended up with a trace on the West side of Lexington.
