Sleet last night, rain today, colder Wednesday
So what was up with the sleet last night? It was due to a process called evaporative cooling. What happened Monday evening was that liquid precipitation began to fall and evaporate in the dry air that enveloped the ‘Burg. Since evaporation requires energy this process “robbed” heat from the surrounding air and cooled the air temperature a couple thousand feet above the surface down to below freezing. When the air column finally moistened to the point where evaporation stopped the precipitation fell all the way to the ground as sleet. That’s why we saw frozen stuff when our surface air temperature was well above the freezing point.
At any rate today’s rain is mostly over with as another surface low tracks west and north of us pulling warm air into the region and pushing our high temperature to the 60 degree (F) mark under clearing skies this afternoon. This short-lived warmth will disappear when the cold front associated with this system pushes through here around midnight tonight leaving behind breezy conditions that will last into most of the day Wednesday. The westerly winds will quickly usher in another round of cooler air with highs Wednesday through Friday topping out in the low 40′s. The weekend will bring another push of warm air and a few showers Saturday.
A good explanation of one of the factors for our warm winter is provided at this link to a NOAA website. The Arctic Oscillation (AO) is one of the parameters forecasters keep an eye on as a key indicator of upcoming winter temperatures for the East Coast. The value of the AO this winter has remained mainly positive which corresponds to warmer than average temperatures for the ‘Burg, but some recent model runs are indicating that the AO may slide into the negative range over the latter part of January. Could that mean an extended period of colder weather is on the way? I’m in the “show me” camp regarding that, especially after a check of the long range GFS model solution indicates continued warm conditions through the end of January.