By Skip Rigney

December is the first month of meteorological winter, and it looks like it will get off to an appropriately cold start in Pearl River County. Temperatures on Monday and Tuesday nights will likely be near the freezing mark for the first time since February.

For ease of record keeping, meteorologists divide the seasons into three month chunks with winter in the Northern Hemisphere being December, January, and February. Plenty of other folks prefer to define the seasons based on the earth’s orientation relative to the sun. They say that the three months of winter begin when the North Pole is tilted farthest away from the sun. This year that will occur on December 21st at 4:02 a.m.

Regardless of which of those definitions you prefer, Monday and Tuesday are forecast to feel more like winter and less like fall.

For most of November, the jet stream, that river of high winds found three to seven miles above ground, has stayed to our north or to our  west. However, computer weather models show the jet stream moving southward over the eastern United States beginning Sunday and continuing through mid-week.

The new jet stream position will help drive cold air at the surface southward out of Canada toward the Gulf Coast.

Whether or not I need to cover the plants on my back porch, will depend on two things. First will be how much cold air pushes this far south. Second, even if there is a good push of cold air, early morning temperatures drop lower when skies are clear, humidity is low, and winds are calm.

Those conditions are ideal for what meteorologists call radiational cooling. When radiational cooling is working well, more of the sun’s energy captured by the ground during the previous day is lost.

Also, when radiational cooling is optimized, the temperature of the ground and other solid objects, such as windshields, may fall below freezing, even though temperatures at the official thermometer height four to six feet above ground are several degrees warmer. That’s why frost can occur when the official air temperature is in the middle 30s.

If you have your own tender vegetation that might need protection, best to check online with the National Weather Service office for our area on Sunday and Monday for their latest forecast at www.weather.gov/LIX/. For the first freeze of the season, the forecasters in Slidell who are responsible for Pearl River County are likely to issue a Freeze Watch or Freeze Warning.

A watch means conditions are favorable for a freeze within the next 12 to 48 hours.

A warning indicates more forecaster certainty and is usually issued within 12 to 24 hours of the event. When temperatures are expected to be 28 degrees or less, watches and warnings will refer to a “hard” freeze.

From now until December 17th, the public has the opportunity to provide feedback to the National Weather Service regarding their freeze watch and warning products. A link to the online survey is available at the top of the webpage at the NWS Slidell website, www.weather.gov/LIX/.