By Skip Rigney

An air mass from the Arctic brought a hard freeze warning for Friday night. By Saturday evening, the complete disappearance of clouds and wind will set the stage for an even colder night, probably the coldest since last February.

The air near ground-level in Pearl River County this weekend has made quite a journey. Last weekend it was above the ice-covered far-northern reaches of Canada with a temperature of nearly 30 degrees below zero. By mid-week that same air had blown southward over the Great Lakes. It continued on its merry way toward the Gulf Coast behind a cold front that moved through our area on Thursday. Those positions are according to calculations from the NOAA Air Resources Laboratory air parcel trajectory model “HYSPLIT” (www.arl.noaa.gov/hysplit/)

As it moved southward, the air gained heat from the progressively warmer and warmer land surface and from the higher angle of daytime sunshine, while at the same time it cooled the underlying land. Today (Saturday), the temperature of this once-Arctic air will have risen to a veritably toasty 50 degrees over south Mississippi.

However, Saturday night, in the absence of sunshine to warm the air and in the absence of clouds to act as a blanket, heat will rapidly be lost to space. The temperature of the earth’s surface and the adjacent layer of air will plummet.

With even a little bit of wind, warmer air from several hundred feet above the surface would mix downward in the turbulence, keeping temperatures near the ground a little warmer. However, the center of a surface high pressure system will be on top of us Saturday night, which means calm conditions and no downward mixing of warmer air allowing the near-surface layer to get colder and colder.

All of those factors are why National Weather Service forecasters expect temperatures in Pearl River County to fall below freezing for about twelve hours Saturday evening through Sunday morning. Water pipes that aren’t well insulated have already lost a lot of heat over the past few days. The long duration of freezing temperatures with lows bottoming out in the lower to middle 20s means a risk of some of those pipes freezing.

As on many clear, calm mornings, the coldest areas are likely to be in the northern part of the county and in lower elevations, such as river, creek, and stream bottoms, into which the coldest air slowly drains.

Sunshine on Sunday will pump some heat back into the environment. However, a forecast of calm conditions and a scarcity of clouds for most of Sunday night probably means another freeze, although not as cold as Friday and Saturday nights.

Abundant clouds will return to south Mississippi Monday, followed by several hours of rain Monday night as low pressure forms along the old front in the Gulf of Mexico, and a new cold front sags southward down the Mississippi River Valley.

Passage of this next cold front, probably on Tuesday, will bring an end to the rain along with a reinforcing shot of cold air for the middle of the upcoming week. Expect highs in the 50s and more lows near freezing from Wednesday through the end of the week.