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Preparation for the hard freeze on Saturday evening – Picayune Item

By Skip Rigney

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

The air near the ground in Pearl River County this weekend has been on quite the journey. Last weekend it was over the extreme north of Canada covered in ice with a temperature of nearly 30 degrees below zero. By mid-week, that same air had been blowing south over the Great Lakes. It continued its merry way towards the Gulf Coast behind a cold front that passed through our region on Thursday. These positions are consistent with NOAA Air Resources Laboratory “HYSPLIT” air parcel trajectory model calculations (www.arl.noaa.gov/hysplit/)

As it moved south, the air gained heat from the increasingly hot earth’s surface and higher daytime sunshine angle, while cooling the land below. -lying. Today (Saturday) that once arctic air will have reached a truly toasty 50 degrees over southern Mississippi.

However, on Saturday night, with no sun to warm the air and no clouds to act as a blanket, heat will quickly be lost to space. The temperature of the earth’s surface and the adjacent layer of air will drop.

With even a little wind, the warmer air several hundred feet above the surface mixed downward in the turbulence, keeping temperatures near the ground a bit warmer. However, the center of a surface high will be overhead on Saturday evening, meaning calm conditions and no downward mixing of warmer air allowing the near surface layer to become increasingly warmer. cold.

All of these factors explain why National Weather Service forecasters expect temperatures in Pearl River County to drop below freezing for about twelve hours from Saturday night through Sunday morning. Poorly insulated water pipes have already lost a lot of heat in recent days. The long duration of freezing temperatures with lows reaching the low to mid-20s means a risk of some of these pipes freezing.

As on many clear, calm mornings, the coldest areas are likely to be in the northern part of the county and at lower elevations, such as the bottoms of rivers, streams, and creeks, in which the coldest air cold flows slowly.

Sunday’s sun will pump some heat into the environment. However, a forecast of calm conditions and sparse cloud for most of Sunday night likely means another freeze, but not as cold as Friday and Saturday nights.

Heavy clouds will return to southern Mississippi on Monday, followed by several hours of rain Monday night as low pressure builds along the old front in the Gulf of Mexico and a new cold front subsides south into the Mississippi River Valley.

The passage of this next cold front, probably Tuesday, will put an end to the rain as well as a reinforcement of cold air for the middle of the week to come. Expect highs in the 50s and further lows near zero from Wednesday through the end of the week.