Wintery air arrives later this week

Published 7:00 am Tuesday, December 6, 2016

After several days of gray, showery weather, today and Wednesday will be dry with at least a few peeks of sun, especially on Wednesday.

Afternoon temperatures will be in the mid to upper 60s. Then get ready for much colder dry air to blow in on Thursday.

Friday is forecast to be an even colder day. Although skies will be fair, temperatures Friday morning will start below freezing and will struggle to get out of the 40s during the day Friday.

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This cold air mass began to build over the weekend in northern Alaska and the Yukon Territory of northwestern Canada. By Sunday morning, temperatures on Alaska’s North Slope had fallen to 25 to 40 degrees below zero. Yes, below zero.

Cold air is more dense than warm air. This means that cold air at the earth’s surface is associated with surface high pressure. This was certainly the case Sunday morning in Alaska. Surface pressures were so high that the National Weather Service in Fairbanks, Alaska, issued a special statement for aircraft pilots.

Pilots estimate their altitude by using instruments called altimeters, which measure atmospheric pressure. Because the rate at which pressure decreases as you go higher in the atmosphere is well-known, the aircraft’s altitude can be estimated by measuring the pressure while in flight compared to the altimeter’s setting for the air pressure at the surface.

Sunday’s very high pressures in Alaska could have posed a problem for some aircraft as explained in this statement from the NWS Fairbanks Office: “Very strong high pressure has formed over northeast Alaska this morning and could build even stronger this afternoon.

Altimeter settings could rise to above 31.00 inches over northeast Alaska this afternoon and remain that high into Tuesday.

This could impact aircraft with altimeters limited to 31.00 inches. Pilots will want to take note of the extreme high pressure impacts on their altimeter settings.”

On Sunday a wave in the jet stream over Alaska began to push the cold air southeastward.

By yesterday the cold dome of high pressure had reached Montana. It will continue to surge southeastward across the Great Plains today and Wednesday.

By Wednesday night the cold front marking the leading edge of the arctic air will make its way into our area.

There may be enough moisture near the front to form some showers Wednesday night or Thursday morning, but they won’t be as widespread as the rain we had over the weekend and on Monday.

It will be colder and blustery on Thursday with highs only in the 50s. By Thursday night the centerline of the trough in the upper level jet stream will move to our east, which means winds several miles above us will veer from west to northwest.

That will allow even more cold surface air to filter into the Gulf South.

Of course, the air will have warmed up considerably since it began its journey from Alaska.

But, it will still be chilly by Mississippi standards. Forecasters predict a light to moderate freeze on Friday and Saturday mornings with lows in the middle to upper 20s.

By Sunday we will be riding the temperature and humidity roller coaster back up.

Surface winds shifting to the east and southeast will bring high temperatures by Monday back into the 70s.

By Skip Rigney