By Skip Rigney

Winter arrives in the Gulf South tonight.

Rather than using a single date on the calendar every year to denote the change of seasons, I like to tie the passage of seasons to the actual weather we experience and the evolution of the large scale atmospheric patterns that cause that weather.

Early December is a typical time in the Deep South for the transition from the generally cool nights and mild days of autumn to the more frequent cold snaps of winter.

The key to understanding why the transition this year will become evident tonight can be found in the wind patterns in the upper atmosphere.

The strongest winds are often found six or seven miles above sea level. The middle of last week through Saturday the winds at those altitudes were blowing generally from west to east above the northeast Pacific, across the United States, and above the northwest Atlantic.

Long arrows which depict wind direction on the weather map are called streamlines. When the streamlines are more or less straight from west to east for long distances, meteorologists call the pattern zonal flow.

In zonal flow, air tends to stay in the same latitude “zone” as it transits long distances. Warm air tends to stay to the south, and cold air stays to the north.

However, the flow patterns in fluids such as the atmosphere and ocean don’t usually stay zonal for very long. The rotation of the earth, the strong temperature differences between the poles and the equator and from land to ocean, as well as the chaotic turbulence inherent in fluids, cause wiggles or “waves” to develop in the flow pattern.

This happened Sunday when warm air began to flow northward high above the Northeast Pacific Ocean and also on the other side of the continent above eastern Canadian.

The ridges of high pressure associated with that warm air are forecast to strengthen, shift eastward, one over western North America and the other over Greenland, and lock into position for the rest of this week.

Between these two ridges, the cold air over northern Canada is surging southward into the United States with the ribbon of strongest high-altitude winds, which meteorologists call the polar jet stream, digging southward toward the Gulf of Mexico.

Back down at the earth’s surface, these dynamic changes are reflected in a strong cold front racing our way.

In the moist air ahead of and near the front, showers and thunderstorms are likely today and tonight.

The cold front will pass through late this afternoon. Winds will shift from south, to west, and then to northwest.

Wednesday morning is forecast to dawn raw, cloudy, windy, and cold. Temperatures will start in the 40s and will probably not climb much above 50.

Small ripples or waves in the upper wind flow in combination with surface low pressure developing in the western Gulf may cause occasional periods of rain Wednesday through early Friday.

A second, reinforcing surge of cold, drier air will move in on Friday, finally ending our rain chances.

Saturday, and Sunday mornings will be frosty with lows near or below freezing. Those days should be sunny, but the cold air mass will only warm into the middle to upper 50s for afternoon highs.

This week’s weather will definitely remind us that Christmas will soon be here.