Kevin Myatt is The Roanoke Times' weather columnist.

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Oct. 15, 2004

LOTS OF BLUSTER

The new upper low I talk about below has turned into more of a blustery scenario than a rainy one, as a deep surface low tightens over the Great Lakes region. Snow showers appear less likely in the higher elevations, as it now looks like the deepest cold air will arrive after the showery moisture has departed. Still, you can't rule out the first flake or two of the season on some high mountaintop along the Virginia-West Virginia line.

Still, this weather system is part of the ongoing switch from warm season to cool season. The core of the real cold stuff is bottled up in Alaska, though, and it may stay there awhile. Until that comes down, we'll be spared real wintry stuff. I think most of us would like some time for autumn anyway before winter sets in.

Almost every time it rains now, it means that another piece of winter air has dropped in upstairs.

A series of upper-level low pressure areas are digging south in the jet stream, each one pulling cold air a little bit farther south. Just as hurricanes serve to take heat out of the tropics and disperse it northward, the upper-level lows and their associated surface cold fronts help transport increasingly chilly air over the far northern regions southward.

It's a procession we'll be watching with more and more interest as the days get shorter, for, in time, it's these upper-level lows that will trigger any threats of snow and ice.

For the first time this season, the four-letter "s" word that some of you consider a curse and many others a blessing enters the forecast for a few areas of Southwest Virginia over the weekend. It's a scant mention and an iffy proposition, but it's there. More on that later in this column.

First, the matter of upper-level lows. Weather people often talk about upper level lows, but we do little to explain them.

Put most simply: An upper level low is a whirlpool of cold air aloft.

Air currents typically whiz from west to east high above the surface. The most vigorous of these currents are called jet streams, the rapid streams of air roughly six miles up that steer weather systems. Generally speaking, jet streams form the boundary between cool air to the north and warm air to the south. It's this contrast that provides the energy for jet streams.

(There can be two or three branches across the country, so I use the term "jet streams" as a plural above. Usually, there's only one, so I'll use it in singular below.)

As chunks of cold air drop south, or faster pieces of wind enter the jet stream, they pull the jet stream farther south. Depicted on a map, this appears to be an arc in the jet stream. This is called a low pressure trough.

If the trough becomes deep enough, or the energy entering it is strong enough, a part of it can begin to spin counterclockwise (the spin is given to it by the earth's rotation, or the Coriolois effect, a subject to explore in full another day.) It becomes a full-fledged upper level low-pressure system. The upper-level low serves as an impetus for precipitation development. It pulls in moist winds from over the oceans, and surface level warmth and moisture begins rising into the cold air pocket, condensing the moisture into dense clouds.

If the spin becomes vigorous enough, an upper-level low can break off entirely from the body of the jet stream and become a "cut-off low" that goes nowhere fast, spinning incessantly in one place. The upper level spin can also get lower atmospheric levels spinning likewise, potentially spawning a surface low. It's when an upper-level low and a surface low get working in tandem that the most significant storm systems develop. Most major winter storms, large-scale rain systems and severe weather outbreaks — and weather systems that do some of each — in the United States occur in this manner.

If you're reading this on Friday morning, it's probably raining on you, or has recently rained. This would have been caused by the most recent of these upper level lows digging into the jet stream, which is sagging farther and farther south as each new upper low pulls it down, bringing winter a little closer each time. A similar one dragged through Wednesday.

For the first time this season, the cold being pulled down may be enough that some of the highest elevations in our area may see some snow showers this weekend. Northwest winds behind a cold front will enhance the potential for some snow, blowing uphill on the west-facing slopes and squeezing out moisture in a rite of the winter season called upslope snow showers.

One step at a time, one day at a time, one low at a time, Old Man Winter gets closer.