First frost almost on schedule

Temperatures in the mid to upper 30s, little or no wind, and clear skies are good ingredients for a light frost, and that's what the National Weather Service is predicting for the immediate Charlotte area Saturday morning.

A frost advisory, which means scattered light to moderate frost, is in effect for Mecklenburg, Union, Cabarrus, Rowan, Catawba, Lincoln, Cleveland and Gaston counties of North Carolina, and for Chester and York counties of South Carolina.

A freeze watch, meaning actual freezing temperatures, has been posted for Iredell, Alexander, Caldwell, Burke, McDowell and Rutherford counties.

The chance of freezing temperatures in Iredell County is probably to the north of Interstate 40, so the southern part of the county is more likely to get frost than a freeze.

No advisories have been posted for Stanly County in North Carolina or for Lancaster County in South Carolina, but scattered frost is possible there, too -- especially in the Lancaster County "panhandle."

First, a little class on frost.

Frost is caused when water vapor in the air freezes. If you see frost on an object, that means the temperature on that object reached 32 degrees.

Temperatures are not uniform. There can be a difference of a few degrees within 100 or 200 feet. Low-lying area tend to be cooler.

So it's possible to have an official air temperature of 35 degrees, yet see frost on some objects. Likewise, it can drop to 32 degrees officially, yet frost doesn't form.

If frost develops Saturday morning in Charlotte, it will be six days early, technically. That's because the N.C. Cooperative Extension Service, out of N.C. State University, says the average first frost in Charlotte (at the airport) comes Nov. 5. But there is a variation of 13 days on either side of that date, so an Oct. 30 frost is not really out of the ordinary.

According to the Extension Service, here are some other average first frost dates for the area:

Albemarle: Oct. 25.

Concord: Nov. 2.

Gastonia: Nov. 1.

Hickory: Oct. 20.

Lenoir: Oct. 21.

Monroe: Oct. 25 (this seems early, given that Monroe is southeast of Charlotte; but the National Weather Service's official observer is in a rural area south of Monroe, so it's possible measurements are made in a low-lying area).

Morganton: Oct. 18.

Salisbury: Oct. 26.

Shelby: Oct. 25.

Statesville: Oct. 19.

Wadesboro: Nov. 4.

The only South Carolina dates I could find came from a seed company, which said the average first frost date in Chester, S.C., is Oct. 20. That seems too early to me, so I discounted that. I'd assume the average date in Rock Hill and Lancaster would be somewhere around the Nov. 5 date.

TROPIC TROUBLE: The tropics are not finished for 2010. Tropical Storm Shary, a relatively weak system, is expected to move into the open Atlantic, encounter cooler water and strong wind shear, and dissipate this weekend.

But there is a stronger system in the deep southern Caribbean that is likely to become a named tropical storm, according to the National Hurricane Center. That system will affect Aruba, Bonaire and Curacao in the next day or two, then slide into the southern Caribbean.

Some long-range forecasters say this system eventually could be a Gulf of Mexico player by late next week. But late-season storms in the Caribbean are difficult to predict, so I wouldn't put much stock in any forecasts yet.