The National Weather Service has upped the ante for an expected snowfall likely to begin after nightfall today and continue until dawn on Saturday, with the heaviest amounts after midnight.

Based on computer models indicating a strengthening weather system approaching the region from the Ohio Valley, the government forecasters issued a winter storm watch calling for a snow and sleet mix of 4 to 8 inches in South Berkshire, and a potential for 7 inches or more in North Berkshire, defined as above the Massachusetts Turnpike.

After a period of rain or a wintry mix this afternoon, the precipitation should change over to all snow in the evening, falling at the rate of an inch an hour at times as temperatures drop to the mid- and upper 20s. Hazardous travel is likely to result, according to meteorologist Hugh Johnson at the NWS office in Albany, N.Y.

Other forecasters were more cautious, with Paul Caiano of WNYT Channel 13 and WAMC Northeast Public Radio anticipating 2 to 4 inches from the storm. AccuWeather.com called for a 3 to 6 inch snowfall total.

"Being this could be the first significant widespread snowfall event of the season and the confidence level for heavy snow (7 inches or more) was right around 50 percent, we went ahead and issued a winter storm watch," Johnson explained in an online post. "It will be a fast moving system, generating a quick and impressive shot of moisture into cold air."

Pittsfield city officials report that the tonight's tree lighting celebration and First Fridays Artswalk are to go on as scheduled beginning at 5 p.m.

After the snow ends on Saturday morning, a cold, dry weekend is expected before another chance of a wintry mix Sunday night into Monday, followed by a very cold work week thanks to an Arctic air mass settling over the region.

At Pittsfield Municipal Airport, only a half-inch of snow has been measured since Nov. 1, compared to a season-to-date average of 8 inches.

To contact Clarence Fanto:

cfanto@yahoo.com or (413) 637-2551.

On Twitter: @BE_cfanto