WASHINGTON — Economic growth is gaining momentum, with factories busy and service firms expanding, but one critical area still lags: job creation.

The Labor Department will issue its January jobs report today, and economists are forecasting that it will show only modest hiring. Employers are expected to add a net total of 146,000 new jobs. That’s barely enough to keep up with population growth. The unemployment rate is likely to tick up to 9.5 percent from 9.4 percent in December.

Some analysts are more optimistic and think the job gains could be larger, after several positive economic reports were released Thursday. The service sector, which employs nearly 90 percent of the work force, expanded at the fastest pace in five years last month, retail sales increased, and factory orders grew in December.

“Activity across the broader economy is picking up,” said Neil Dutta, an economist at Bank of America Merrill Lynch. “Momentum is improving.”

The harsh winter weather, however, is a wild card and may temper January’s employment gains.

Even broad economic growth may not mean a sharp increase in new jobs anytime soon.

Dutta, for example, points out that companies are spending much more on new computers, machines and other equipment than on hiring. That trend is likely to continue this year, he said, making it harder to bring down the jobless rate.

It takes about 125,000 new jobs a month just to keep up with population growth. The economy needs to add more than double that amount to make a significant dent in the unemployment rate. In the past three months, job gains have averaged only 128,000.