As millions of Americans flock to the stores or browse the web to search for holiday gifts, they may also want to keep a close eye on the weather in the coming weeks.

While much of the country has experienced tranquil weather conducive to shopping during the early stages of the holiday season, a pattern change is underway. Much colder air is arriving across the central and eastern U.S. this week, and there will be opportunities for snow to develop during the middle of the month.

For those who tend to wait until the last minute or haven't had the chance to hit the stores, any storms that may form could cause shipping delays or poor travel weather for shoppers headed to shopping malls and other retail outlets.

People wait in line for a Best Buy store to open for a Black Friday sale on Thanksgiving Day, Thursday, Nov. 23, 2017, in Overland Park, Kan. (AP Photo/Charlie Riedel)

AccuWeather's long-range forecasting team is confident that below-normal temperatures will remain throughout much of the East during the next two weeks.

With all of the cold air ahead, it could become a very active period with several chances for snow for the Northeast, Ohio Valley, Great Lakes and Midwest. In addition to a possible storm, or two, in the Northeast later this week, AccuWeather meteorologists are monitoring the potential for a system along the East Coast around Dec.12-13.

Lake-effect snow showers may bring slippery travel around the Great Lakes during the second week of December, while parts of the western Great Lakes could see a stronger lake-effect event during the window of Dec. 11-13.

"Shoppers would be smart to make their purchases and ship as soon as possible, and not wait until it's too late," said AccuWeather Founder, President and Chairman Dr. Joel N. Myers. "We see that typically 5 percent of holiday purchases are shipped last-minute, and we hope that won't be the case this year. For the purposes of convenience, planning and most of all safety, we advise shoppers and travelers alike to download the AccuWeather app and closely monitor changing weather conditions this holiday season."

Myers cautioned that with AccuWeather forecasting wintry and snowy conditions for many parts of the country around Dec. 21, shoppers who wait until then to send holiday packages may risk possible delays which could affect shippers and shipping hubs.

Consumers should also be mindful of slick or slippery conditions as they venture out to shop.

New analysis of proprietary models, which examine weather and economic data, by AccuWeather's D3 Data Driven Decisions analytics group predicts that weather conditions will provide a boost in shopping sales this holiday season.

While the weather will become more volatile, up until this point, temperatures have been seasonable and have allowed plenty of opportunities for consumers to spend.

"Our findings suggest colder, seasonable weather will promote holiday shopping in the northern tier of the country," said AccuWeather Business Intelligence Analyst and Meteorologist Tim Loftus, adding that only during extreme cold would shoppers be more likely to limit their shopping trips.

The holiday shopping season has gotten off to a strong start thanks to a higher-than-expected number of shoppers over the Thanksgiving holiday weekend and good weather, according to the National Retail Foundation.

"From good weather across the country to low unemployment and strong consumer confidence, the climate was right, literally and figuratively, for consumers to tackle their holiday shopping lists online and in stores," said NRF President and CEO Matthew Shay.

Loftus said that on a national level, the higher the maximum average temperature, the more people will shop.

More than 174 million Americans shopped in stores and online during the holiday, surpassing the projected number of 164 million, the National Retail Federation stated. About 20 million more consumers shopped this year, compared to the 2016 Thanksgiving holiday weekend.

"Coming off the heels of this year's Black Friday and then Cyber Monday -- which turned out to be the largest online shopping day in history, resulting in nearly $1 billion more than last year's -- the momentum is set to continue this trend through the holiday season," Myers said. "Obviously, less stormy weather means more in-store sales, and more storms will benefit online retailers."