Brace yourselves: "Mobile-geddon" is coming.

Starting Tuesday, all mobile Google searches will favor sites that have "mobile-friendly" Web sites. That move could prompt major changes to how Web sites are ranked on your smartphone and tablet -- and have potentially harmful consequences for many businesses, particularly small retailers, that haven't designed their sites to look particularly good on small screens.

This apocalypse, rather uniquely, was scheduled.  Google announced in February on its Webmaster Central Blog that it would tweak its search algorithm to favor Web properties that are "mobile-friendly." Characteristics of a smartphone-ready site include large text that's easy to read on small screens, well-spaced links, and mobile-friendly plug-ins.

(If you want to test your own site, plug the address in here.)

"This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results," the company said in its February blog post.

Brian England, owner of British American Auto Care in Columbia, Md., said that most of the visitors to his repair shop’s Web site come via Google. While he has already optimized his site for smartphones, England feels it is generally taxing—but crucial—to keep with Google’s algorithm changes.

England says he meets regularly with his marketing specialist, only to find that things that worked one month didn’t quite get the same results the next month.

“One of the challenges has been every time Google does a little tweak, suddenly all your analysis work, you have to rework it,” England said.

Google Webmaster Trends analyst Zineb Ait Bahajji  told Search Engine Land last month that the change has the potential to affect search even more than Google's previous "Panda" and "Penguin" updates. Those were aimed at catching "spammy" links but tripped up some news organizations and businesses who found themselves lower on Google search results.

So why another update, specifically focused on mobile? The company explained it like this:  "[Users] will find it easier to get relevant, high quality search results that are optimized for their devices."

On paper, that all sounds great. Mobile traffic now makes up an estimated 60 percent of all Web traffic, according to a 2014 comScore report. And more and more of Google's advertising revenue depends on mobile traffic. Those trends certainly indicate that a majority of consumers have switched over from desktop to mobile when it comes to the way they think about the Web.

But businesses haven't. And that's a problem-- particularly small and medium-sized businesses that simply haven't had the resources or knowledge to keep up with user search trends. (Google's Webmaster Central Blog, after all,  isn't exactly a must-read destination for most people, and almost certainly not for the boutique down the street.)

Roughly one-fifth of small businesses reported having a mobile-friendly Web site in the National Small Business Administration's 2013 technology survey; 18 percent had no Web site at all. A Google/Ipsos survey, also from 2013, put the number of small businesses with no Web site significantly higher, at 55 percent.

And if businesses fall down Google's results for not having a mobile site, that could be a serious problem for them and for any consumer looking for them. The top Google search result gets 33 percent of the traffic, according to a 2013 study by Chitika. If you fall to the second page? Forget it: even the 11th place site on Google results get only 1 percent of the clicks.

Lars Schmidt, founder of Amplify Talent, a consulting firm in Reston, Va., said Google’s move could be “make or break” for small businesses like his, particularly if those companies use search results as a cornerstone of their marketing tactics.

"I think it was important before they made this announcement. I think it’s essential now,” Schmidt said. “Now it has the risk of costing you business."

Google search algorithms do correct themselves over time. But if you want to support that cute little store around the corner, be patient when you're looking for its Web site. Or --if you really want to help -- you may want to point its proprietor in the direction of a talented Web designer.