It's wildly inaccurate - until it isn't.

The Federal Reserve Bank of Atlanta's computer-driven GDPNow estimator provides one of the most closely-followed forecasts of quarterly U.S. economic growth: It's routinely cited by by stock-market watchers on business channels like CNBC and in publications including TheStreet, the Wall Street Journal, Bloomberg News and the Associated Press.

Historically, GDPNow's forecasts have fluctuated wildly during any given quarter, reflecting not just the emergence of new data -- the inputs to the computer program -- but also the inherent impossibility of perfectly modeling an economy fraught with unpredictable humans, not to mention the natural world.

Yet a close examination of the GDPNow tool's statistical accuracy shows that, right around the end of the quarter, it usually gets pretty close to the official figure reported about a month later by the Commerce Department for growth in gross domestic product. On average, it gets roughly within 0.6 percentage point -- closer than the mean forecast of Wall Street (human) economists.

All this matters now because, with a little less than a month to go before the Commerce Department's next scheduled report on U.S. economic growth, the Atlanta Fed's GDPNow is currently forecasting fourth-quarter GDP growth of 2.7%. Such a reading would represent a marked slowdown from the third-quarter pace of 3.4%, confirming the fears of analysts and traders that growth is slipping as the effect fades from President Donald Trump's tax cuts last year.

With the U.S. budget deficit set to swell by about 25% next year to almost $1 trillion, pushing the national debt past an unprecedented $22 trillion, there's little hope of further economic stimulus from lawmakers. And the Federal Reserve's recent string of increases in short-term borrowing costs, designed to keep inflation at bay, only serves as an additional brake on consumer and business activity.

Some economists worry that the stock market's drop last year, with a 6% decline in the Standard & Poor's 500 Index, could hurt confidence, putting additional downward pressure on growth, as businesses refrain from new investments in plants, equipment and technology.

Watch below: Preparing For a Rough Economy? Here's How to Invest For 2019

The continuing partial government shutdown that began late last year - the result of Trump's insistence on funding for a wall along the southern U.S. border to prevent illegal immigration from Mexico - could prove a further drag on confidence, not to mention the direct effect on some federal employees if they miss paychecks due in coming weeks.

In fact, the Commerce Department's Bureau of Economic Analysis, which produces the quarterly GDP reports, has itself been hit by the government shutdown, so it's unclear if the fourth-quarter GDP report will be released on time. According to a Dec. 22 press release, the bureau's services "will not be available during a lapse except to extent funded by other than current year annual appropriations." Press officials there didn't immediately return a call for comment.

Trump, who is wrangling with Congressional leaders to resolve the standoff, has blamed Federal Reserve Chairman Jerome Powell - his own appointee - for raising interest rates too quickly, and unnecessarily. In a Dec. 24 tweet, the president used a golf analogy to denigrate the central banker, comparing him with a bad putter. The Federal Reserve also has been using the proceeds from maturing Treasury bonds and mortgage-backed securities to pay down its obligations -- an additional source of tighter monetary conditions.   

Ian Shepherdson, chief economist at the forecasting firm Pantheon Macroeconomics, wrote in a report on Wednesday, Jan. 2, that he doesn't expect a recession this year. But there's a lot to worry about.

"Strong nerves will be required," he wrote. "Growth is set to be hit by (take a deep breath) slower growth in China and Europe, the fading of the impact of the tax cuts, falling oil prices, the lagged effect of previous increases in interest rates, the Fed's balance sheet run-down and the ongoing trade war, especially with China. The government shutdown isn't helping."

Bank of America economists put it succinctly in a report in early December: "We are returning to a low-2% economy."

Trust the computer. Even the humans concur.

Follow us on Twitter @TheStreet; TheStreet: Investing Strategies on Youtube or check out our podcasts on SoundCloud at TheStreetLive.

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