The bottom is falling out of economic forecasters’ expectations for U.S. economic growth in the final months of 2012. And if some of the more bearish estimates prove accurate, it will be the weakest rate of growth since the start of 2011.

Macroeconomic Advisers had been projecting a 1.4 percent annual rate of GDP growth in the fourth quarter. On Friday it revised that estimate down to 1.1 percent Friday morning--and then again to 0.8 percent Friday afternoon. J.P. Morgan slashed its number to 1.5 percent from 2 percent. Tom Porcelli of RBC cut his estimate all the way from 1 percent to 0.2 percent.

The damage from Hurricane Sandy is one factor -- but only one - -in weak expectations for fourth-quarter growth

The downgrades are driven by a disappointing report on personal income and consumption Friday morning, coupled with a revision to third-quarter gross domestic product numbers Thursday. That's even aside from the risk that the ongoing debate over the looming "fiscal cliff" might damage consumer spending or business investment in December.

Hurricane Sandy, which devastated large chunks of the Northeast at the end of October, is partly at fault. Sandy affected some of the most populous areas in the nation, so its impact on economic data is bigger than most hurricanes, though it is all but impossible to gauge the exact impact. It dragged on some forms of economic activity (retail sales and industrial production in the areas where things shut down amid power outages, for example) and will bolster others (such as investment spending for rebuilding destroyed homes).

Overall, Michael Feroli of J.P. Morgan estimates Sandy will drag “a few tenths of a percent” off fourth-quarter GDP growth.

But there’s more to the weak fourth quarter than Sandy. One driver of the 2.7 percent growth rate in the third quarter was businesses building up their inventories. As they try to pull those inventories down in the fourth, that will subtract from growth.

Add that up, and Porelli of RBC argues that the risk of a negative number on fourth-quarter GDP is “nontrivial, to say the least.”