Prepare to be excited about a blockbuster economic report on Friday morning — but not too excited.

Preliminary data from the Commerce Department is expected to show that United States gross domestic product grew at an annual rate of more than 4 percent in April through June. Some economists think growth may have topped 5 percent — a figure reached only once in the eight years of the Obama administration as the economy recovered from the recession.

While he said he didn’t know the actual number, President Trump didn’t wait to herald the rosy news. At an event in Iowa on Thursday, he said he was expecting a very strong result, noting predictions that ran to 5 percent or higher. “It could be very close,” he said. “Could even happen.”

“We’ll take anything with a four in front,” he added.

[Update: On Friday the government said that the economy grew at a 4.1 percent annual rate last quarter.]

Even a number starting with a four, though, will almost certainly be misleading. Several one-time factors — including a surge in exports tied, at least in part, to Mr. Trump’s trade policies — probably combined to pump up growth in the second quarter. Those effects won’t last, and economists expect growth to slow in the second half of the year. Pretty much no one outside the White House thinks a growth rate of 4 percent is sustainable in the long term.