Analysts forecast that U.S. auto sales in June should be a single-digit percentage point higher than the same month a year earlier, though some now see a slightly weaker second half of 2016.

Automakers report June sales on Friday.

Kelley Blue Book, TrueCar and LMC Automotive are looking for vehicle sales to rise 5% or so for the month. predicted an increase of about 3.8%.

Yet analysts remain divided about prospects for the second half. Some forecast enough strength to push 2016 into record territory, breaking the 17.4 million sales mark for 2015. Others think that Brexit and other macroeconomic factors could slow automotive sales.

"There certainly is a higher probability of having a slightly down year than there was a month ago," Jeff Schuster, an analyst with research firm LMC Automotive, told Bloomberg News. "It's no longer just a leveling off -- it's a potential contraction in the second half of the year."

U.S. auto sales are headed for the possibility of an unprecedented seventh straight year of expansion, a streak beyond any in the country's automotive history. Automakers are eager to keep the momentum going as long as possible; they point to low interest rates, low unemployment and the depth of the last downturn as factors stimulating sales.

Great Britain's decision to leave the European Union, initially greeted by a global selloff in equities, could turn into a negative factor for U.S. auto sales, according to RBC Capital Markets.

"We can't say the selloff in automaker and supplier stocks from Brexit uncertainties has been completely unjustified," Joseph Spak, auto analyst, wrote for RBC Capital Markets. "We foresee meaningful negative revisions to 2017 and beyond forecasts. The exact magnitude remains uncertain as there are still more questions than answers."

Mustafa Mohatarem, General Motors' (GM) - Get Report  chief economist, told reporters Wednesday: "I'll tell you what I told Mary Barra (GM's chief executive officer): We're going to let the dust settle" before coming to conclusions about what Brexit will mean for the auto industry.

Analysts surveyed by Bloomberg forecast that major automakers, with the exception of GM, will post gains for June. The projected seasonally adjusted annual selling rate for the month could be 17.2 million, up from the 17 million rate a year ago. Last month, the rate was 17.5 million vehicles.

Kurt McNeil, GM vice president of sales operations, noted on Wednesday that the No. 1 U.S. automaker in terms of sales has been curtailing low profit sales to rental and other fleets in an effort to improve the automaker's profitability. The tactic likely cost GM U.S. market share in June, although the automaker said it is gaining share of purely retail sales to individual customers.

Toyota (TM) - Get Report , along with GM, is expected to report fewer vehicles sold in June, according to, while Ford (F) - Get Report and the other major producers are thought to have sold more vehicles in June than a year earlier.

Doron Levin is the host of "In the Driver Seat," broadcast on SiriusXM Insight 121, Saturday at noon, encore Sunday at 9 a.m.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.