Updated at 2:50 p.m.: Revised to add Hegar's additional comments, reactions of other state leaders, budget experts

AUSTIN -- Texas' booming economy means lawmakers will have almost $9 billion more in general-purpose state revenue for the next two-year budget, but Comptroller Glenn Hegar warned Monday that the fiscal future is cloudy because of uncertainty in U.S and global economies.

"The economy has been extremely robust," Hegar said, issuing his revenue estimate for the 2020-21 cycle.

But the Republican chief tax collector noted that legislators will have to plug holes in the current budget they passed in 2017 before they start applying some of the additional $8.9 billion of general revenue to new initiatives.

He mentioned IOUs in the Medicaid health insurance program for the poor, state costs for Hurricane Harvey and other unfunded liabilities.

Hegar also said that while the Texas economy "will continue to outpace" that of other states, he sees a "cloudy" fiscal picture.

"The U.S. and global economies are uncertain and any slowdown will affect Texas," he said, noting that stock markets have been turbulent and interest rates have been rising.

Also, trade tensions with China and President Donald Trump's renegotiating of the North American Free Trade Agreement pose unique risks to Texas, which "has more international trade than the other 49 states," Hegar said.

"That uncertainty can cause a question mark of where we're going as a state economy," he said.

Hegar said he expects "continued but slowing expansion."

Going into last session, Hegar projected a 2.7 percent decrease in general-purpose revenue, after accounting for a voter-approved diversion of sales taxes to highways. For the session that begins Tuesday, he's giving budget writers a cheerier forecast -- $119.1 billion in general-purpose revenue, an 8.1 percent increase over the current cycle.

That includes a $4.2 billion cash balance when he closes the books on this cycle on Aug. 31, he said.

Hegar's forecast sets a ceiling on what lawmakers may spend this session as they write the state's next two-year budget.

Rainy day fund available

They also can tap into the rainy day fund. They are expected to do so, at least to defray between $1 billion and $2 billion of state costs from Hurricane Harvey.

Hegar said the reserve, which currently has about $12.5 billion, is expected to grow to $15.4 billion by Aug. 31, 2021, before any drawdowns by lawmakers.

Compared with past revenue estimates, Monday's was "above average," he said. But he cautioned that the double-digit increases in sales tax collections the state has posted in recent times are probably a thing of the past. Sales tax yields more than half of general-purpose revenue.

"We've had [revenue] growth as though we've had an economic boom, but we don't forecast a boom to continue ... because of those uncertainties" about the national and international economies, Hegar said.

Perhaps the brightest spot in his revenue estimate is a guess that the oil production tax will spin off $7.4 billion in state revenue over the next two years, an increase of 11.1 percent over this cycle.

With unpaid bills and a desire to boost state funding of public schools while curbing property taxes, lawmakers face "a tight balancing act, as always," said Hegar, a former state representative and senator from the Houston suburb of Katy.

Confidence in boosting funding

On Monday, Gov. Greg Abbott and Lt. Gov. Dan Patrick promptly issued written statements hailing Hegar's forecast as evidence of the Texas economy's continued good health.

Even so, "Texans expect their government to live within its means, while funding its priorities," Abbott said.

Patrick, the Senate's presiding officer, said he and the senators "are committed to property tax reform, increasing teacher pay and school finance reform. We are confident we can accomplish these goals."

Rep. Dennis Bonnen, the House's likely next speaker, had no comment. Though Bonnen is expected to be elected to lead the chamber on Tuesday, the Angleton Republican has avoided commenting on issues, saying that would be presumptuous.

House Democratic Caucus Chairman Chris Turner of Grand Prairie said the revenue estimate shows lawmakers have enough cash to improve teacher pay, "address health care costs for current and retired educators," reduce college costs and improve Medicaid managed care "to better serve medically fragile Texans."

Talmadge Heflin of the conservative Texas Public Policy Foundation said Hegar's forecast "indicates there is taxpayer money available to fund key budget items while lowering the school maintenance and operations property tax."

Dick Lavine of the progressive Center for Public Policy Priorities, though, said Hegar's estimate will allow modest improvements in school funding but isn't rosy enough for leaders to consider tax reductions.

"There's enough to keep current services going but not so much that tax cuts should be a priority," he said.