OPATOVAC (Croatia) — The migrants coming into Europe through the Western Balkans in recent months have been resourceful and adaptable enough to slip around unfriendly police, raging rivers, hostile borders and razor-wire fences. But there is one thing they cannot evade, and that is the looming winter.

Perhaps as soon as late October and certainly by the end of November, the season will shift in the Balkans. Finger-numbing rain, a fall fixture, will descend into snow and freezing winds, complicating and even endangering the arduous journeys starting from Syria and other war-torn nations into the heart of Western Europe.

An unofficial humanitarian corridor, which had been operating for more than a month with the unacknowledged cooperation of the nations involved, had kept asylum-seekers steadily moving through the region, and the summer’s squalid backups at blocked borders had nearly vanished.

But the situation was thrown into flux again with the announcement by Hungary on Friday that it intended to close its border with Croatia.

As long as the migrants kept moving, the countries along the route were able to deal with the number of migrants passing through, refugee officials and aid workers said. And as long as there was a steady flow, the opportunity for tragedy from the impending cold was lessened.

But with fall winds carrying the first hints of frost, and the situation along the borders unresolved, the migrants, aid workers and government officials are anxiously looking ahead. If the numbers increase drastically or, worse, if the border closings expand, there would be an almost immediate backup that would quickly repopulate border camps within a week — some of them open-air, others mostly comprising unheated tents.

“For now, it is OK,” said Mr Uros Jovanovic, manager of a new processing centre being set up in a former psychiatric hospital near the Serbo-Croatian border. “But in 20 days or so, it is going to be very cold here.”

The looming threats have kept migrants on the move, hurrying from border to border to try to reach their destinations — most often Germany and Sweden.

“I am scared, everybody is scared,” said Mr Ali Lolo, 35, a clothing store manager from Damascus, Syria, who waited with his family last week beneath a weather-rippled tarp at the encampment here, where only a few tents are heated. “We are worried they will close the border, but we are also worried about winter. We must get where we are going before the snows fall.”

In previous years, the flow of migrants into Europe slowed to a trickle as winter approached, largely because the Mediterranean becomes especially treacherous in cold months. But with more migrants avoiding the once-popular sea route from Libya to southern Italy in exchange for a shorter, but still potentially dangerous, sea crossing from Turkey to the nearest Greek islands, the numbers flowing into Europe continue in the thousands, with a record 5,800 registered in just one day this month on Serbia’s border with Macedonia.

“The fear of borders closing and winter approaching is just making for a rush, rush, rush,” said Mr Mette Petersen, regional spokesman for the Office of the UN High Commissioner for Refugees.

Governments throughout the region are struggling to find disused buildings, like military barracks or schools, that can be quickly converted into heated housing for migrants, should the path forward become blocked. And aid workers are also trying to assemble a stockpile of blankets, heavy coats and winterised tents, just in case the worst happens.

Migration officials warned that it may be a matter of days until the system collapses under the strain, as the migrants keep crossing from Turkey into Greece and through the Balkans, many saying they want to get ahead of the winter and the possibility of further border closings. Slovenia has estimated it can manage things if Croatia admits up 2,500 migrants a day. But Croatia has asked Slovenia to increase its daily admittance and take in at least 5,000 migrants. Austria, Slovenia’s northern neighbor, insists it cannot handle more than 1,500 people entering the country daily.

“We are facing huge demands from Croatia and severe limitations from Austria,” said Mr Bostjan Sefic, a senior official at the Slovenian Interior Ministry.

The only way that the human flow will run smoothly through Slovenia is if authorities are able to “balance the number of migrants entering the country with the number of those leaving it,” Mr Sefic said.

In Austria, where thousands are requesting asylum while the majority press on toward Germany, officials at the largest reception camp outside Vienna hope to have all of the tents down by the end of the month and shift migrants into heated buildings.

“We were freezing,” said Fadi, 42, a former hair salon owner who arrived at the camp on Oct 5, and who declined to give his last name to protect his family in Syria. “We didn’t sleep. We did exercises all night long to keep warm.”

So far the coldest weather in central Croatia has been last Monday night’s 42 degrees, but the long-range forecast calls for temperatures to fall below freezing this week.

With the open borders, the system seemed to be working.

“You should have seen it here a few weeks ago,” said Ms Ivana Marisavyevic, the Red Cross coordinator in the camp beside the main train station in Belgrade, Serbia. “There were many, many times more people here than there are now. Now, most just stay for a few hours before catching the next bus.” THE NEW YORK TIMES