Winter is upon us.

Two feet of snow blanketed Mauna Loa and Mauna Kea by late Friday afternoon, closing roads and trails to both summits, and another 2 feet could fall by Sunday, a National Weather Service meteorologist said.

“Visitors were delighted, especially those who don’t realize we get snow in Hawaii,” said Hawai‘i Volcanoes National Park spokeswoman Jessica Ferracane.

At lower elevations heavy rain, thunderstorms and flooding will continue this weekend as two weather systems move across the islands.

Matt Foster, lead forecaster at the National Weather Service, estimated that at least a couple of feet of snow had fallen from Thursday to late Friday afternoon at and above the 11,000-foot elevation, with another 6 to 12 inches likely between Friday night and this morning.

“It shuts off, and it’s back on Sunday for maybe another foot of snow,” he said.

The weather service issued a winter storm warning Friday until 6 p.m. today.

“There’ll be a little bit of a pause as activity shifts to the western end of the state tomorrow,” Foster said.

An upper-level system was moving across the eastern end of the state Thursday and Friday, causing the atmosphere to destabilize and bringing thunderstorms and snow to the summits, he said.

Hawaii County Civil Defense urged residents and visitors to monitor weather forecasts and to prepare for flash flooding. A flash flood watch, in effect until Sunday afternoon, replaced Friday’s warning. Campers and hikers are urged to postpone outings until the weather improves.

The first system will bring wet weather through this morning to windward areas of Maui and Hawaii counties, as well as the Koolaus on Oahu.

Then moisture will push up from the southeast, bringing some heavy showers and thunderstorms this afternoon to Oahu and Kauai, primarily in leeward and interior sections.

Foster said it’s hard to pinpoint the specific timing and locations of these events, saying, “Random things pop up here and there.”

On Sunday, as the second system moves east, there’s a good chance the precipitation and flooding will recur on Hawaii island and Maui.

Hawai‘i Volcanoes National Park closed the summit of Mauna Loa for day use and overnight camping. Also closed was Thurston Lava Tube, the floor of which was flooded, requiring the power to be shut off, and the Kahuku Unit, also due to flooding and a road closure.

The Mauna Loa summit closure applies to areas above the Red Hill Cabin. Hikers are permitted to go to Red Hill but are advised to use caution and carry appropriate gear.

Several Big Island roads were closed Friday due to flooding, including Kahikopele Road in upper Puna and Kamehameha Avenue in Hilo. Inbound traffic on Hilo Bayfront Highway was diverted.

On Kauai one lane of Kuhio Highway, which flooded near the Hanalei Bridge, was closed for several hours until about 5:40 p.m. Friday, police said.

The state Department of Health issued a brown- water advisory for Oahu, warning the public to keep out of floodwater and storm runoff due to overflowing cesspools, sewer manholes, chemicals, animal fecal matter, pathogens and dead animals associated with flood debris.