Part of Yosemite National Park closed amid flooding fears

Officials at Yosemite National Park announced Tuesday that part of the park will be closed later this week amid flooding that is forecast in the area as a heat wave in the West causes snow to rapidly melt.

According to officials, most of Yosemite Valley will close at 10 p.m. PDT Friday and will not reopen until at least May 3.

They noted that the closure could last longer, depending on conditions.

“Reservations for lodging and campgrounds in eastern Yosemite Valley will automatically be canceled and refunded,” a post on the park’s Twitter account read.

“Wilderness permits can be rescheduled to alternate trailheads as space allows.”

More information about the closure, including a map of the area, is available on the park’s website at nps.gov/yose.

Flood alerts issued for Yosemite

Just two months ago, Yosemite National Park was buried under 15 feet of snow, homes in Mammoth Lakes had snow depths higher than their roofs, and some ski resorts received so much snow that you could barely see the tops of 35-foot-tall ski lifts.

Now those regions are about to deal with the opposite issue: Too much sunshine and warmth, which is adding an unwelcome boost to the springtime snowmelt process and threatening to overwhelm the region’s rivers and streams.

“We have built up an epic snowpack, (and) temperatures are starting to rise,” FOX Weather meteorologist Britta Merwin said.

“And we have a Flood Watch for the Sierra Nevada and moving into the foothills, and we’re already starting to eat away at that snowpack as we speak.”

Those Flood Watches are now in effect and lasting into at least the start of May for much of the Sierra Nevada as temperatures zoom about 10 to 20 degrees above average.

The areas of Reno, Tahoe, Yosemite Valley, Yosemite National Park outside the valley and Mammoth Lakes — all areas with plenty of winter snowfall — are included in the watch.

How hot will it get in the West?

Temperatures will likely reach their highest levels of the season later in the week as a ridge of high pressure strengthens along the West Coast. 

In California’s San Francisco Bay Area, temperatures in the city could reach the upper 70s, while upper 80s to near 90 degrees is likely in the warmer East Bay areas.

The warmth spreads into the Sacramento and San Joaquin valleys, with highs in Sacramento and Hanford expected in the lower to mid-90s from Thursday through Saturday. 

“By Friday, we have 82 degrees in Susanville (California) — that’s well above 6,000 feet,” Merwin said.

“So this is going to zap our snowpack.”

Even the Pacific Northwest gets in on the heat, with highs reaching the mid-to-upper 70s around Seattle and mid-80s around Portland — both far and away their warmest temperatures of the year so far.

239 billion gallons of snowmelt in Yosemite

Up in the Sierra, temperatures will reach into the 50s through the 70s, depending on elevation.

The FOX Forecast Center predicts highs in the mid-70s for Yosemite Falls this week, just two months after parts of Yosemite National Park were buried in 15 feet of snow and were forced to close for over three weeks.

Some areas around Yosemite National Park saw around 240% of the snow they would normally receive in a winter.

“According to the National Park Service, the snowpack in the watershed around Yosemite Valley is releasing 239 billion gallons of water,” FOX Weather’s Max Gorden said.

“(It’s) creating waterfalls where there’s normally dry rock and swelling the Merced River. Some campsites have started to flood, and water continuously flows over some roads with the ground so saturated. Landslides have also damaged some sections of roads.”

Park officials warned if a heat wave melts the remaining snow too quickly, they might have to close more of the park.

“We can’t know what exactly is going to happen, but we have a lot of data,” Scott Gediman, Yosemite National Park Ranger, told FOX Weather.

“And so we know at certain levels what will be impacted, but there’s a lot of health, life and safety things (we have to watch for).”

Gediman said that means if first responders would be unable to transport people out in case of emergency or if some facilities are closed down, they’d have to shut down the park.

Flooding concerns could spread into Central California river deltas

Right now, lower-elevation rivers in the Central California River deltas are expected to run high but stay within or close to flood banks.

“The Sacramento-San Joaquin River Delta — that’s where this (water) is all going and then eventually dumps out into the Pacific (Ocean),” Merwin said.

“But this water could stack up too quickly and lead to some flooding.”

Rivers in the San Joaquin River valleys are currently in “action” stage, asking area residents and emergency managers to monitor the rivers for potential flooding as water levels run high.

“There could be a change to this (river) forecast,” Merwin said. “Some of the Sierra, we’ve never seen this type of a snowpack, so it’s hard to tell what it’s going to look like when it starts to melt.”

Rivers — and river dangers — running high with cold water temperatures

But it’s not only that the rivers will be running high and fast from the melting snow – they’ll be quite cold with the freshly melted snow, leaving officials concerned people will seek relief from the heat by jumping or perhaps falling into dangerously cold waters while boating or rafting.

“As unpredictable as water can be, it is very attractive this time of year,” FOX Weather meteorologist Amy Freeze said.

“People are going to want to get near it, (and) this is going to be a season where we will have catastrophes because people get too close to this fast-flowing water and they didn’t expect (the chill) when they get inside (the water) or get too close to it.”

The National Weather Service in Reno, Nevada, warned that with water temperatures only in the 40s and 50s, loss of dexterity can occur in just five minutes, while even strong swimmers will lose muscle control in 10 minutes. 

But 20% of those who fall or jump into cold water die in the first 60 seconds due to cold-water shock.

“Wearing a life jacket will significantly increase the chances of survival,” NWS Reno forecasters said.

Cooler weather returns next week

This summer preview returns to more springlike weather next week as the ridge of high pressure fades and temperatures drop closer to more typical early-May readings, perhaps even drifting a bit below average again across the West with a return of rain showers to California.