Yosemite weather can be unpredictable. Here's how to prepare for summer travel

San Francisco Chronicle​

2 days ago

Yosemite may look like a perfect summer postcard, but its weather can turn fast — and conditions vary dramatically by elevation. Afternoon heat, sudden storms and fast-fading daylight can catch visitors off guard, especially those expecting the kind of consistent, dry warmth typical elsewhere in California.
As peak tourist season continues, here's what you need to know to stay prepared.
Why the weather in Yosemite can be tricky
In elevation, Yosemite spans from roughly 2,000 feet to over 13,000 feet, creating sharp differences in temperature, wind and storm potential. Most summer days are warm and sunny, with low humidity and no rainfall. But that dryness can be deceptive.
'Many people think they can escape the heat by heading into the Sierra, but Yosemite Valley can still hit triple digits,' said Stephen McCoy, a meteorologist with the National Weather Service in Hanford (Kings County). 'That surprises a lot of visitors, especially during a heat wave, when the valley acts like a granite bowl and traps the warmth.'
In the highest-altitude areas, afternoon thunderstorms are common, especially from late July into September, when monsoonal moisture surges into the Sierra Nevada. These fast-building storms can bring lightning, gusty winds and brief but intense downpours.
By late summer, wildfire risk increases. Lightning strikes can spark new fire starts, and smoke from regional fires often drifts into the park, especially when wind and terrain trap it in the valley. Dry lightning is a particular concern during early North American monsoon surges.
This is the park's heat zone. July, August and September highs often top 90 degrees, with triple-digit spikes during heat waves. Exposed trails like the Mist Trail amplify the heat through glare and the granite's radiant warmth. Rain is scarce in summer, with only occasional afternoon thunderstorms. While flash floods are rare, they're possible along river corridors during intense downpours.
During late summer fire events, light winds and frequent overnight temperature inversions mean smoke pools at valley level and lingers. Nights are cooler, with lows often in the 50s and occasional dips into the 40s by September.
Tuolumne Meadows (6,000-10,000 feet)
Located on Yosemite's Tioga Road, Tuolumne Meadows enjoys milder summer temperatures than Yosemite Valley in the daytime. Highs usually stay in the 60s or low 70s and frost is possible overnight, even in July.
In summer, cumulus clouds build after noon near Tioga Pass and Mount Dana, often leading to brief, localized thunderstorms. The open landscape offers little protection, a major risk when storms roll in. These storms usually last an hour or two, bringing rain, small hail and lightning. While summer snowfall is rare, thunderstorms can drop small graupel or hail, sometimes coating the ground in white. Winds here are more active than in the valley, which helps clear wildfire smoke more efficiently.
High Sierra backcountry (above 10,000 feet)
Even on clear days, highs usually don't break the 70s, and overnight temperatures regularly fall below freezing. Many camps above 10,000 feet, like Vogelsang High Sierra Camp, start the day near 30 degrees.
The high country is Yosemite's thunderstorm factory, with storms often bringing heavy downpours. Lightning is abundant in these storms. Many of Yosemite's high peaks have scars from lightning strikes. Dry lightning is a particular concern early in the thunderstorm season as it can ignite wildfires in remote areas.
As snow melts from higher elevations, the ground warms and temperatures peak in late July or August, though even then, a 75-degree day is rare. Snow can still fall in late summer, and early autumn storms sometimes bring snow to the highest peaks.
How to prepare for the weather
The most important thing in staying safe is knowing that Yosemite's weather depends on where and when you are. Forecasts for Yosemite Valley won't help much if you're headed to Tuolumne or the backcountry. Check the National Weather Service's Yosemite zone forecasts, which is available for both the valley and the higher elevations before you go, and pay attention to elevation-specific updates.
'Hikers should always be prepared for extended periods of heat and carry enough water to stay hydrated,' said McCoy. 'Heat has been the leading cause of weather-related deaths nationally for decades. But thunderstorms are also a serious hazard as lightning can strike more than 10 miles from where it's raining, and intense downpours can happen fast.'
Cell service can be limited. Download maps and forecasts before heading out, and consider keeping a radar app on hand for real-time updates in areas with coverage. Above all, start early. Heat builds fast in the valley, and high-elevation storms are most likely from noon to 4 p.m.
Smoke and air quality are harder to forecast, but if fires are burning nearby, expect visibility and breathing conditions to deteriorate, especially in the valley.
Even during heat waves, rivers like the Merced can run dangerously cold, often below 60 degrees. That's cold enough to sap muscle control in minutes and has been linked to multiple fatalities in recent years.
Yosemite, much like the rest of California, has had a cool and dry summer season to this point. However, heat and thunderstorm activity are expected to increase in August and September. Afternoon thunderstorms remain likely in the high country, and fire weather concerns will only grow as fuels dry out.