Graupel storm hits the White Mountains, leaves a coating of white

August 24, 2012

"Graupel" covers the tops of the White Mountains on Wednesday, Aug. 22. Photo/Leslie Willoughby

It was supposed to be a hot, sunny week up here.

All week.

All the meteorological models showed relentless sun. None showed thunder, lightening, rain.

Yet at about 1:30 a.m. on Wednesday, Eastern Sierra residents sprang from their beds after a crash of thunder started off a spectacular lightning storm that lit the night sky for hours and dumped an inch of rain.

By 7 a.m., when the clouds had cleared, the White Mountains were cloaked in a thick, white blanket that looked very much like snow.

But it was not snow. Rather, it was very high altitude snowflakes coated in moisture and frozen, falling from the sky in tiny, round balls that look like hailstones but are actually something called “graupel.”

It was not supposed to be there.

Every model—from the National Weather Service to Howard Sheckter’s website to the Weather Channel to Weather Underground—got it wrong.

“I rarely see the models screw up this badly in a 24-hour period,” said Sheckter, Mammoth’s well-known amateur weather forecaster.

“It was not because nocturnal storms were unexpected, but they were expected farther south of us, with the forecast for Wednesday actually calling for isolated to widely scattered thunderstorms.”

“That system was expected to come in over Paso Robles some 150 miles to the south of Monterey Bay producing Tuesday night’s storm effects well into the south of us,” Sheckter said.

Instead, the system came in over Monterey Bay, almost directly west of Mammoth, he said.

“This is what actually happened,” Sheckter said. “As the system approached Monterey Bay, a weak upper jet, cyclonically curved, of some 50 knots at roughly 35,000 feet, produced strong vertical motion (rising air), and the lift necessary for the thunderstorms that developed over the Southern Sierra, which then moved north over the Central Sierra (where Mammoth is).      

“There was deeper moisture in places over the White Mountains and Nevada, with additional supercooled air in a richer moisture environment. Although the freezing level was approximately 17,000 feet over the Whites at the time, the strong convection that was initiated by the upper jet produced several inches of graupel above 12,500 feet.  

“Thunderstorms here in the Sierra are fairly rare after midnight, as most of the daytime heating has dissipated,” he said. “They can only develop with the aid of some dynamic lifting process such as what occurred Tuesday night.”

“Graupel,” according to a Wikipedia entry, is defined as “precipitation that forms when super-cooled droplets of water are collected and freeze on a falling snowflake, forming a 2–5 mm (0.079–0.197 in) ball of rime. Strictly speaking, graupel is not the same as hail or ice pellets, although it is sometimes referred to as small hail.”

Graupel, then, is moisture that forms around a snowflake at very high elevations, then precipitates to the ground in a small, roundish ball.

“Under some atmospheric conditions, snow crystals may encounter supercooled water droplets,” Wikipedia continued.

“These droplets … can exist in the liquid state at temperatures as low as −40 °C (−40 °F), far below the normal freezing point. Contact between a snow crystal and the super-cooled droplets results in freezing of the liquid droplets onto the surface of the crystal.

“Graupel forms fragile, oblong shapes and falls in place of typical snowflakes in wintry mix situations, often in concert with ice pellets. Graupel is also fragile enough that it will typically fall apart when touched.”

If the Whites, which are closer to the monsoon moisture than the Sierra, would have been several thousand feet higher, the moisture could have been snow, Sheckter said.