Lake has great impact on storms, weather

Its impact is so severe that it is one of the few weather phenomena recognizable by its initials: the DLE, dreaded lake effect.

Powered by differences in temperature between the Great Salt Lake and its surroundings, the DLE can send blizzards howling into Salt Lake City, Magna, Tooele or other nearby communities. The DLE is as drenching and as concentrated as a torrent from a fire hose.Lake-effect storms threaten Utah's capital and surrounding cities with such deadly danger that Jim Steenburgh, assistant professor of meteorology at the University of Utah, hopes to head a field study about them next February. The research would focus on the DLE and the way precipitation patterns are affected by the Wasatch Mountains.

If approved for funding, the Intermountain Precipitation Experiment would use mobile atmospheric laboratories from the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory. It would employ a pair of portable Doppler radars. NOAA's P-3 "hurricane hunter" aircraft would fly into the hearts of violent snowstorms.

Forecasters peer at their data and try to decide if a DLE will happen and what direction it will go if it does.

"We look at what the temperature difference is from what the water is, up to about 6,000 feet above the lake," said William J. Alder, meteorologist in charge of the National Weather Service regional headquarters on North Temple.

When the conditions are right for the DLE, the fire hose can deliver its precipitation to certain areas while ignoring the next county. A northwest flow of wind over the lake can let a heavy band of snow fly into Salt Lake County; a flow from the west puts it in Davis County; a storm coming in from the north can slam Tooele County.

In October 1984, the DLE dumped a couple of feet of snow in Salt Lake City, snarling traffic. Salt Lake City International Airport had a foot and a half, while 2 feet blanketed the Olympus Cove area. Alder recalls a "tremendous amount of damage." Trees and limbs crashed, bringing down power lines, and schools closed for the day.

Six to eight storms per year can be blamed on the DLE, Alder said.

Contrary to what many believe, the effect doesn't pick up a heavy load of moisture from the Great Salt Lake. It may add some water content, but that is not its more serious impact. Instead, temperature differences help channel precipitation already in the storm fronts, with intense storms striking some unlucky city in the vicinity.

The most likely seasons for lake-effect snowstorms are spring and fall, when trees are covered with leaves. Branches and trunks are more likely to break when a lot of leaves collect the snow that accumulates.

Often, the water of the Great Salt Lake is like a huge insulating blanket. It stays cooler than the land during the summer and warmer during the fall. In the fall, a cold blast can arrive when the air above the lake is still relatively warm.

Then in the winter, the lake gets colder. Its high salt content lets the water temperature plummet below the 32 degrees at which fresh water freezes. In the winter or spring, storm fronts can sweep in with temperatures in the 50s, while the air above Great Salt Lake has been cooled to the 30s or 40s.

The important factor in the DLE is the temperature difference, which can set up ranks of clouds that send wind and precipitation flying along narrow corridors.

In fact, Alder said, the lake adds to the instability of local weather even during the summer. Because of temperature differences, thunderheads can form over the lake at night and drift toward Davis and Weber counties.

One of the not-so-dreaded effects of the lake is a zephyr that acts something like a sea breeze on the coast.

"The lake has a lot of influences on the weather," Steenburgh said. "The lake has a really big, pronounced effect on local wind circulation. . . . Both the lake and the topography affect that wind regimen."

As much as 10 percent of Salt Lake Valley's total annual precipitation may be from lake-effect storms, he estimates.

In 1994, the National Weather Service installed a new Doppler radar at Promontory Point, Box Elder County, which is giving meteorologists their clearest look ever at DLE storms.

Scott Halvorson, who earned his master's degree at the University of Utah, carried out research using Doppler radar images going back to 1994. Steenburgh said the studies showed elongated storms can form on the lake, setting up features called mid-lake bands.

Sometimes they extend on an axis from the northwest to the southeast on the lake, shoving precipitation into Salt Lake Valley. At other times they can form from north to south, dumping the moisture into Tooele Valley.

"Sometimes they move, sometimes they're fairly stationary," Steenburgh said. "The ones that make the most news are the ones that stay in the same spot."