Lecture 11 11 18
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Lecture 11 (11/18). Winter Storms and Lake Effect Snow. Winter Storms. A winter storm is just a low pressure area (storm) that moves across the U.S. with particular cold air behind it. Important to forecast a couple days in advance so public can prepare, close airports, close schools, etc.

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Lecture 11 (11/18)

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Lecture 11 (11/18)

Winter Storms and

Lake Effect Snow

Winter Storms

  • A winter storm is just a low pressure area (storm) that moves across the U.S. with particular cold air behind it.

  • Important to forecast a couple days in advance so public can prepare, close airports, close schools, etc.

Winter Storms

  • Snowfall almost always occurs in the cold sector of a cyclone (NW of cold front and N of warm front).

  • Most common just to the North or Northwest of the center of the cyclone.

  • Exception: Upslope snow in Denver – when moist, cold air blows in from Kansas, it must go uphill (2000 ft)

  • This is often enough to induce clouds and snow

Precipitation Types

  • Freezing Rain

    • Snow melts in warm layer

    • The cold layer below is too shallow and water only freezes on contact

  • Sleet

    • Snow melts in warm layer

    • Refreezes in the lower cold layer

  • Snow

    • Begins as snow and layer is cold enough to remain frozen

Blizzards (not the thing at DQ)

  • Blizzard = A snowstorm with winds of at least 35 mph, and visibility reduced to less than 1/4 mile with snow and blowing snow

  • A severe blizzard has 45 mph or greater winds

  • Blizzards can make “white out conditions” where visibility is so bad you cannot tell the ground from the sky (everything is white)

  • Also make snow drifts

Snow Drifts

  • Snow drifts happen when the wind blows snow and deposits it into mounds

  • Remember the snow fence example in the book

  • In extreme cases, snow drifts can get as high as 40+ feet!

A Typical Snow Drift

Not a typical Snow Drift

Start shoveling lady!

Lake Effect Snow

  • Lake Effect Snows (LES) most common in late fall through early winter

  • Cold arctic air (typically associated with strong surface high pressure from Canada) flows over the warm lakes

  • Heat and moisture from the lake surface is transferred into the boundary layer (the layer where the air from the ground is mixed into the air above it.)

LES process

  • Surface heating beneath cold air above destabilizes the lowest 1 to 2 km of the atmosphere

  • Initiates convection

  • If the air resides over a lake long enough, it picks up enough moisture to condense water vapor and form clouds

  • Condensation and freezing in the clouds releases latent heat which adds to buoyancy

Moving towards shore

  • The warm, moist, unstable air moves onshore

  • Rougher surface beneath and topography (if there is any) decelerates the air and induces convergence winds coming together)

  • Enhanced lift at the shore

  • Cumulus and stratocumulus clouds precipitate

Results of LES

  • Land breeze circulation occurs (circulation from land to water at surface and water to land aloft)

  • All of this produces heavy snow squalls in narrow regions

  • Skies can be clear just a few miles away

  • If these heavy snow resides in same area for hours or even days you can get huge snow accumulations

LES Picture

LES Climatology

Forecasting Lake Effect Snow

  • Lake effect snow is seriously under-forecasted by the models

  • If the models predict lake-effect snow, it will most likely happen

  • Determine the wind direction: examine the 850mb analysis or forecast.

  • Add 13 deg F to the 850mb temp over the lake (to calculate potential temp and determine stability of air between lake and 850

Rule of Thumb

  • If temp at 850 + 13° > temp of lake then you won’t get LES situation (since air is stable)

  • Cyclonic curvature favors lake effect snow of winds

  • If the 850 mb height contours curve to the left as you go downstream, strong lake-effect snow is more likely

For Next Time

  • Do your homework.

  • Read Chapter 8 (Thunderstorms and Tornadoes). I know, I’m really getting into wishful thinking.

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