Lecture 11 (11/18)

1. Lecture 11 (11/18) Winter Storms and Lake Effect Snow

2. 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.

3. 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

4. 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

5. 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

6. 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!

7. A Typical Snow Drift

8. Not a typical Snow Drift Start shoveling lady!

9. 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.)

10. 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

11. 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

12. 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

13. LES Picture

14. LES Climatology

15. 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

16. 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

17. For Next Time • Do your homework. • Read Chapter 8 (Thunderstorms and Tornadoes). I know, I’m really getting into wishful thinking.