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Meteo 3: Chapter 16

Meteo 3: Chapter 16. Winter Weather Read pages 619-630, 632-640, 643-648. Snow. Almost all precipitation begins as snow in clouds as long as ice nuclei are present (think Bergeron) The temperature structure of the atmosphere determines the precipitation type at the ground

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Meteo 3: Chapter 16

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  1. Meteo 3: Chapter 16 Winter Weather Read pages 619-630, 632-640, 643-648

  2. Snow • Almost all precipitation begins as snow in clouds as long as ice nuclei are present (think Bergeron) • The temperature structure of the atmosphere determines the precipitation type at the ground • Snowflakes can be a single ice crystal or several ice crystals stuck together • Supercooled water: Water existing in its liquid state below 32°F

  3. Snow terminology • Snow squall: Heavy, brief burst of snow • Snow shower: Moderate, brief burst of snow • Snow flurries: Very light, brief snow shower with no accumulation • Above three categories are analogous to “convective” precipitation (cumuliform clouds) • Long-duration (stratiform) snows categorized based upon visibility • Blizzard: At least 3 consecutive hours with winds >= 35 mph & falling/blowing snow reducing visibility to < .25 mi • Ground blizzard if no falling snow

  4. Can it snow with ground temperatures above freezing? • YES!!! • It takes time for falling snowflakes to melt • An above freezing layer ~1000 ft thick near ground needed for melting • Can have snow with surface temperatures at 40F

  5. “Calling sleet hail is tantamount to comparing roast beef to Swiss cheese” Sleet (Ice Pellets)

  6. Leaves objects coated in ice Dangerous…can down wires, trees, can cause auto accidents Freezing Rain See CD Animation

  7. Forecasting Precipitation Type: 1000-500 mb Thickness (5400 m “critical” thickness)

  8. Forecasting Precipitation Type: 850 mb Temperature

  9. A Note on Forecasting Precipitation Type • 850 mb temperature is a better indicator of precipitation type because it’s a direct measure of temperature in the lower troposphere • 5400 m 1000-500 mb critical thickness represents 50%-50% probability of liquid versus frozen precipitation

  10. Alberta Clippers- Mostly light snow due to limited moisture, fast speed

  11. Jet stream pattern leading to east coast cold and snow

  12. Sharp upper-level trough…leads to upper-air DIV and pressure lowering “Bomb” if low deepens >= 12 mb in 24 hours Arctic high pressure center in SE Canada enhances NE wind, which funnels in cold air & moisture for overrunning Ingredients for Northeast Winter Storms

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  14. More Nor’ Easter Facts • Named as such because of strong NE winds associated in advance of low • Most frequent between December & March • Besides causing heavy snow, ice, and rain, can lead to coastal flooding and beach erosion

  15. Lake-Effect Snow

  16. Lake-Effect Snow • Primarily generated in late fall-mid winter as cold air moves over warm Great Lake waters • Cold air heated and moistened as it passes over lakes • Snowfall can be extreme (2-4 inches per hour, several feet over a day or two) • Some enhancement from frictional convergence at lake shore • Enhanced when warmed, moistened air encounters higher terrain…forced lifting = more snow • Heaviest snow usually occurs slightly inland from lake shore

  17. More on Lake-Effect Snow • The longer the wind trajectory over the lake, the more heating and moistening of arctic air occurs, the heavier the snow • Falls in narrow bands aligned with the wind direction • Bands relatively narrow => forecasting challenge

  18. Ocean-Effect Snow

  19. Winter Weather Forecasting is Tough! • Precipitation type? • Enhanced bands of heavy snowfall • Imperfect forecast models predict precipitation in terms of liquid equivalent • Converting that to snowfall is challenging • General rule: “ 10 inches snow = 1 inch water” • At temperatures well below freezing, this ratio can be significantly higher • Difficult to measure snow, as snow compacts on the ground • Snow/no snow boundary can be very sharp

  20. Image taken by the MODIS instrument on NASA's Terra satellite at 11:40 CST December 26, 2004.

  21. Wind-Chill Temperature • Air flowing by the skin moves a thin layer of warm air in contact with the skin away • The wind-chill is an apparent temperature, describing the enhanced sensation of cold as a result of the wind • Frostbite can occur on exposed skin in 30 minutes or less when wind-chill values are less than -20°F

  22. Wind Chill Temperature

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