Meteo 3: Chapter 16Meteo 3: Chapter 16

Winter WeatherWinter Weather

Read pages 619-630, 632-640, Read pages 619-630, 632-640, 643-648643-648

SnowSnow

Almost all precipitation begins as snow in clouds Almost all precipitation begins as snow in clouds as long as ice nuclei are present (think Bergeron)as long as ice nuclei are present (think Bergeron)

The temperature structure of the atmosphere The temperature structure of the atmosphere determines the precipitation type at the grounddetermines the precipitation type at the ground

Snowflakes can be a single ice crystal or several Snowflakes can be a single ice crystal or several ice crystals stuck togetherice crystals stuck together

Supercooled water: Water existing in its liquid Supercooled water: Water existing in its liquid state below 32state below 32°F°F

Snow terminologySnow terminology

Snow squall:Snow squall: Heavy, brief burst of snow Heavy, brief burst of snow Snow shower:Snow shower: Moderate, brief burst of snow Moderate, brief burst of snow Snow flurries:Snow flurries: Very light, brief snow shower with no Very light, brief snow shower with no

accumulationaccumulation– Above three categories are analogous to “convective” precipitationAbove three categories are analogous to “convective” precipitation

(cumuliform clouds)(cumuliform clouds)

Long-duration (stratiform) snows categorized based upon Long-duration (stratiform) snows categorized based upon visibilityvisibility

Blizzard:Blizzard: At least 3 consecutive hours with winds >= 35 At least 3 consecutive hours with winds >= 35 mph & falling/blowing snow reducing visibility to < .25 mimph & falling/blowing snow reducing visibility to < .25 mi– Ground blizzardGround blizzard if no falling snow if no falling snow

Can it snow with ground temperatures above freezing?Can it snow with ground temperatures above freezing?

YES!!!YES!!!– It takes time for falling snowflakes to meltIt takes time for falling snowflakes to melt– An above freezing layer ~1000 ft thick near An above freezing layer ~1000 ft thick near

ground needed for meltingground needed for melting– Can have snow with surface temperatures at Can have snow with surface temperatures at

40F40F

Sleet (Ice Pellets)Sleet (Ice Pellets)

““Calling sleet hail is Calling sleet hail is tantamount to tantamount to comparing roast beef comparing roast beef to Swiss cheese”to Swiss cheese”

Freezing RainFreezing Rain

Leaves objects coated Leaves objects coated in icein ice

Dangerous…can down Dangerous…can down wires, trees, can cause wires, trees, can cause auto accidentsauto accidents

See CD AnimationSee CD Animation

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

Forecasting Precipitation Type: 850 mb Forecasting Precipitation Type: 850 mb TemperatureTemperature

A Note on Forecasting Precipitation TypeA Note on Forecasting Precipitation Type

850 mb temperature is a better indicator of 850 mb temperature is a better indicator of precipitation type because it’s a direct measure of precipitation type because it’s a direct measure of temperature in the lower tropospheretemperature in the lower troposphere

5400 m 1000-500 mb critical thickness represents 5400 m 1000-500 mb critical thickness represents 50%-50% probability of liquid versus frozen 50%-50% probability of liquid versus frozen precipitationprecipitation

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

Jet stream pattern leading to east coast cold and Jet stream pattern leading to east coast cold and snowsnow

Ingredients for Northeast Winter StormsIngredients for Northeast Winter Storms

Sharp upper-level trough…Sharp upper-level trough…leads to upper-air DIV and leads to upper-air DIV and pressure loweringpressure lowering

““Bomb” if low deepens >= Bomb” if low deepens >= 12 mb in 24 hours12 mb in 24 hours

Arctic high pressure center Arctic high pressure center in SE Canada enhances in SE Canada enhances NE wind, which funnels in NE wind, which funnels in cold air & moisture for cold air & moisture for overrunningoverrunning

Nor’easters

• responsible for Northeast’smost memorable snowstorms

• track northeastward alongEast Coast

• land-water temperaturegradient important for cycloneintensification

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More Nor’ Easter FactsMore Nor’ Easter Facts

Named as such because of strong NE winds Named as such because of strong NE winds associated in advance of lowassociated in advance of low

Most frequent between December & MarchMost frequent between December & March Besides causing heavy snow, ice, and rain, can Besides causing heavy snow, ice, and rain, can

lead to coastal flooding and beach erosionlead to coastal flooding and beach erosion

Lake-Effect SnowLake-Effect Snow

Lake-Effect SnowLake-Effect Snow

Primarily generated in late fall-mid winter as cold air Primarily generated in late fall-mid winter as cold air moves over warm Great Lake watersmoves over warm Great Lake waters– Cold air heated and moistened as it passes over lakesCold air heated and moistened as it passes over lakes

Snowfall can be extreme (2-4 inches per hour, several Snowfall can be extreme (2-4 inches per hour, several feet over a day or two)feet over a day or two)

Some enhancement from frictional convergence at Some enhancement from frictional convergence at lake shorelake shore

Enhanced when warmed, moistened air encounters Enhanced when warmed, moistened air encounters higher terrain…forced lifting = more snowhigher terrain…forced lifting = more snow– Heaviest snow usually occurs slightly inland from lake shoreHeaviest snow usually occurs slightly inland from lake shore

More on Lake-Effect SnowMore on Lake-Effect Snow

The longer the wind trajectory over the lake, the The longer the wind trajectory over the lake, the more heating and moistening of arctic air occurs, more heating and moistening of arctic air occurs, the heavier the snowthe heavier the snow

Falls in narrow bands aligned with the wind Falls in narrow bands aligned with the wind directiondirection– Bands relatively narrow => forecasting challengeBands relatively narrow => forecasting challenge

Ocean-Effect SnowOcean-Effect Snow

Winter Weather Forecasting is Tough!Winter Weather Forecasting is Tough!

Precipitation type?Precipitation type? Enhanced bands of heavy snowfallEnhanced bands of heavy snowfall Imperfect forecast models predict precipitation in terms of Imperfect forecast models predict precipitation in terms of

liquid equivalentliquid equivalent– Converting that to snowfall is challengingConverting that to snowfall is challenging– General rule: “ 10 inches snow = 1 inch water”General rule: “ 10 inches snow = 1 inch water”– At temperatures well below freezing, this ratio can be significantly At temperatures well below freezing, this ratio can be significantly

higherhigher

Difficult to measure snow, as snow compacts on the Difficult to measure snow, as snow compacts on the groundground

Snow/no snow boundary can be very sharpSnow/no snow boundary can be very sharp

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

Wind-Chill TemperatureWind-Chill Temperature

Air flowing by the skin moves a thin layer of warm Air flowing by the skin moves a thin layer of warm air in contact with the skin awayair in contact with the skin away

The wind-chill is an apparent temperature, The wind-chill is an apparent temperature, describing the enhanced sensation of cold as a describing the enhanced sensation of cold as a result of the windresult of the wind– Frostbite can occur on exposed skin in 30 minutes or Frostbite can occur on exposed skin in 30 minutes or

less when wind-chill values are less than -20°Fless when wind-chill values are less than -20°F

Wind Chill TemperatureWind Chill Temperature