WINTER WEATHER Winter Storms Precipitation Type Lake Effect Snow Wind Chill Cold Air Damming Winter

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WINTER WEATHERWinter Storms Precipitation
TypeLake Effect SnowWind Chill Cold Air
DammingWinter Storm Reconnaissance

• MSC 243 Lecture 12
• 11/19/09

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Winter Storm Frequency (JFM)
Typical storm tracks in the USA http//wxpredicti
on.com/storm_track2_files/slide0001.htm
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Winter storm formation Ingredients

• Favorable jet stream position aloft.
• Cold polar air coming from north.
• Sharp temperature gradient.
• Availability of moisture.

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Norwegian Cyclone Model

• Initial frontal boundary separating cold and
  warm air.
• A wave forms along the front as an upper-air
  disturbance in the jet stream moves over it.
• Front develops a kink where wave develops.

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Isobars
Isotherms
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Precipitation Type
Precipitation type depends on the critical
thickness and freezing level.
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Critical Thickness Contour
The dashed blue line is the 540 decameter
thickness line. As a general rule of thumb
expect the precipitation type north of this line
to be snow over land.
? Snow in NE on Thanksgiving?
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Snow and Rain

• Snow and rain are generally the easiest to
  predict.
• If temperatures are all below freezing from 700mb
  to the surface, it will be snow.
• If temperatures are all above freezing from 850mb
  to the surface, it will be rain.

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Sleet / Ice Pellets

• Sleet is rain that freezes before reaching the
  ground.
• Temperatures aloft need to be above freezing, but
  below freezing in roughly the lowest 100mb of the
  atmosphere.

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An Ice Pellet Sounding
Melting Layer
Freezing Layer
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Freezing Rain

• Freezing rain is like sleet, except the near
  surface layer of cold air is not thick enough to
  freeze the rain before it hits the ground.
• Rain falls, but the surface temperature is below
  freezing, so the rain freezes after hitting the
  surface.

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A freezing rain sounding
Melting Layer
Freezing Layer
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Factors Affecting Precipitation Type

• Thermal Advection
• Warm advection acts to warm the environment and
  hence melt any freezing precipitation.
• Vertical Motion
• Upward motion causes adiabatic cooling in the
  column (sometimes rain changes to snow)
• Diabatic Effects
• In borderline situations, evaporative cooling
  near the surface could lower the temperature to
  below freezing (if the wet-bulb is below
  freezing). Precipitation reaching the ground
  would then be frozen. Melting is similarly
  important.

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Precipitation Type Summary
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Lake Effect Snow
http//www.erh.noaa.gov/buf/lakeffect/indexlk.html

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Lake Effect Snow Formation
http//rammb.cira.colostate.edu/visit/les/conceptu
al.asp
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Where will the snow fall?

• Lake effect snow will fall on the downstream side
  of the lake.

Winds from 270o will result in snowfall under
lake effect conditions on the east side of the
lake
MEAN 850-700mb WIND
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How much snow will fall?

• If winds blow across the lake the long way, there
  will be a greater amount of precipitation than if
  they blow across the short way.

MEAN 850-700mb WIND
MEAN 850-700mb WIND
Heavy Snowfall Expected
Light Snowfall Expected
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Types of Lake Effect Snow
Single Band
http//www.cira.colostate.edu/ramm/visit/les/singl
e.asp
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Types of Lake Effect Snow
http//www.cira.colostate.edu/ramm/visit/les/types
.asp
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Key ingredients for lake effect snow (where there
is no synoptic forcing)

• 1. Critical difference between lake
  temperature and 850mb temperature is 15C.
• 2. Small vertical wind shear ( lt30o difference
  in wind direction and lt10 kt difference in speed
  between 850mb-500mb).
• 3. Absence of dry air.

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Wind Chill

• Wind Chill is based on the rate of heat loss from
  exposed skin caused by wind and cold. As the wind
  increases, it draws heat from the body, driving
  down skin temperature. Therefore, the wind makes
  it FEEL much colder. If the temperature is 0F and
  the wind is blowing at 15 mph, the wind chill is
  -19F. At this wind chill temperature, exposed
  skin can freeze in 30 minutes.

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Wind Chill
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Cold Air Damming
Cold air damming is the term used to describe the
buildup of cold air along the eastern edge of a
mountain range. Cold air damming helps to
maintain a low overcast cloud deck, and cold
surface temperatures. Its onset and decay are not
well forecast by the models.
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Cold Air Damming

• Definition
• The phenomenon in which a low-level cold air mass
  is trapped topographically. Often, this cold air
  is entrenched on the east side of mountainous
  terrain.
• Cold Air Damming often implies that the trapped
  cold air mass is influencing the dynamics of the
  overlying air mass, e.g. in an overrunning
  scenario where warm air gets forced up over the
  cold air and condenses.
• Effects on the weather
• Cold temperatures
• Freezing precipitation
• Extensive cloud cover

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Effects of Cold Air Damming
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Cold Air Damming

• Classification of Events
• Classical Cold advection associated with large,
  strong surface high
• At the mountain slope Evaporative cooling of
  precipitation into pre-existing dry, stable air.
• Hybrid Combination of above 2 types
• Climatology of Cold Air Damming
• Most common and strongest in winter months
• Occur along east slopes of mountain ranges

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Cold Air Damming

• Elements of a CAD Event
• Inversion present
• Overrunning warm, moist air
• Upper-level forcing influences precipitation type
  and distribution
• May include a barrier jet
• Contributors to the Cold Air
• Cold advection from a polar anticyclone
• Adiabatic cooling as air ascends slopes
• Diabatic cooling via melting or evaporation

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Effects of Damming
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Winter Storm Reconnaissance