Transcript Lecture 13: Winter Weather

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
 Snowflakes can be a single ice crystal or several
ice crystals stuck together
 Supercooled water: Water existing in its liquid
state below 32°F
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
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
Sleet (Ice Pellets)
 “Calling sleet hail is
tantamount to
comparing roast beef
to Swiss cheese”
Freezing Rain
 Leaves objects coated
in ice
 Dangerous…can down
wires, trees, can cause
auto accidents
See CD Animation
Forecasting Precipitation Type: 1000-500 mb
Thickness (5400 m “critical” thickness)
Forecasting Precipitation Type: 850 mb Temperature
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
Alberta Clippers- Mostly light snow due to
limited moisture, fast speed
Jet stream pattern leading to east coast cold and
snow
Ingredients for Northeast Winter Storms
 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
Nor’easters
• responsible for Northeast’s
most memorable snowstorms
• track northeastward along
East Coast
• land-water temperature
gradient important for cyclone
intensification
-
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
Lake-Effect Snow
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
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
Ocean-Effect Snow
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
Image taken by the MODIS instrument on NASA's Terra satellite at 11:40
CST December 26, 2004.
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
Wind Chill Temperature