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Lecture 22 Weather Hazards and Polar Lows
Polar low seen in
an infrared image
from a polar
orbiting satellite
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Hazards associated with Midlatitude Cyclones
Weather Hazards in Winter Storms
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Winter Storm - Hazards• High Winds
– Nor’easter, Blizzard Conditions, Turbulence
• Frozen Precipitation
– Blizzard Conditions, Snow, Sleet and Freezing
Rain, Aircraft Icing, Riming, Snow Avalanches
• Heavy Rain
– Flooding, Flash Floods, Mud Slides
• Large Ocean Swell and Waves
– High Surf, Storm Surge
• Severe Thunderstorms
– Tornados, Large Hail, High Winds, Lightning
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Severe T-storms
Tornados
High winds
Severe T-storms
Tornados
High winds
Sleet and ZR
Blizzard
High winds
Where are the hazards?
Weather Hazards
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High Winds
Where do we find the strongest winds in a midlatitude cyclone?
• Surface
– Where the pressure gradient is largest, often on NW side
of storm.
– Near convective clouds (including tornados)
– Near the fronts
• Upper Air
– Where the pressure gradient is largest
– In pulses superimposed on the jet stream
– these pulses are sometimes called jet streaks– turbulence is found near jets in regions of large wind
shear
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Where do the Hazards Occur?
Nor’easters
• Strong low pressure systems moving up Atlantic seaboard
• Strong winds and heavy precipitation (blizzards).
Blizzard conditions occur N and NW of surface-low center.
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Hazards – High Winds
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Precipitation Patterns
Where does air rise
• Along the fronts
• In areas of convection
• Around surface the low center
Radar View of
Precipitation
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Hazards – Sleet and Freezing Rain
The Rain – Snow Line
• North of the warm
front there is a
transition from rain to
snow called the rain-
snow line.
• Freezing rain and
sleet fall on the warm
side of this line.
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Hazard – Freezing Rain
Kansas City, MO, January 31, 2002.
Frozen precipitation causes
significant disruption of
transportation and resulting
economic impact.
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Freezing
Rain
Television Tower
Raleigh, NC
January 1990
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‘93 Blizzard
Blizzard conditions include heavy snowfall and blowing snow with strong winds. Note the tight spacing of isobars at left.
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Hazards – Blizzard
North Dakota: A March 1966 blizzard nearly buried utility poles.
Blizzard conditions include winds !35 mph
and visibility less than 1/4 mi.
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Hazards – Heavy Snow
Snowstorms leave a band of heavy snow in their wake.
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Hazards – Heavy Snow
Death in blizzards comes from exposure to cold.
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Hazard – Snow Avalanche
Western U.S. mountains generally record over 100,000 avalanches every winter. Some avalanche slides can reach speeds of over 100 miles per hour.
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Hazard – Snow Avalanche
In US roughly 25 deaths per year due to snow avalanches.
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An ice storm in New York also caused flooding of water front properties in the region, January 1998. .
Hazards – Heavy Rain and Flooding
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Flash floods often cause fatalities by drowning when people try to drive across flood waters.
Hazards – Heavy Rain and Flooding
Oahu flood, November 1995
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Hazards – Large Waves
Peggotty Beach, Massachusetts
February 9, 1978
Oahu, North Shore
January 1998.
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Hazards – Large Swell
Surf's up!
Heavy surf on the Columbia
River bar tests a Coast Guard
vessel approaching the mouth of
the Columbia River.
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Hazards – Severe Thunderstorms
Large Hail
Tornados
High Straight-line winds
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Severe Thunderstorms
associated with a cold
front.
Weather Hazards
in Winter Storms
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Where do the Hazards Occur?
Severe Thunderstorms
Produce:
• Tornados
• Large Hail
• High Straight-line Winds
(and Lightning)
Occur where jet stream crosses the cold front.
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Severe T-storms
Tornados
High winds
Severe T-storms
Tornados
High winds
Sleet and ZR
Blizzard
High winds
Questions?
Three areas with distinct hazards in winter storms.
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What is a Polar Low?A small-scale (typically 200-300 mi in diameter) cyclone that forms in
a cold air mass poleward of major jet streams or frontal zones.
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NOAA-5 infrared satellite photograph of a polar low over the Bering Sea and Bristol bay; at 21:27 GMT on 19 January 1979.
Small ScalePolar low seen in
an infrared image
from a polar
orbiting satellite
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Polar Low Characteristics
• Non-polar frontal low-pressure systems
• Cyclonic wind circulation (in Northern Hemisphere)
• Anticyclonic outflow at upper levels
• Warm Core
• Winds decrease with height
• Vertical structure, spiral or symmetric about clear "eye"
• Sensible and latent heat from the ocean primary
energy source
• Formation over high latitude oceans adjacent to snow
or ice covered surfaces
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Forecast Challenge
Polar Low over the Barents Sea area at
0240 UTC 13 December 1982.
• Small scale - 200 - 300 mi in diameter
• Data sparse habitat - over high latitude oceans
• Rapid development - ~12 hour spin up
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Weather Related Hazards:
• High winds (up to 90 kt observed)
• Large waves and swell
• Low visibility, heavy snow, icing of ships
• Thunderstorms and in some cases tornados
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Form to the North and West
of the Polar Front
Schematic ModelPolar lows do not form on the polar front
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Comma Clouds
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Comma Cloud
This one produced tornados in CA!
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a) Schematic depiction of a series of developing comma clouds forming within a 500 mb cold-core low (L). b) NOAA-7 infrared-satellite image at 1508 PST, 16 January 1982, showing high clouds associated with the incipient comma cloud Case 1 and a mature comma cloud that preceded it.
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Schematic Model
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Schematic Model
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Schematic Model
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Prerequisites for Formation
• Cold temperatures in the upper troposphere
• Air passing from snow or ice covered surface over the open ocean
leading to enhanced surface sensible and latent heat fluxes
• Curved flow (large vorticity or spin) at the surface and aloft
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Sea-surface temperature and extent of the ice edge (shaded area) for 9-12 February 1984
Prerequisites for Formation
Habitat
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Paths of polar lows over the Norwegian Sea 1978-1982. Closed circles indicate point of origin, open circles indicate final position.
Polar Low
Climatology
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Mean sea-surface temperature (°C) and extent of the ice edge (heavy dashed line) for the Bering Sea and the North Pacific Ocean; 20 year mean (1957-1978) for January.
Polar Low Habitat
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Histogram of the number of days per month on which polar lows were observed in polar-orbiting satellite imagery over the Gulf of Alaska or the Bering Sea during the period 1975-1983.
Polar Low
Climatology
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Polar low and cloud streets over the Barents Sea: NOAA-7 infrared satellite photograph for 0320 GMT, 22 November 1983
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Polar Low over the Norwegian Sea area at 1340 UTC
27 February 1984.
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Polar Low over the Norwegian Sea area at 1340 UTC
27 February 1984.
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Sea-level pressure analysis of Polar Low over the
Norwegian Sea area at 1340 UTC 27 February 1984.
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Polar Lows over Mediterranean Sea
26 January 1982
25 January 1982
Polar lows are sometimes called extra tropical hurricanes.
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NOAA-6 enhanced infrared-satellite photograph of a polar low near the ice edge just west of the Palmer Peninsula, Antarctica at 18:25 GMT on 15 march 1985.
Polar Low in Southern Hemisphere
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NOAA-5 infrared satellite photograph of a polar low and cloud streets over the Bering Sea at 22:24 GMT on 23 January 1979.
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NOAA-4 infrared satellite photograph of polar low and cloud streets over the Gulf of Alaska at 20:21 GMT on 22 March 1975.
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Summary
• Conditions favorable to Polar Low Development
– Strong cold air advection over water.
– Cold temperatures at 500 mb - unstable air
• Forecast Challenge
– Small scale - subsynoptic
– Data sparse habitat - over high latitude oceans
– Rapid development - ~12 hour spin up
• Weather Related Hazards
– High surface winds (up to 90 kt observed)
– Large waves and swell
– Low visibility, heavy snow, icing of ships
– Thunderstorms and in some cases tornados
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Questions?
Why can’t I show you a nice satellite loop of a polar low?
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