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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