Factors and environmental conditions leading to severe weather

Factors and environmental conditions leading to severe weather

Global atmospheric circulation

Planet-scale wind patterns that move heat from the tropics toward the poles and help establish prevailing winds, jet streams, and storm tracks.

Hadley, Ferrel, and Polar cells

The three broad circulation cells in each hemisphere: Hadley in the tropics, Ferrel in the midlatitudes, and Polar near the poles. Their boundaries help create prevailing winds and jet streams.

Jet stream

A narrow band of very strong upper-level wind, usually flowing west to east near strong temperature contrasts. It steers weather systems and can help storms strengthen.

Polar jet

The highly variable jet stream near the boundary between cold polar air and warmer midlatitude air. It strongly influences U.S. winter storms and severe-weather patterns.

Subtropical jet

A jet stream near about 30 degrees latitude that can carry moisture and add upper-level winds to developing storm systems.

Jet streak

A local maximum of wind speed within a jet stream. Rising motion is favored in certain entrance and exit regions because air can spread apart aloft.

Why the jet stream blows west to east

Unequal heating creates a poleward pressure-gradient force aloft; Earth’s rotation turns the moving air through the Coriolis effect, producing mainly westerly flow. Conservation of angular momentum also contributes

Rossby waves

Large north-south bends in the jet stream that form ridges and troughs and control the movement of major weather systems.

Zonal flow

A relatively straight west-to-east upper-level pattern that moves weather systems quickly and limits large north-south temperature changes.

Meridional or amplified flow

A strongly wavy upper-level pattern with large ridges and troughs. It allows cold air to move south, warm air to move north, and storms to slow or strengthen.

Upper-level ridge

An elongated area of higher heights aloft, commonly associated with sinking air, warming, and quieter weather.

Upper-level trough

An elongated area of lower heights aloft, commonly associated with colder air aloft and rising motion ahead of the trough.

Cyclogenesis

The formation or strengthening of a low-pressure system, often helped by upper-level divergence, surface convergence, and strong temperature contrasts.

Low-level convergence

Air flowing together near the surface. Because it cannot enter the ground, it is forced upward and may initiate clouds and thunderstorms.

Upper-level divergence

Air spreading apart high in the atmosphere. This removes air from the column and encourages rising motion and falling pressure below.

Gulf of Mexico low-level jet

A fast current of air a short distance above the ground that often strengthens at night and transports warm, humid Gulf air into the Great Plains.

Four ingredients for organized severe thunderstorms

Moisture, instability, lift, and vertical wind shear. Weakening or removing one ingredient usually lowers the severe-weather threat.

Atmospheric stability

The atmosphere's resistance to vertical motion; it describes whether lifted air tends to sink back or continue rising.

Stable atmosphere

Lifted air becomes colder and denser than the surrounding air and tends to sink. Stable conditions suppress tall thunderstorm clouds.

Unstable atmosphere

Lifted air remains warmer and less dense than the surrounding air and continues to rise, favoring deep clouds and thunderstorms.

Conditional instability

Air may become strongly buoyant after it rises enough to become saturated; this is common in thunderstorm environments.

Environmental lapse rate

The rate at which the surrounding air temperature changes with height. A rapid temperature decrease with height favors instability.

Steep lapse rates

Temperature decreases rapidly with height, increasing instability and supporting stronger updrafts, large hail, or damaging winds.

Temperature inversion

A layer where temperature increases with height. It is very stable and can prevent surface air from rising.

CAPE

Convective Available Potential Energy: an estimate of the energy available to rising air. Larger CAPE can support stronger updrafts, but severe weather also requires lift and often wind shear.

CIN

Convective Inhibition: the energy barrier that prevents warm surface air from rising freely. Lift or heating must overcome it before storms form.

Capping inversion or cap

A warm, stable layer above moist surface air. A moderate cap can allow energy to build, but a cap that never breaks prevents thunderstorms.

Air-mass source region

A large area where air remains long enough to gain the temperature and moisture characteristics of the land or water below.

Continental Polar air mass (cP)

Cold, dry air that forms over high-latitude land and often moves south behind a cold front.

Maritime Polar air mass (mP)

Cool, moist air that forms over cold northern oceans and can produce clouds, showers, or mountain snow.

Maritime Tropical air mass (mT)

Warm, humid air that forms over tropical oceans. Gulf and Atlantic mT air supplies moisture for many U.S. severe storms.

Continental Tropical air mass (cT)

Hot, dry air from deserts or plateaus. It is commonly found west of a Plains dryline.

Air-mass modification

The change in an air mass as it moves over a different surface and gains or loses heat and moisture.

Cold front

The leading edge of advancing cold air. Its steep lifting can produce a narrow line of thunderstorms, gusty winds, and a rapid temperature change.

Warm front

The leading edge of advancing warm air. Warm air rises gradually over cooler air, usually producing widespread clouds and steady precipitation.

Stationary front

A boundary that moves very little because neither air mass advances. Repeated storms along it can cause flash flooding.

Occluded front

A boundary formed when a cold front catches a warm front and lifts the warm air away from the surface.

Warm sector

The wedge of warm, humid air between a warm front and a cold front in a midlatitude low. It is often the main severe-thunderstorm region.

Surface trough

An elongated area of relatively low pressure that is not necessarily a front. Convergence along it can focus showers and thunderstorms.

Squall line

A long line of organized thunderstorms, commonly along or ahead of a cold front, with damaging straight-line winds as a major hazard.

Dryline

A boundary between humid air and much drier air, identified mainly by a sharp dew-point change rather than a large temperature change.

How a dryline helps storms form

Surface winds converge near the moisture boundary and force humid air upward. Storms may form when the lift is strong enough to break the cap.

Typical dryline location

The southern and central Great Plains in spring, with moist Gulf air to the east and hot, dry desert or plateau air to the west.

Dryline daily movement

It commonly moves east during afternoon heating and retreats west after sunset as the lower atmosphere cools.

Signs of a dryline passage

A sharp drop in humidity, a wind shift, clearer skies, and sometimes a temperature rise.

Drylines and severe weather

Supercells may develop along or just east of a dryline where moisture, instability, lift, and wind shear overlap.

Triple point

The meeting point of a low-pressure system, a front, and a dryline or another front. Lift, moisture, and changing winds may be concentrated there.

Wind shear

A change in wind speed or direction over distance, especially with height.

Speed shear

Wind speed changes with height while the direction remains similar.

Directional shear

Wind direction changes with height, helping create horizontal rotation that an updraft can tilt vertically.

Low-level wind shear

A strong change in wind in the lowest part of the atmosphere. It can increase the rotation available to a thunderstorm.

Deep-layer wind shear

The overall wind change through much of the troposphere. Strong deep-layer shear separates updrafts from downdrafts and organizes long-lived storms.

Why wind shear supports supercells

Shear tilts the updraft and keeps it separated from rain-cooled downdrafts, allowing the storm to persist and sometimes rotate.

Wind shear and storm type

Weak shear favors short-lived pulse storms; stronger shear favors organized multicells, squall lines, and supercells.

Mesocyclone

A rotating updraft inside a supercell. It increases tornado potential, but most mesocyclones do not produce tornadoes.

Atmospheric river

A long, narrow corridor that transports large amounts of water vapor, often from warm ocean regions toward the midlatitudes.

Pineapple Express

An atmospheric river that carries warm, humid air from near Hawaii toward the West Coast of North America.

Atmospheric-river effects

Atmospheric rivers can provide valuable rain and mountain snow, but a strong or slow-moving event may cause flooding and debris flows.

Atmospheric-river duration

How long concentrated moisture transport remains over an area. A stalled event may cause more flooding than a stronger but brief event.

Atmospheric-river orographic enhancement

Moist atmospheric-river air rises over coastal mountains, cools, and condenses, greatly increasing windward rain or snow.

Fetch

The uninterrupted distance wind travels across water. Longer fetch gives the air more time to gain heat and moisture and allows larger waves to develop.

Sea surface temperature

The temperature of the ocean's upper surface. Warm water increases evaporation and heat transfer into the atmosphere.

Ocean heat content

The total heat stored through the upper ocean. Deep warm water can support a tropical cyclone better than a thin warm surface layer.

Latent heat from water

Evaporation stores energy in water vapor. When the vapor condenses in clouds, the released heat strengthens rising air and storm convection.

Cold wake

Cooler water left behind a tropical cyclone after winds mix or pull colder subsurface water toward the surface.

Storm-induced upwelling

Strong storm winds bring deeper, colder water toward the surface, reducing the heat available to the storm.

Storm speed and ocean cooling

A slow-moving storm spends more time mixing and upwelling water, so it is more likely to cool the ocean below and weaken itself.

Warm-core ocean eddy or Loop Current

A region of deep warm water. A hurricane crossing it may strengthen because mixing does not quickly expose colder water.

Lake-effect snow

Snow that forms when cold air crosses a relatively warm, unfrozen lake, gains heat and moisture, becomes unstable, and rises downwind.

Lake-breeze boundary

A daytime boundary where cooler lake air moves inland and meets warmer air. Convergence along it can initiate or strengthen thunderstorms.

Storm surge and shallow coastal water

Onshore winds pile water against the coast. A broad shallow shelf or funnel-shaped bay allows water to build higher than a steep, deep coast.

Windward side

The side of a mountain facing the incoming wind, where air rises, cools, and often produces more clouds and precipitation.

Leeward side

The downwind side of a mountain, where air commonly descends, warms, and dries.

Orographic lift

Air forced upward by mountains or elevated terrain. Moist unstable air can form stronger thunderstorms or heavier precipitation as it rises.

Rain shadow

A dry area on the leeward side of a mountain where descending air warms after losing moisture on the windward slope.

Upslope flow

Wind moving from lower terrain toward higher terrain. Moist upslope flow can produce persistent clouds, heavy rain or snow, and sometimes severe storms.

Lee cyclogenesis

Formation or strengthening of a surface low on the downwind side of a mountain range, especially east of the Rocky Mountains.

Downslope wind

Air descending a mountain slope accelerates, compresses, warms, and dries; it may produce damaging winds and suppress clouds.

Chinook wind

A warm, dry downslope wind east of the Rocky Mountains that can cause rapid warming and snowmelt.

Santa Ana wind

A strong, dry offshore wind descending toward Southern California. It lowers humidity and greatly increases wildfire danger.

Gap wind

Wind accelerated through a mountain pass, canyon, or gap because air is squeezed through a narrow opening.

Terrain channeling

Valleys, passes, and ridges steer or accelerate winds, which can locally increase convergence, storm inflow, wind shear, or damaging gusts.

Terrain-triggered convection

Elevated ground heats and forces air upward, making mountain slopes and ridges common locations for afternoon thunderstorm development.

Land weakening of tropical cyclones

After landfall a tropical cyclone loses its warm-water energy source, encounters greater friction, and often draws in drier air, so it usually weakens.

Mountain disruption of tropical cyclones

High terrain blocks and distorts low-level inflow and greatly increases friction, often weakening a tropical cyclone faster than flat land.

Coastal mountains and atmospheric rivers

Coastal ranges force moist air upward, intensifying atmospheric-river rain and snow and increasing flood, debris-flow, and avalanche risk.

Scenario — Hurricane crosses deep warm Gulf water

A hurricane moves over the Loop Current and rapidly strengthens. The deep warm water has high ocean heat content, so storm mixing does not quickly bring cold water to the surface.

Scenario — Hurricane stalls over one area

A hurricane slows greatly over the ocean and begins weakening. Its winds repeatedly mix and upwell colder water, producing a cold wake and reducing its heat supply.

Scenario — Second hurricane follows the same track

A second hurricane crosses the path of a recent storm and strengthens less than expected. The first storm left a cold wake with less available ocean heat.

Scenario — Warm surface but shallow warm layer

Sea surface temperature is high, but colder water lies just below it. A strong hurricane may mix up that colder water and weaken because ocean heat content is not very high.

Scenario — Warm ocean but strong atmospheric shear

A tropical cyclone is over warm water but fails to strengthen. Warm water supplies energy, but strong vertical wind shear disrupts the storm's circulation.

Scenario — Long fetch across a Great Lake

Cold air blows along the long axis of an unfrozen Great Lake. The long fetch lets the air gain more heat and moisture, producing a stronger lake-effect snow band downwind.

Scenario — Great Lake becomes ice covered

Lake-effect snow decreases after extensive ice forms because the ice blocks much of the heat and moisture transfer from the lake to the air.

Scenario — Lake-breeze collision

Two lake-breeze boundaries meet over warm land on a humid afternoon. Their convergence forces air upward and can trigger thunderstorms.

Scenario — Hurricane approaches a shallow Gulf Coast shelf

Storm surge becomes especially high because strong onshore winds can pile water efficiently over the broad, shallow shelf.

Scenario — Atmospheric river reaches the Sierra Nevada

Moist Pacific air is forced up the mountains, causing much heavier rain or snow on windward slopes than along nearby lowlands.

Scenario — Moist east winds climb the Rockies

Humid air is forced upslope, cools, and condenses. Heavy precipitation or thunderstorms may form on the eastern slopes.

Scenario — Surface low develops east of the Rockies

A new low forms on the leeward side of the mountains. This is lee cyclogenesis and can strengthen fronts, winds, and severe-weather potential on the Plains.

Scenario — Plains severe-weather setup

Gulf moisture moves north while hot, dry plateau air spreads east above it and a dryline forms. Terrain has helped place moisture, instability, lift, and shear close together.

Scenario — Thunderstorm crosses rugged mountains

The storm may weaken or change direction because terrain blocks inflow, increases friction, and disrupts the organized circulation.