La Niña

La Niña, a phase of ENSO, is a periodic cooling of surface ocean waters in the eastern tropical Pacific along with a shift in convection in the western Pacific further west than the climatological average. These conditions affect weather patterns around the world. The preliminary CPC definition of La Niña is a phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature departure from normal.

Lag

1) The measure of the time between the center of mass of precipitation to the center of mass of runoff (on the hydrograph); basin lag is a function of not only basin characteristics, but also of storm intensity and movement. Some hydrologic texts define lag from the center of mass of rainfall to the hydrograph peak.
2) The time it takes a flood wave to move downstream.

Lake Breeze

A thermally produced wind blowing during the day from the surface of a large lake to the shore, caused by the difference in the rates of heating of the surfaces of the lake and of the land.

Lake Effect Snow

Snow showers that are created when cold, dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.

Lake Effect Snow Advisory

This product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not because of a low pressure system) may pose a hazard or it is life threatening. The criteria for this advisory varies from area to area.

Lake Effect Snow Squall

A local, intense, narrow band of moderate to heavy snow squall that can extend long distances inland. It may persist for many hours. It may also be accompanied by strong, gusty, surface winds and possibly lightning. Accumulations can be 6 inches or more in 12 hours.

Lake Effect Snow Warning

This product is issued by the National Weather Service when pure lake effect snow (this is where the snow is a direct result of lake effect snow and not because of a synoptic storm or low pressure system) may pose a hazard or it is life threatening.

Lake Effect Storm

A fall or winter storm that produces heavy but localized precipitation as a result of temperature differences between the air over snow-covered ground and the air over the open waters of a lake.

Lakeshore Flood Advisory

See Coastal/Lakeshore Flood Advisory.

Lakeshore Flood Watch

See: COASTAL/LAKESHORE FLOOD WATCH

Lakeshore Flooding

See COASTAL/LAKESHORE FLOODING

LALs

(L)ightning (A)ctivity (L)evels.
LAL 1 - No thunderstorms.
LAL 2 - Few building cumulus with isolated thunderstorms.
LAL 3 - Much building cumulus with scattered thunderstorms. Light to moderate rain.
LAL 4 - Thunderstorms common. Moderate to heavy rain reaching the ground.
LAL 5 - Numerous thunderstorms. Moderate to heavy rain reaching the ground.
LAL 6 - Dry lightning (same as LAL 3 but without the rain).

Laminar

Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

Laminar Flow

Streamline flow in which successive flow particles follow similar path lines and head loss varies with velocity to the first power.

Land Breeze

A coastal breeze at night blowing from land to sea, caused by the difference in the rates of cooling of their respective surfaces.

Landfall

The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water. Compare direct hit, indirect hit, and strike.

Landspout

[Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Lapse Rate

The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.

Large Scale

(Synoptic Scale) Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems.

Last Update

The time and date in which the forecast was issued or updated. The forecast may be updated at any time as weather conditions warrant.

LAT

Latitude- The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.

Latent Heat

Heat absorbed or released during a change of phase at constant temperature and pressure.

Latent Heat Flux

The flux of heat from the earth's surface to the atmosphere that is associated with evaporation or condensation of water vapor at the surface; a component of the surface energy budget.

Latitude

(abbrev. LAT) The location north or south in reference to the equator, which is designated at zero (0) degrees. Lines of latitude are parallel to the equator and circle the globe. The North and South poles are at 90 degrees North and South latitude.

LAWEB

(Great Lakes Weather Broadcast) - A National Weather Service product containing an observation summary prepared to provide Great Lakes mariners with a listing of weather observations along or on the Lakes.

Layer Composite Reflectivity Average

This WSR-88D radar product displays the average reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.

Layer Composite Reflectivity Maximum

This WSR-88D radar product displays the maximum reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). Currently, the low layer extends from the surface to 24,000 feet, the mid layer extends from 24,000 feet to 33,000 feet, and high layer extends above 33,000 feet. It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.

Layered Haze

Haze produced when air pollution from multiple line, area or point sources is transported long distances to form distinguishable layers of discoloration in a stable atmosphere.

LCD (Local Climatological Data)

This National Climatic Data Center (NCDC) publication is produced monthly and annually for some 270 United States cities and it's territories. The LCD summarizes temperature, relative humidity, precipitation, cloudiness, wind speed and direction observation.

LCL

1. Abbreviation for "local" or "locally"

2. Lifting Condensation Level - the level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.

LDS

Lightning Detection System

LDT

Local Daylight Time.

Leader

The streamer which initiates the first phase of each stroke of a lightning discharge. The first stroke is led by a steeped leader, which may be preceded by a pilot streamer. All subsequent strokes begin with a dart leader.

Leader Spot

In solar-terrestrial terms, in a magnetically bipolar or multipolar sunspot group, the western part precedes and the main spot in that part is called the leader.

Lee

The side or part that is sheltered or turned away from the wind, such as with a mountain.

Lee Wave

The wavelike effect, characterized by severe updrafts and downdrafts, that occurs in the lee of a mountain range when rapidly flowing air is lifted up the steep front of a mountain range. Compare mountain wave.

Leeside Low

Extratropical cyclones that form on the downwind (lee) side of a mountain chain. In the United States, they frequently form on the eastern side of the Rockies and Sierra Nevadas.

Leeward

The side away from the wind. Compare windward.

Left Exit Region

Used interchangably with Left Front Quadrant; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.

Left Front Quadrant

Used interchangably with Left Exit Region; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.

Left Mover

A thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm.

Length

In hydrologic terms, the distance in the direction of flow between two specific points along a river, stream, or channel.

Lentic System

In hydrologic terms, a nonflowing or standing body of fresh water, such as a lake or pond

Lenticular Cloud

A very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.

Levee

(Dike) In hydrologic terms, a long, narrow embankment usually built to protect land from flooding. If built of concrete or masonary the structure is usually referred to as a flood wall. Levees and floodwalls confine streamflow within a specified area to prevent flooding. The term "dike" is used to describe an embankment that blocks an area on a reservoir or lake rim that is lower than the top of the dam.

Level of Free Convection

(LFC) - The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

LEWP

Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line. The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of a LEWP.

LFC

An acronym for Level of Free Convection- the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere.

LFT

Lift

LGT

Light

LGWV

Long Wave

LI

Lifted Index. A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

Lid

(Also called cap.) A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent. This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC. The intensity of the cap is measured by its convective inhibition. The term capping inversion is sometimes used, but an inversion is not necessary for the conditions producing convective inhibition to exist.

Lid

A layer of warm air several thousand feet above the earth's surface which suppresses or delays the development of thunderstorms.

LIFR

Low Instrument Flight Rules

Lifted Index

(abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

Lifting Condensation Level

(LCL) - The level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.

Light Bridge

In solar-terrestrial terms, it is observed in white light, a bright tongue or streaks penetra- ting or crossing sunspot umbrae. The appearance of a light bridge is frequently a sign of impending region division or dissolution

Lightning

(abbrev. LTNG) A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground or between the ground and a cloud.

Lightning Channel

The irregular path through the air along which a lightning discharge occurs. A typical discharge of flash between the ground and the cloud is actually a composite flash which is composed of several sequential lightning strokes, each of which is initiated by a leader and terminated by a return streamer.

Lightning Discharge

The series of electrical processes by which charge is transferred along a channel of high ion density between electrical charge centers of opposite sign. This can be between a cloud and the Earth's surface of a cloud-to-ground discharge.

Lightning Stroke

Any of a series of repeated electrical discharges comprising a single lightning discharge (strike). Specifically, in the case of a cloud-to-ground discharge, a leader plus its subsequent return streamer.

Likely

(abbrev. LKLY) In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

Limb

In solar-terrestrial terms, the edge of the solar disk.

Limb Flare

In solar-terrestrial terms, a solar flare seen at the edge (Limb) of the sun.

Limnology

In hydrologic terms, the branch of hydrology that pertains to the study of lakes

Line Echo Wave Pattern

(abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.

Line Source

An array of pollutant sources along a defined path that can be treated in dispersion models as an aggregate uniform release of pollutants along a line. Example: the sum of emissions from individual cars traveling down a highway can be treated as a line source. Compare area source and point source.

Liquid Water Equivalent

Same as Water Equivalent; the liquid content of solid precipitation that has accumulated on the ground (snow depth). The accumulation may consist of snow, ice formed by freezing precipitation, freezing liquid precipitation, or ice formed by the refreezing of melted snow.

Lithometeor

Atmospheric phenomena which affect the state of the atmosphere. They constitute dry particles that hang suspended in the atmosphere, such as dust, smoke, sand, and haze.

Lithosphere

In hydrologic terms, that part of the earth which is composed predominantly of rocks (either coherent or incoherent, and including the disintegrated rock materials known as soils and subsoils), together with everything in this rocky crust.

Littoral Zone

In hydrologic terms, the area on, or near the shore of a body water

Live Capacity

In hydrologic terms, the total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation storage and exclusive flood control storage.

LIVV

Lifted Index Vertical Velocity

LKLY

Likely- In probability of precipitation statements, the equivalent of a 60 or 70 percent chance.

LLJ

Low Level Jet - A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer). The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

LLWS

Low Level Wind Shear

LMTD

limited

LN

Line

Loaded Gun (Sounding)

[Slang], a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.

Local Convective Wind

In fire weather terminology, local thermally driven winds arising over a comparatively small area and influenced by local terrain. Examples include sea and land breezes, lake breezes, diurnal mountain wind systems and columnar convective currents.

Lock & Dam (L&D)

Used in navigable rivers, lock and dams allow for large ships to move from one level of water to another.

Lofting

A pattern of plume dispersion in a stable boundary layer topped by a neutral layer, in which the upper part of the plume disperses upward while the lower part of the plume undergoes little dispersion.

Long Term Retention

Retention of data for 5 years to satisfy requirements for local studies and to support litigation.

Longitude

The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.

Longwave Radiation

A term used to describe the infrared energy emitted by the earth and atmosphere at wavelengths between about 5 and 25 micrometers. Compare shortwave radiation.

Longwave Trough

A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration.

Loop Prominence System

(abbrev. LPS) In solar-terrestrial terms, a system of loop prominences associated with major flares.

Looping

A pattern of plume dispersion in an unstable atmosphere, in which the plume undergoes marked vertical oscillations as it is alternately affected by rising convective plumes and the subsiding motions between the plumes.

LOPRES

low pressure

LORAN

Long Range Navigation, a system of long range navigation whereby latitude and longitude are determined from the time displacement of radio signals from two or more fixed transmitters.

Lotic System

In hydrologic terms, a flowing body of fresh water, such as a river or stream.

Low

A region of low pressure, marked as "L" on a weather map. A low center is usually accompanied by precipitation, extensive cloudiness, and moderate winds. See Cyclone.

Low Frequency

(abbrev. LF) The portion of the radio frequency spectrum from 30 to 300 kHz.

Low Level Jet

(abbrev. LLJ)- A region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).

The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

Low Pressure System

An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.

Low Water Advisory

An advisory to describe water levels which are significantly below average levels over the Great Lakes, coastal marine zones, and any tidal marine area, waterway, or river inlet within or adjacent to a marine zone that would potentially be impacted by low water conditions creating a hazard to navigation.

Lowland Flooding

In hydrologic terms, inundation of low areas near the river, often rural, but may also occur in urban areas.

LP Storm

Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

LPS

Loop Prominence System- In solar-terrestrial terms, a system of loop prominences associated with major flares.

LRG

Large

LSR

Local Storm Report. A product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events

LST

Local Standard Time

LTD

Limited

LTL

Little

LTLCG

Little Change

LTNG

Lightning- A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground or between the ground and a cloud.

LTR

Later

LTST

latest

LVL

Level

LVLS

levels

LWR

Lower

Lysimeter

In hydrologic terms, a device to measure the quantity or rate of downward water movement through a block of soil usually undisturbed, or to collect such percolated water for analysis as to quality

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