NOAA's National Weather Service Weather Forecast Office

 
L

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/Land Breeze: A lake breeze occurs when prevailing winds blow off the water, while a land breeze indicates winds blowing from land to sea. Both are caused by the difference in surface temperature (heating) of the land and water. As a result, a lake breeze occurs during the day while a land breeze happens at night.

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.  In Michigan, the criteria for its issuance is a pure lake effect snow event that is forecasted to produce snow (average of forecast range) greater than 3 inches, but less than warning criteria (6 inches in Lower Michigan and 8 inches in Upper Michigan) in 12 hours.  If the forecaster feels that it is warranted, he or she can issued it for amounts less than the minimum criteria.  For example, it may be issued for the first snow of the season or when snow has not fallen in long while.  In cases where you have a system snow  transitioning to a pure lake effect snow, the criteria for defining this transition is when the mid and high clouds have been stripped away leaving only the stratocumulus clouds behind.

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 low pressure system) may pose a hazard or it is life threatening.  The criteria for this warning varies from area to area. In Lower Michigan, the criteria is a pure lake effect snow event that is forecasted to produce snow greater than or equal to 6 inches in 12 hours, or greater than 8 inches in 24 hours.  In Upper Michigan, the criteria is a pure lake effect snow event that is forecasted to produce snow greater than or equal to 8 inches in 12 hours, or greater than 10 inches in 24 hours.  In cases where you have a system snow  transitioning to a pure lake effect snow, the criteria for defining this transition is when the mid and high clouds have been stripped away leaving only the stratocumulus clouds behind.

Lake St. Clair Forecast:  Until the spring of 1996, the lake forecast for Lake St. Clair was issued as an Open Great Lakes Forecast.  It is now a combination of Open Great Lakes Forecast and the Great Lakes Nearshore Forecast.  This was done due to the size of the lake.  This product includes:  winds (in 8 points of the compass), waves, and significant weather, and sky conditions.  Unlike the Open Great Lakes Forecast, this product does not include a MAFOR at the end of the narrative portion of the lake forecast.  Unlike the Great Lake Nearshore Forecast, this product is issued all year round.  If the forecaster issues a Small Craft Advisory, Gale Warning, and Storm Warning for Lake St. Clair, it will appear as a headline at the beginning of this product.  Like other marine forecast products, it is issued 4 times a day (3-4 AM, 9-10 AM, 3-4 PM, and 9-10 PM) and each forecast will contain 3 time periods covering usually 36 hours.  This product is issued by NWFO Detroit/Pontiac Michigan.

Lakeshore Statement:  The local National Weather Service Offices with Great Lakes responsibility will issue this product to alert the public when their is either a potential or actual reports of minor Great Lakes lakeshore flooding and erosion.  This means that the lakeshore flooding or erosion would not cause too much damage to property, but it would be an inconvenience to living or driving in those areas.

Lakeshore Warning:  The local National Weather Service Offices with Great Lakes responsibility will issue this product to alert the public when there is either a potential or actual reports of major Great Lakes lakeshore flooding and erosion.  If precautions are not taken, this could pose a considerable threat to life and property.

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.

Landspout:  Slang for 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.  The global average rate of temperature change with height in the atmosphere is 6.5 degrees C/km.  The adiabatic  lapse rate (or dry adiabatic lapse rate) is the normal rate of change (9.8 degrees C/km) for a dry parcel of air that is moved up or down and cools or warms as the pressure changes.  The wet (moist) adiabatic lapse rate (4.9 degrees C/km) is the rate at which satuarted air cools as it ascends.

LARC (Limited Automatic Report Collector):  An electronic device that interfaces a river or precipitation gage with a telephone line making it possible for remote computers to call a gaging site and retrieve data. Eventually LARCs will Replace DARCs.

Large-scale:  See synoptic-scale.

LAWEB:  An acronym for Great Lakes Weather Bulletin.  This consists of weather reports from Canadian and United States lighthouses, automated stations located adjacent to The Great Lakes, and buoys from 0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC.  These reports are carried as part of the appropriate Continuous Marine Broadcasts for 3 hours after the time of the observation.

Layer Composite Reflectivity Average (LRA):  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 (LRM):  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.

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.  Most monthly publications also contain the 3-hourly weather observations for that month and an hourly summary of precipitation.  Annual LCD publications contain a summary of the past calendar year as well as historical averages and extremes.The LCD contains 3-hourly, daily, and monthly values. The annual issue contains the year in review plus normals, means and extremes.

LCL:  An acronym for Lifted Condensation Level.  See Lifted Condensation Level.

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.

Lee Effect:  The effect of topography on winds to the lee (downwind) side of an obstacle such as a steep island, cliff, or mountain range.

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/Windward: Leeward is on the side facing the direction toward which the wind is blowing. On the other hand, windward is on the side facing the direction away from the wind.

Left Front Quadrant (or 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. Also, see entrance region and 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. Also, see right mover.

Length:  The distance in the direction of flow between two specific points along a river, stream, or channel.

Lentic System:  A nonflowing or standing body of fresh water, such as a lake or pond. Compare lotic system.

Levee (Dike):  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): It is the height at which a parcel of air lifted dry-adiabatically until saturated (this is the Lifting Condensation Level) and moist-adiabatically thereafter would first become warmer (less dense) than the surrounding air. At this point, the buoyancy of the parcel would accelerate upward without further need for forced lift.

LEWP:  An acronym for Line Echo Wave Pattern.  See Line Echo Wave Pattern.

LFC:  An acronym for Level of Free Convection.  See Level of Free Convection.

LFWS:  A generic term for any type of Local Flood Warning System.

LI:  An acronym for Lifted Index.  See Lifted Index.

Lid: see Cap Strength

Lifted Index (LI): It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet) denotes the LI. Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the "Showalter Index (SWI)". The greater negative values of LI indicate energy available for parcel ascent. The following table shows what LI values mean:
 

Light Icing:  The rate of ice accumulation that may create a problem if the flight is prolonged in this environment (over one hour).  Occasional use of de-icing equipment removes/prevents accumulation.  It does not present a problem if de-icing/anti-icing equipment is used.  This standard of reporting this type of icing was based on a recommendation set forth by the subcommittee for Aviation Meterorological Services in the Office of the Federal Coordinator for Meteorology in November 1968.  The convetion has been to designate icing intensity in terms of its operational effect upon reciprocating engine, straight wing transport aircraft as used by commuter operators.

Lifting Condensation Level (LCL): It is the height at which a parcel of air becomes saturated when lifted dry-adiabatically.

Lightning: A sudden visible flash of energy and light caused by an electrical discharges from thunderstorms.

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 Flash: The total luminous phenomenon accompanying a lightning discharge. It may be composed of one to a few tens of strokes that use essentially the same channel to ground.

Lightning Ground Flash Density: The number of cloud-to-ground flashes per unit time per unit area.

Lightning Ignition Efficiency:  Ignition efficiency is calculated by algorithm, using the National Fire Danger Rating System (NFDRS) 100 hour fuel moisture and specialized fuel type maps for the western United States to indicate probability of ignition, given occurrence of lightning.

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:  A National Weather Service precipitation descriptor for a 60 or 70 percent chance of measurable precipitation (0.01 inch).  When the precipitation is convective in nature, the term numerous will occasionally be used.  See Precipitation Probability (PoP).

Lilapsophobia:  The fear of tornadoes and hurricanes.

Limited Automatic Report Collector (LARC): An electronic device that interfaces a river of precipitation gage with a telephone line making it possible for remote computers to call a gaging site and retrieve data.

Limnology:  The branch of hydrology that pertains to the study of lakes.

Limnophobia:  The fear of lakes.

Line Echo Wave Pattern (LEWP): A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate. A meso-high pressure area is usually present behind the accelerating thunderstorms. A meso-low pressure area is usually present at the crest of the wave. 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.

Lining:  A coating of concrete, rubber, or plastic to a canal, tunnel, shaft or reservoir to provide water-tightness, prevent erosion, reduce friction, or support the periphery of the structure.

Lithosphere:  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:  The area on, or near the shore of a body water.

Live Capacity:  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.

Live Fuel Moisture -- Greenness Maps:  Four vegetation greenness maps are derived weekly from Normalized Difference Vegetation Index (NDVI) data observed by satellites and provided by the EROS Data Center, U.S. Geological Survey. These maps have a 1 kilometer (.6 mile) spatial resolution. Maps with historical references (RG and DA) are based on the years 1989 through 1995. The derived maps are:

Visual Greenness Maps:  These maps portray vegetation greenness compared to a very green reference such as an alfalfa field or a golf course. The resulting image is similar to what you would expect to see from the air. Normally dry areas will never show as green as normally wetter areas.
 
Relative Greenness Maps:  These maps portray how green the vegetation is compared to how green it has been historically (since 1989). Because each pixel is normalized to its own historical range, all areas (dry to wet) can appear fully green at some time during the growing season.
 
Departure from Average Greenness Maps:  These maps portray how green each pixel is compared to its average greenness for the current week of the year.
 
Live Moisture Maps:  These maps portray experimental live vegetation moisture with values ranging from 50 to 250 percent of dry weight.

LLJ:  An acronym for Low Level Jet.  See Low Level Jet.

Loaded Gun (Sounding):  Slang for 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 Flooding:  Flooding conditions over a relatively limited (localized) area.
 
Local Flood Warning System (LWFS):  A general designator for a network of stream and rain gages implemented by a community or local government to monitor hydrologic events as they occur. LFWS gages are either read manually by spotters or fitted with radio transmitter to communicate data to a computerized base station (see IFLOWS and ALERT).

Log and Safety Boom:  A net-like device installed in a reservoir, upstream of the principal spillway, to prevent logs, debris and boaters from entering a water discharge facility or spillway.

Logarithmic Scale:  10^x, where x is a number.

Long Term Storage Dams:  Reservoirs used for recreational use or storage of irrigation, municipal or industrial water. Because water is impounded on a "permanent" basis, the design of these dams is more complex than for tailings or flood control detention dams. A long term storage dam may include an impermeable core surrounded by shell materia, have many types of drains and filters, outlet works, with gates and valves, seepage collection boxes, and possibly several spillways. The capacity of the spillway is dependant upon the downstream hazard potential.

Longwave Trough:  A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

Look Angle:  A given radar's "perspective" on a storm; i.e., the angle at which its antenna beam hits it. By using more than one radar with different look angles, multiple Doppler analysis can be performed to retrieve the 2D and 3D winds.

Lotic System:  A flowing body of fresh water, such as a river or stream. Compare lentic system.

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 Clouds: The bases of these clouds range from near the surface to about 6,500 feet in middle latitudes. These clouds are almost entirely of water, but the water may be supercooled at sub-freezing temperatures. Low clouds at sub-freezing temperatures can also contain snow and ice particles. The two most common members of this family are stratus and stratocumulus.

Low Drifting (DR): A descriptor used to describe snow, sand, or dust raised to a height of less than 6 feet above the ground.

Lowland Flooding:  Inundation of low areas near the river, often rural, but may also occur in urban areas.
 
Low Level Jet (LLJ): It often forms at 1-1.5 km under the exit region (the place just ahead of a speed maximum) of an upper-level jet (ULJ) streak. It has a strong isallobaric component due to the pressure rise/fall pattern under the right exit/left exit region of the jet streak, respectively. It is usually at a significant angle with respect to the ULJ and has a strong southerly component. This enhances the warm, moist air ahead of an upper level front-jet system. This is common over the Plains states at night during the warm season (spring and summer).  Another type of a LLJ is called a "Nocturnal Jet". See Nocturnal Jet.

Low Water Statements:  The local National Weather Service Offices with Great Lakes responsibility will issue this product to alert mariners when the Great Lake water levels are too low for safe navigation.

LP Storm (or LP Supercell) - 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.

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

Lygophobia:  The fear of darkness.

Lysimeter:  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.