Laminar - Smooth, non-turbulent. Often used to describe cloud
formations which appear to be shaped by a smooth flow of air
travelling in parallel layers or sheets.
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.
Left Front Quadrant (or Left Exit Region) -
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
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. See also right mover.
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.
Lifted Index (or 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
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.
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).
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
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.
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 (Fig.
5). 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