ECMWF - European Centre for Medium-Range Weather Forecasting.
Operational references in forecast discussions typically refer to
the ECMWF's medium-range forecast model. See MRF, UKMET.
Elevated Convection - Convection occurring within an elevated layer,
i.e., a layer in which the lowest portion is based above the earth's
surface. Elevated convection often occurs when air near the ground
is relatively cool and stable, e.g., during periods of isentropic
lift, when an unstable layer of air is present aloft. In cases of
elevated convection, stability indices based on near-surface
measurements (such as the lifted index) typically will underestimate
the amount of instability present. Severe weather is possible from
elevated convection, but is less likely than it is with
Energy Helicity Index (or EHI) -
An index that incorporates
vertical shear and instability, designed for the purpose of
forecasting supercell thunderstorms. It is related directly to
storm-relative helicity in the lowest 2 km (SRH, in m2/s2) and CAPE
(in j/kg) as follows:
EHI = (CAPE x SRH)/160,000.
Thus, higher values indicate unstable conditions and/or strong
vertical shear. Since both parameters are important for severe
weather development, higher values generally indicate a greater
potential for severe weather. Values of 1 or more are said to
indicate a heightened threat of tornadoes; values of 5 or more are
rarely observed, and are said to indicate potential for violent
tornadoes. However, there are no magic numbers or critical threshold
values to confirm or predict the occurrence of tornadoes of a
Enhanced V - A pattern seen on satellite infrared photographs of
thunderstorms, in which a thunderstorm anvil exhibits a V-shaped
region of colder cloud tops extending downwind from the thunderstorm
core. The enhanced V indicates a very strong updraft, and therefore
a higher potential for severe weather.
Enhanced V should not be confused with V notch, which is a radar
Enhanced Wording - An option used by the SPC in tornado and severe
thunderstorm watches when the potential for strong/violent
tornadoes, or unusually widespread damaging straight-line winds, is
high. The statement "THIS IS A PARTICULARLY DANGEROUS SITUATION WITH
THE POSSIBILITY OF VERY DAMAGING TORNADOES" appears in tornado
watches with enhanced wording. Severe thunderstorm watches may
include the statement "THIS IS A PARTICULARLY DANGEROUS SITUATION
WITH THE POSSIBILITY OF EXTREMELY DAMAGING WINDS," usually when a
derecho event is occurring or forecast to occur. See PDS watch.
Entrance Region - The region upstream from a wind speed maximum
in a jet stream (jet max), in which air is approaching (entering)
the region of maximum winds, and therefore is accelerating. This
acceleration results in a vertical circulation that creates
divergence in the upper-level winds in the right half of the
entrance region (as would be viewed looking along the direction of
flow). This divergence results in upward motion of air in the right
rear quadrant (or right entrance region) of the jet max. Severe
weather potential sometimes increases in this area as a result. See
also exit region, left exit region.
Equilibrium Level (or EL) - On a sounding, the level above the
level of free convection (LFC) at which the temperature of a rising
air parcel again equals the temperature of the environment. (See
Fig. 6, sounding.)
The height of the EL is the height at which thunderstorm updrafts no
longer accelerate upward. Thus, to a close approximation, it
represents the height of expected (or ongoing) thunderstorm tops.
However, strong updrafts will continue to rise past the EL before
stopping, resulting in storm tops that are higher than the EL. This
process sometimes can be seen visually as an overshooting top or
The EL typically is higher than the tropopause, and is a more
accurate reference for storm tops.
Eta Model - One of the operational numerical forecast models
run at NCEP. The Eta is run twice daily, with forecast output out to
Exit Region - The region downstream from a wind speed maximum
in a jet stream (jet max), in which air is moving away from the
region of maximum winds, and therefore is decelerating. This
deceleration results in divergence in the upper-level winds in the
left half of the exit region (as would be viewed looking along the
direction of flow). This divergence results in upward motion of air
in the left front quadrant (or left exit region) of the jet max.
Severe weather potential sometimes increases in this area as a
result. See also entrance region, right entrance region.