Occluded Mesocyclone - A mesocyclone in which air from the
rear-flank downdraft has completely enveloped the circulation at low
levels, cutting off the inflow of warm unstable low-level air.
Orographic - Related to, or caused by, physical geography (such
as mountains or sloping terrain).
Orographic Lift - Lifting of air caused by its passage up and
over mountains or other sloping terrain.
Orphan Anvil - [Slang], an anvil from a dissipated
thunderstorm, below which no other clouds remain.
Outflow Boundary - A storm-scale or mesoscale boundary
separating thunderstorm-cooled air (outflow) from the surrounding
air; similar in effect to a cold front, with passage marked by a
wind shift and usually a drop in temperature. Outflow boundaries may
persist for 24 hours or more after the thunderstorms that generated
them dissipate, and may travel hundreds of miles from their area of
origin. New thunderstorms often develop along outflow boundaries,
especially near the point of intersection with another boundary
(cold front, dry line, another outflow boundary, etc.; see triple
Overhang - Radar term indicating a region of high
reflectivity at middle and upper levels above an area of weak
reflectivity at low levels. (The latter area is known as a weak-echo
region, or WER.) The overhang is found on the inflow side of a
thunderstorm (normally the south or southeast side).
Overrunning - A weather pattern in which a relatively warm
air mass is in motion above another air mass of greater density at
the surface. Embedded thunderstorms sometimes develop in such a
pattern; severe thunderstorms (mainly with large hail) can occur,
but tornadoes are unlikely.
Overrunning often is applied to the case of warm air riding up over
a retreating layer of colder air, as along the sloping surface of a
warm front. Such use of the term technically is incorrect, but in
general it refers to a pattern characterized by widespread clouds
and steady precipitation on the cool side of a front or other
*Overshooting Top (or Penetrating Top) -
protrusion above a thunderstorm anvil, representing a very strong
updraft and hence a higher potential for severe weather with that
storm. A persistent and/or large overshooting top (anvil dome) often
is present on a supercell. A short-lived overshooting top, or one
that forms and dissipates in cycles, may indicate the presence of a
pulse storm or a cyclic storm. See Figs. 3 (HP storm), 5 (LP storm),
and 7 (supercell).