Mammatus Clouds - Rounded, smooth, sack-like protrusions
hanging from the underside of a cloud (usually a thunderstorm
anvil). Mammatus clouds often accompany severe thunderstorms, but do
not produce severe weather; they may accompany non-severe storms as
well. See Figs. 3 (HP storm), 5 (LP storm), and 7 (supercell).
MCC - Mesoscale Convective Complex. A large MCS, generally
round or oval-shaped, which normally reaches peak intensity at
night. The formal definition includes specific minimum criteria for
size, duration, and eccentricity (i.e., "roundness"), based on the
cloud shield as seen on infrared satellite photographs:
Size: Area of cloud top -32 degrees C or less: 100,000 square
kilometers or more (slightly smaller than the state of Ohio), and
area of cloud top -52 degrees C or less: 50,000 square kilometers or
Duration: Size criteria must be met for at least 6 hours.
Eccentricity: Minor/major axis at least 0.7.
MCCs typically form during the afternoon and evening in the
form of several isolated thunderstorms, during which time the
potential for severe weather is greatest. During peak intensity, the
primary threat shifts toward heavy rain and flooding.
MCS - Mesoscale Convective System. A complex of thunderstorms
which becomes organized on a scale larger than the individual
thunderstorms, and normally persists for several hours or more. MCSs
may be round or linear in shape, and include systems such as
tropical cyclones, squall lines, and MCCs (among others). MCS often
is used to describe a cluster of thunderstorms that does not satisfy
the size, shape, or duration criteria of an MCC.
Medium Range - In forecasting, (generally) three to seven
days in advance.
Meridional Flow - Large-scale atmospheric flow in which the
north-south component (i.e., longitudinal, or along a meridian) is
pronounced. The accompanying zonal (east-west) component often is
weaker than normal. Compare with zonal flow.
*Mesocyclone - A storm-scale region of rotation, typically
around 2-6 miles in diameter and often found in the right rear flank
of a supercell (or often on the eastern, or front, flank of an HP
storm). The circulation of a mesocyclone covers an area much larger
than the tornado that may develop within it.
Properly used, mesocyclone is a radar term; it is defined as a
rotation signature appearing on Doppler radar that meets specific
criteria for magnitude, vertical depth, and duration. Therefore, a
mesocyclone should not be considered a visually-observable
phenomenon (although visual evidence of rotation, such as curved
inflow bands, may imply the presence of a mesocyclone).
Mesohigh - A mesoscale high pressure area, usually associated
with MCSs or their remnants.
Mesolow (or Sub-synoptic Low) -
A mesoscale low-pressure
center. Severe weather potential often increases in the area near
and just ahead of a mesolow.
Mesolow should not be confused with mesocyclone, which is a
A regional network of observing stations (usually
surface stations) designed to diagnose mesoscale weather features
and their associated processes.
Mesoscale - Size scale referring to weather systems smaller
than Squall lines, MCCs, and MCSs are examples of mesoscale weather
*Microburst - A small, concentrated downburst affecting an
area less than 4 kilometers (about 2.5 miles) across. Most
microbursts are rather short-lived (5 minutes or so), but on rare
occasions they have been known to last up to 6 times that long.
Mid-level Cooling - Local cooling of the air in middle levels of
the atmosphere (roughly 8 to 25 thousand feet), which can lead to
destabilization of the entire atmosphere if all other factors are
equal. Mid-level cooling can occur, for example, with the approach
of a mid-level cold pool.
Moderate Risk (of severe thunderstorms) - Severe thunderstorms are
expected to affect between 5 and 10 percent of the area. A moderate
risk indicates the possibility of a significant severe weather
episode. See high risk, slight risk, convective outlook.
Moisture Advection - Transport of moisture by horizontal
Moisture Convergence - A measure of the degree to which moist
air is converging into a given area, taking into account the effect
of converging winds and moisture advection. Areas of persistent
moisture convergence are favored regions for thunderstorm
development, if other factors (e.g., instability) are favorable.
Morning Glory - An elongated cloud band, visually similar to
a roll cloud, usually appearing in the morning hours, when the
atmosphere is relatively stable. Morning glories result from
perturbations related to gravitational waves in a stable boundary
layer. They are similar to ripples on a water surface; several
parallel morning glories often can be seen propagating in the same
MRF - Medium-Range Forecast model; one of the operational
forecast models run at NCEP. The MRF is run once daily, with
forecast output out to 240 hours (10 days).
Multi-cell(ular) Thunderstorm -
A thunderstorm consisting of
two or more cells, of which most or all are often visible at a given
time as distinct domes or towers in various stages of development.
Nearly all thunderstorms (including supercells) are multi-cellular,
but the term often is used to describe a storm which does not fit
the definition of a supercell.
*Multiple-vortex (or Multi-vortex) Tornado -
a tornado in
which two or more condensation funnels or debris clouds are present
at the same time, often rotating about a common center or about each
other. Multiple-vortex tornadoes can be especially damaging. See
Mushroom - [Slang], a thunderstorm with a well-defined anvil
rollover, and thus having a visual appearance resembling a mushroom.