Weather Glossary: M's

1. To provide follow-up information on Special Marine Warnings and to cancel all or part of a warning.
2. To describe short duration, non-severe, but potentially hazardous conditions which sustained winds or frequent gusts are less than 34 knots for 2 hours or less. Short-lived increases in winds, although below threshold for Special Marine Warnings, that may make small craft handling difficult especially for inexperienced boaters.
3. To provide information for a variety of conditions not covered by warnings or routine forecasts (e.g., low water conditions, dense fog, etc.).
4. To discuss increasing or decreasing winds and to convey details on possible later warnings.

1. Uncorrelated Shear - It is a sufficiently strong circulation detected on only one elevation angle within a thunderstorm.
2. 3-Dimensional Correlated Shear - It is a sufficiently strong circulation detected on two or more elevation angles within a thunderstorm, but less than two of the features are symmetrical. It is displayed on the radar display as a thin yellow circle.
3. Mesocyclone - It is a sufficiently strong circulation detected on two or more elevation angles which are symmetrically linked within a thunderstorm.

A storm-scale region of rotation, typically around 2-6 miles (3-10 km) 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.

It will appear as a yellow solid circle on the Doppler velocity products. 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).

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°C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52°C or less: 50,000 square kilometers or more.
• 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.

When conditions actually begin to shape up for severe weather, SPC often issues a Mesoscale Discussion (MCD) statement anywhere from roughly half an hour to several hours before issuing a weather watch.

SPC also puts out MCDs for hazardous winter weather events on the mesoscale, such as locally heavy snow, blizzards and freezing rain (see below). MCDs are also issued on occasion for heavy rainfall, convective trends, and other phenomena, when the forecaster feels he/she can provide useful information that is not readily available or apparent to field forecasters.

MCDs are based on mesoscale analysis and interpretation of observations and of short term, high resolution numerical model output.

The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential).

Severe thunderstorm MCDs can help you get a little extra lead time on the weather and allow you to begin gearing up operations before a watch is issued.

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.

One of 2 categories of downbursts (the other category is called a macroburst). This downburst has an affected outflow area of less than 2.5 miles (4 kilometers) wide and peak winds lasting less than 5 minutes. They may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.

They can be sub-classified into either dry or wet microburst depending on how much (or little) rain accompanies the microburst when it reaches the ground. 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.

1. Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed, or
2. Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as Moderate Chop.

1. the weight of water which the soil, after saturation, will retain against a centrifugal force 1,000 times the force of gravity, to
2. the weight of the soil when dry.

These thunderstorms are organized in clusters of at least 2-4 short-lived cells. Each cell generates a cold air outflow and these individual outflows combine to form a large gust front.

Convergence along the gust front causes new cells to develop every 5 to 15 minutes. The cells move roughly with the mean wind. However, the area (storm) motion usually deviates significantly from the mean wind due to discrete propagation (new cell development) along the gust front.

The multicellular nature of the storm is usually apparent on radar with multiple reflectivity cores and maximum tops.

Severe multicell thunderstorms occur in environments possessing moderate vertical wind shear and moderate to large positive buoyancy (CAPE).

The moderate vertical wind shear leads to the development of a non-symmetric surface convergence pattern associated with the thunderstorm outflow, with the strongest convergence taking place on the downwind from the storm's motion.

New cells developing along this enhanced convergence zone tend to move in the same direction as the older cells, which increases the time that the new cells will experience low-level convergence and tap into the warm, moist inflow air. Cell regeneration in the convergence zone along an outflow boundary is the primary physical mechanism for maintaining a multicell storm. Moderate to large buoyancy ensures that strong updrafts are possible.

Multicell thunderstorms usually develop in an environment where the vertical wind shear is characterized by a straight-line hodograph. The name describes well what it looks like on the hodograph. The Bulk Richardson number for a multicellular thunderstorm environment is typically above 50.

These thunderstorms can become severe producing hail greater than 1" (2.5 cm) hail, wind gusts greater than 58 mph (93 km/h), and possibly a tornado. 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.