A squall is a sudden, sharp increase in wind speed which is usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to an increase in the sustained winds over a short time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong mid-level height falls, or mid-level tropospheric cooling, which force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
In most parts of the country, squalls are called subasko characterized by heavy rains driven by blustery winds. Local fishermen at sea are often on the lookout for signs of impending squalls on the open water and rush to shore at its early early hints.
A squall line is an organized line of thunderstorms. It is classified as a multi-cell cluster, meaning a thunderstorm complex comprising many individual updrafts. They are also called multi-cell lines. Squalls are sometimes associated with hurricanes or other cyclones, but they can also occur independently. Most commonly, independent squalls occur along front lines, and may contain heavy precipitation, hail, frequent lightning, dangerous straight line winds, and possibly funnel clouds, tornadoes and waterspouts. Squall lines require significant low-level warmth and humidity, a nearby frontal zone, and vertical wind shear from an angle behind the frontal boundary. The strong winds at the surface are usually a reflection of dry air intruding into the line of storms, which when saturated, falls quickly to ground level due to its much higher density before it spreads out downwind. In England, a squall associated with tempestuous weather is known as a blunk. Significant squall lines with multiple bow echoes are known as derechos.
There are several forms of mesoscale meteorology, including simplistic isolated thunderstorms unrelated to advancing cold fronts, to the more complex daytime/nocturnal Mesoscale Convective System (MCS) and Mesoscale Convective Complex (MCC), to squall line thunderstorms.
The main driving force behind squall line creation is attributed to the process of in-filling of multiple thunderstorms and/or a single area of thunderstorms expanding outward within the leading space of an advancing cold front.
The leading area of a squall line is composed primarily of multiple updrafts, or singular regions of an updraft, rising from ground level to the highest extend of the troposhere, condensing water and building a dark, ominous clouds to one with a noticeable overshooting top and anvil (thanks to synoptic scale winds). Because of the chaotic nature of updrafts and downdrafts, pressure perturbations are important.
Pressure perturbations within an extent of a thunderstorm are noteworthy. With buoyancy rapid within the lower and mid-levels of a mature thunderstorm, one might believe that low pressure dominates in the mesoscale environment. However, this is not the case. With downdrafts ushering colder air from mid-levels, hitting ground and propagating away in all directions, high pressure is to be found widely at surface levels, usually indicative of strong (potentially damaging winds).
Wind shear is an important aspect to measuring the potential of squall line severity and duration. In low to medium shear environments, mature thunderstorms will contribute modest amounts of downdrafts, enough to turn will aid in create a leading edge lifting mechanism - the gust front. In high shear environments created by opposing low level jet winds and synoptic winds, updrafts and consequential downdrafts can be much more intense (common in supercell mesocyclones). The cold air outflow leaves the trailing area of the squall line to the mid-level jet, which aids in downdraft processes.
As thunderstorms fill into a distinct line, strong leading-edge updrafts - occasionally visible to a ground observer in the form of a shelf cloud, appear as an ominous sign of potential severe weather.
Beyond the strong winds because of updraft/downdraft behavior, heavy rain (and hail) is another sign of a squall line. In the winter, squall lines can occur albeit less frequently - bringing heavy snow and/or thunder and lightning - usually over inland lakes (i.e. Great Lakes region).
Following the initial passage of a squall line, light to moderate stratiform precipitation is also common. A Bow echo is frequently seen on the northern and southern most reaches of squall line thunderstorms (via satellite imagery. This is where the northern and southern ends curl backwards towards the middle portions of the squall line, making a "bow" shape. Bow echoes are frequently featured within supercell mesoscale systems.
The northern end of the squall line is commonly referred to as the cyclonic end, with the southern side rotating anticyclonically. Because of the coriolis force, the northern end may evolve further, creating a "comma shaped" mesolow, or may continue in a squall-like pattern.
As supercell or multi-cell thunderstorms disappate because of a weak shear, poor lifting mechanisms: (e.g. considerable terrain or lack of daytime heating. The squall line associated gust front may outrun the squall line, the synoptic scale low may fill - leading to a weaking of a cold front, or the thunderstorm has exhausted its updrafts, becoming purely a downdraft dominated system. The areas of disappating squall line thunderstorms may be regions of low CAPE, low humidity, insufficient wind shear, or poor synoptic dynamics (e.g. an upper level low filling) leading to frontolysis.
From here, a general thinning of a squall line will occur: within, winds decaying with time, outflow boundaries weakening updrafts substantially, and clouds losing their thickness.