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Thundersnow, also known as a winter thunderstorm or a thundersnowstorm, is an unusualkind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically falls in regions of strong upward motion within the cold sector of an extratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorm, but the top of the cumulonimbus cloud is usually quite low. In addition to snow, graupel or hail may fall as well.
Thundersnow, while relatively rare anywhere, is more common with lake-effect snow in the Great Lakes area of the United States and Canada, the midwestern United States, and the Great Salt Lake. Thundersnow also occurs in Halifax, Nova Scotia, sometimes several times per winter season. Bozeman, Montana also sees thundersnow more often than average with these storms typically occurring in April or May. In the United States, March is the peak month of formation; on average, only 6.4 events are reported per year.
The British Isles and other parts of northwestern Europe occasionally report thunder and lightning during sleet or (usually wet) snow showers during winter and spring. It is also common around Kanazawa and the Sea of Japan, and even around Mount Everest. Low-pressure events in the eastern Mediterranean that originate from polar origin cause copious thundersnow occurrences during winter storms, especially over the elevated provinces of Israel and Jordan, including Amman and Jerusalem. When such storms happen at areas intended for skiing, the mountains are often evacuated for safety.
The South Region of Brazil registered episodes of thundersnow in 1984 and 2005, in the state of Santa Catarina, and in August 2011, in some municipalities of the highland region of Serra Gaúcha, in the southern state of Rio Grande do Sul.
One unique aspect of thundersnow is that the suspended and deposited snowfall act as an acoustic suppressor of the sound of thunder. The thunder from a typical thunderstorm can be heard many miles away, while the thunder from thundersnow can usually only be heard within a 2–3-mile (3.2–4.8 km) radius from the lightning.[ citation needed ]
There are usually three causes of thundersnow:
Lake effect thundersnow occurs after a cold front or shortwave aloft passes over a body of water. This steepens the thermal lapse rates between the lake temperature and the temperatures aloft. A difference in temperature of 25 °C (45 °F) or more between the lake temperature and the temperature at about 1,500 m (4,900 ft) (the 850 hPa level) usually marks the onset of thundersnow, if surface temperatures are expected to be below freezing. However several factors, including other geographical elements, affect the development of thundersnow.
The primary factor is convective depth. This is the vertical depth in the troposphere that a parcel of air will rise from the ground before it reaches the equilibrium (EQL) level and stops rising. A minimum depth of 1,500 m (4,900 ft) is necessary, and an average depth of 3,000 m (9,800 ft) or more is generally accepted as sufficient. Wind shear is also a significant factor. Linear snow squall bands produce more thundersnow than clustered bands; thus a directional wind shear with a change of less than 12 °C (54 °F) between the ground and 2,000 m (6,600 ft) in height must be in place. However, any change in direction greater than 12 °C (54 °F) through that layer will tear the snow squall apart. A bare minimum fetch of 50 km/h (31 mph) is required so that the air passing over the lake or ocean water will become sufficiently saturated with moisture and will acquire thermal energy from the water.
The last component is the echo top or storm top temperature. This must be at least −30 °C (−22 °F). It is generally accepted that at this temperature there is no longer any super cooled water vapour present in a cloud, but just ice crystals suspended in the air. This allows for the interaction of the ice cloud and graupel pellets within the storm to generate a charge, resulting in lightning and thunder.
Synoptic snow storms tend to be large and complex, with many possible factors affecting the development of thundersnow. The best location in a storm to find thundersnow is typically in its NorthWest quadrant (in the Northern Hemisphere, based on observations in the Midwestern United States), within what is known as the "comma head" of a mature extratropical cyclone.Thundersnow can also be located underneath the TROWAL, a trough of warm air aloft which shows up in a surface weather analysis as an inverted trough extending backward into the cold sector from the main cyclone. In extreme cases, thunderstorms along the cold front are transported towards the center of the low-pressure system and will have their precipitation change to snow or ice, once the cold front becomes a portion of the occluded front. The 1991 Halloween blizzard, Superstorm of 1993, and White Juan are examples of such blizzards featuring thundersnow.
Similar to the lake effect regime, thundersnow is usually witnessed in terrain in the cold sector of an extratropical cyclone when a shortwave aloft moves into the region. The shortwave will steepen the local lapse rates, allowing for a greater possibility of both heavy snow at elevations where it is near or below freezing, and occasionally thundersnow.
Thundersnow produces heavy snowfall rates in the range of 5 to 10 cm (2 to 4 in) per hour. Snowfall of this intensity may limit visibilities severely, even during light wind conditions. However, thundersnow is often a part of a severe winter storm or blizzard. Winds of above tropical storm force are frequent with thundersnow. As a result, visibilities in thundersnow are frequently under 2/5th of a mile. Additionally, such wind creates extreme wind chills and may result in frostbite. Finally, there is a greater likelihood that thundersnow lightning will have a positive polarity, which is associated with a greater destructive potential than the more common negatively-charged lightning.
A blizzard is a severe snowstorm characterized by strong sustained winds of at least 56 km/h (35 mph) and lasting for a prolonged period of time—typically three hours or more. A ground blizzard is a weather condition where snow is not falling but loose snow on the ground is lifted and blown by strong winds. Blizzards can have an immense size and usually stretch to hundreds or thousands of kilometres.
Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. The lower layer of air, heated up by the lake water, picks up water vapor from the lake and rises up through the colder air above; the vapor then freezes and is deposited on the leeward (downwind) shores.
A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form. In temperate continental climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. Very rarely, they may form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeastern United States.
An Alberta clipper, also known as a Canadian clipper, is a fast moving low pressure area weather system which generally affects the central provinces of Canada and parts of the Upper Midwest, Great Lakes, and New England, precipitating a sudden temperature drop and sharp winds. Alberta clippers take their name from Alberta, the province from which they appear to descend, and from clipper ships of the 19th century, one of the fastest ships of that time.
A panhandle hook is a relatively infrequent winter storm system whose cyclogenesis occurs in the South to southwestern United States from the late fall through winter and into the early spring months. They trek to the northeast on a path towards the Great Lakes region, as the southwesterly jet streams are most prevalent, usually affecting the Midwestern United States and Eastern Canada. Panhandle hooks account for some of the most memorable and deadly blizzards and snowstorms in North America. The name is derived from the region of surface cyclogenesis in the Texas panhandle and Oklahoma panhandle regions. In some winters, there are no panhandle hook storms; in others, there are several.
The 2004 Christmas Eve United States winter storm was a rare weather event that took place in Louisiana and Texas in the United States on December 24, 2004, before the storm moved northeast to affect the coastal sections of the Mid-Atlantic states and New England in the succeeding few days. This was a different storm from the historic event that struck the Midwest and southern Canada around December 23 from another cyclone which preceded this storm. The event involved a thin band of snowfall with unusually cold temperatures for the middle Texas coast, and caused dozens of varied weather records to be shattered. It was the most significant snow for the Texas Gulf Coast, and deep South Texas, since February 1895.
A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.
A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and Mesoscale Convective Complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
A snowsquall, or snow squall, is a sudden moderately heavy snow fall with blowing snow and strong, gusty surface winds. It is often referred to as a whiteout and is similar to a blizzard but is localized in time or in location and snow accumulations may or may not be significant.
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.
The Early Winter 2006 North American storm complex was a severe winter storm that occurred on November 26, 2006, and continued into December 1. It affected much of North America in some form, producing all kinds of severe weather including a major ice storm, blizzard conditions, high winds, extreme cold, a serial derecho and some tornadoes.
Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects of severe weather, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal severe weather phenomena include blizzards (snowstorms), ice storms, and duststorms.
A cold front is the leading edge of a cooler mass of air, replacing at ground level a warmer mass of air, which lies within a fairly sharp surface trough of low pressure. It forms in the wake of an extratropical cyclone, at the leading edge of its cold air advection pattern, which is also known as the cyclone's dry conveyor belt circulation. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is less, a broad shield of rain can move in behind the front, which increases the temperature difference across the boundary. Cold fronts are stronger in the fall and spring transition seasons and weakest during the summer.
Classifications of snow describe and categorize the attributes of snow-generating weather events, including the individual crystals both in the air and on the ground, and the deposited snow pack as it changes over time. Snow can be classified by describing the weather event that is producing it, the shape of its ice crystals or flakes, how it collects on the ground, and thereafter how it changes form and composition. Depending on the status of the snow in the air or on the ground, a different classification applies.
The December 2009 Midwest blizzard was a powerful extratropical cyclone which was of a category which meteorologists refer to as a cyclogenic bomb, a system which shows a drop in central pressure similar to the rapid intensification cycle of a tropical cyclone, more than 1 mbar per hour for 12 to 24 hours or more. A sustained drop averaging more than 2.5 mbar/h is termed explosive deepening/intensification, and this was the case with this rapidly deepening and intensifying storm as it traversed the Midwest and Ontario and on to Québec, Greenland and vicinity. In many locations wind, snowfall, and precipitation moisture content records dating back to the December 2, 1990 storm, the 1976-1978 period, the 1949 blizzard, or even further back were broken, with barometric pressure records falling as well. Both the central pressure (depth) and rate of change and differential over a given distance (intensity) were remarkable, and both caused hurricane-force winds in places.
Global storm activity of 2006 profiles the major worldwide storms, including blizzards, ice storms, and other winter events, from January 1, 2006 to December 31, 2006. Winter storms are events in which the dominant varieties of precipitation are forms that only occur at cold temperatures, such as snow or sleet, or a rainstorm where ground temperatures are cold enough to allow ice to form. It may be marked by strong wind, thunder and lightning, heavy precipitation, such as ice, or wind transporting some substance through the atmosphere. Other major non winter events such as large dust storms, Hurricanes, cyclones, tornados, gales, flooding and rainstorms are also caused by such phenomena to a lesser or greater existent.
At the start of spring 2016, a major winter storm moved through the High Plains and Midwest, bringing blizzard conditions with snowfall of up to 12–18 inches (30–46 cm) and strong winds as well. Snowstorms like this are typical in these areas at this time of year. The system also brought with it a severe weather threat as well. As it moved to the east, a crippling ice storm scenario was developing as well.
The 2010–11 North American winter season started in late 2010 and ended in mid-2011.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.