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Thundersnow, also known as awinter thunderstorm or athundersnow storm, is athunderstorm in whichsnow falls as the primaryprecipitation instead ofrain. It is considered a rare phenomenon.[1] It typically falls in regions of strong upward motion within the cold sector of anextratropical cyclone. Thermodynamically, it is not different from any other type of thunderstorm, but the top of thecumulonimbus cloud is usually quite low. In addition to snow,graupel orhail may fall as well. The heavy snowfall tends to muffle the sound of the thunder so that it sounds more like a low rumble than the loud, sharp bang that is heard during regular thunderstorms.[2]
Thundersnow can occur during a normal snowstorm that sustains strong vertical mixing which allows for favorable conditions forlightning andthunder to occur. It can also occur from thelake effect or ocean effect thunderstorm which is produced by cold air passing over relatively warm water; this effect commonly producessnow squalls over theGreat Lakes.
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Within the United States, thundersnow is relatively rare but most common in "easternNevada andUtah, thecentral plains, and theGreat Lakes states".[3] Thundersnow also occurs inNova Scotia and in theNortheastern United States, especially inNew England andNew York, sometimes several times per winter season.[citation needed] On December 30, 2019, asevere thunderstorm warning was issued for parts ofMassachusetts for a thunderstorm cell that was producing "lightning, thundersnow, thundersleet, and thunderice".[4] A "really rare" thundersnow storm occurred nearVancouver,British Columbia on December 17–18, 2022.[5]
TheSouth Region ofBrazil registered episodes of thundersnow in 1984 and 2005, in the state ofSanta Catarina, and in August 2011, in some municipalities of the highland region ofSerra Gaúcha, in the southern state ofRio Grande do Sul.[6]
TheBritish Isles and other parts of northwestern Europe occasionally report thunder and lightning during sleet or (usually wet) snow showers during winter and spring.Scotland registered an episode of thundersnow in the early hours of 4 December 2020, the unusual noise causing alarm among local people.[7] TheMet Office warned of thundersnow in Scotland,Wales andnorthern England in early January 2022.[8]
Western Europe has rare occurrences of thundersnow, as on 8 March 2010, when northeasternCatalonia, includingBarcelona, experienced a heavy snowfall accompanied by lightning, with snow depths surpassing 30 centimetres (12 in) in low altitude areas.[9]
In Central Europe, a large-area (non-local) thundersnow occurred on 17 January 2022, when a strong synoptic-scale squall line passed north to south over whole central and eastern Poland, precipitating both granular snow and snowflakes, with discharge intensity exceeding 100 per minute.[10] Other recent occurrences were in Poland and the Czech Republic in January 2023,Germany in January 2021, andNorway andNetherlands as well asAustria in April 2021, with previous occurrences in Norway in January 2019[11] and January 2020.[12]Stockholm experienced thundersnow on 21 November 2022.[13][14] It was detected once again in theNetherlands on 3 January 2026.[15]
Low-pressure events in the easternMediterranean that originate from polar origin cause copious thundersnow occurrences during winter storms, especially over the elevated provinces ofIsrael,Jordan andLebanon, includingAmman andJerusalem. When such storms happen at areas intended for skiing, the mountains are often evacuated for safety.[citation needed]
Thundersnow is also common aroundKanazawa and theSea of Japan, and even aroundMount Everest.[citation needed]
Thundersnow is caused by the same mechanisms as regularthunderstorms, but it is much more rare because cold dense air is less likely to rise.[16]
Lake effect thundersnow occurs after a cold front orshortwave aloft passes over a body of water. This steepens the thermallapse 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 thetroposphere 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° between the ground and 2,000 m (6,600 ft) in height must be in place. However, any change in direction greater than 12° through that layer will tear the snow squall apart. A bare minimumfetch 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 anysuper 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.[17]
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 itsNorthwestquadrant (in theNorthern Hemisphere, based on observations in theMidwestern United States), within what is known as the "comma head" of a matureextratropical cyclone.[18][19] Thundersnow can also be located underneath theTROWAL, a trough of warm air aloft which shows up in asurface weather analysis as an inverted trough extending backward into the cold sector from the main cyclone.[20] 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.[19] The1991 Halloween blizzard,Superstorm of 1993, andWhite 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 anextratropical 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.[21]
Thundersnow produces heavy snowfall rates in the range of 5 to 10 cm (2 to 4 in) per hour. Snowfall of this intensity maylimit visibilities severely, even during light wind conditions. However, thundersnow is often a part of a severewinter storm orblizzard. Winds of abovetropical stormforce are frequent with thundersnow. As a result, visibilities in thundersnow are frequently under 2/5th of a mile. Additionally, such wind creates extremewind chills and may result infrostbite. Finally, there is a greater likelihood that thundersnow lightning will have apositive polarity, which is associated with a greater destructive potential than the more common negatively-charged lightning.[22] That said, lightning is far less frequent in a thundersnow storm than in a summertime storm, and is usually of the cloud-to-cloud variety, rather than a strike that travels to the ground.[2]