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Thundersnow, is an unusual kind of thunderstorm with snow falling as the primary precipitation instead of rain. It typically can occur within the cold sector of an extratropical cyclone or in mesoscale bands not associated with such a cyclone. Thermodynamically, it is no different than any other type of thunderstorm, but the top of the cumulonimbus cloud is usually quite low. In addition to snow, hail may fall as well. Overall, these storms are not as strong as in spring or summer when temperatures are warmer and the air is more unstable.
Lake effect snow
Lake effect thundersnow occurs after a cold front or shortwave trough aloft passes over a body of water. The relatively warm water heats the air near the surface and with cold air aloft, steep lapse rates mean convective development is possible. See the separate article on Lake Effect Snow for more detailed information.
Synoptic snow storms tend to be large and complex, with many possible factors affecting the development of thundersnow. One location in a storm to find thundersnow is typically in its northwest quadrant (in the Northern Hemisphere), within what is known as the "comma head" of a mature extratropical cyclone.
Thundersnow is often located underneath the 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, where there is upper-level frontogenesis and strong upward motion. The convection is produced more by the strong lifting than typical instability.
Other thundersnow storms can occur to the northeast of the cyclone center, ahead of the surface warm front. They are associated with conditional symmetric instability (CSI) sometimes found here and typically occur in the developing stage of an extratropical 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.
Hazards to flight operations
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 300m. 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.
Thundersnow, while relatively rare, is most common in the Great Lakes area of the United States and Canada, the Midwestern United States, the Great Salt Lake and across the Northeastern states and provinces of the United States and Canada. March is the peak month of formation. There are an average of fewer than 10 events per year.
The British Isles and parts of northwestern Europe occasionally report thunder and lightning during sleet or (usually wet) snow showers during winter and spring. Thundersnow 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.
The South Region of Brazil has also registered occasional episodes of thundersnow.
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. Beneath the storm, the sound of thunder can seem unusually loud compared with a normal thunderstorm.