Chapter 18 Thunderstorms are Critical for redistribution of heat and moisture in the atmosphere Occur continuously Provide essential water and agricultural resources 2 4 billion property crop loss in US over past decade NWS requirements for a sever thunderstorm Hail with diameter of 1 inch or larger any thing smaller does not inflict damage Wind damage or gusts of 50knots 58mph or greater And or tornado Lightning occurs in all thunderstorms it is not used to differentiate between sever and non sever Four elements required for thunderstorm formation 1 Source of moisture 2 Conditionally unstable atmosphere 3 Mechanism to trigger updraft 4 Vertical wind shear Destructive thunderstorms develop most often in an environment characterized by large conditional instability and strong vertical wind shear Mechanisms for updraft of lifting include Frontal boundaries Daytime heating Differential heating of air over surfaces with different properties Leading edge of cool air outflow from a past thunderstorm The primary mechanism that triggers the updraft of an airmass thunderstorms is solar heating of the surface Non sever thunderstorms Often called airmass thunderstorms Typically form within an airmass not along a front Triggered by weak boundaries cool air outflow surface heating lifting along slopes Form during hot afternoons Winds do not change with height lifetime time is short about an hour Airmass thunderstorms grow vertically without significant tilt No vertical shear Symmetric anvil In mature stages downdraft falls into updraft cutting off the moisture source In the dissipation stage downdrafts dominate often producing a gust front outflow boundary 1 Anvil the flat top of a thunderstorm formed when updraft hits tropopasue Mammatus clouds often form at base of thunderstorm anvils Airmass thunderstorm would most likely form far away from any frontal boundaries around 4pm Airmass thunderstorms tend to occur in environments where the winds do not increase substantially with height Meso Scale Convective Systems MCS when thunderstorms breed other thunderstorms Progress over a large area Produce damaging winds and occasional weak tornadoes Mesoscale atmospheric processes a few km to a few hundred km or one thunderstorm to a group of thunderstorms Lifecycle of MCS Lifting air along weak boundaries triggers a thunderstorm or two often in late afternoon Storms organize often aligning into an arc shaped squall line where precip falls in a continuous line Squall violent burst of wind Squall lines often have strong straight lines winds Squall line may blow outward producing bow echo on radar Behind the squall typically on W or NW widespread less intense precipitation develops in the stratiform region Squall line a long line of thunderstorms in which adjacent thunderstorm cells are so close together that the heavy precipitation falls in a long continuous line Bow Echo portion of the squall line that bow outward from MCS Trailing stratiform region area of less intense precipitation on rear side of squall line Why is wind shear a key environmental condition for sever thunderstorms Strong winds cause the updraft to tilt and separate from the downdraft allowing the inflow of warm moist air to continue to feed the storm Gust front leading edge of evaporating cooled air 2 Evolution of a Mesoscale Convective System Initially updrafts are upright Rain evaporates as it falls cooling the air into a cold pool Cold pool spreads outward toward the warm moist air feeding the thunderstorm and spreads rearward New updrafts preferentially form along advancing cold pool forming a squall line now tilted Cold pool can become deep and cold enough to rush outward as a gust front which can have a shelf cloud or roll cloud detectable on a radar as a fine line return Air in the evaporation region flows forward toward line of storms creating rear inflow jet Key Features of a MCS squall line Cold pool is the accumulation of rain cooled air near the surface under a thunderstorm A thunderstorm s anvil cloud is composed of ice crystals 3 What develops when cool air generated by evaporation of rain within the downdrafts spreads outward away from the thunderstorm after reaching the surface Gust front Frontal squall lines can persist for the lifetime of a mid latitude cyclone Note the tail of comma cloud associated with the mid latitude cyclone Squall line is normally along leading cold front or upper level front within the extratropical cyclone Many features of frontal squall lines are similar to MCS squall lines Overshooting top the portion of the updraft that penetrates the tropopause Super Cell Thunderstorms Most intense thunderstorms on Earth Always rotate Responsible for majority of most dangerous sever thunderstorm weather and strongest tornadoes in US Hail as large as grapefruit Extreme winds Flash flooding Updrafts typically 45 90 mph some 100mph Same 4 requirements as other thunderstorm CAPE convective available potential energy quantifies a parcel s buoyancy Moist layer at the surface feeds the updraft providing fuel necessary to 4 drive the rapidly growing supercell Supercells typically first form where airmass boundaries intersect Key features of a Supercell Mesocyclone 5 10km Tilted updraft Overshooting top Anvil Rain free base Rear flanking line Bounded weak echo region Wall cloud Mammatus Virga 5 You are outdoors in spring in the Central Plains A supercell thunderstorm is traveling from SW to NW and passes directly over you In what order would you observe the different precipitation types Virga light rain moderate rain heavy rain hail SUMMARY CH 18 The National Weather Service determines a thunderstorm is severe if it meets at least one of the following criteria hail with diameter inch or larger wind damage or gusts over 50 kts or a tornado Boundaries are important in thunderstorm development because lifting occurs along the boundaries Wind shear is important to a severe thunderstorm because vertical wind shear separates the updraft and downdraft preventing precipitation from falling into the updraft It can also induce rotation in a thunderstorm Airmass thunderstorms can be expected to form during afternoon far from frontal boundaries MCSs are important to the Central Plains because they provide summer rainfall which is important for agriculture MCSs are 200 to 300 miles in diameter and typically last of 6 12 hours The most common types of severe weather with MCSs are straightline winds and weak tornadoes Squall