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Severe Weather Severe Weather AT 351 Lab 10 April 2, 2008 Weather Fatalities - 2006 1 Types of Severe Weather Thunderstorms Hail Lightning Flood Tornado Severe Wind (Straight-Line Winds) Thunderstorm distribution 2 Favorable Conditions Instability Restricting Cap Initial Lift Shear Fuel Instability You know this part already Steep lapse rate Means warm, moist air near the surface Colder air above it Needs to be pulled from a sounding 3 Sources of Lift Convective lifting Boundaries Fronts Drylines Outflow boundaries Orographic Convergence Shear Because of the way a thunderstorm works, it needs to be tilted to remain strong Therefore, winds need to change with height Two kinds of shear Speed Shear: Wind is faster as you go up Directional Shear: Wind changes direction with height 4 Vertical Wind Shear Change of wind speed and/or direction with height Weak vertical wind shear: short-lived since rainy downdraft quickly undercuts and chokes off the updraft Sheared environments are associated with organized convection Vertical Wind Shear 5 Fuel Just like anything else, a storm needs fuel to survive The fuel for a storm is just a continued supply of what started it Storm needs to remain in areas of warm, moist air If storm moves into a colder region, it will die Restricting Cap If the atmosphere is unstable all the way up, you get a constant updraft More effective is when the energy is held back and released all at once Stable layer near the surface that supresses convection As ground heats during the day, energy builds up until it can “break the cap” Also called “capping inversion” Remember CIN? 6 Back to the Skew-T Meteorologists have made up a lot of numbers that are designed to tell how favorable the weather is for a storm Some deal with instability Some deal with shear Some deal with both Let’s discuss a few Remember CAPE? Convective Available Potential Energy Instability is great for convection CAPE measures how unstable it is, and over what depth It is an integrated quantity that tells how much energy is available for the storm 7 Lifted Index A very simple number Compares actual temperature at 500 mb to theoretical parcel temperature If actual temperature is colder: unstable LI = Parcel – Environmental If LI < -6, considered very favorable 8 Severe WEAther Threat A single number that takes a lot of other things into account Temperature Dewpoint Stability Wind Shear SWT > 400 = very favorable Thunderstorms Form in unstable environment (typically in mT air mass) Triggered by Surface heating Terrain Frontal boundaries Important for heat and energy balance Act to stabilize the atmosphere 9 Types of Thunderstorms Thunderstorms come in many varieties Likelihood of severity proportional to storm lifetime NWS definition of severe (one or more of the following elements ¾” or larger diameter hail 50 kt or greater winds tornadoes Thunderstorm ingredients Triggering mechanism Large convective instability Low-level moisture Vertical shear 10 Single cell thunderstorms Also referred to as ordinary, pulse, or air mass thunderstorms Typically do not produce severe weather Three stages Cumulus Mature Dissipating Life span: ~45-60 min. Cumulus Stage Warm moist air rises, condenses Latent heat release keeps air in cloud warmer than environment Grows to a towering Cu Cloud particles grow larger, begin to fall No precipitation at surface 11 Mature Stage Marked by appearance of downdraft Falling cloud drops evaporate, cooling the air Storm is most intense during this stage Cloud begins to form anvil May have an overshooting top Lightning and thunder may be present Gust front forms Downdraft reaches the surface and spreads out in all directions Gust front forces more warm, humid air into the storm Downdraft Like the exhaust of a car, the storm needs to get rid of the cold air that it took the energy from Cold air plunges to the ground and spreads out Creates “gust front” or “outflow boundary” Can provide lift for new storms 12 Entrainment Downdrafts are cool not only because of evaporation of precipitation but also due to entrainment of cooler, unsaturated air from outside of the thunderstorm 13 Downbursts Strong downdrafts Microburst: diameter of downdraft is 4 km or less (sub-thunderstorm scale outflow feature) Macroburst: diameter greater than 4 km (on scale of entire cold air outflow field) Produce winds comparable to weak tornadoes Downbursts 14 Gust fronts An area of high pressure created at the surface by cold heavy pool of air from downdraft called a mesohigh Gust front: leading edge of cold air from downdraft Passage noted by calm winds followed by gusty winds and a temperature drop then precipitation Convergence region between cold outflow and warm, moist inflow Gust fronts Can generate new cells Leads to multi-cell storms Production of shelf and roll clouds 15 Overshooting tops 16 Overshooting tops Dissipation Stage Usually follows mature stage by ~15-30 min Gust front moves out away from the storm, and most air is no longer lifted into the storm. Downdrafts become dominant Low level cloud drops can evaporate rapidly, leaving only the anvil as evidence of the storms existence 17 Types of Storms Single Cell Multi-Cell Bow Echo Squall Line Supercell Single Cell Storms Normally weak and short lived Rarely severe Pose a relatively low risk to the public 18 Multi-Cell Storms Cluster of storms moving as a single unit Stronger wind shear than the ordinary cell case More organized multi-cells Bow Echoes Squall Lines New cells tend to form on the upwind (W or SW) edge of the cluster, with mature cells located at center and dissipating cells found along the downwind (E or NE) portion of the cluster Updraft competition for warm, moist low-level air so not incredibly strong and have short life spans Multicell Storms Cell 1 dissipates at time=0 while cell 2 matures and becomes dominant Cell 2 drops heaviest precipitation at time=10 as cell 3 strengthens Severe multicell storms typically produce a brief period of hail and/or downbursts during and immediately after the strongest updraft stage 19 Dissipation Mature Growing Multicell Thunderstorm Flash Floods Producer 20 Flash Floods Except for heat related fatalities, more deaths occur from flooding than any other weather hazard Most are vehicle-related Occur from thunderstorms that repeatedly move over the same area or heavy rains from tropical cyclones Flood Safety DROWNING IS THE NUMBER ONE CAUSE OF FLOOD DEATHS DO NOT walk through flowing water DO NOT drive through a flooded area STAY AWAY from power lines 21 Big Thompson Flood July 31, 1976 In canyon of Big Thompson River, received as much as 30.5 cm (12”) of rain in 4 hours (from 6:30 – 10:30pm) This area usually only receives about 40.5 cm (16”) of rain in an entire year Killed 145 people Big Thompson Flood Synoptic conditions Cold front moved through earlier in day, remaining to the S near Denver Weak inversion associated with the front kept cumulus from growing earlier in the afternoon Strong SE flow behind cold front pushed unusually moist air upslope along mountain range Moist, conditionally unstable air penetrated inversion and grew to large multicell storms Extremely weak upper level winds 22 Big Thompson Flood Squall Lines Multicell storms can form as a line of storms extending for hundreds of km, called a squall line Squall lines often form along or just ahead of a cold frontal boundary (called pre-frontal squall lines) Supercells may be embedded within prefrontal squall lines Leading line of thunderstorms may be followed by large region of stratiform precipitation where the anvil cloud trails behind the main storm. 23 Squall Lines Solid or broken line of thunderstorms with a continuous, well-developed gust front on the leading edge New updrafts form on downwind side, where line is moving into unstable inflow air Squall Lines • May be triggered as a line or may organize into a line from a cluster of cells • For a given CAPE, strength and longevity increases with increasing depth and strength of vertical wind shear 24 Squall Lines When vertical wind shear is strong and deep, the squall line may be composed primarily of supercells Southern end of a mature squall line and breaks in the leading line of convection are favored locations for supercells Bow Echoes Bow Echo – a bowed convective line (25 – 150 km long) with a cyclonic circulation at the northern end and an anticyclonic circulation at the southern end (Fujita) Can produce long swaths of damaging winds Form in conditions of large instability and strong low level shear Observed both as isolated convective systems or as substructures within much larger convective systems (such as a squall line) 25 Bow Echoes Storm develops with strong jet streak coming in from behind Bows the storm out forward Can contain very strong winds and potentially tornadoes Bow Echoes 26 Squall Lines A bow echo is part of a larger group called a squall line Refers to a strong line of storms that have outrun the cold front or dry line that initiated them Mesoscale Convective Complexes • Individual storms can grow and organize into a large convective system (weak upper level winds) • can grow to the size of a state • moves slowly (less than 20 kt) • life span greater than 12 hours • New storms grow as older ones dissipate • Provide widespread precipitation • Can spawn severe weather • hail, high winds, flash floods, tornadoes • Formation • usually during
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