CH21: Severe Weather Earth Science Spring, 2016

Thunderstorms A thunderstorm is a form of weather with lightning, thunder, wind, and rain Earth Science Fall, 2014

Forecasting Thunderstorms Surface and upper air observations to find low level moisture and instability. It determines how winds aloft might storm development. Satellite imagery is used to track the movement of weather systems Computer model data determines where favorable thunderstorm formation might be

Conditions Needed for Storms Ingredients needed the most are moisture from the Gulf of Mexico or crops and air mass instability. Warm fronts as well form storms but not as strong A lifting mechanism like a cold front coming out of the Rockies or winds off the ocean or mountain Daytime heating

Life Cycle-Shaped Like an Anvil Feet 40,000 10,000 20,000 30,000 Cumulus stageMature stageDissipating stage Masses of moisture are lifted upwards Two winds converge forcing air upwards The moisture rapidly cools into liquid drops of water because of the adiabatic temp change The air is forced to spread out because it can’t rise any farther A storm may dissipate because of a downburst causing the inflow to be cut off and cause an outflow boundary

Classification SupercellMulticell Thunderstorm that are generally weak and usually not severe Air-mass Lifting source is uneven heating of the surface The most common type Strong cold fronts Moderate-sized hail, flash flooding, and weak tornadoes Very large, severe storm strong, rotating updraft At least 90 percent of the time severe Destructive tornadoes The most powerful thunderstorm

Squall line Severe thunderstorms along or ahead of a cold front Hail, frequent lightning, strong straight-line winds Possibly tornadoes Shelf Cloud in front of the winds A Derecho is a widespread and long-lived line of storms Bow echo Weather Underground

Lightning 140,000 mph 54,000 °F Electrons are stimulated during a storm: electrons are moving in clouds down the air in a zigzag motion Electrons travel to ground, causing a lightning bolt to strike and travel from ground up Opposites attract so that protons on ground start moving to the surface

Ground to Cloud Types The best known and second most common type of lightning Discharge between cloud and the ground. Cloud-to-cloud lightning Discharges occur between areas of cloud

Pea0.25 inches inches Dime Quarter 1.00 inch 1.50 inches Ping-pong ball 1.75 inches Golf ball Tennis ball 2.50 inches 2.75 inches Baseball Softball 4.50 inches Volleyball8.00 inches The largest recorded hailstone in the United States 1.93 pounds Hail formation requires strong upward motion of air (updraft) and a low freezing level As hail is pushed upward it grows as droplets of water hit it. This happens until the updraft can’t support the hail. Then it falls to the ground.

Vertical draft Updraft Downdraft Warm air will rise until it reaches air warmer or less dense than itself Low pressure area will attract air from the surrounding area. It will move to the center and then rise High pressure region will attract air from the surrounding area. It will move towards the center and sink Both can cause airplane crashes

DownburstMicroburst / Column of sinking air producing damaging straight-line winds High winds that can knock over fully grown trees. A couple of seconds to several minutes. Is capable of producing damaging straight-line winds of over 150 mph

Heat burst Gusty winds, rapid increase in temperature, and decrease in dew point Typically occur during night-time Associated with dying thunderstorms Caused when rain evaporates into a parcel of cold dry air high in the atmosphere. They descends rapidly, warming due to compression. Very rare Temperatures reach 90 °F Reports up to 120 °F

Tornadoes

Tornado A tornado is a violent, dangerous, rotating column of air May 1999 Come in many shapes and sizes Warm and moist inflow to power it. May 1999 The strongest ever. 318 MPH winds Most have wind speeds less than110 mph Tornadoes are usually located on a hook echo at the southwest part of the storm. Doppler reflecting off debris swept up in the tornado. Wedge Tornado August 24, 2006

Tornadoes-When and Where Do They Occur? Tornadoes can occur in any state and any time of year They typically occur from March to August The most common tornadoes are in “Tornado Alley” “Tornado Alley” - Arkansas, Iowa, Kansas, Louisiana, Minnesota, North Dakota, Ohio, Oklahoma, South Dakota, and Texas

A Tornado that contains two or more small vortexes as part of the main vortex The only times they are visible are when the tornado is first forming.

Watches Tornado watch Issued when conditions are favorable for a tornado to form Map of weather advisories Issued when weather conditions are favorable for the development of severe thunderstorms. Tornadoes can still form. Severe thunderstorm watch (NWS website)

Warnings Tornado warning Issued after a tornado or funnel cloud has been spotted or if radar indications of tornado formation. (Red box) Tornado emergency A large violent tornado expecting to cause significant damage and a high likelihood of numerous fatalities. Is issued when trained storm spotters or a radar indicate a strong thunderstorm. (Yellow Box) 58 mph wind and 1 inch hail Severe thunderstorm warning

Tornado Classifications: The Enhanced Fujita Scale

Tornado Safety What To DO: Get indoors Get to the lowest point in your house AWAY from windows Get to the lowest point in your house AWAY from windows No basement, get in your bathtub If time, put something soft and heavy over you (mattress/blanket) If time, put something soft and heavy over you (mattress/blanket) If time, grab your animals If time, grab your animals If Caught Outside: Get to a low lying area (ditch/river/etc.) Lay as flat as you can and protect your head/face/neck Lay as flat as you can and protect your head/face/neck

Bands of Heavy Rainfall Warm Ocean