Lecture 26 Severe Thunderstorms NWS Criteria Environment Structure Climatology 1 Severe Thunderstorms NWS Criteria Environment Structure Climatology 2
Severe Thunderstorms: NWS Criteria NWS Criteria: to qualify as a severe thunderstorm at least one of the following must be present: Large Hail > 3/4 Inch Strong straight line winds >50 kt Presence of a Tornado 3 The Severe Thunderstorm Environment 4
Air Mass vs Severe Thunderstorms Environment: Air Mass thunderstorms form in regions of relatively light winds and light wind shear. Thus they form away from fronts and jet streams. LOW WIND SHEAR HIGH WIND SHEAR IN AN AIR MASS NEAR FRONTS OR JETSTREAMS Air Mass Thunder Storm Severe Thunderstorm 5 The Severe Thunderstorm Environment 6
Squall line A series of thunderstorms that form an extended line. 7 Supercell Thunderstorms 8
Severe Thunderstorm Structure 9 Super Cell Structure 10
Super Cell Structure Shelf cloud at leading edge of downdraft region. 11 Super Cell Structure Shelf cloud at leading edge of downdraft region. 12
Super Cell Structure Shelf cloud at leading edge of downdraft region. 13 Gust Front Outflow Boundary 14
Outflow Boundary Triggers New Thunderstorms 15 Severe Thunderstorms NWS Criteria: to qualify as a severe thunderstorm at least one of the following must be present: Large Hail > 3/4 Inch Strong straight line winds >50 kt Presence of a Tornado 16
Large Hail 17 Large Hail Hail Damage One Colorado hailstorm caused $625,000,000 damage in July 1990. Hailstones have high density and fall fast (up to 90 mph) Hail damages Crops Cars, trucks Livestock 18
Large Hail > 3/4 inch diameter 19 Large Hail Large hail forms in clouds with High supercooled water content Very strong updrafts Large hailstones make several trips up into the cloud layering tells about hailstone history 20
Strong Straight-line Winds >50 knots (57 MPH) 21 Strong Straight-line Winds Strong straight-line winds in thunderstorm are often the result of precipitation produced downdrafts called downbursts. 22
Downburst Downbursts can reach wind speeds of ~200 mph! Damage to forests and man made structures. Cause of numerous aviation disasters. 23 Downbursts Downbursts can reach wind speeds of ~200 mph! Damage to forests and man made structures. Cause of numerous aviation disasters. 24
Downburst 25 TORNADOS 26
27 Lecture 26 Part II Tornados Environment Storm Structure Life Cycle Source of Spin Forecasting Climatology Damage Marilee Thomas of Beaver City, NE took this photograph of her daughter Audra about two miles from a Furnas County tornado in April 1989. 28
1st Tornado Photograph 29 Tornado Environment Strong winds aloft (300mb, >25,000 ft), usually associated with the leading (east) side of a trough. Cool, dry air at mid levels (~18,000 ft, ~500 mb), usually brought by winds out of the southwest. Warm humid air at low levels (~5000 ft, 850 mb) brought in by strong gusty southerly winds. 30
Tornado Environment 31 Tornadic Thunderstorm Structure A mesocyclone is a rotating vortex in conjunction with the updraft in a supercell storm. Supercells develop mesocyclones by tilting environmental and/or locally generated horizontal vorticity. 32
Tornadic Thunderstorm Structure 33 Tornadic Thunderstorm Structure 34
Tornado Life Cycle Wall Cloud 35 Tornado Life Cycle The wall cloud usually exhibits a lot of rotation. Wall Cloud 36
Tornado Life Cycle The wall cloud usually exhibits a lot of rotation. 37 Tornado Life Cycle The wall cloud usually exhibits a lot of rotation. 38
Tornado Life Cycle The wall cloud usually exhibits a lot of rotation. 39 Funnel Formation Tornados usually develop out of the wall cloud, which is located at the bottom of the updraft portion of the storm. 40
Young Monster Tornado Early stage of large tornado; stubby funnel 41 Mature Tornado Rotation has reached the ground. Debris cloud kicked up by the winds is visible. 42