Severe weather – origins and prediction Ross Reynolds Department of Meteorology University of Reading

US National Weather Service definition of a severe thunderstorm * wind gust > 50 knots (25 m/s) &/or * hail of diameter > ¾ inch &/or * tornado (funnel cloud reaching surface)

Source of winds thunderstorms: localised frontal depressions: large-scale tropical cyclones: large-scale

Hail production strength of updraft (can be >50 msˉ¹) vertical extent of updraft (residence time of hailstone) water vapour & supercooled water concentration in updraft 1cm diameter falls at ~ 9ms¹ 8cm “ ~ 48ms¹

Fairly common springtime High Plains hail damage !

Recipe for severe storm surface heating vertical wind shear low-level stable layer – temp inversion cold & dry upper troposphere

The dry line (TX, NM, OK)

Vertical wind shear & temperature inversion

Severe thunderstorm

Downbursts from rapidly descending, rain-loaded air Derechos straight line winds associated with a line of thunderstorms

Recipe for a supercell as for the severe thunderstorm + growth of deep convection in an environment within which the wind changes direction with height

Supercell (long-lived, rotating storm)

South Dakota supercell

Nebraska supercell

Tornado – only occurs when a funnel cloud or debris cloud is in contact with the ground

UK tornadoes Spanish plume

Stream of hot, dry air moves north from meseta; overruns higher theta W air north of Pyrenees & is overrun by colder, drier air

Fujita scale

Doppler on wheels

Mobile mesonet

Doppler windfield

Phased array radar 20 to 30 second frequency of simultaneous scanning of multiple targets (NEXRAD 6 to7 minutes). Increase warning from 10 to 20 minutes. Developed from US Navy SPY-1 radar ($10,000,000 funding from Navy to adapt).

Tornado watch 29 May 2007

SEL4 URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 304 NWS STORM PREDICTION CENTER NORMAN OK 1230 PM MDT TUE MAY THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF NORTHEAST COLORADO NORTHWEST KANSAS SOUTHWEST NEBRASKA EFFECTIVE THIS TUESDAY AFTERNOON AND EVENING FROM 1230 PM UNTIL 800 PM MDT. TORNADOES...HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS. THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 55 STATUTE MILES NORTH AND SOUTH OF A LINE FROM 10 MILES NORTHWEST OF DENVER COLORADO TO 25 MILES EAST NORTHEAST OF MCCOOK NEBRASKA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU4). REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS. DISCUSSION...STRONG/ISOLATED SEVERE STORMS CONTINUE TO DEVELOP/INTENSITY OVER PARTS OF SWRN NEB AND ALONG THE FRONT RANGE IN CO. ACTIVITY IS ASSOCIATED WITH COLD FRONTAL BOUNDARY THAT WILL MOVE SWD/SEWD THIS AFTERNOON INTO MODERATELY UNSTABLE AIR MASS. MLCAPE IS J/KG OVER SWRN NEB ATTM...AND BETWEEN J/KG OVER ERN CO. DEEP LAYER SHEAR IS KT WHICH IS FAVORABLE FOR STRONG ROTATING UPDRAFTS AND THE POSSIBILITY OF ISOLATED TORNADOES. AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM MOTION VECTOR MCCARTHY Top of Page/All Current Watches/Forecast Products/Home Weather Topics: Watches, Mesoscale Discussions, Outlooks, Fire Weather, All Products, Contact UsNOAA / National Weather Service National Centers for Environmental Prediction Storm Prediction Center 120 David L. Boren Blvd. Norman, OK U.S.A. Page last modified: May 29, 2007Disclaimer Credits GlossaryTop of PageAll Current WatchesForecast ProductsHome WatchesMesoscale DiscussionsOutlooksFire WeatherAll ProductsContact UsNOAA National Weather Service National Centers for Environmental Prediction Storm Prediction Center Credits Glossary

Hurricane season

Hurricane structure

2006 hurricane tracks

Hurricane genesis SST>26  C to 60m deeply unstable troposphere moist middle troposphere (5-7km) low vertical windshear (1-9km) large low level relative vorticity

Hurricane prediction National Hurricane Centre, Miami use range of global models + statistical problem with eye landfall two days ahead for large-scale evacuations asymmetry in damage; worse on forward right quadrant wind, rain, surge