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
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
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 29 2007 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 2000-2500 J/KG OVER SWRN NEB ATTM...AND
BETWEEN 500-1500 J/KG OVER ERN CO. DEEP LAYER SHEAR IS 40-45 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 23030. ...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 73072 U.S.A.
spc.feedback@noaa.gov
Page last modified: May 29, 2007Disclaimer
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