Severe weather

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Severe weather origins and prediction

• Ross Reynolds
• Department of Meteorology
• University of Reading

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

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• Source of winds
• thunderstorms localised

frontal depressions large-scale
tropical cyclones large-scale
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• Hail production
• strength of updraft (can be gt50 ms¹)
• vertical extent of updraft (residence
• time of hailstone)
• water vapour supercooled water concentration
  in updraft
• 1cm diameter falls at 9ms¹
• 8cm 48ms¹

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• Fairly common springtime High Plains hail
  damage !

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

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• The dry line (TX, NM, OK)

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• Vertical wind shear temperature
• inversion

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• Severe thunderstorm

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

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

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• Supercell (long-lived, rotating storm)

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• South Dakota supercell

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• Nebraska supercell

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

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• UK tornadoes
• Spanish plume

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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

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• Fujita scale

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• Doppler on wheels

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• Mobile mesonet

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• Doppler
• windfield

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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).

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Tornado watch 29 May 2007
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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
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  Fire Weather, All Products, Contact UsNOAA /
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  May 29, 2007DisclaimerCreditsGlossary

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• Hurricane season

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• Hurricane structure

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• 2006 hurricane tracks

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• Hurricane genesis
• SSTgt26?C to 60m
• deeply unstable troposphere
• moist middle troposphere (5-7km)
• low vertical windshear (1-9km)
• large low level relative vorticity

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• 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