Download Webinar Slides in PDF

VDEM March Webinar
Understanding Tornadoes and other
Severe Thunderstorm Phenomena
Bill Sammler
Warning Coordination Meteorologist
National Weather Service – Wakefield, VA
Severe Thunderstorms
• Definition – Produce one
or more of the following:
– Hail ≥ 1 Inch in Diameter
(quarter size)
– Sustained Winds or Gusts
≥ 58 mph (50 knots)
– Produces Wind Damage –
Trees Down, Power Lines
Down, Structural Damage,
etc.
– Produces a Tornado
Thunderstorm Development
Main Ingredients
• Moisture/Instability – “CAPE”
– Moisture/Heat in Lowest 5000 ft/2 km
– Steep Lapse Rates (temperature change with height)
– Dry Air Aloft – Classic
• Forcing Mechanism – Upward Motion
– Front/Dry Line – Low Level Convergence
– Upper Level Energy (Short Wave Trough)
• Favorable Wind Fields
– Mid/Upper Level Jet “Crossing” Low Level Jet
– Upper Level Divergence
Convective Available Potential
Energy (CAPE) Explained
Dulles Airport – June 29-2012 Derecho
8 am Friday
8 pm Friday
Temperature
Parcel
Trajectory
DEW
POINT
CAPE
Cold Fronts and
Warm Fronts –
Primary Forcing
Mechanisms
Cold Front
Cold front
Cold
Warm
Cold air (more dense/heavier) is displacing warm air (less
dense/lighter), pushing it upward rapidly.
Pressure and weather changes are rapid and dramatic
6
Copyright 1992, USA TODAY. Reprinted with permission.
Cold Front
7
Warm Front
Warm front
Warm
Cold
Warm air (less dense/lighter) replacing cold air (more
dense/heavier). Warm air slides over Cold air.
Slope of front is gentle. Mostly “stable” precip rain, snow,
sleet or freezing rain falls. Thunderstorms are possible.
8
Copyright 1992, USA TODAY. Reprinted with permission.
Warm Front
9
Severe Thunderstorm Types
• Single Cell – Also known as
Supercells
• Lines - Also Known as Squall
Lines
• Multi-Cell clusters
Supercell Thunderstorms
Source: NWS Louisville, KY
Weak Echo Region (WER)
802
am
831am
am
812
821 am
Cherrystone Tornado - July 24, 2014
Supercell Thunderstorms
April 1, 1998
Supercell Thunderstorms
Source: NWS Louisville, KY
Tornado Signatures on Radar
Couplets of Opposite Motion Close Together
Motion TOWARD the Radar in GREEN/BLUE
Motion AWAY FROM the Radar in RED/YELLOW
Bertie Tornado April 16, 2011
Reflectivity
Doppler Velocity
Tropical Tornadoes
• Generally Weaker than
“Typical” Tornadoes
• Circulations Often ShortLived
• Circulations Shallow <10,000 Feet deep
• Not all Tropical Systems
Produce Tornadoes
Irene Tornado Circulations
Lowest Elevation
+6000 feet
+2500 feet
+4000 feet
Not All Radar Signatures
Produce Tornadoes
X
X
Squall Lines
• Squall Line Characteristics
–Damaging Winds on Leading
Edge
–Often Move at 50+ mph
–Common in Eastern U.S.
• Seen 15-20+ Days per Year in
Mid-Atlantic
–Radar Signature - Bow Echo
Squall Lines – Contd.
January 7, 1995 – Radar
Reflectivity (2:20am EST)
Squall line with
Multiple Segments
Squall Lines Contd.
January 7, 1995 – Doppler
Velocity (2:05am EST)
Downburst Winds ≥
50 kt (58 mph)
Squall line Depicted
in Wind Field
Local Radar (DC-LMA Lightning)
June 29 Derecho
Source: Dr. Scott Rudlosky,
NOAA/NESDIS
NWS Sterling Radar
Animation
8:57 – 10:48 PM
Severe Thunderstorms
Bow Echo Evolution Scenario
Severe Thunderstorms
Bow Echo Example
Squall line moving ~
60 mph
June 16, 1998
Radar Reflectivity
Downburst Winds ≥
60 mph
June 16, 1998
Doppler Velocity
Thunderstorms
Gust Front/Shelf Cloud
Squall Lines
Visual Clue = Shelf Cloud
Using Radar During
Severe Weather
Static Imagery vs. Loops
• Loops very
beneficial for
defining
trends/movement
• Generally better to
loop radar than to
view static images
The Things You Can See on Radar
The Things You Can See on Radar
The End!!
Are There Any Additional
Questions?
[email protected]