Severe Storms !!
To be classified as “severe”, need ONE of 3
ingredients:
1. Windspeed > 58 mph (93 kmph)
2. Hail > 0.75 inches (1.9 cm)
3. Tornado
Types of severe storms
1. Mesoscale Convective System (MCS)
• Cluster of thunderstorms acting as one system;
• 10 – 1000 km (6.2 – 620 miles)
• More than one updraft and downdraft
• Self-propagating: downdraft of one storm leads to
growth of another storm
• May last 12 hours – several days
2. Supercell
• Intense storm with one updraft zone
MCS
MCS
supercell
MCS’s occur…
1. in a line: Squall Line
2. in an oval or round shape :
Mesoscale Convective Complex (MCC)
Squall line
Line of severe storms
•Ave. 300 miles long
•Parallel to and 180-300 miles ahead of cold
front
•Last up to 10 hours
•In warm sector of mid-latitude (wave)
cyclone
Squall line requirements:
1.Line of converging moist air,
low-level (1000-2300 ft) jet stream
often occur at night during radiational inversion when air aloft is cut
off (by the inversion) from surface friction; if connected to a
source of moisture (e.g., Gulf of Mexico), brings in moisture to
jet
2. Upper level divergence zone of trough
produces vigorous lifting of warm, moist air
3. Vertical wind shear
(change in speed or direction)
Wind speed relative to ground
Wind speed relative to storm speed and direction
Results of wind shear:
• Strong winds aloft push updrafts ahead of downdrafts
• Rising air feeds more moisture into storm
• Updrafts tilted
• Rain produces downdrafts
• Downdrafts reach ground as wedge of cold, dense air:
GUST FRONT
• SHELF CLOUD
Mesoscale Convective Complex (MCC)
MCS in oval or round shape
>100,000 km2
Does NOT require vertical wind shear
MCC requirements:
1. Weak upper level winds
2.
mid-level trough (divergence aloft)
3. Infusion of warm, humid air from low level jet
(usually at night)
Duluth 2102 flood story
more
IR satellite video (IR national, 500 mb, radar)
Derecho
Cluster of downbursts in northern end of squall
line
Bow Echo: radar signature
• Bowed-out line of T-storms, embedded in squall
line, associated with axis of strong winds, creating
straight line wind damage at surface
Derecho damage, West Virginia, 2012
Derecho, bow echo
Cluster of storms in bow echo is 25-75 miles long
Strongest wind at “surge” region, strong REAR
INFLOW JET
• Fast stream of slanting, descending air drawn into
rear of bowing storm pattern
BWCA Blowdown
July 4, 1999
Began as MCC over western Plains (Montana, S.
Dakota)
Organized into MCS, drifted across Dakotas
Weakened early morning July 4
30-40 miles west of Fargo, changed into squall
line with bow echo
91 mph wind in Fargo
Squall line tracked across Minnesota
Two paths across Minnesota
1. Southern : started in western MN, near Detroit
Lakes
• Hail
• 60 mph winds
• Small tornadoes
• Reached north of Duluth by noon
2. Northern:
Bemidji 9:30 am :
75 mph winds
1 inch hail
Hibbing 11:30 am:
81 mph winds
Ely 12:30:
developed into bow echo
no wind speeds recorded in wilderness
leveled 478,000 acres
damage 12 miles wide by 30 miles long
20 injured
“(un) happy campers”
6000 mile journey
Storm crossed central Ontario, through southern
Quebec, into New York and New England, moved
off coast of Maine, followed perimeter of a High
over the Atlantic, came back onto shore north of
Myrtle Beach, S. Carolina (July 6), then on to
Georgia, Alabama, Mississippi, Gulf of Mexico
Ingredients:
Warm temps at 700 mb, creating inversion “cap”
Cap trapped low level moisture SOUTH of storm
path as colder air moved in from North