20. Flash Flooding
• ____________- a sudden local flood of great
volume and short duration
• Of all of the phenomena that thunderstorms
create, flash flooding is the deadliest in the
U.S. (ave. of 140 deaths per year)
21. Flash Flooding
• What causes flash flooding?
Two primary ways:
1. ________________________
2. ________________________
23. What to do when flash flooding occurs
• NEVER drive across a flooded road, no matter
how deep the water is
• Water has tremendous momentum when it is in
motion, a cubic meter of water weighs 1.1 tons
• Just a few inches of water rushing across a
road can be enough to sweep you away.
24. Hail
• Hail- formed when ice crystals fall into the updraft,
accumulate supercooled droplets, and are then are forced
upward and completely freeze. Hail falls out of a storm, when
it is heavier than the updraft can sustain in the air
25. Hail
• Hail forms in nearly every thunderstorm, but
rarely reaches the ground. This is because
hail is rarely large enough to make it to the
surface before it evaporates.
• What kinds of updrafts do you need for large
hail?
• For ____________ size hail – 55-60 mph
updrafts
• For ____________ size hail- 100 mph
updrafts
26. Hail Records
• Coffeyville, Kansas, in
1970. The largest
hailstone ever
documented (until just
recently), it weighed
1.67 pounds, and
spanned 5.67 inches
27. • Largest hailstone on record fell in June of 2003 in
Aurora Nebraska, measuring 7 inches in diameter,
weighing over a 1.3 lbs.
28. Larger stones are believed to have fallen…the largest
impact crater was measured to be just under 12 inches
across!
30. Microbursts
• ____________- straight-line bursts of
concentrated wind (diameter less than 4 km
• Created by ________________________:
• Dry air is pulled into the storm, and the
precipitation rapidly evaporates to moisten the
air…. Leading to tremendous cooling (takes
energy to evaporate)
31. Microbursts
• Where does all of this cold air go?
• The cold air very rapidly falls to the surface,
greatly enhancing the downdraft, and tends to
‘splash’ when it hits the ground
• Microbursts are also a threat to aviation
42. Damage from Tornadoes
Enhanced Fujita Scale
Wind
Speed
EF 0 65-85
mph
EF 1 86-110
mph
EF 2 111-135
mph
EF 3 136-165
mph
EF 4 166-200
mph
EF 5 Over 200
mph
46. Tropical Cyclone Structure
• Eye - descending air in the center of a tropical cyclone
• Eyewall - The innermost band of thunderstorms which surround
the eye (maximum latent heating)
• Rainbands - the spiral bands of thunderstorms that extend
outward from the hurricane center (100’s of miles)
48. The Eyewall
• Aircraft flown into
hurricanes get an
excellent view of
the eyewall pictured
here
49. Ingredients for hurricane
formation
• ____________________
(SST’s)- greater than 26°C
(79°F)
• ____________________
• __________throughout
troposphere
(very little vertical shear)
• need __________, a trigger
• tropical African wave
• ITCZ
• typically form between 5°-20°
latitude, not on equator.... why?
51. Where does a hurricane get its
energy?
• __________
• __________ (largest
contributor)
• Initially, need an unstable
environment: cold air aloft,
warm air at the surface, this
leads to the growth of
thunderstorms
• Equivalent of 21 atomic
bombs of energy used PER
SECOND !!
53. Hurricane Development
1. Intense latent heating from convection heats the column
of air near the center of the storm
• lowers the pressure at the surface (low pressure)
• generates divergence (high pressure) aloft
2. lower surface pressure increases the pressure gradient at
low levels - generates stronger surface winds.
3. stronger convergence into storms center
4. enhance convection - back to #1
• the above is a positive feedback loop - enhancing the
55. Hurricane Size
• Typical hurricanes are about __________although they can vary
considerably, as shown in the two enhanced satellite images below.
Size is not necessarily an indication of hurricane intensity.
Hurricane Andrew (1992), one of the most devastating hurricane of
DANNY
FRAN
56. Hurricane Strength-
Saffir-Simpson Scale
• Category 1 Hurricane — winds 74-95 mph (64-82 kt)
No real damage to buildings. Damage to unanchored mobile homes. Some damage to poorly constructed
signs. Also, some coastal flooding and minor pier damage.
- Examples: Irene 1999 and Allison 1995
• Category 2 Hurricane — winds 96-110 mph (83-95 kt)
Some damage to building roofs, doors and windows. Considerable damage to mobile homes. Flooding
damages piers and small craft in unprotected moorings may break their moorings. Some trees blown down.
- Examples: Bonnie 1998, Georges(FL & LA) 1998 and Gloria 1985
• Category 3 Hurricane — winds 111-130 mph (96-113 kt)
Some structural damage to small residences and utility buildings. Large trees blown down. Mobile homes
and poorly built signs destroyed. Flooding near the coast destroys smaller structures with larger structures
damaged by floating debris. Terrain may be flooded well inland.
- Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy 1965
• Category 4 Hurricane — winds 131-155 mph (114-135 kt)
More extensive curtainwall failures with some complete roof structure failure on small residences. Major
erosion of beach areas. Terrain may be flooded well inland.
- Examples: Hugo 1989 and Donna 1960
• Category 5 Hurricane — winds 156 mph and up (135+ kt)
Complete roof failure on many residences and industrial buildings. Some complete building failures with
small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures
near the shoreline. Massive evacuation of residential areas may be required.
- Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935
58. Storm Surge
• Storm Surge- water that is pushed toward the shore by the force of the
winds swirling around the storm This advancing surge combines with the
normal tides to create the hurricane storm tide, which can increase the
mean water level 15 feet or more.
• In addition, wind waves are superimposed on the storm tide. This rise in
water level can cause severe flooding in coastal areas, particularly when
the storm tide coincides with the normal high tides.
59. Storm Surge
• Primarily determined by the slope of the beach
gradual slope –
steep slope –
• The primary cause of death in non-U.S. locations
Gradual Slope Steep Slope
60. Storm Surge
• Power of Hurricane Camille's storm surge
seen on the next two slides:
Three-story apartment building directly
across the highway from the beach at Pass
Christian, Mississippi
Only category 5 hurricane to make direct
landfall in the U.S.
63. High Winds
• Understanding the category
of storm greatly helps
understand the wind
damage
• Example: A cat 4 storm is
likely to do 100 times the
damage as a cat 1
64. Fresh Water Flooding
• The ____________________from hurricanes in
the U.S. in the past 30 yrs.
• Rains are generally heaviest with slower
moving storms (less than 10 mph).
• To estimate the total rainfall in inches, one rule of
thumb is to divide 100 by the forward speed of the
hurricane in miles per hour (100/forward speed =
estimated inches of rain).
• North Carolina receives __________of annual
rainfall from tropical cyclone systems
65. Tornadoes
• Because hurricanes have tremendous
rotation associated with them, it is not
surprising that tornadoes are often spawned
in the rainbands and can cause additional
damage and fatalities
• Tornadoes can make even weak hurricanes
very deadly
66. Where is the worst location to
be?
• The right-front
quadrant of the
storm
68. Hurricane Movement
Tropical storm and
hurricane tracks
during the 2005
season shows a wide
range of tracks
Because they are so
powerful, hurricanes
are very difficult to
forecast
69. Hurricane Movement
• Hurricane Gordon
(1994)
• A very erratic track
• Storms are very hard
to predict when the
steering winds (winds
around the hurricane)
are very weak
