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Hurricanes and Nor’Easters

Severe Thunderstorms and Tornadoes

Hurricane Basics Severe Tropical Cyclones

Tropical cyclones with winds greater than 74 mph (120 km/h)

Most destructive weather on earth

Typhoons

Tropical Cyclones

Hurricanes Hurricanes

From Wikipedia

Hurricane Formation

Formed over ocean water > 26o C (80o F)

Convection (Warm, moist, rising air.) and Latent Heat

From NWS Jetstream

Hurricane Formation Tropical Disturbance

some T-storms

Tropical Depression

Organized T-storms (wind speed 0-38 mph)

Tropical Storm

Organized rotation (wind speed 39-73 mph)

name given

Hurricane Structure

Eye Wall –strongest T-storm and winds

Eye – calm descending air

Feeder Bands /Rain bands - storms spiral in towards center

Storm Surge- dome of water pushed by the winds

From NWS Jetstream

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Hurricane Damage Storm Surge – may exceed 20 feet

Winds – may exceed 150 mph

Waves – may exceed 50 feet

Flooding – Rain intensity and speed of the storm may cause flooding

Tornadoes – occasional weak tornadoes may occur

Saffir–Simpson Hurricane Scale

Category Wind speed Storm surge

mph (km/h)

ft (m)

5 ≥156 (≥250)

>18 (>5.5)

4 131–155 (210–249)

13–18 (4.0–5.5)

3 111–130 (178–209)

9–12 (2.7–3.7)

2 96–110 (154–177)

6–8 (1.8–2.4)

1 74–95 (119–153)

4–5 (1.2–1.5)

Hurricane Rating Scale

Hurricane Damage Category One Hurricane: Damaging winds are expected. Some damage to building structures could occur,. Some

damage is likely to poorly constructed signs. Loose outdoor items will become projectiles, causing additional damage. Persons struck by windborne debris risk injury and possible death. Numerous large branches of healthy trees will snap.. Many areas will experience power outages with some downed power poles. Category Two Hurricane: Very strong winds will produce widespread damage. Some roofing material, door, and window damage of buildings will occur. Considerable damage to mobile homes. A number of glass windows in high rise buildings will be dislodged and become airborne. Numerous large branches will break. Many trees will be uprooted or snapped. Extensive damage to power lines and poles will likely result in widespread power outages that could last a few to several days Category Three Hurricane: Dangerous winds will cause extensive damage. Some structural damage to houses and buildings will occur with a minor amount of wall failures. Mobile homes and poorly constructed signs are destroyed. Persons struck by windborne debris risk injury and possible death. Many trees will be snapped or uprooted and block numerous roads. Near total power loss is expected with outages that could last from several days to weeks. Category Four Hurricane: Extremely dangerous winds causing devastating damage are expected. Some wall failures with some complete roof structure failures on houses will occur. All signs are blown down. Complete destruction of mobile homes Extensive damage to doors and windows is likely. Numerous windows in high rise buildings will be dislodged and become airborne. Most trees will be snapped or uprooted. Fallen trees could cut off residential areas for days to weeks. Electricity will be unavailable for weeks after the hurricane passes. Category Five Hurricane: Catastrophic damage is expected. Complete roof failure on many residences and industrial buildings will occur. Some complete building failures with small buildings blown over or away are likely. All signs blown down. Complete destruction of mobile homes). Severe and extensive window and door damage will occur. Nearly all windows in high rise buildings will be dislodged and become airborne. Severe injury or death is likely for persons struck by wind-blown debris. Nearly all trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months.

“Winter Hurricane”

Nor’Easter Basics Mid-Latitude

Cyclone forms off the Carolina Coast

Brings Snow, Rain and Ice to Mid Atlantic states

Severe damage and beach erosion to the NC coast.

Nor’Easter Formation Low pressure often forms over the Gulf

Stream

Storm increases intensity with greater temperature contrast in air masses

Pressure drop of 24mB in 24 hrs – Bomb cyclone

Strongest winds from the NE give the storm its name

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Storm with lightning, thunder, and strong winds T-Storm Formation Ingredients:

Moisture

Unstable air

Cold air over warm allows more rapid convection

Development of T-Storm

Cumulus stage Warm moist air

forms a large cumulus cloud

T-Storm Development Mature stage Violently rising air

forms cumulonimbus cloud Heavy rain, wind, hail, and

lightening possible

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T-Storm Development

Dissipating Stage Down drafts cool

and slow convection

Types of T-storms Single cell

Multi-cell

Multi-cell line (Squall line)

Super cell

A thunderstorm will be called SEVERE if: Hail > ¾”

Winds in excess of 57 mph

Tornado

Single Cell Storm Multi Cell Storm

Squall Line Super Cell Storm

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Smallest and most violent short lived cyclones associated with thunderstorms

Tornado Formation Middle and high altitude wind shear

causes rotation (roll)

Directional Shear

Speed Shear

Convection (Updrafts) causes roll to become vertical

Wind speeds may reach over 300 mph.

Tornado on Doppler RADAR

Tornado Formation

Tornado Alley

From Wikipedia

Tornado Rating Scale Fujita Scale