1. EASC 11 Forecasting, Weather Maps, and Severe Storms Forecasting
Daily weather forecasts are based on recent weather maps and computer models of the atmosphere as well as satellites and commercial aircraft
A computer model of the atmosphere contains data on wind, temperature, pressure, humidity, clouds, and precipitation
Not a perfect predictor – still needs human interpretation, and cannot account for all variables
Satellites and radar are used to fill in data in areas where there is little information available (where few people are living, over oceans, etc.)

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Radar uses the echoes of radar waves off of precipitation particles to “see” the shape of precipitation areas
“Doppler” radar uses the Doppler effect: particles moving toward the radar have a shorter wavelength and particles moving away from the radar have a longer wavelength (this is the same effect that causes a siren to sound higher-pitched when moving towards to you, and lower-pitched when moving away from you).

3. EASC 11 Thunderstorms and Tornadoes
Thunderstorms are small-area storms formed by the strong upward movement of warm, unstable moist air and are characterized by cumulonimbus clouds, lightening and thunder, usually rainfall, and high winds; may also bring hail and tornadoes
Thunderstorms may be km across, and km deep
Two types:
Air Mass Thunderstorms (local)
Form within a warm moist air mass
Start when ground surface is strongly heated (spring and summer)
Usually single storms lasting less than an hour
Widely scattered, unpredictable, form mainly over land

4. EASC 11 2. Frontal Thunderstorms
Usually form in warm moist air on or ahead of cold fronts
MAY also occur in warm fronts
MAY occur as squall lines ahead of the front
Lines of thunderstorms can be hundreds of km long, and can last for several hours
Occur most often in spring and summer
Electricity in a Thunderstorm
All thunderstorms produce lightning – the discharge of electricity from one cloud to another
Thunder ALWAYS comes after lightning because light travels faster than sound – you can estimate how far the storm is by counting how much time between the flash of lightening and the roar of thunder (1 second = about 1km)

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Tornadoes
Most of the world’s tornadoes occur in the Mississippi Valley and Great Plains of the US
A tornado is a narrow, funnel-shaped column of spiral winds that extends down from the cloud base and touches the ground
A tornado travels with its parent thunderstorm at km/h BUT wind speed WITHIN the tornado is km/h
The funnel is usually less than 500 m across at ground level, and “carves” an irregular path less than 2.5 km long
The tornado funnel is a mix of cloud and dust
Tornadoes over water are called waterspouts, and are usually weaker than land-based tornadoes

6. EASC 11 Severe Weather Watches and Warnings
Severe thunderstorm: wind gust at least 80 km/h; hail 2 cm diameter or larger; funnel clouds or tornado
These are most frequent in spring and early summer in the late afternoon
Winter Storms
Caused by strong mid-latitude lows IF there is enough moisture AND it is cold enough to snow
Mid-latitude lows are strongest in winter along the East coast, strongest in the spring over continental North America
Blizzard: is a snowstorm with high winds and low temperatures that can result in heavy snows, mixed rain and snow, freezing rain, or rain

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Cyclonic Storms
A Hurricane is an intense tropical low-pressure area with winds 120 km/h or greater and heavy rains
A storm surge forms when a hurricane piles up water along the coast then blows it inland
In a hurricane, wind spirals toward the centre (low pressure area)
There is heavy rain in a hurricane that that grows stronger over warm water, and weaker over land or cool water
In the centre of the hurricane is the EYE – an area of sinking air (high pressure) that is km across; there is no rain and almost no wind, and only stratocumulus or cirrus clouds
Outside the eye is a wall of thunderstorms
Hurricanes are km in diameter, and high winds may stretch over an area 800km across, 1600 km long

8. EASC 11 Naming and Forecasting Hurricanes
Before 1953, hurricanes identified by date
From , hurricanes were given female names
After 1979, hurricanes given male and female names
Names alternate beginning with A
Weather satellites and aircraft are used to monitor storm activity, and if severe conditions are expected coastal areas are evacuated

9. EASC 11 Sources and Tracks of Tropical Storms
Form from tropical lows along the ITCZ (intertropical convergence zone – that band of low pressure along the equator)
Storms get energy from heat and water vapour from the sea surface, and from condensing water vapour
Hurricanes may also be called “Tropical Cyclones”, and “Typhoons” (in the Pacific)
Hurricanes tend to do more damage on the East coast of North America, but also occur fairly frequently over the Pacific Ocean
Hurricanes are most common in the late summer and early fall in the Northern Hemisphere, because the ITCZ is further north