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.)

EASC 11 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).

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 10-20 km across, and 10-15 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

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)

EASC 11 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 40-65 km/h BUT wind speed WITHIN the tornado is 360-500 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

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

EASC 11 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 15-50 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 300-600 km in diameter, and high winds may stretch over an area 800km across, 1600 km long

EASC 11 Naming and Forecasting Hurricanes Before 1953, hurricanes identified by date From 1953-1979, 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

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