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Meteorology 1010Supplement to Chapters 9-11

This PowerPoint is not a substitute for reading the textbook and taking good notes in class.

This PowerPoint presentation is the first of two that supports Quiz #4, Fall Semester, 2013.

Powerpoint presentations should be viewed as a study guide.

Jet StreamsMeridional vs Zonal

When the jet stream provokes storms by

mixing cool/dry with warm/wet storminess

can occur.

Mid-latitude storms often begin with

mixing of cool/dry air and warm/wet air as

the jet stream(s) undulates more

north/south, helping to mix differing air

masses.

November 18 Salt Lake weather report indicated that westerly flow was going to be more ‘zonal’ rather than

‘meridional’ so there would not be any increasing intensity that otherwise might occur if ‘storm track’

flow dipped north or south to pick up greater moisture and/or greater contrast.

Sunday’s tornadoes in Illinois were unusual because of more-than-normal moisture and heat – November

usually exhibits snow storms, with some wind.

Notice that eastward-moving storm is typical - counter-clockwise air in the mid-latitudes

Frontal Weather

This one is a cold front. A warm front would have similar features, but more gradual.

Notice that the cold-front side is more vigorous than the warm front side (right side).

Cold front chasing warm air

Warm front chasing cool air

Faster-moving cold front gradually overtakes warm air and causes warm air to lift.

Life of a Midlatitude Cyclone

Notice that the cold front side shows more severe radar returns – more vigorous lifting and more severe weather.

Relatively Cold Air

Relatively Warm Air

Notice that in a mid-latitude cyclone, cold and warm air don’t mix at first.

As Coriolis force helps turn the air, mixing begins as warm, humid air lifts over cooler, drier air that is more heavy.

Rising air provokes condensation, precipitation and strong winds.

At the end, warm air is temporarily stable above cold air below.

Westerly winds

Warm, wet air rises above cool/dry air classic “frontal” storm.

Here we see how the jet stream (with storm track) helps pull low and high pressure cells toward each other.

The difference between warm/wet and cool/dryhelps produce rising air, high wind, precipitation, hail, lightning.

If you click quickly on the next six slides you can see how a mid-latitude cyclone can develop.

Figure 9.18a

Figure 9.18b

Figure 9.18c

Figure 9.18d

Figure 9.18e

Figure 9.18f

Air-Mass Thunderstorms

Mountainous regions, such as the Rockies and the Appalachians, experience a greater number of air-mass thunderstorms.

Because Utah is fairly dry, we have enough heat for dust devils and brief air-mass thunderstorms.

For really severe weather, heat and humidity are needed – not just “cash”, but “credit”

Severe Thunderstorms

• Severe thunderstorms:– Heavy downpours– Flash flooding– Straight line wind gusts– Hail, lightning– Wind shear– Can overshoot (enter stratosphere)– Downdraft preceding (gust front)

Supercell Thunderstorms

• Supercells – These storms can produce extremely dangerous weather.

• They consist of a single, powerful cell that can extend to heights of 20 km or more.

• The clouds can measure 20–50 km in diameter.

• Mesocyclone:– Vertical winds may cause the updraft to rotate, which

forms a column of cyclonically rotating air.– Tornadoes often form.

• Squall lines – a line of thunderstorms that may hundreds of miles long – cooler/drier air on one side -- warmer/wetter air on the other side.

ColderDrier air

Warmer, more humid air

Supercell Thunderstorms

• Mesoscale convective complexes (MCC):– An MCC consists of many individual

thunderstorms. – It is organized into a large oval to circular cluster. – They cover an area of at least 100,000 km2.– It is a slow-moving complex that may last for 12

hours or more.– MCCs tend to form mainly in the Great Plains.

Tornadoes

The Development and Occurrence of Tornadoes

• Tornado development

• Profile of a tornado:– Average diameter 150–600m– Travels ~45 kph – 28mph (Nov 13 storms moved at 60+ mph)– Path about 26 km long– Most travel to the NE– Exist between < 3 min to > 3 hours– Wind speeds between < 150 kph to > 500 kph – November 17, 2013 storm winds estimated at 190 mph

Based on this chart, can we use single events to support the theory of global warming?

No. However, a persistent pattern of more severe weather is consistent with other findings about climate change.

Tornado Destruction

• Tornado intensity

2013 tornado in Oklahoma probably produced 300 mph wind.

A November 2013 tornado reached nearly 200 mphEF4

Tornado Destruction

Tornado Forecasting

• Tornado watches and warnings:– Watches alert the public.

• Tornadoes are possible and conditions are favorable.• They usually cover an area of about 26,000 km2.• Watches can last 3 hours or longer.

– Warnings are issued when a tornado is actually sighted or conditions are just right.

• There is a high probability of imminent danger.• They are usually for a much smaller area. • Warnings are in effect for a much shorter period,

usually 30–60 minutes.

News reports stated that:

“Predictions are saving lives.”

The truth is - - they weren’t genuine predictions, just better quality warnings.

Oddly, hurricanes are so big that we can only generalize about where, what and when - - just a forecast.

Oddly also, tornadoes are so small and transient that they are very difficult to predict - - we can forecast better, but

not really predict.

Tornado ForecastingDoppler effect:

1. radar reflections arriving more quickly indicate the target is getting closer.

2. Radar reflections arriving more slowly indicate the target is getting further away.

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