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Copyright © 2013 Pearson Education, Inc.

The Atmosphere:

An Introduction to

Meteorology, 12th

Lutgens • Tarbuck

Lectures by:

Heather Gallacher,Cleveland State University

Chapter 12: Weather Analysis and Forecasting



The Weather Business: A Brief Overview

� National Weather Service (NWS) is a branch of the

National Oceanic and Atmospheric Administration

(NOAA).

� The mission statement of NWS is to provide

weather and climate forecasts and warnings to the

U.S., territories, and waters for the protection of life

and property and enhancement of the economy.


� National Centers for Environmental Prediction:

� This branch of the NWS collects weather data from all

over world.

� Information is disseminated to 125 regional Weather

Forecast Offices, which prepare local forecasts.

The Weather Business: A Brief Overview

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Weather Analysis

� Weather analysis:

� The forecaster must collecting, transmit, and compile

billions of pieces of observational data.


Weather Analysis

� Gathering data:

� The World Meteorological Organization (WMO), which is

part of United Nations is responsible for the international

exchange of weather data.

� Surface observations:

� The NWS and the Federal Aviation Administration (FAA)

operate ~900 automated surface observation systems (ASOS).

� Worldwide, more than 10,000 land observation stations, 7000

ships at sea, and hundreds of data buoys and oil platforms

report conditions four times daily.


Weather Analysis

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Weather Analysis

� Observations aloft:

� Weather balloons are launched twice daily at midnight and

noon GMT.

� They contain radiosondes, which are lightweight instrument

packs measuring temperature, humidity, and pressure.


Weather Analysis

� Weather maps: pictures of the atmosphere

� The collected information is displayed in a format that can

be comprehended easily by forecasters.

� The information is placed on synoptic weather maps, which

means “coincident in time.”


Weather Forecasting Using Computers

� Numerical weather prediction:

� This technique uses mathematical models.

� It begins by entering temperature, wind speed and

direction, pressure, and humidity.

� The NWS produces generalized forecast maps, called

prognostic charts (or progs).

� A statistical analysis of progs compares the accuracy of

previous forecasts.

� This approach is known as Model Output Statistics (MOS).

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� Ensemble forecasting:

� This method produces a number of forecasts using the

same computer model.

� It alters initial conditions while remaining within error

range of the observational instruments.



� Role of the forecaster:

� After progs are sent to the WFOs, forecasters blend

information with local conditions and regional weather

to produce site-specific forecasts.

� Forecasters use their knowledge and judgments based

on experience.

� They will sometimes add extra detail to the model

forecasts.



Other Forecasting Methods

� Persistence forecasting:

� This technique is based on the tendency of weather to

remain unchanged for hours or days.

� Climatological forecasting:

� This method uses climate data (average weather statistics

over many years).

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� Analog method:

� This is based on the assumption that weather repeats itself.

� Forecasters attempt to find weather patterns.

� Pattern recognition is an important tool, which helps

improved short-range, computer-generated forecasts.

� Trend forecasting:

� This method determines the speed and direction of fronts,

cyclones, clouds and precipitation.

� Forecasters try to extrapolate the future position of

weather phenomena.


� Upper level maps:

� These maps are generated twice daily.

� They are drawn at 850-, 700-, 500-, 300-, and 200-millibar

(mb) levels.

Upper Airflow and Weather Forecasting

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� 850- and 700-millibar

maps:

� The 850-mb map finds

areas of cold-air and

warm-air advection.

� Winds at the 750-mb

level are used to predict

movement of weather

producers.



� 500-mb maps:

� This level if found approximately 5.5 km above sea level.

� These maps show upper level lows best.



� 300- and 200-mb maps:

� These maps represent zones near the top of

the troposphere.

� The jet stream is best observed at these levels.


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� The connection between upper-level flow and

surface weather:

� Zonal patterns depict quickly moving westerlies (west to

east).

� Meridional patterns consist of slow moving north to south

flows, typically with heavy precipitation.





� An extreme winter


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� Long-range forecasts:

� The Climate Prediction Center, a branch of the NWS,

produces 30- and 90-day outlooks.



Forecast Accuracy

� Percentage probability:

� Rainfall is the only aspect of a forecast expressed this way.

� The prediction of occurrence is fairly accurate.

� The predictions of amount, time, and duration are not

as reliable.

� Short- and medium-range forecasts:

� Accuracy has improved over the decades.

� Beyond 8 days:

� Predictability of day-to-day weather is of low accuracy.


Satellites in Weather Forecasting

� TIROS 1:

� Launched in 1960, this was the first weather satellite.

� Since then, more than 30 versions have been launched

through 2009.

� Polar-orbiting:

� These satellites orbit over the poles.

� Geostationary:

� These satellites orbit west to east over the equator.

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� What types of images do weather satellites provide?

� Geostationary Operational Environmental Satellites

(GOES) provide visible, infrared, and water-vapor images.

� They track large weather systems.

� Visible light imagery records the intensity of light reflected

from cloud tops and other surfaces, to help define shapes,

patterns, and thicknesses.

� Infrared images are obtained from radiation emitted

from objects.

� They help to determine which clouds are likely to produce

heavy precipitation.



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� Visible:

� Only during day

� Higher resolution

� No temperatures

� Infrared:

� Day or night

� Shows temperature of

cloud tops

� Lower clouds—warmer

� Higher clouds—cooler



� Water-vapor images:

� Most of Earth’s radiation with a wavelength of

6.7 micrometers, is emitted by water vapor.


End Of Chapter 12

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