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Introduction Forecasting methods Forecast types Forecasting tools Weather maps Practice
Weather forecast
A weather forecast (TMA)
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Introduction Forecasting methods Forecast types Forecasting tools Weather maps Practice
The purpose of weather analysis
The aim of weather analysis is to diagnose the weather situation in
three dimensions
The analyses provide a complete picture of the physical state of the
atmosphere
Meteorologists use the analyses to forecast the future displacements
and developments of weather systems
In this topic, we focus to understanding some of the basic analytical
methods used for forecasting purposes:
⇒ Surface charts (pressure analysis)
⇒ Tephigram
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Forecasting methods
Availability of powerful computers and sophisticated observing
equipment has improved significantly the science of weather prediction
Weather forecasting has a history:
Historical forecasting
⇒ predicting the weather by assuming that what happened in the past will
happen again this time
⇒ This is one of the methods used to predict the tracks of hurricanes
Different societies also used to forecast weather using various
traditional ways (e.g. weather lore)
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Forecasting methods ...
Persistence forecasting
⇒ Assume that it will not change for a little while
⇒ Often reliable over periods of minutes or hours
⇒ In most locations, persistence forecasting doesn’t work well for periods longer
than a couple of hours
Trend forecasting
⇒ Assume that a current tendency or trend will continue, or that an observed
cycle will repeat
⇒ It is a form of extrapolation - extending a defined rate of change into the
future
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Introduction Forecasting methods Forecast types Forecasting tools Weather maps Practice
Forecasting methods ...
Synoptic weather forecasting
⇒ The term synoptic means generalized, or taking a view of the whole.
⇒ Done by assembling weather maps of large regions from observed and
reported data at numerous stations (station models)
⇒ The defining tool of synoptic forecasting is the weather maps
Numerical forecasting
⇒ Numerical forecasting is more like digital communications or imagery, in
which the variables are bro- ken up into small parcels or packets, and the
behaviour of the whole system is deduced by brute force of calculation, based
on the laws of physics
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Time range of forecasts
Nowcasting
⇒ It is a very short range forecast for up to a few hours (usually not more than
6 hours)
⇒ Involves both subjective and definitive interpretations of surface
observations, satellite imagery, and Doppler radar information
⇒ Rely on trend method of forecasting, with human experience and pattern
recognition coming into play
Short-range forecasts
⇒ Weather forecasts that range from about 12 hours to a few days (generally 3
days or 72 hours)
⇒ Various techniques: satellite imagery, Doppler radar, surface weather maps
and upper-air winds
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Time range of forecasts ...
Medium-range forecast
⇒ Extends from about 3 days up to 8 days (192 hours) into the future
⇒ Base entirely on computer-derived products, such as forecast charts and
statistical forecasts
⇒ A forecast that extends beyond 3 days is often called an extended forecast
Long-range forecasts
⇒ A forecast that extends beyond about 8 days
Apart from using certain forecasting methods, forecasters also can use
various forecasting tools to analyse weather systems
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Introduction Forecasting methods Forecast types Forecasting tools Weather maps Practice
Meteogram
Figure 2 : Example of a meteogram
A chart showing how one or more
weather variables has changed at a
station over a given period of time
Represents how air temperature,
dew point, and sea-level pressure
have changed over the past five days
It may illustrate how these same
variables are projected to change
over the next five days
Exercise: Customize a meteogram
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Vertical sounding
Figure 3 : A Thermodynamic chart
Analysis needs to provide a picture
of 3-D structure of the atmosphere
Sounding provides a 2-D vertical
profile of temperature, dew point,
and winds
Helpful for a short-range forecast
that covers a small area
It can also aid in predicting fog, air
pollution alerts, and the downwind
mixing of strong winds
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Thickness chart
Figure 4 : A thickness layer
It analyse temperatures at different
altitudes and how they are changing
It can help identify air masses and
locate fronts
Low thickness lines correspond to
cold air, and high thickness lines to
warm air
A prognostic thickness chart can
help predict the daily max and min
temperature
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Thickness Chart
Figure 5 : A thickness layer
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Analysis of pressure
Distribution of atmospheric pressure is significantly related to the
weather phenomena and the changes in weather situation
Analysis of pressure is done by drawing isobars on the chart
Satisfactory analysis base only on a succession of weather charts, not
on a single chart
Analysis is preceded by plotting all observed data on the chart using a
station model
Then use isobars, usually drawn for every 5 mb and the direction of
the wind field to join places with equal pressure
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Analysis of pressure ...
Pressure analysis can involve
⇒ examining the forms of clouds and precipitation, to indicate the fronts
between the various air masses, and whether the air masses are stable or
unstable
⇒ identify the various air masses, fronts, and cyclones with those indicated on
the preceding map (historical sequence)
The final chart that meteorologists produce after interpreting the
weather patterns and correcting any errors is called an analysis
The final forecast chart representing the atmosphere at a specified
future time is called a prognostic chart
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Levels of analysis
Table 1 : Levels of analysis
Analysis chart Altitude Elements to be spotted and tracked
Surface frontal systems, highs and lows;
cloudiness, precipitation, wind, and fog
850 mb 1500 m high moisture values
convergent winds
700 mb 3000 m moisture, temperature advection
dry, warm layer
500 mb 5600 m steering flow for cyclonic storm systems,
cold advection
300 mb 9180m core of jet stream
high pressure and light, divergent wind
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Introduction Forecasting methods Forecast types Forecasting tools Weather maps Practice
Practice
Study the weather information contained in the plotted surface
charts. Identify the basic weather systems: highs, lows (ITCZ). Also
discuss the basic flow on the eastern Africa region
Use the sounding data from an upper air station to plot on the
provided Tephigram chart
Useful resources:
⇒ http://wxmaps.org/
⇒ http://weather.uwyo.edu/upperair/
ndettoel@2016 ENV 111: Introduction to Meteorology
