Weather%20Forecasting%20and%20Analysis

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Chapter 13 Weather Forecasting and Analysis
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Weather forecasting by the U.S. government began
in the 1870s when Congress established a National
Weather Service under the authority of the Army
Signal Corps.
The National Oceanic and Atmospheric
Administration (NOAA) was established in 1970 to
include a number of environmental agencies,
including the National Weather Service.
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The Meteorological Service of Canada
(MSC), located in Downsview, Ontario, assumes all
forecasting duties for that country and provides
local and regional information to its 14 regional
weather centers.
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Forecasts based on long-term information in the
absence of any data about current weather are
called climatological forecasts. The reliability
of a climatological forecast depends on
year-to-year variability in weather conditions
for the forecast day.
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A persistence forecast relies completely
on current conditions with no reference to
climatology. This simple procedure might work for
a little while but will eventually fail to catch
changes in weather.
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The analog approach tries to recognize
similarities between current conditions
and similar well-studied patterns from
before, assuming that what happened sometime in
the past provides a clue about the future.
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Numerical weather forecasting is based
on computer programs that attempt to mimic
the actual behavior of the atmosphere. The
numerical models typically used in weather
forecasting are very large and can only be run on
the most powerful computers, so-called
supercomputers.
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Quantitative forecasts specify the amount of
the forecast variable. For example, forecasts of
the expected high or low temperature are
quantitative because a value for the forecast
variable is provided.
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Qualitative forecasts provide only a categorical
value for the predicted variable. For example, in
a forecast of rain, the predicted variable is
assigned to a particular class or category.
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In a probability forecast, the chance of some
event is stated. The most common example is the
probability-of-precipitation forecast (PoP
forecast).
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Forecast quality refers to the agreement between
forecasts and observations. Forecast value
refers to the utility of a forecast and
necessarily depends on the application of a
forecast to a particular problem or decision.
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The quality of a forecast concerns forecast
accuracy. That is, on average, how close is the
forecast value to the true value?
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Forecast bias concerns systematic over- or
under-prediction. A biased forecast method
is one whose average forecast is above or below
the true average.
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The mean absolute error (MAE), ignores the sign
(positive or negative) of the errors. That is,
over- and under-predictions are treated the same.
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Forecast skill is defined as the improvement a
method provides over what can be obtained using
climatology, persistence, or some
other no-skill standard.
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Hydrogen-filled balloons carry weather instrument
packages called radiosondes. Radiosondes tracked
by radar are called rawinsondes.
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The general procedure for all numerical models
includes the following three phases The
analysis phase, in which observations are used to
supply values corresponding to the starting state
of the atmosphere for all the variables carried
in the model.
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The prediction phase begins with values delivered
by the analysis phase using governing equations
to obtain new values a few minutes into the
future. The process is then repeated, using the
output from the first step as input for the next
set of calculations.
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In the post-processing phase, conditions forecast
by the model at regular intervals are represented
in grid form for mapping and other display
purposes.
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Ensemble forecasting involves a number
of different model runs performed for the same
forecast period starting with slightly different
initial values.
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If two model runs are made with slightly
different initial values, the results might be
very different after a week or so. This
behavior is now known to be typical of many
natural and human systems, and is referred to as
chaos.
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Long-range forecasts include climatology,
statistics, numerical models, and subjective
judgment to produce forecasts for periods ranging
from a week to the limits of technical
feasibility.
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Surface maps of prevailing conditions present a
general depiction of sea level pressure
distribution and the location of frontal
boundaries.
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Station models offer detailed knowledge of
the conditions at a particular location with over
a dozen weather elements represented on each
station model.
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An 850 mb map.
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A 500 mb map.
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A 300 mb map with isotachs (lines of equal wind
speed).
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Visible images view the atmosphere by
registering the intensity of reflected shortwave
radiation and are available only during the
daytime.
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Infrared images are based on measurements
of longwave radiation emitted (not reflected)
from below.
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Water vapor image obtained from GOES 8 satellite.
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Radar images observe the internal cloud
conditions by measuring the amount of radiation
backscattered by precipitation (both liquid and
solid).
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Vertical profiles of temperature and dew point
observed by radiosondes are plotted on
thermodynamic diagrams (pseudo-adiabatic charts).
On the Stuve chart, air temperature is scaled
along the horizontal axis and pressure is on a
nearly logarithmic vertical axis. The straight,
solid lines are dry adiabats and the dashed,
slightly curved lines are wet adiabats, showing
temperature changes in a rising saturated parcel.

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The lifted index combines the average humidity
in the lowest kilometer of the atmosphere, the
predicted maximum temperature for the day, and
the temperature at the 500 mb level into a single
number. The K-index uses values of
temperature and dew point at the surface and
the 850, 700, and 500 mb levels.
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The next chapter examines human effects air
pollution and heat islands.