Chapter 13 Weather Forecasting and Analysis. 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
Weather Forecasting and Analysis
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.
located in Downsview, Ontario,assumes all
forecasting duties for that country and
provides local and regional information
to its 14 regional weather centers.
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.
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.
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.
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,
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.
categorical value for the predicted variable.
For example, in a forecast of rain,
the predicted variable is assigned
to a particular class or category.
some event is stated.
The most common example is 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.
That is, on average, how close is
the forecast value to the true value?
over- or under-prediction.
A biased forecast method is
one whose average forecast is
above or below the true average.
the sign (positive or negative) of the errors.
That is, over- and under-predictions
are treated the same.
a method provides over what can be obtained
using climatology, persistence, or some other
instrument packages called radiosondes.
Radiosondes tracked by radar are called rawinsondes.
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.
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.
forecast by the model at regular intervals
are represented in grid form for mapping
and other display purposes.
different model runs performed for
the same forecast period starting with
slightly different initial values.
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.
numerical models, and subjective judgment to produce
forecasts for periods ranging from a week to
the limits of technical feasibility.
a general depiction of sea level pressure distribution
and the location of frontal boundaries.
conditions at a particular location with over a dozen
weather elements represented on each station model.
the intensity of reflected shortwave radiation and
are available only during the daytime.
longwave radiation emitted (not reflected) from below.
by measuring the amount of radiation backscattered
by precipitation (both liquid and solid).
and dew point observed by
radiosondes are plotted on thermodynamic diagrams
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.
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.
air pollution and heat islands.