Welcome to Weather World's complete and informative instructions
on how to make your own weather forecasts. The information that
you read below has been compiled by many meteorologists, and it's
these techniques that these same meteorologists use. Also,
Weather World has an instructions page on weather observations
incase you don't know how to properly record weather data and the
sort.
Weather forecasting is a science and an art. To make a good
forecast you must know some meteorology and have a hunch what is
going to happen based on experience. Forecasting temperature and
precipitation will be discussed here since they are the most
relevant parameters to the public.
Before attempting to perform a weather forecast, one should
understand
a few key concepts in atmospheric science. These concepts
include:
The following discussion will assume the reader is familiar with
these
concepts.
The first thing you should know about the place you are
forecasting for is
the climatology (average high/low temperature). Climatology is
rarely a correct
forecast for a given day but you might think twice about
forecasting a high
that is 20 degrees higher than the average high (unless you're
absolutely
sure).
Persistence means that tomorrow's weather will be the same as
today's. In
many situations, persistence is the best forecast to make--for
example, in
the Southeastern U.S. in summertime, when there are few fronts or
low pressure
systems moving through to change the weather.
One of the most important techniques for making a forecast is to
use weather
maps to estimate the speed of movement of air masses, fronts, and
high and low
pressure systems. For example, to make a low and high temperature
forecast for
tomorrow, find the current location of the air mass that you
expect to be over
the forecast site tomorrow. What were the high and low
temperatures in the
region that that air mass affected today? Remember to adjust your
forecast for
differences in latitude, possible acceleration/deceleration or
intensification/deintensification of storm systems, and local
effects such as
topography, bodies of water, and the urban heat island effect.
If you are located in the region you are forecasting for (as
is most often
the case) remember to look out the window.
The first thing you want to know about your precipitation
forecast location
is what season is it. Summer precipitation is generally on a
smaller scale and
can be locally heavy (thunderstorms). Winter precipitation is
generally more
widespread, and light to moderate. If the precipitation is in the
form of snow
remember liquid equivalent is roughly 10 inches of snow equals 1
inch of water.
In making precipitation forecasts for an approaching winter storm
be careful of
the possibility of the change from rain to snow or vice versa.
Rain adds to the
precipitation amount much faster than snow.
In making your temperature forecast remember the high temperature
generally
occurs during the late afternoon. Know the high temperature for
the air mass
moving into your area and account for modification (clouds,
winds, large water
bodies etc.). Light winds and clear skies allow for greater
heating during the
day. Look at observations for your location as well as upstream
(usually west)
of your location. The low temperature generally occurs early in
the morning.
Nighttime cloud cover or high humidity slows down the rate of
cooling
significantly.
Currently, hourly updated images and animations are available on
the Weather
Underground Groundhog Gopher server for the files shown
below. The animations are in the directory
"Weather_Animations" and the still images are in
directory "Interactive Weather Maps. (Exception: interactive
satellite images are not yet available; satellite images can be
found in the Weather_Images directory.)
Update time in
Image minutes past the hour
---------------------------------------------
Precipitation 10
Temperature 17
Relative Humidity 22
Wind Vectors 25
Pressure 30
Heat Index 35
Infrared Satellite 55
Visible Satellite 57
The following images are generated once per day, and can be
found in
directory Weather_Images:
Image Update time
-----------------------------------------------------
Average_high_temperature.gif 6:30pm EDT
Average_low_temperature.gif 6:30pm EDT
Forecast_low_tonight.gif 12:10am EDT
Forecast_high_tomorrow.gif 12:10am EDT
The heat index and wind chill images are not particularly
useful for
forecasting, and will not be discussed here. Perhaps the most
useful forecast
tools available via Blue-Skies are the Quicktime animations;
these allow one to
gauge the speed with which air masses, fronts, and pressure
systems are moving.
Temperature map - Note other areas that have the same
temperature as your
region. A rapid change in temperature sloping roughly north-south
is a good
indication of a cold front. Check to see where the air mass
coming into your
area is coming from. Is warm or cold air moving in?
Pressure map - Low pressure areas are probably close to a
front and therefore
precipitation. High pressure areas are probably not associated
with
precipitation. If there is a rapid change in pressure over a
short distance,
strong winds will result.
Humidity map - High humidity should be located along and
ahead of a front. Low humidity should be located behind a front
and near highs.
Precipitation map - Shows the location and intensity of
precipitation. This map
is usually the best guidance for a precipitation forecast.
Satellite imagery - shows where the clouds (and therefore
fronts and low
pressure systems) are located. If images from the last few hours
are looped the
motion of the atmosphere is seen so you can see air mass
movement.
Wind Vectors - shows the direction and speed (length of
arrow) of the wind. You
can analyze the wind vectors and find out where the high and low
pressure
systems are located. Winds circulate counterclockwise around a
low and
clockwise around a high.
Average High and Low temperature map- These maps show the
average high and low temperature across the U.S. for a given
date, based on long-term climatological averages.
Forecast High and Low temperature maps - These maps show
the forecast high and low temperatures for the U.S. as predicted
by a computer model known as MOS(Model Output Statistics). They
serve as a good reference with which to compare one's forecasts.
Don't get caught up to much in computer model guidance, though.
The computers can't predict weather all by themselves (otherwise
there would be no such thing as a weather forecaster), so use
them as a guide. The best forecast is made by utilizing data from
the computer and applying your own meteorology skills in making a
forecast.
Although it is more difficult, one can make a forecast using only
the weather text information available via the Weather World web
site. A good place to start is the National Weather Summary,
which gives one an overview of the significant weather over the
past 24 hours as well as an indication of where significant
weather will be occurring during the next 24 hours. Next, one
should look at the climatic data summary for the city of
interest. This gives the precipitation and high and low
temperature for the previous day, as well as the normal highs,
lows, and precipitation from long-term climatology. Next, one
should consult the current weather observations for a region of
interest and plot one's own weather map using a blank U.S. map.
Ideally, one should plot two or more weather maps over a several
hour period to get an idea of the motion of air masses, fronts,
and weather systems. Analyze the map to look at the weather and
think about what will happen in the future. You may want to
contour the data so you can visualize what's going on. Lastly,
one should consult the National Weather Service forecast for the
selected city and nearby cities, unless you consider this
"cheating".
Teachers may want to call upon additional resources to help in
preparing a
weather forecast. One good source is The Weather Channel. It's on
all the time
and the maps are quite easy to comprehend. For those without
cable television
is the PBS television show, AM Weather. It airs each weekday at
7:15 A.M. A
teacher might want to videotape and show a few minutes of either
program to
augment the Weather Underground data, particularly when using
only the
text-based UM-WEATHER information. A universal date source that
is available
any time and anyplace is a newspaper's weather map. The USA Today
has an
excellent color weather page.
The reason why you make a particular forecast is very important.
Making a
correct forecast for the wrong reasons is of no value. If you
make a wrong
forecast for the right reasons you learn from it and become more
likely to make
a right forecast for the right reasons. If your forecast varies
considerably
from other people's that's not necessarily a bad sign (as long as
you have a
legitimate reason for making your forecast). When justifying your
forecast tell
what guidance you used and what was indicated by it.
Examples:
There are several methods of scoring in a forecast contest. For
max/min
temperature and precipitation forecasting here's the simplest
method. Scoring
is like golf, the lower score the better. Keep a cumulative score
of the people
in the forecast and the person with the lowest score is in first
place.
TEMPERATURE - If your forecast is within 3 degrees of the
actual value then you are "in the window" and don't
receive any points. For every degree after 3 you are off you
receive one point. Example: You are 10 degrees off on your
forecast, so you receive 7 points.
PRECIPITATION - For precipitation you must select one of the
following
categories:
The forecast is generally made in a 24 hour period midnight to
midnight. If
you forecast the correct category you receive no points. One
category off is 3
points and for each additional category off there after add 3
points. Example:
You predict category 1 and the actual category is 3, so you
receive 6 points.
An additional method for keeping precipitation score is the
Briers score.
This is based on predicting the percent probability of
precipitation. The
forecaster assigns a probability of precipitation between 0% and
100% for a
given 24-hour period. The Briers score is computed as the square
of the
difference between predicted and observed percent probabilities.
For example,
over a 3-day period forecaster "A" predicts a 30%
chance of precipitation each
day. Meanwhile, the more adventurous forecaster "B"
predicts a 0% chance of
precipitation each day. Precipitation actually occurs on one of
the 3 days.
Here's the cumulative Briers score for each forecaster:
Forecaster "A"
Forecast Actual
Precipitation Precipitation Briers score
30% 0% (30 - 0 )**2 = 900
30% 100% (30 - 100)**2 = 4900
30% 0% (30 - 0 )**2 = 900
Total Briers score for the 3-day period: 6700
Forecaster "B"
Forecast Actual
Precipitation Precipitation Briers score
0% 0% (0 - 0 )**2 = 0
0% 100% (0 - 100)**2 = 10000
0% 0% (0 - 0 )**2 = 0
Total Briers score for the 3-day period: 10000
So, the more daring Forecaster "B" was hurt by going
all-out attempting to
make a perfect forecast (a lower Briers score is better).
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