Weather Forecasting
Information Courtesy of:
University of Illinois at Urbana-Champaign
(http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/home.rxml)
Forecasting Methods: Persistence
Method
• Simplest Method.
• Assumes conditions won’t
change from time of
forecast.
• Works well with patterns
that change very little, or
with slow moving
weather patterns.
• Works best for short-term
forecasts.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/mth/prst.rxml
Forecasting Methods: Trends Method
• Involves determining
speed and direction of
pattern movement.
• If patterns speed up,
slow down, change
direction or change
intensity, the trends
method won’t work as
well.
Example: a storm system is 1000 miles west of
your location and moving to the east at 250 miles
per day, using the trends method you would
predict it to arrive in your area in 4 days.
Mathematics
(1000 miles / 250 miles per day = 4 days)
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/mth/trnd.rxml
Forecasting Methods: Other Methods
• Climatology: averaging weather statistics over
many years to forecast weather.
• Analog Method: examining the current
weather and thinking back to a past date
where the weather was similar and then
assuming the weather will behave the same
way again.
• Numerical Weather Prediction: using complex
computer programs (forecast models)to
predict weather based on equations and
formulas.
Surface Features
•
•
•
•
•
•
•
Anticyclones
Cyclones
Cold Fronts
Warm Fronts
Stationary Fronts
Occluded Fronts
Dry lines
Anticyclones
• A high-pressure center
where pressure has been
measured to be highest
to relative to its
surroundings.
• Moving any direction
away from the “high” will
result in decrease in
pressure.
• High pressure centers are
generally centers of
anticyclones.
• High-pressure centers are
represented by a blue
“H”.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/hgh.rxml
Think about it: Air diverges outward from
a surface high. With air moving away
from this region, air must sink to replace
it. This sinking motion generally leads to
fair skies and no precipitation.
Anticyclones
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/hgh.rxml
• Winds flow clockwise
around a high pressure
center in the northern
hemisphere.
• Temperatures dependent
upon location relative to
high.
• Northerly winds
(approaching high) tend to
bring colder temperatures
• Southerly winds (backside
of high) bring warming
trend.
Cyclones
• Cyclone is an area of low
pressure around which
winds flow
counterclockwise (in
northern hemisphere).
• Moving away from the
low pressure center will
result in increasing
pressure.
• Air converges into low
pressure center causing
air to rise.
– Rising motion may
produce
clouds/precipitation
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/lw.rxml
Cyclones
• A “Low” is represented by
a red “L”
• As cyclone approaches,
the likelihood of
clouds/precipitation
increases
• Southerly winds
(approaching cyclone)
likely to result in warmer
temperatures.
• Northerly winds (backside
of Low) typically result in
cooling trend.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/lw.rxml
Cold Fronts
• Cold Front: transition zone
where cold air mass is
replacing warmer air mass.
• Air mass behind a cold
front is generally cooler
and drier than before the
front.
• If a cold front is
approaching, precipitation
is possible just before and
while the front passes.
• Behind the front: clearing
skies, cooler temperatures,
lower relative humidity.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/cfrnt.rxml
-In this picture, temperatures ahead of
the front are 55 and 62 degrees where
behind the front, temperatures are 28
and 31 degrees.
Cold Front Diagram
• As the cold air mass propagates, it lifts the warmer less dense air ahead of it (red
arrows).
• The air cools as it rises and the moisture condenses to produce clouds and
precipitation ahead of and along the cold front.
• In contrast to lifting along a warm front, upward motions along a cold front are
typically more vigorous, producing deeper clouds and more intense bands of
showers and thunderstorms.
• These bands are often quite narrow (a couple hundred kilometers across) and
move rapidly just ahead of the cold front.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/cfrnt.rxml
Warm Fronts
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/wfrnt.rxml
-In the picture above, temperatures ahead
of the front are 28 and 31 degrees where
the temperatures behind the front are 55
and 62 degrees. This is a typical of warm
fronts.
• Warm Front: transitions
zone where warm air mass
is replacing a cold air mass.
• Air behind a warm front is
generally warmer and more
moist than air ahead of it.
• Before the front: light
rain/winter precipitation
possible
• After the front: clearing
skies, warmer temperatures
and higher relative
humidity.
Warm Front Diagram
• The surface of the warm front extends vertically into the
atmosphere, sloping up and over colder air ahead of it.
• Warm air rides up and over cold air mass, cooling as it rises,
producing clouds/precipitation before the front.
• Since lifting is gradual and steady, wide spread and lightintensity precipitation is typical before a warm front.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/wfrnt.rxml
Stationary Fronts
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/sfrnt.rxml
-Stationary fronts are
represented by alternating blue
and red lines with blue triangles
(pointed towards warmer air)
and red semicircles (pointed
towards colder air).
• Stationary Front: a front that
is not moving
• Heavy amounts of
precipitation possible if
stationary front is nearby and
a low pressure center is
approaching.
• Generally noticeable change
and wind shift crossing from
one side of the front to
another.
Occluded Fronts
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/ofrnt.rxml
-Occluded fronts are represented by a
solid purple line with alternating
triangles and semicircles pointing the
direction the front is moving. On
colored weather maps, they are a solid
purple line.
• Occluded Front: develops
when a cold front catches
a warm front.
• With passage of an
occluded front, weather
conditions will likely turn
from cool to cold.
• Winds will swing around
from easterly to westerly
or southwesterly with
rain/snow showers
possible.
Dry Line (a moisture boundary)
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/sfc/dfrnt.rxml
Drylines are typically found
east of the Rocky Mountains
because it separates the
warm, moist air mass to the
east from the hot, dry air
mass to the west.
• Dry Line: a boundary
separating a moist air mass
from a dry air mass. (also
called Dew Point front).
• If a dry line is approaching
your region, predict much
drier air after the boundary
moves through.
• Storms are possible as the
dry line approaches.
• Temperatures tend to rise
after the dry line passes.
Cloud Cover
• When skies are clear,
more heat reaches the
earth’s surface causing
warmer temperatures.
• When it’s cloudy, some
of the sun’s rays are
reflected off the cloud
droplets back into
space causing cooler
temperatures.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/tmps/cld.rxml
-If cloud cover is expected during the day,
predict cooler temperatures, but at night, cloud
cover keeps temperatures warmer because less
heat escapes back into the atmosphere.
Temperature Advection
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/tmps/ta.rxml
-For example, above, assume that a temperature
forecast is being made for the northern station.
The northern station is cooler than the southern
station, but the wind directions are the same, out
of the south. The wind will transport the warmer
air located at the southern station towards the
northern station, so expect the temperature at
the northern station to rise.
• Temperature Advection:
refers to change in
temperature caused by
movement of air by wind.
• To forecast using
temperature advection,
look at wind direction at
your forecasting site and
the temperatures
upstream (direction from
which the wind is
blowing.
• There is both cold and
warm advection.
Effects of Snow Cover and Wind
Snow Cover
Wind
-Wind forecast tip: on a windy night,
some warmer air is mixed
downward to the surface, which
prevents the temperature from
dropping as quickly as they would
on a clear night. Predict warmer
temperatures than on a clear night.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/tmps/snc.rxml
-Snow cover forecast tip: predict
lower daytime and nighttime
temperatures than normal.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/tmps/wnd.rxml
Precipitation
• When the dew point is
close to the
temperature, the air is
nearly saturated and
precipitation is possible.
• When temperature and
dew point are further
apart, it means the air is
quite dry.
Predicting Rain Vs. Snow
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fcst/prcp/rs.rxml
Summary
• By combining all of the techniques given, you
should be able to create more accurate
forecasts.
• Now let’s practice your forecasting!