Download Upper Airflow and Weather Forecasting

Chapter 12: Weather Analysis and Forecasting
The Atmosphere:
An Introduction to
Meteorology, 12th
Lutgens • Tarbuck
Lectures by:
Heather Gallacher,
Cleveland State University
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The Weather Business

National Weather Service (NWS) is a branch of the
National Oceanic and Atmospheric Administration
(NOAA).

NWS provides weather and climate forecasts and
warnings to U.S.A and its waters for safety and
economic value.
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The Weather Business

National Centers for Environmental Prediction:
 NWS collects weather data from all over world.
 Shared with 125 Weather Forecast Offices for local
forecasts.
 Forecasters collect and compile billions of pieces of
observational data.
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Weather Analysis
 The World Meteorological Organization (WMO), is part of
United Nations, responsible for international exchange of
weather data.
 Surface observations:
 The NWS and the Federal Aviation Administration (FAA)
operate ~900 automated surface observation systems (ASOS).
 Worldwide, more than 10,000 land observation stations, 7,000
ships at sea, and hundreds of data buoys and oil platforms
report conditions four times daily.
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Weather Analysis
Remember
what was
missing from
the “big six”?
<humidity and air
pressure are implied, but
not given>
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Weather Analysis
Weather balloons are launched twice daily at midnight and
noon GMT.
 They contain radiosondes, which are lightweight instrument
packs measuring temperature, humidity, and pressure.
Fixed surface
stations are
complemented
by mobile
observations
aloft
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Not measuring wind,
cloudiness, precipitation

Weather maps: pictures of the atmosphere
 The collected information is displayed in a format that can
be comprehended easily by forecasters.
 The information is placed on synoptic weather maps, which
means “coincident in time.”
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What can be surmised from
comparing these two ‘pictures’
of the atmosphere?
First of all, one map
is surface air
pressure; the other
is air pressure at
about what altitude?
Notice that the midwestern storm has low
pressure at both height
locations.
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Notice falling air
pressure in western
Canada – moisture
moving inland
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Notice dewpoint and
actual temperature
in various places.
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Probably
represents
‘synoptic’
change in air
pressure
Note the
wind
flags
near low
pressure
Air pressure aloft
– somewhere at
least 18,000 feet
above surface
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Notice how little
pressure change is
occurring at this
time in the
subtropics
– distance between
isobars.
Weather Forecasting Using Computers
 Mathematical models begin with temperature, wind speed
and direction, pressure, and humidity.
 The NWS produces generalized forecast maps, called
prognostic charts (or progs).
 A statistical analysis of progs compares the accuracy of
previous forecasts.
 This approach is known as Model Output Statistics
(MOS).
Does not include cloud
cover and precipitation.
The last two are more
about results than
causes.
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Weather Forecasting Using Computers

Ensemble forecasting:
 This method produces a number of forecasts using the
same computer model.
 It alters initial conditions while remaining within error
range of the observational instruments.
What might “auto-correlation” have to do
with modeling and forecasting?
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Weather Forecasting Using Computers

Role of the forecaster:
 After progs are sent to the WFOs, forecasters blend
information with local conditions and regional weather
to produce site-specific forecasts.
 Forecasters use their knowledge and judgments based
on experience. (such as 250 years of ‘Farmer’s Almanac’ records)
 They will sometimes add extra detail to the model
forecasts.
Like, maybe they will
look out the window and
quickly change their
forecast!
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Other Forecasting Methods

Persistence forecasting:
 This technique is based on the tendency of weather to
remain unchanged for hours or days.

Climatological forecasting:
 This method uses climate data (average weather statistics
over many years).
Auto-correlation
Compare these methods to Utah County
high school method:
“Today will be like yesterday. Tomorrow
will be like today.”
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Other Forecasting Methods
“Connect the dots”
Draw isolines across points of
known value.
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Could this map be
depicting percent
cloud cover for a
certain period of
time?
Other Forecasting Methods

Analog method:
 This is based on the assumption that weather repeats itself.
 Forecasters attempt to find weather patterns.
 Pattern recognition is an important tool, which helps improved
short-range, computer-generated forecasts.

Trend forecasting:
 This method determines the speed and direction of fronts,
cyclones, clouds and precipitation.
 Forecasters try to extrapolate the future position of weather
phenomena.
Consider the “cone” pathway of
tornadoes and hurricanes.
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Upper Airflow and Weather Forecasting

Upper level maps:
 These maps are generated twice daily.
 They are drawn at 850-, 700-, 500-, 300-, and 200-millibar
(mb) levels.
Divergence
Convergence
H
L
L
H
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In a complete thermo-cycle
pathway, there are four main
pressure zones.
(like a Hadley cell)
Upper Airflow and Weather Forecasting

850- and 700-millibar maps:
 The 850-mb map finds areas of
cold-air and warm-air advection.
 Winds at the 750-mb level are
used to predict movement of
weather producers.
Notice the position of the L.
Notice the implied direction
of the cold/warm pair.
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Upper Airflow and Weather Forecasting

500-mb maps:
 This level is found approximately 5.5 km above sea level.
What are we seeing here?
The altitude in meters at
which air pressure of 500 mb
is found. Ordinarily, at what
altitude would that be?
<slightly above 18,000 feet>.
Convert meters to feet.
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Upper Airflow and Weather Forecasting

300- and 200-mb maps:
 These maps represent zones near the top of
the troposphere.
 The jet stream is best observed at these levels.
Fluids that are moving are
associated with lower pressure.
The jet stream is fast-moving
and located well above the
midway point of standard
atmospheric pressure.
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About
160
mph.
Jet streams can be called the
“storm track”
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Upper Airflow and Weather Forecasting

The connection between upper-level flow and surface
weather:
 Zonal patterns depict quickly moving westerlies (west to east).
 Meridional patterns consist of slow moving north to south flows,
typically with heavy precipitation.
Why?
North/South jet flow helps mix cold/dry
with warm/wet. Slower speed allows
more time for precipitation to fall.
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Upper Airflow and Weather Forecasting
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Upper Airflow and Weather Forecasting

Notice how the jet stream in this picture is more “meridional” (north/south) than
“zonal” (east/west flow). The result is more mixing of cold/dry with warm/wet –
provoking more severe weather.
Delivering
cold/dry air to
a region of
warm/wet air.
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Upper Airflow and Weather Forecasting

Long-range forecasts:
 The NWS Climate Prediction Center, produces 30- and
90-day outlooks.
Observing trade wind can help forecast an El
Nino condition, which would portend wetter
than usual conditions in the western USA.
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Forecast Accuracy

Percentage probability:
 Rainfall is the only aspect of a forecast expressed this way.
 The prediction of occurrence is fairly accurate.
 The predictions of amount, time, and duration are not
as reliable. <My prediction for Jordan Campus
flags/precip are similar.>

Short- and medium-range forecasts:
 Accuracy has improved over the decades.

Beyond 8 days:
 Predictability of day-to-day weather is of low accuracy.
 Notice the use of “prediction” when talking about
“forecast”.
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Satellites in Weather Forecasting

TIROS 1:
 Launched in 1960, this was the first weather satellite.
 Since then, more than 30 versions have been launched
through 2009.

Polar-orbiting:
 These satellites orbit over the poles.

Geostationary:
 These satellites orbit west to east over the equator.
 Explain the difference.
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Satellites in Weather Forecasting
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