Weather Forecasting - II
Review
• The
forecasting of weather by high-speed computers is known as
numerical weather prediction. Mathematical models that describe
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how atmospheric temperature, pressure, winds and moisture will
change with time are programmed into the computer. The computer
then draws surface and upper-air charts, and produces a variety of
forecasts called progs.
After a number of days, flaws in the computer models – atmospheric
chaos and small errors in the data – greatly limit the accuracy of
weather forecasts.
Available to the forecaster are a number of tools that can be used
when making a forecast, including surface and upper-air maps,
computer progs, meteograms, soundings, and satellite information.
Ensemble forecasting is a technique based on running several
forecast models (or different versions of a single model), each
beginning with slightly different weather information to reflect the
errors in the measurements (similar to BCS computer averages).
Ensemble forecasting
• medium-range forecasts
A technique to improve
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♦ Run different models
♦ Run the same model
with slightly different
initial conditions
Superimpose all results
on the same graph
♦ “Spaghetti plot”
The most common
outcome is probably the
most likely one
Forecasting Tools
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• temperature, due point T and wind.
AWIPS – Advanced Weather
Interactive Processing System
Meteogram – collection of observations
at a given station over some time
Sounding: vertical profile of the
Forecasting Methods (1)
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• Steady state forecast (trend method). If we know the
Persistent forecast: (short term forecast) when there is
no reason for the weather conditions to change.
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speed and the direction of the weather system, we can
extrapolate to its future location assuming same speed.
♦ Example: a cold front, located 90 km due west from
here, is moving eastward at 30 km/hr. Prediction: a
cold front will pass through in exactly 3 hours.
Analogue forecast (pattern recognition): “I have seen
these atmospheric conditions before and based on what
happened back then, I can predict tomorrow’s weather”.
Statistical forecast: the forecast is made based on
♦ Numerical model’s forecast for weather elements
(humidity, cloud cover, wind direction, temperature).
♦ Statistically weighted analogue forecasts based on
those
“The chance of rain is 60%”
• It will rain over 60 % of the forecasted area. NO
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• receive a measurable amount of rain. YES
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There is a 60% chance that it will rain somewhere
within the forecasted area. NO
It will rain 60% of the time. NO
There is 60% chance that any random location will
If you stay at one place there is a 60% chance that
you will see rain. YES
Forecasting Methods (2)
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• Probability forecast: based on historical data. “What is
Climatological forecast: Based on information for the
typical weather conditions at a given location for a given
season.
the chance to snow in Gainesville on Christmas?”
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Forecasting Methods (3)
Weather type. Uses
general criteria such
as the position of the
subtropical highs,
upper-level flow,
prevailing storm
track…
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In general: For a
forecast to show
skill, it must be
better than a
persistence
forecast or a
climatological
forecast
The weather over the weekend
Type of Forecasts
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• Short-range forecast: (2-3 days) variety of techniques
Very short-range forecasts (up to 6 h): based on
satellite images and Doppler radar. Not likely to include
numerical models.
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- satellite images, Doppler radar, wind and pressure
maps of the surface and aloft … also numerical models –
usually beyond 12 hrs.
Medium range forecast: (3-8 days) Mostly based on
computer generated weather predictions.
Long-range forecast (up to 16 days). It is not very
accurate. Gives mostly the trends of the weather and
the global expectations.
Extended forecast – one that goes beyond 3 days
Outlook – how the average weather conditions for a
particular month or a season compare to normal ones.
90-day outlook for
(a) precipitation and (b) temperature
Above normal
Below normal
Below normal
Above normal
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Predicting the weather
from local signs.
Signs of an approaching cold front: hot
and humid air, pressure quickly dropping.
Signs of an approaching warm front: cool weather, high
clouds that are thickening and lowering, decreasing
pressure, precipitation starting with snow (if cold enough)
changing to sleet, freezing rain and rain.
Signs of an approaching tropical
storm: decreasing pressure,
increased cloud cover, the
direction and the speed of the
cloud motion (winds).
Weather folklore
• When halo rings the moon or sun,
rain's approaching on the run
• When windows won't open,
TRUE
and the salt clogs the shaker,
The weather will favor
the umbrella maker!
• A cow with its tail to the West
TRUE
makes the weather best,
A cow with its tail to the East
makes the weather least
• Cats and dogs eat grass before a rain.
TRUE
FALSE
The Station Model – Appendix B
• Temperature=77 F
• Dewpoint =68 F
• Pressure (mbar) = 998.
• Pressure tendency the last 3 h.
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The pressure here has fallen .3 mb
the last 3 hours.
Cloud cover: it's mostly black showing that this station is mostly cloudy.
Wind barb: it points to where the wind is coming from. The two ticks
represent 20 knot winds.
Present weather: a thunderstorm is occurring at the station.
Pressure tendency: the 1st hour the pressure was steady, then fell the
last two hours.
Past weather (one hour ago). In this case it was a light rain shower.
Example
Using surface maps to predict the
weather.
Current weather
Pressure Tendency
Predicted front position
Upper level map
What actually happened
• Skip
the last sections “A forecast for six cities” and “A meteorologist
makes a prediction”