Weather Forecasts: From Tragedy to
Triumph
Cliff Mass
University of Washington
2012: Hurricane Sandy
125 dead, 60+ billion dollars damage
Well predicted over a week ahead
of time
ECMWF Forecast of Sea Level Pressure
1938 Hurricane: Similar in
Strength to Sandy
Nearly a thousand died
Not forecast the day before
1962 Columbus Day Storm
Not forecast the day before either
Seattle Times
Jan 1993: Inauguration Day
Storm: Near Perfect Forecast
Something Has Changed
Before 1990 the National Weather Service got
virtually every major storm wrong, even the
day before.
After 1990, they gave good warnings for
nearly all.
P
Forecast Skill Improvement
NCEP operational S1 scores at 36 and 72 hr
over North America (500 hPa)
75
e
r
o
c
s
National Weather Service
65
"useless forecast"
55
36 hr forecast
1
S
72 hr forecast
45
Forecast
Error35
10-20 years
Better
"perfect forecast"
25
15
1950
1960
1970
1980
Year
Year
1990
2000
Skill Improvements (ECMWF)
Major improvements, mainly due to satellite data and
improved models
The Revolution in Weather
Prediction Technology
The Key Technology of Modern
Weather Forecasting is
Numerical Weather Prediction
Numerical Weather Prediction
• The basic idea is that if you can determine
the current state of the atmosphere (known
as the initialization) , you can predict the
future using the equations that describe the
physics of the atmosphere.
• These equations can be solved on a threedimensional grid.
The
“Primitive”
Equations
Numerical Weather Prediction
• Numerical weather prediction is limited by the
available computer resources.
• As computer speed increases, the number of
grid points can be increased.
• More (and thus) closer grid points means we
can simulate (forecast) smaller scale features.
National Weather
Service Weather
Prediction Computer
NGM,
80 km,
1995
2007-2008
4-km MM5
Real-time
1.33 km resolution available on
the UW web site
But just as important has been the
weather data revolution, with
satellites giving us three
dimension data over the entire
planet
Example: The Pacific Data Void
No Longer Exists
Cloud Track Winds
Better than Star
Trek!
NOAA Polar
Orbiter Weather
Satellite
Satellite Sensors Provide Thousands of High
Quality Vertical Soundings Daily over the Pacific
Cosmic GPS Satellites Provide
More Soundings!
We are now starting to see
frequent examples of forecast
skill past one week:
Hurricane Sandy is only one
example
Observed
180 hr (7.5 days)
Forecast Skill Will Continue to
Extend Further in Time…with
limits (about 2 weeks)
• More satellite assets will provide a far better
description of the atmosphere.
• Better models and higher resolution
• Better data assimilation: how we use the
observations to produce an initialization for
our models.
Increasing Resolution and Better
Models Will Not Be Enough
The Next Major Revolution in
Numerical Weather Prediction
Will Come Elsewhere
The Transition from
Deterministic to Probabilistic
Prediction
A Fundamental Problem
• The way we have been forecasting
has been essentially flawed.
• The atmosphere is a chaotic
system, in which small differences
in the initialization…well within
observational error… can have
large impacts on the forecasts,
particularly for longer forecasts.
• Not unlike a pinball game….
A Fundamental Problem
• Similarly, uncertainty in our model physics
(e.g., clouds and precipitation processes) also
produces uncertainty in forecasts.
• Thus, all forecasts have some uncertainty.
• The uncertainty generally increases in time.
This is Ridiculous!
Forecast Probabilistically
• We should be using probabilities for all our
forecasts or at least providing the range of
possibilities.
• There is an approach to handling this issue
that is being explored by the forecasting
community…ensemble forecasts
Ensemble Prediction
• Instead of making one forecast…make
many…each with a slightly different
initialization or different model physics.
• Possible to do this now with the vastly
greater computation resources that are
available.
Ensemble Prediction
•Can use ensembles to give the
probabilities that some weather
feature will occur.
• Ensemble mean is more accurate
than any individual member.
•Can also predict forecast skill!
•When forecasts are similar, forecast
skill is generally higher.
•When forecasts differ greatly,
forecast skill is less.
Prediction!
• The meteorological profession is rapidly
gaining the ability to produce highresolution probabilistic weather forecasts
AND analyses.
• Probabilistic forecasts and analyses will be
available for a wide range of weather
parameters.
The Nowcasting Revolution
AMS Nowcasting Definition
A description of current weather and a
short-term forecast varying from minutes
to a few hours; typically shorter than most
operational short-range forecasts.
American Meteorological Society’s
Glossary of Weather and Climate
During the past decade or so the
geographical and temporal detail
the weather profession can
provide has greatly increased.
• High resolution forecasting, NWS forecasts
on a 2.5 km grid, radar data, satellite
imagery, huge numbers of surface stations,
and now probabilistic prediction!
Example:
The Pacific
Northwest
Based on 72
different
networks
3000-4000
observations
per hour
over WA and
OR
Traditional Approaches of Weather
Information Dissemination Are Incapable
of Delivering the Specificity and Detail
Meteorologists Can Provide
Typical TV weathercasters have only 2.5 minutes!
Many of us worried about this
problem in the 90’s but now the
solution is literally at hand
Smartphones are Ideal for
Weather Data Delivery!
• Lots of bandwidth
• They know where they are, so forecast
information can be tailored to the user
• Substantial computational capacity.
There are now thousands of weather
apps for smartphones…and the best
are yet to come!
Some Tragedies Continue
Little Improvement in Forecasting the Intensity of Hurricanes
Tragedy: The U.S. is Now in
Second or Third Place in Global
Weather Prediction
The End