A History of Modern Weather
Forecasting
The Stone Age
• Prior to approximately 1955, forecasting was
basically a subjective art, and not very skillful.
• Observations were sparse, with only a few scattered
ship reports over the oceans.
• The technology of forecasting was basically
subjective extrapolation of weather systems, in the
latter years using the upper level flow (the jet
stream).
• Local weather details—which really weren’t
understood-- were added subjectively.
Upper
Level
Chart
The Development of NWP
• Vilhelm Bjerknes in his
landmark paper of 1904
suggested that NWP was
possible.
– A closed set of equations
existed that could predict
the future atmosphere
(primitive equations)
– But it wasn’t practical then
because there was no
reasonable way to do the
computations and sufficient
data for initialization did
not exist.
Numerical Weather Prediction
• The advent of digital computers in the late
1940s and early 1950’s made possible the
simulation of atmospheric evolution
numerically.
• The basic idea is if you understand the current
state of the atmosphere, you can predict the
future using the basic physical equations that
describe the atmosphere.
Numerical Weather Prediction
One such equation is Newton’s Second Law:
F = ma
Force = mass x acceleration
Mass is the amount of matter
Acceleration is how velocity changes with time
Force is a push or pull on some object (e.g.,
gravitational force, pressure forces, friction)
This equation is a time machine!
Numerical Weather Prediction
Using a wide range of weather observations we
can create a three-dimensional description of the
atmosphere… known as the initialization
Numerical Weather Prediction
•This gives the distribution of mass and
allows us to calculate the various forces.
•Then… we can solve for the acceleration
using F=ma
•But this gives us the future…. With the
acceleration we can calculate the velocities in
the future.
•Similar idea with temperature and humidity.
Numerical Weather Prediction
• These equations can be solved on a three-dimensional grid.
•
• As computer speed increased, the number of grid points could
be increased.
• More (and thus) closer grid points means we can simulate
(forecast) smaller and smaller scale features. We call this
improved resolution.
NWP Becomes Possible
• By the 1940’s there was an extensive upper
air network, plus many more surface
observations. Thus, a reasonable 3-D
description of the atmosphere was possible.
• By the mid to late 1940’s, digital
programmable computers were becoming
available…the first..the ENIAC
The Eniac
First NWP
• The first successful numerical prediction of
weather was made in April 1950, using the
ENIAC computer at Maryland's Aberdeen
Proving Ground
• The prediction was for 500 mb height,
covered North America, used a twodimensional grid with 270 points about 700
km apart.
• The results indicated that even primitive
NWP was superior to human subjective
prediction. The NWP era had begun.
Evolving NWP
• Early 50s: one-level baratropic model
• Late 50s: Two-level baroclinic QG model (just
like Holton!)
• 1960s: Primitive Equation Models of increasing
resolution and number of levels.
• Resolution increases (distance between grid points
decrease): 1958: 380 km, 1985: 80 km, 1995: 40
km, 2000: 22 km, 2002: 12 km
A Steady Improvement
• Faster computers and better understanding of
the atmosphere, allowed a better representation
of important physical processes in the models
• More and more data became available for
initialization
• As a result there has been a steady increase in
forecast skill from 1960 to now.
P
Forecast Skill Improvement
NCEP operational S1 scores at 36 and 72 hr
over North America (500 hPa)
National Weather Service
75
S1 score
65
"useless forecast"
55
36 hr forecast
72 hr forecast
45
Forecast
Error 35
10-20 years
Better
"perfect forecast"
25
15
1950
1960
1970
Year
1980
Year
1990
2000
The National Weather Service
Forecaster at the Seattle National Weather Service Office