Numerical Model Guidance
-how Meteorologists construct their weather forecasts-
Forecasters Rely on Computer Models
With the explosion of computer technology scientists have developed many computer models to help
forecasters analyze and predict the weather.
Computer models depend on the fact that mathematical equations can describe the physical changes that
govern the weather, just as equations describe movements of the solar system well enough for solar and lunar
eclipses to be predicted years, even centuries, in advance.
Equations describing the solar system are complicated. Still, mathematicians and astronomers have been able
to predict eclipses for centuries. The atmosphere's equations are much more complex. Solving them had to
wait not only for more knowledge, but also for computers.
And, even with the fastest computers, meteorologists can't forecast day-to-day weather for more than about a
week ahead of time. The atmosphere is just too complex, among other reasons.
When researchers began developing the first computer models for the earth's atmosphere in the 1950s, they
worked with computers that were extremely limited compared with today's. As a result, the first models were
overly simplified, but still provided valuable insight into the atmosphere's future state. As computer
technology advanced, the complexity of the forecast models increased and more of the dynamical and
physical factors influencing the atmosphere were taken into account. This improved forecasts.
These models were never intended to replace human forecasters. Instead, the models were developed as aids.
Human forecasters study the output from models over a long time period and compare the forecast output
from the models to the actual verification for the forecasted time period. This is how model biases, model
strengths, and model weaknesses are determined. Often, human forecasters will modify model output
based on past experience in forecasting the weather and physical and dynamical reasons.
Many new models are currently being developed. Some of the models being developed will help forecast
tropical features, such as hurricanes. Other models of the future will help forecast smaller scale features, such
as thunderstorms and severe weather outbreaks. Once these models are developed, forecasters will be able to
issue better and more timely warnings and advisories.
In the United States, the National Weather Service's National Centers for Environmental Predictions (NCEP)
runs the computer models. All forecasters, both National Weather Service and private companies, including
Weather Services Corp., which does USA TODAY's forecasts, rely on these models.
The Computer Models that Meteorologists Use
Global Models

European Models (UKMET and ECMWF). Forecasters use two European models to forecast in
the medium range time period. Medium range means issuing a forecast beyond 2 days. One of
models, known as the UKMET model, was developed in the United Kingdom. The other model,
known as the ECMWF, is based in the European Meteorological Center. Both models also give
forecasts for the entire northern hemisphere like the MRF.

Medium Range Forecast Model (MRF). This model used to be known as the GFS. MRF is one of
the main models forecasters use for the medium range time period beyond 48 hours into the future.
The MRF model forecasts for the entire northern hemisphere, unlike the national models, which only
forecast for North America. The MRF is primarily used for the medium range time period from 60 to
240 hours into the future. A newer version of the MRF became operational during the fall of 1995,
but it is way too early to tell if the new version performs better than the old one.
It is still too early to tell which of the medium range models performs the best. Each of the medium range
models has its own strengths and weaknesses.
National Models



Aviation Model (AVN). One of the oldest operational models used by forecasters today is the
Aviation Model, or AVN. The AVN model was developed primarily to aid in forecasting for
aviation. The AVN gives short range forecasts like the NGM and ETA models do, but it also
forecasts well into the medium range with forecasts up to 72 hours into the future. The AVN also
tends to perform better than the other models in certain weather situations, such as a strong low
pressure area near the East Coast of the USA.
ETA Model. The ETA model is a newer model. The ETA gives a more detailed forecast over the
USA but only provides a forecast up to 4 days. Therefore, it is a short range model The ETA model
was named after the ETA coordinate system, which is a mathematical coordinate system that takes
into account topographical features such as mountains. As a result of using this coordinate system
and the higher resolution, the ETA model has a much more accurate picture of the terrain across the
USA. It is still too soon to determine whether or not the ETA model gives a more accurate forecast
than the NGM model for all the forecast variables, but according to Dr. Ronald McPherson, Director
of NCEP, the ETA model has outperformed all the other models in forecasting amounts of
precipitation. This noted improvement is very important and useful for many applications such as
hydrology and flood forecasts. By the way, the name, ETA is the seventh letter of the Greek alphabet.
Nested Grid Model (NGM). The NGM is a short range model that forecasts variables such as
temperature at various levels of the atmosphere, amount of precipitation, position of upper level
toughs and ridges, and the position of surface high and low pressure areas. In the nested grid model
and others like it, the atmosphere is divided into squares, or a grid, for various levels of the
atmosphere. Grids with smaller squares are "nested" inside larger ones for areas forecasters are
particularly interested in, such as North America. The smaller the grids, the more calculations that
have to be made and the bigger the computer needed. The NGM produces forecasts every 6 hours
from 0 hours to 48 hours into the future.
Three roads will improve forecasts
Those who use weather forecasts regularly have noted tremendous improvements over the past couple of
decades. Will the improvement continue?
Ronald McPherson, director of the National Center for Environmental Prediction (NCEP), says the answer
depends on three factors:
Advances in computer technology. When researchers developed the first mathematical models of the
atmosphere, computer technology was limited. As a result, the equations of the first models were overly
simplified. As computers improved, the model equations could be made more complex. This resulted in
better model forecasts. The NCEP plans on improving the national models' resolutions to 4 kilometers by the
year 2002. This goal is based on predicted advances in computer technology in the next five years. If these
advances come about sooner or later than expected, then the improved models will be developed sooner or
later than planned.
Advances in understanding the physics of the ocean and the atmosphere. In order to develop more
accurate equations for the computer models, scientists must better understand what is going on in the
atmosphere. Meteorology has grown enormously as a profession and a scientific field. Many more scientists
have devoted their entire careers to understanding atmospheric and oceanic physics and how they interact.
Advances in atmospheric observations. Good observations are crucial to improving model performance
and for atmospheric studies. For example, if you have a really good atmospheric model, but bad
observations, the model will create a bad forecast no matter how advanced its equations. Scientists have been
making atmospheric observations for centuries. As technology has rapidly improved, the ability to observe
has also improved. New technology, such as satellites, have enabled meteorologists to make observations
over locations such as the Pacific Ocean, which were not possible using land-based instruments. NCEP is
currently overhauling and modernizing its observation network, which will lead to much improved
observations in the near future.
Improvements are expected in all three areas. McPherson and other atmospheric scientists are optimistic
about a continuing improvement of weather forecasts.
You are the future!
Your Assignment
Go to the following site: http://www.nws.noaa.gov/
On the left hand side there is a heading titled “Forecast Models.” Click on the option below it titled
“Numerical Models.”
Around the middle of the page you will see a table depicting 3 models, the ETA, the GFS, and the NGM.
The ETA and GFS are updated 4 times a day. The NGM is updated 2 times a day. The times listed are
universal in nature and correspond with Greenwich, England time. When we observe daylight savings time
(October through April--when we turn our clocks back…remember fall back, spring ahead…), their time is 5
hours ahead of ours. From April through September their time is 4 hours ahead of ours. Therefore, a
universal time of 00Z (midnight) along the prime meridian means it is either 7 or 8pm here in the Eastern
Time Zone. Note: time is “Z” is the same as time in “UTC.”
The 11 o’clock news cast is often watched for its weather report at the end of the newscast. Meteorologists
have usually just completed analysis of the 00Z data in which their computers began giving them data at 7 or
8pm. This data takes a couple of hours to complete its report. Therefore, the data run is not complete until
around 9 or 10pm then the meteorologist studies it carefully so he or she it prepared to go on the air with it at
11pm. The 10 o’clock meteorologists really have to rush!
…………………………………………………………………………………………………………………...
Place all of your answers on your answer sheet!!!
Practice problems. Do not use the computer:
1. The pretend date is December 12th. At what time will the 18Z model data begin coming from the
computers in the Eastern Time Zone (this is where we live)? [(6 am) (1pm) (2pm) (6pm)]
2. You will be analyzing the 06Z (UTC) computer model data. At what time did this data begin coming
from the computers in the Eastern Time Zone? [(around 11pm) (around 1am) (around 6am) (around
11am)]
Use your computer. You should be on the indicated site and sub sites listed above.
3. On the ETA model under 06Z, click on the “fine” icon. Next, under the column titled, “MSLP 1000 –
500mb” select “024.” This is the projected weather for 24 hours into the future from an initial time when the
model was put out by the computer. Analyze the top of the map. Write out the complete date and time
(Eastern Time Zone) for the projected weather forecast.
4. Sketch the location of the low and high pressure systems with their respective pressures in units of
millibars (mb) on the blank USA map within your answer sheet. Note: the map on your answer sheet
covers less geographic area than the map you on your computer.
5. Click “Back” 2 times! On the GFS model under 06Z, click on the “fine” icon. Next, under the column
titled, “MSLP 1000 – 500mb” select “024.” This is the projected weather for 24 hours into the future from
an initial time when the model was put out by the computer. Analyze the top of the map. Write out the
complete date and time (Eastern Time Zone) for the projected weather forecast.
6. Sketch the location of the low and high pressure systems with their respective pressures in units of
millibars (mb) on the blank USA map within your answer sheet.
7. Are there any differences between the ETA map and the GFS map for 24 hours into the future? If so,
what are they? Be specific!!!
8. Click “Back” 2 times! On the ETA model under 06Z, click on the “fine” icon. Next, under the column
titled, “MSLP 1000 – 500mb” select “084.” This is the projected weather for 84 hours into the future from
an initial time when the model was put out by the computer. Analyze the top of the map. Write out the
complete date and time (Eastern Time Zone) for the projected weather forecast.
9. Sketch the location of the low and high pressure systems with their respective pressures in units of
millibars (mb) on the blank USA map within your answer sheet.
10. Click “Back” 2 times! On the GFS model under 06Z, click on the “fine” icon. Next, under the column
titled, “MSLP 1000 – 500mb” select “084.” This is the projected weather for 84 hours into the future from
an initial time when the model was put out by the computer. Analyze the top of the map. Write out the
complete date and time (Eastern Time Zone) for the projected weather forecast.
11. Sketch the location of the low and high pressure systems with their respective pressures in units of
millibars (mb) on the blank USA map within your answer sheet.
12. Are there any differences between the ETA map and the GFS map for 84 hours into the future? If so,
what were they? Be specific!!!
13. Click “Back” once! Find the heading titled “Tot Pcpn 60 hrs”. Select “084.” This is the projected total
precipitation from the time period of 24 hours into the future until 84 hours into the future. How many hours
does this encompass?
14. Analyze this precipitation map and view the key on the left for color clarification. Show with an outline
and various shaded areas where precipitation is expected during this 60 hour time frame on the blank USA
map within your answer sheet.
15. Click “Back” 2 times! On the ETA model under 06Z, click on the “fine” icon. Find the heading titled
“Tot Pcpn 60 hrs.” Select “084.” This is the projected total precipitation from the time period of 24 hours
into the future until 84 hours into the future. How many hours does this encompass?
16. Analyze this precipitation map and view the key on the left for color clarification. Show with an outline
and various shaded areas where precipitation is expected during this 60 hour time frame on the blank USA
map within your answer sheet.
17. Are there any differences between the ETA and the GFS total precipitation forecast model from 24 to 84
hours into the future? If so, what are they? Be specific!!!
Earth and Space Science
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Answer Sheet
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