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Q: accuracy of weathermen ( Answered 5 out of 5 stars,   1 Comment )
Question  
Subject: accuracy of weathermen
Category: Miscellaneous
Asked by: gargazons-ga
List Price: $15.00
Posted: 04 Jan 2006 00:44 PST
Expires: 03 Feb 2006 00:44 PST
Question ID: 428846
Any good data out there on how accurate weathermen are in their
forecasts, preferably in the form of some sort of national
composite/estimate? As for what defines "accurate", that may depend on
how the data would define it, so I'm open to interpretations on
that...
Answer  
Subject: Re: accuracy of weathermen
Answered By: tlspiegel-ga on 04 Jan 2006 10:02 PST
Rated:5 out of 5 stars
 
Hi gargazons,

Thank you for another very interesting question.  

Simply put, when we want to know today's weather, we look outside.
When we want to know tomorrow's weather, we listen to a forecast! 
Weathermen predict the future several days in advance - with their
accuracy diminishing the farther their forecast extends.  Reliability
declines with the length of the forecast.


"Ask a Scientist - Weatherman error
http://www.newton.dep.anl.gov/askasci/gen99/gen99198.htm

Question: We undestand that weathermen predict weather by a dopplar radar.
But if the dopplar radar is such a good system, why are the weathermen 
wrong so many times?


Answer 1:
Predicting weather patterns is not an easy job.  The dopper radar does
help, and satellites play a large part as well.  It's just that the
weather is extremely dynamic; changing all the time.  Usually, the
forecaster isn't that far off from the actual weather.  It could be
raining at your house, but not at the place where the weather report
was
centered (i.e. some major city a little ways off).  Sometimes, they
are wrong, and this can be caused by misreading/misinterpreting the
data they recieve, or simply the weather changed suddenly.  It's
a constant job to predict the weather.

Answer 2:
Forecasters are actually right most of the time - the bad forecasts
get more attention and stick in your mind. But in answer to your question
...Doppler radar is a fantastic new tool for OBSERVING weather and
making short term forecasts and warnings (weather that is occurring
now or in the next hour or two). But longer range forecasts for the
next day or 2 are based on computer models of the atmosphere. Weather
reports from the ground, from satellite and from weather balloons 
are fed into the computer. The computer then generates a package
a weather maps depicting differnt parameters such as temperature
moisture, pressure, wind for different heights in the atmosphere.
Meteorologists use this computer guidances along with the latest
weather reports and satellite and radar to make a forecast. There
are many variables in the mathematical equations the computers use.
Also the weather reports that are fed into the computer are spaced
across the U.S. (and around the world) but there may be several
hundred miles between upper air weather balloon reports. So the
computer models may miss some small scale features. Though the 
technology is improving, weather is not an exact science."

=========

WSJS - 3 Degree Guarantee
http://www.wsjs.com/features/three_degree/

"Joe Bastardi is an Expert Senior Forecaster at AccuWeather and has
been the chief forecaster for WSJS for close to 20 years.

His daily discussion is the most extensive in the business and has
helped establish him as the industry's go-to guy for long-range and
hurricane forecasting."

[edit]

"In 2004, Joe missed the 3 Degree Guarantee only 28 times - a better
than 90% accuracy rating."

=========

An article I found at The Daily Reporter: To determine accuracy of
weather forecasts, ?just plug in the numbers?
http://www.intellovations.com/DailyReporter.pdf 

referred to ForecastWatch.com:
http://www.customweather.com/accuracy/2004_POP_study.html

"ForecastWatch.com is the only company that provides ongoing weather
forecast accuracy information to meteorologists, utilities and energy
companies, the agriculture industry, and anyone whose reputation or
business depends on being right about the weather. In 2003,
ForecastWatch.com released the results of the largest public weather
forecast accuracy study undertaken to that point."

[edit]

"How Accuracy Was Measured

The accuracy of a probability of precipitation (POP) forecast is
difficult to evaluate. If a forecast states that there is a 30% chance
of precipitation, is it right if it doesn't rain? The only way to
evaluate the accuracy of a probability forecast is to look at a large
number of forecasts that predicted a 30% chance of rain or snow, and
see how often it actually rained or snowed. Then, do the same for all
other forecasted percentages, from 0% to 100%, and see how closely it
matches the forecast.

Unfortunately, it takes a lot of forecasts to get a good picture of
accuracy. This is because for every POP or POP range evaluated, a
statistically valid sample of actual precipitation events must be
captured. For this study, each POP forecast, from 0% to 100% was
evaluated individually, not grouped into ranges (say 10%-20% chance,
20%-30% chance, etc.).

From February 2004 through January 2005, ForecastWatch.com collected
nearly one and a half million POP forecasts from CustomWeather's
website, MyForecast.com. These forecasts were from one- to five-days
out, and spanned 839 locations within the United States. Each of the
POP forecasts was grouped together. Then, for each location on each
day, the actual number of precipitation events was counted.
ForecastWatch.com compared the forecasts to the National Weather
Service ASOS weather network. An actual daily observation was
considered a precipitation event if more than a trace of either rain
or snow was reported by the station.

POP Accuracy Results
The results of the POP weather forecast accuracy evaluation can be
seen in Figure 1. The dashed line indicates the ?perfect? forecast. A
perfect forecast means that for each probability of precipitation
forecast, precipitation was observed exactly that percent of time. For
example, precipitation was observed exactly 50% of the time a 50%
chance of precipitation was forecast.

The blue line is the actual observed precipitation percent for each
POP forecast value. If the blue line is above the dashed line, it
indicates it rained or snowed more often than forecast. For example,
it actually rained or snowed 23.8% of the time when a 20% probability
of precipitation was forecast. If the blue line is below the dashed
line, it indicates it rained or snowed less often than forecast.

(see Figure 1: CustomWeather's POP Forecast versus Actual
Precipitation Event Percentage.)

As Figure 1 demonstrates, there is a high correlation between
CustomWeather's POP forecast and what actually occurs. The average
error between forecast and actual precipitation probability was 3.51
percentage points. The greatest error occurs when CustomWeather
forecasts 100% probability of precipitation. Only 94.47% of the time
did it actually rain or snow. The least error occurs at the 80%
probability of precipitation forecast. With an error of only 0.12
percentage points, it actually rained or snowed 80.12% of the time
that forecast was made. CustomWeather's POP forecasts are accurate and
closely reflect what actually happened."

=========

How good are the weather forecasts?
http://weather.slimyhorror.com/

(read article and see charts)

"The weather predicted by the BBC for four days time was just 30-40%
accurate - i.e. they get it wrong around two times out of three."

=========

utahweather.org
http://www.utahweather.org/UWC/forecasting_utahs_weather/forecasting_utahs_weather.html

"The Importance of Accurate Forecasts

The United States experiences more severe storms and flooding than any
other country in the world. In a typical year it experiences 10,000
violent storms, 5,000 floods, 1,000 tornadoes and several hurricanes.
In an effort to save lives and property, American meteorologists use
the most powerful computers and scientific instruments available to
predict future weather conditions and phenomena.

However, even with extensive training and modern equipment,
meteorologists sometimes make inaccurate predictions. Why? Because
computer forecast models are highly dependent on the data that goes
into them, and accurate real-time data on changing atmospheric
conditions are not yet available for everywhere around the world.
Nevertheless, today's weather forecasts are much more accurate than
they were ten years ago.

Accuracy of Short and Medium-Range Forecasts (1-7 Days in the Future) 

During the 1980's and 1990's, the accuracy of short-range (1-3 days)
and medium-range (4-7 days) weather forecasts improved significantly.
In fact, in 1993, NOAA stated that for much of the United States,
"Today's 3-to-5 day forecasts are as good as 1-to-2 day forecasts of a
decade ago." (A Change in the Weather Service, U.S. Department of
Commerce, NOAA, November 1993.)

When it comes to predicting Utah's daily weather and atmospheric
phenomena, short-range weather forecasts are remarkably accurate. In
1996, William J. Alder, then Meteorologist-in-Charge of the Salt Lake
City National Weather Service Forecast Office stated:

"The National Weather Service is given the responsibility by law to
issue forecasts for the protection of life and property due to
hazardous or severe weather. We rely on the media-through radio and
television weathercasts-to disseminate our watches, warnings,
forecasts and other weather information. The Salt Lake City National
Weather Service Forecast Office employs...professional meteorologists
and...weather specialists. We work around-the-clock shifts to maintain
a close watch on the weather, and issue up to the minute warnings and
forecasts. On the average, we feel our general forecasts have the
following rates of accuracy:

[See Time Periods Accuracy Rate]

[edit]

"Accuracy of Long-Range Outlooks (Beyond 7 Days)

Weather predictions issued for periods longer than seven days are
known as long-range outlooks. Advances in computer power and
technology have now make it possible for the National Weather Service
to produce meteorological forecast charts of possible future weather
conditions out to 16 days. For example, the Global Forecast System
(GFS) is a computer model that currently predicts general weather
patterns and conditions out to 16 days. According to R. Clayton
Brough, ABC4 (KTVX) Climatologist, "When such [long-range] computer
models are used in conjunction with known climatological patterns and
probabilities they can generate accuracy levels of 55% to 65% relative
to general weather patterns--such as drier or wetter or cooler or
warmer than normal conditions--over an expected area and period of
time." Similarly, Paul Douglas, Chief Meteorologist for WCCO-TV (CBS)
in Minnesota, state in his popular book Restless Skies (2004, page
191), that "The GFS model can crunch atmospheric conditions up to 240
hours into the future, with a steady drop in accuracy over time.
Recent studies suggest that there is some reliability to computerized
weather prediction (better than a 50-50 flip of the coin) for a
specific location as much as 14 days into the future."

Weather predictions issued by the government for periods beyond seven
days are rather general in nature and are usually called "outlooks".
These outlooks are based on overall expected weather patterns--such as
warmer or colder than normal or wetter or drier than normal
conditions.  Outlooks for 8-14 Days, and Outlooks for 30-90 Days are
regularly issued by the Climate Prediction Center of the National
Weather Service."

(continue reading article)

=========

Weather Forecasting - A policy statement of the American
Meteorological Society as adopted by the Council on 13 January 1991
http://twm.co.nz/wforcstg.html

"The most impressive gain in forecast accuracy in recent years has
been in prediction for the 1 to 5 day range. A number of factors have
contributed to the increase in accuracy. Foremost among these has been
the further development of numerical prediction models, based on the
laws of physics, that are able to forecast the formation and movement
of the large high and low pressure systems that govern day-to-day
weather changes in middle and high latitudes. These models have
undergone steady improvement since their introduction more than a
quarter century ago. The improvement has been made possible in large
measure by the growth in the capacities of the computers that are
required for carrying out the vast number of calculations involved in
a numerical forecast."

=========

Lastly, a bit of humor for you.  :)

The Weather Network - Weather Forecasts
http://www.theweathernetwork.com/inter/ask/ask10.htm

"Of course, almanacs are most noted for their weather predictions,
whose accuracy, by the way, is not any more accurate than pure chance.
On occasion, of course, the almanac weather prediction has turned out
to be right on target. A classic example was the forecast for July 13,
1816. when it accurately predicted snow in New England.

According to almanac lore, the manuscript was about to go to press
when someone realized that no prediction was made for July 13. the
editor was home ill, so a copy boy was sent to get a weather
prediction for the missing date. Put in anything you want, he was
told, so the young man inserted, rain, hail and snow. Sure enough,
parts of New England had all three on that July 13th. While the editor
denied making such a ridiculous forecast for the summer ahead, when it
really did snow on that day, he changed his tune and took full credit,
saying told you so."

=========

keyword search:

weatherman accuracy forecast
accuracy of weather prediction
accuracy of weather forecasts united states

=========

Best regards,
tlspiegel
gargazons-ga rated this answer:5 out of 5 stars
Beautiful! The link to forecastwatch.com was just the thing I was looking for.

Comments  
Subject: Re: accuracy of weathermen
From: tlspiegel-ga on 04 Jan 2006 15:41 PST
 
Hi gargazons,

Thank you for the 5 star rating.  :)

Best regards,
tlspiegel

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