Forecasting Utah's Weather

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:

Time Periods Accuracy Rate

12-hour forecasts: 95%

24-hour forecasts: 85%

36-hour forecasts: 75%

48-hour forecasts: 70%

3-5 day forecasts: 65%

"In addition, our daily temperature forecasts are much better than they were a few decades ago. For example, the number of days in a year when the forecasted high temperature for Salt Lake City for the next day was off by more than 10 degrees has dropped from a total of 61 error days in 1949 to less than 10 error days since 1987. Here are the facts:

Years    # of Error Days

1950     55

1960     41
1970     22

1980     17

1990     8

(For more information on this subject, see the article Are Forecasters Better Today? Probably. Deseret News, April 3, 1994, p.B-2.)."

Likewise, R. Clayton Brough, Climatologist for ABC4 (KTVX) News, stated in 1996 that "...Spring weather patterns are perhaps the most difficult to forecast when trying to time the passage of cold fronts and the rapid weather changes associated with such fronts. Yet, during the spring months of March to May, [ABC4] weathercasters predicted tomorrow's (24-hour) major weather events (such as rain, snow, wind, or cloud cover) and particularly those relating to dry or wet conditions with an average accuracy of 95%...."  (Utah's Comprehensive Weather Almanac, p.82-83.)

Check out this website for the accuracy ratings of various short-range forecasts: http://forecastadvisor.com/

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.

According to the Encyclopedia of Climatology (1987), the National Weather Service "...Monthly (30-day) and seasonal (90-day) temperature forecasts in temperate latitudes would be right about 60% on the average, while precipitation forecasts would rate less than 55%. All of these scores are higher than could be obtained by predicting values of the previous period to persist unchanged."

Beyond outlooks, the best long-range guides to future weather and climate conditions that science can presently offer are:

1. Analyses and summaries of past weather conditions--such as the numerous climatic charts for Utah cities and other locations found in the book Utah's Weather and Climate (1996), and the climatological statistics found on the website for Climate Summaries for Utah Cities and Locations.

2. Daily or weekly probability charts and summaries--such as those for Salt Lake City and St. George in the book Utah's Weather and Climate (1996), Bet on the Weather, KSL-TV's Wasatch Front Outdoor Weather Planner, and IAMAT's World Climate Planning Charts.

3. Statistical projections--such as those based on what the weather has done in the past and, therefore, what it could do in the future if it followed a similar pattern. Example: Utah's Droughts of the Past 500 Years.

4. Questionable predictions--such as those made by some atmospheric professionals and private consulting companies who state they can predict daily weather events beyond 16 days with a stated degree of accuracy.  Example: Experimental Long Range Weather Predictions

Current Forecast Models and Methods

There are several major computer models that are used by Utah meteorologists to predict daily weather changes. These models can be found on various websites, including those of the: National Centers for Environomental Prediction and the Salt Lake City National Weather Service Forecast Office.

These computer models sometimes vary considerable in predicting future weather conditions. This is because they emphasize different atmospheric variables and use different mathematical equations to arrive at their predictions. Because of this, weather forecasters not only have to determine what computer model(s) best describes the present state of the atmosphere (which they do by using current weather observations and satellite data), but they also have to decide what model(s) they feel presents the most reasonable prediction for the future. Obviously, this can be very difficult, time consuming, and sometimes open to individual interpretation, experience and judgment.

New Improvements in Weather Forecasting (Doppler Radar)

The latest tool that meteorologists use for short-range forecasting is the Doppler Weather Radar. Today, there is a nationwide network of these radars that help forecasters accurately predict severe storms and hourly weather trends. Two of these radars are in Utah. One is on Promontory Point and provides coverage for the Wasatch Front and northern portions of the state. The second is in the mountains east of Cedar City and covers much of the southern Utah.

These radars are specifically designed to detect clouds and precipitation, but what makes them unique is their ability to detect air motions within and around those elements (thus the Doppler label). This ability gives forecasters a wealth of information on the severity of storms and has led to greater accuracy in warnings of severe storms.

Another new system that is used by National Weather Service Forecast offices is the AWIPS computer system. AWIPS (or Advanced Weather Interactive Processing System) streamlines the forecast process by combining a number of separate data sets and communication systems into one. Using the latest technology, this system allows meteorologists to manipulate these data sets in a myriad of combinations for use in the analysis of the weather, resulting in more timely and accurate warnings and forecasts.

Local Media and the National Weather Service

In an effort to protect lives and property, Utah television weathercasters and NWS meteorologists regularly share weather information, forecasts and products. This professional sharing of weather information has resulted in good communication and a number of co-sponsored projects between Utah television weathercasters and government meteorologisits.

Much of the information for this section originally appeared in the copyrighted book Utah's Weather and Climate, edited by Dan Pope and Clayton Brough, in 1996. UCCW Directors have received permission from the copyright owners of this book to reproduce such information on its website and to revise and updated it where appropriate.