Weather Forecasting Using the University of Michigan Weather Underground

Adapted to the World Wide Web from a file on the University of Michigan Gopher. Another adaptation was done on a file describing techniques for making weather observations at the schools so the students can contribute their own observations to the online database.

Weather forecasting is a science and an art. To make a good forecast you must know some meteorology and have a hunch what is going to happen based on experience. Forecasting temperature and precipitation will be discussed here since they are the most relevant parameters to the public.

Before attempting to perform a weather forecast, one should understand a few key concepts in atmospheric science. These concepts include:

Synoptic scale (large scale) meteorology: Air masses, fronts, and pressure systems.
Mesoscale (local scale) meteorology: Effects of topography, bodies of water, the urban heat island, etc.
Vertical structure of the atmosphere: How and why pressure, temperature, and density decrease with altitude.

The following discussion will assume the reader is familiar with these concepts.

GENERAL HINTS ON WEATHER FORECASTING

CLIMATOLOGY

The first thing you should know about the place you are forecasting for is the climatology (average high/low temperature). Climatology is rarely a correct forecast for a given day but you might think twice about forecasting a high that is 20 degrees higher than the average high (unless you're absolutely sure).

PERSISTENCE

Persistence means that tomorrow's weather will be the same as today's. In many situations, persistence is the best forecast to make--for example, in the Southeastern U.S. in summertime, when there are few fronts or low pressure systems moving through to change the weather.

ESTIMATING MOVEMENT OF AIR MASSES, FRONTS, AND PRESSURE SYSTEMS

One of the most important techniques for making a forecast is to use weather maps to estimate the speed of movement of air masses, fronts, and high and low pressure systems. For example, to make a low and high temperature forecast for tomorrow, find the current location of the air mass that you expect to be over the forecast site tomorrow. What were the high and low temperatures in the region that that air mass affected today? Remember to adjust your forecast for differences in latitude, possible acceleration/deceleration or intensification/deintensification of storm systems, and local effects such as topography, bodies of water, and the urban heat island effect. If you are located in the region you are forecasting for (as is most often the case) remember to look out the window.

PRECIPITATION FORECASTING

The first thing you want to know about your precipitation forecast location is what season is it. Summer precipitation is generally on a smaller scale and can be locally heavy (thunderstorms). Winter precipitation is generally more widespread, and light to moderate. If the precipitation is in the form of snow remember liquid equivalent is roughly 10 inches of snow equals 1 inch of water. In making precipitation forecasts for an approaching winter storm be careful of the possibility of the change from rain to snow or vice versa. Rain adds to the precipitation amount much faster than snow.

TEMPERATURE FORECASTING

In making your temperature forecast remember the high temperature generally occurs during the late afternoon. Know the high temperature for the air mass moving into your area and account for modification (clouds, winds, large water bodies etc.). Light winds and clear skies allow for greater heating during the day. Look at observations for your location as well as upstream (usually west) of your location. The low temperature generally occurs early in the morning. Nighttime cloud cover or high humidity slows down the rate of cooling significantly.

FORECASTING WITH THE USE OF IMAGES: BLUE-SKIES

Currently, hourly updated images and animations are available on the Weather Underground Groundhog Gopher server for the files shown below. The animations are in the directory "Weather_Animations" and the still images are in directory "Interactive Weather Maps. (Exception: interactive satellite images are not yet available; satellite images can be found in the Weather_Images directory.)

                        Update time in
     Image              minutes past the hour
---------------------------------------------
  Precipitation                10
  Temperature                  17
  Relative Humidity            22
  Wind Vectors                 25
  Pressure                     30
  Heat Index                   35
  Infrared Satellite           55
  Visible Satellite            57

The following images are generated once per day, and can be found in directory Weather_Images:

         Image                    Update time
-----------------------------------------------------
Average_high_temperature.gif        6:30pm EDT
Average_low_temperature.gif         6:30pm EDT
Forecast_low_tonight.gif            12:10am EDT
Forecast_high_tomorrow.gif          12:10am EDT

The heat index and wind chill images are not particularly useful for forecasting, and will not be discussed here. Perhaps the most useful forecast tools available via Blue-Skies are the Quicktime animations; these allow one to gauge the speed with which air masses, fronts, and pressure systems are moving.

Temperature map - Note other areas that have the same temperature as your region. A rapid change in temperature sloping roughly north-south is a good indication of a cold front. Check to see where the air mass coming into your area is coming from. Is warm or cold air moving in?

Pressure map - Low pressure areas are probably close to a front and therefore precipitation. High pressure areas are probably not associated with precipitation. If there is a rapid change in pressure over a short distance, strong winds will result.

Humidity map - High humidity should be located along and ahead of a front. Low humidity should be located behind a front and near highs.

Precipitation map - Shows the location and intensity of precipitation. This map is usually the best guidance for a precipitation forecast.

Satellite imagery - shows where the clouds (and therefore fronts and low pressure systems) are located. If images from the last few hours are looped the motion of the atmosphere is seen so you can see air mass movement.

Wind Vectors - shows the direction and speed (length of arrow) of the wind. You can analyze the wind vectors and find out where the high and low pressure systems are located. Winds circulate counterclockwise around a low and clockwise around a high.

Average High and Low temperature map - These maps show the average high and low temperature across the U.S. for a given date, based on long-term climatological averages.

Forecast High and Low temperature maps - These maps show the forecast high and low temperatures for the U.S. as predicted by a computer model known as MOS (Model Output Statistics). They serve as a good reference with which to compare one's forecasts. Don't get caught up to much in computer model guidance, though. The computers can't predict weather all by themselves (otherwise there would be no such thing as a weather forecaster), so use them as a guide. The best forecast is made by utilizing data from the computer and applying your own meteorology skills in making a forecast.

FORECASTING USING TEXT ONLY: UM-WEATHER

Although it is more difficult, one can make a forecast using only the weather text information available via the Weather Underground's UM-WEATHER program. A good place to start is the National Weather Summary, which gives one an overview of the significant weather over the past 24 hours as well as a indication of where significant weather will be occurring during the next 24 hours. Next, one should look at the climatic data summary for the city of interest. This gives the precipitation and high and low temperature for the previous day, as well as the normal highs, lows, and precipitation from long-term climatology. Next, one should consult the current weather observations for a region of interest and plot one's own weather map using a blank U.S. map. Ideally, one should plot two or more weather maps over a several hour period to get an idea of the motion of air masses, fronts, and weather systems. Analyze the map to look at the weather and think about what will happen in the future. You may want to contour the data so you can visualize what's going on. Lastly, one should consult the National Weather Service forecast for the selected city and nearby cities, unless you consider this "cheating".

ADDITIONAL SOURCES OF WEATHER INFORMATION

Teachers may want to call upon additional resources to help in preparing a weather forecast. One good source is The Weather Channel. It's on all the time and the maps are quite easy to comprehend. For those without cable television is the PBS television show, AM Weather. It airs each weekday at 7:15 A.M. A teacher might want to videotape and show a few minutes of either program to augment the Weather Underground data, particularly when using only the text-based UM-WEATHER information. A universal date source that is available any time and anyplace is a newspaper's weather map. The USA Today has an excellent color weather page.

JUSTIFYING YOUR FORECAST

The reason why you make a particular forecast is very important. Making a correct forecast for the wrong reasons is of no value. If you make a wrong forecast for the right reasons you learn from it and become more likely to make a right forecast for the right reasons. If your forecast varies considerably from other people's that's not necessarily a bad sign (as long as you have a legitimate reason for making your forecast). When justifying your forecast tell what guidance you used and what was indicated by it. Examples:

Nighttime lows over central Minnesota were in the low 50's last night; I expect this airmass to be over Detroit tonight, based on my analysis of the motion of the associated high pressure system from the weather maps I plotted using hourly observations from UM-WEATHER. Therefore, I am forecasting lows in the mid to upper 50's here tonight (expecting warmer temperatures here than Minnesota because we're farther south).
U.S. Precipitation movie shows a cold front approaching at 30 mph, with heavy rain associated with it. However, front is 900 miles away, so it should not affect us in the next 24 hours. Therefore, I gave only a 10% chance of precipitation for the next 24 hours.

FORECAST CONTEST SCORING

There are several methods of scoring in a forecast contest. For max/min temperature and precipitation forecasting here's the simplest method. Scoring is like golf, the lower score the better. Keep a cumulative score of the people in the forecast and the person with the lowest score is in first place. TEMPERATURE - If your forecast is within 3 degrees of the actual value then you are "in the window" and don't receive any points. For every degree after 3 you are off you receive one point. Example: You are 10 degrees off on your forecast, so you receive 7 points. PRECIPITATION - For precipitation you must select one of the following categories:

0 - Trace
.01 - .50 inches
Greater than .5 inches

The forecast is generally made in a 24 hour period midnight to midnight. If you forecast the correct category you receive no points. One category off is 3 points and for each additional category off there after add 3 points. Example: You predict category 1 and the actual category is 3, so you receive 6 points.

An additional method for keeping precipitation score is the Briers score. This is based on predicting the percent probability of precipitation. The forecaster assigns a probability of precipitation between 0% and 100% for a given 24-hour period. The Briers score is computed as the square of the difference between predicted and observed percent probabilities. For example, over a 3-day period forecaster "A" predicts a 30% chance of precipitation each day. Meanwhile, the more adventurous forecaster "B" predicts a 0% chance of precipitation each day. Precipitation actually occurs on one of the 3 days. Here's the cumulative Briers score for each forecaster:

Forecaster "A"

Forecast          Actual
Precipitation  Precipitation     Briers score
     30%            0%          (30 - 0 )**2  = 900
     30%          100%          (30 - 100)**2 = 4900
     30%            0%          (30 - 0 )**2  = 900

     Total Briers score for the 3-day period:   6700

Forecaster "B"

Forecast          Actual
Precipitation  Precipitation     Briers score
     0%            0%          (0 - 0 )**2  = 0
     0%          100%          (0 - 100)**2 = 10000
     0%            0%          (0 - 0 )**2  = 0

     Total Briers score for the 3-day period:   10000

So, the more daring Forecaster "B" was hurt by going all-out attempting to make a perfect forecast (a lower Briers score is better).