Weather Forecasts

Objectives:

Modern weather forecasting is based on the process of numerical weather prediction (NWP). In a computer (or numerical) model of the atmosphere, the physical laws that govern fluid motions (Newton’s laws of motion, the first law of thermodynamics, conservation of mass, etc.) are put into computational forms for computer calculation. Supercomputers distribute an initial set of data throughout a three-dimensional grid that represents the atmosphere from the surface to the upper stratosphere. The physical relationships between variables (e.g. temperature, pressure, winds, water vapor) are used to take the initial values of these quantities a short step forward in time at all the grid points. Those new values are then used to step ahead once more. This process is repeated in leap-frog fashion into the future. As the length of the forecast period (from the starting point) lengthens, small errors magnify and the detail in expected specific values of weather variables declines.

The starting values for these calculations at all grid points are weather observations. By international convention, weather observations are taken at 0000Z and 1200Z at both the surface and in the upper atmosphere, worldwide. These observational data are exchanged and collected at national weather centers. The U.S. National Centers for Environmental Prediction, National Oceanic and Atmospheric Administration (NCEP/NWS/NOAA), collect these data and input them to the computer forecast process. After running the computer program to simulate times into the future of 6, 12, 18, 24 hours, and longer, the predicted values throughout the atmosphere are related to the surface temperatures, dewpoints, cloud cover, wind, and precipitation probabilities that are familiar components of weathercasts. This computer-generated information is distributed to local NOAA/NWS Forecast Offices.

The final step is taken by NWS meteorologists at local forecast offices. They use their experience and knowledge of local influences on the weather to adjust the computer information to the final forecasts disseminated to television and radio outlets, Internet sites and newspapers. Also, private sector providers utilize the NWS weather data and products to produce value-added products for the special needs of their customers.

After completing this investigation, you should be able to:

• Describe the general elements of a weather forecast.
• Compare the forecasts available to the public by NWS forecast offices with resulting weather conditions.

Making Forecasts:

Forecasts of weather conditions are provided for the U.S. via maps and descriptive summaries similar to those available from the course website (Watches, Warnings, Advisories and Forecasts). Forecast maps are provided for various parameters and in varying degrees of detail out to the following 16 days beginning with surface and upper-air observations at either 0000 UTC (00Z) or 1200 UTC (12Z). Forecast maps give predicted positions of surface weather features (high- and low-pressure centers and fronts) along with areas and types of expected precipitation for a certain future (valid) time, specified in the lower left margin of each map. These maps show the conditions at 6-hour intervals during the period or by animation for the entire period.

Figure 1 is the surface weather map for 12Z 11 DEC 2013 (7 AM EST December 11th). This is a broad depiction of the actual weather conditions across the country when a forecast cycle was initiated. These and other surface observed values along with upper air conditions and satellite data were distributed across a three-dimensional grid on an imaginary Earth and stepped forward in time utilizing the physical laws governing fluid behavior.

Figure 1. Surface weather map for 12Z 11 DEC 2013.

1. At map time, national conditions depicted on this surface weather map showed that generally there was (were):[(patches of scattered, generally light precipitation across the Great Lakes region into the Northeast)(fair weather and mostly clear to partly cloudy skies over the Southeast, West and north-central portions of the U.S.)(both of these situations)].

2. The Figure 1 weather map showed Tallahassee, Florida with a temperature (10 o’clock position of the station model) of [(39)(44)(51)] degrees Fahrenheit. Tallahassee’s station model also reported clear skies (station circle not shaded). High pressure marking the center of a generally cool but fair air mass was shown to the north of Tallahassee.

3. A general eastward movement of weather systems [(would)(would not)] suggest that any precipitation across the southern portion of the country might move into the Tallahassee area and produce precipitation in the next couple of days or so.

Figure 2 is a combination display of forecasts available from the course webpage for a period of two and one-half days into the future following the state-of-the-atmosphere conditions shown in Figure 1. The Watches, Warnings, Advisories and Forecasts section of the homepage, NWS Short-Range Forecast Maps provides maps and a loop of their motion over the next day and a half or so. The time interval is 6 hours between the first four maps and 12 hours thereafter. Figure 2a (top panel) is an example of a national map showing the forecast positions of major weather systems and their associated precipitation locations and types. Symbols for precipitation types can be found from the Extras section, Surface Station Model link on the course website. Areas of expected precipitation are outlined by a heavy green line for probabilities of 30 to 50% while the area is dashed for 50 to 100% likelihoods.

Figure 2. (a) Short-range forecast map for 00Z 15 DEC 2013 and (b) quantitative precipitation estimate for the 12-hour period ending at that same time.

4. Figure 2a shows a [(warm)(cold)(stationary)] located over the Tallahassee area at 00Z 15 December 2013 (7 pm EST Saturday evening).

5. From Figure 2a for the Tallahassee area at 00Z 15 December, there is a likelihood of: [(rain or thunderstorms)(snow)].

Figure 2b (bottom panel) is an example of the Quantitative Precipitation Estimates products which show the forecast of amounts of liquid precipitation anticipated in 12-hourly intervals from forecast weather systems. The particular map displays the precipitation amounts expected during the 12 hours ending at 00Z 15 December 2013 (7 pm Saturday evening) resulting from the general position of the weather systems shown in part a.

6. From the Figure 2b interval shadings and the notation of the maximum precipitation amounts at locations marked with an “X”, the Florida panhandle, including the Tallahassee area, could expect to receive as much as [(0.01)(0.1)(0.5)(2.0)] inches of precipitation during the 6-hour period ending at 00Z 15 December 2013.

Figure 3 is an example of the forecast webpage from the NWS Forecast Office at Tallahassee, FL from 3:53 PM EST, 11 December 2013. These forecast sites can be accessed from the course website as noted in Investigation 13A or from clicking on the Forecast Office circle from the Climate section of the webpage, Local NWS Offices. As noted in Investigation 13A, current conditions are also shown. Here we will examine the forecast for Tallahassee for Saturday night, 14 December (the local equivalent time period of 00Z 15 Dec.), almost three and one-half days into the future from the time of the initial conditions.

Figure 3. NWS Forecast Office, Tallahassee, FL webpage from 11 DEC 2013.

7. The small graphical depictions cover a period of approximately the next [(one day)(four days)(seven days)].

8. The graphical forecasts section is generally divided into [(morning and afternoon)(day and night)] segments.

9. In the Figure 3, 7-Day Forecast (text) section, the forecast weather conditions for the first two days of the period can also include: [(wind)(probability of precipitation)(both of these)].

From Figure 3, note the forecast weather conditions (7-day Forecast) for Saturday night, 14 Dec., approximately 75 hours following the initial conditions on which the forecast was based. (The pictorial and text forecasts are highlighted with red boxes.)

10. Low temperature: [(46)(58)(63)] °F. (Actually, this will probably occur early on Sunday morning.)

11. Cloud cover (given the likely weather situation noted): [(clear)(partly cloudy)(cloudy)].

12. General probability of precipitation: [(none given)(30%)(80%)].

One might also wish to know more detailed weather conditions for several days into the future at hourly intervals for specific event planning. That is also available from the NWS forecast pages, further down the right-hand column of the page called the “Hourly Weather Graph”. Clicking that thumbnail link of the webpage brings up an hourly graph of forecast weather conditions for the chosen site.

Figure 4 is a portion of the 3-day History of weather data for Tallahassee covering the period from 10:53 AM EST 14 December to 10:53 AM 15 December 2013. The hourly weather conditions at Tallahassee cover the 24-hour period including the time of the forecast period noted previously.

Figure 4. Abridged 3-day weather history from Tallahassee, FL at 10:53 AM EST 15 December 2013.

13. Note the forecast conditions in Figure 3 for Tallahassee for Saturday, 14 December. The temperature columns report the maximum and minimum temperatures that occurred in each 6-hour period over the day. The maximum for the entire period shown is given with the 12:53 PM line as 71 °F. This is only two degrees different from the forecast high for Saturday, close considering the weather conditions that occurred and the seventy-five hour lead time involved. In the same sets of values, the minimum 6-hour temperature given, which occurred at 6:53 AM on Sunday morning, 15 December, was [(52)(59)(61)] °F.

14. Observe the weather conditions reported for the hourly times prior to 18:53 EST (essentially 1900 EST or 7 PM (00Z 15 DEC) on the 14th. Light or heavy rain and/or a thunderstorm was reported in [(0)(1)(3)(5)] of the hourly lines.

15. The far right column of the report lists total precipitation amounts for 6-hour periods. The 18:53 EST report showed that [(0)(0.14)(0.86)(1.57)] inches fell in the 6-hour period prior to 18:53 EST on 14 December 2013, essentially the time frame within the quantitative precipitation forecast from Figure 2b for the Tallahassee area.

16. Compared the forecast Saturday high and Saturday night low temperatures and the 12-hour precipitation total values from Figures 2 and 3 with the conditions that actually occurred more than three days following as given in Figure 4. These values [(were)(were not)] close enough to provide useful planning for activities. General planning for several days into the future for many activities and events calls for necessary preparations to minimize damages and excessive costs; these forecasts would provide such information.

“The National Weather Service (NWS) provides weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas, for the protection of life and property and the enhancement of the national economy. NWS data and products form a national information database and infrastructure which can be used by other governmental agencies, the private sector, the public, and the global community.” (NOAA/NWS Mission Statement)

The gathering of weather information is therefore a process that is conducted primarily so the NWS can fulfill its forecasting and warning mission. The Internet now forms an exceptional vehicle to disseminate those forecasts, warnings, and other weather and climate information to the public in text and graphic formats. The following Internet address: http://www.weather.gov/ () is the location for the latest official watches and warnings from the National Weather Service. The areas on this U.S. map that currently have active warnings, watches, advisories, and/or special weather statements are colored according to the legend below the map. Figure 5 is the watches and warnings map for 19 December 2013 as an example.

Figure 5. NWS Watches and Warning Map for 19 DEC 2013.

17. At http://www.weather.gov/ (), find your location on the National Weather Service interactive map and click on it. The resulting webpage shows the local NWS forecast office’s (location listed below the “headlines”) area of responsibility for issuing weather warnings and forecasts. From the regional map, click on your approximate location. Examine the forecast products that are displayed. Under the graphical forecasts of the next few days are plain-language forecasts. These text forecasts are presented out to [(3)(5)(7)] days in the future.

18. Scroll down the page and click on one of the thumbnail maps for the National Digital Forecast DataBase. The “Daily View” tab shows maps can be advanced or backed within each 12-hour period for the length of the forecasts presented in text. Individual maps for most weather elements (e.g., temperature) are shown at [(3-)(6-)(12-)] hour intervals within each period. Some products are for longer periods and there are tabs for choosing a Weekly View or for Loops. The looping feature can be used to produce a vivid display of diurnal temperature cycles or day-to-day changes as weather systems pass.

A warning is a statement issued by the National Weather Service indicating that a specified hazardous weather or hydrologic event is imminent or actually occurring. The intention of these warnings is to urge the public to take immediate appropriate action for personal safety. Warnings may be issued for flash floods, hurricanes, severe thunderstorms, tornadoes, or winter storms. Often these warnings are carried by television stations interrupting broadcasts or as text and graphics on-screen or via radio. Special NOAA Weather Radio receivers can be triggered by the local NWS forecast office to signal the issuance of a warning and alert the public.

All weather warnings, watches, advisories, and statements are posted on the maps you have been examining. Browse the national map (http://www.weather.gov/ ()) and check on several watches and warnings to familiarize yourself with these products.

Official NWS forecasts are updated several times daily and made available to private meteorological companies and the media. Whether you receive your weather forecast from radio, TV, the Internet, or newspapers, the original source for the information is your local NWS forecast office.

Suggestions for further activities: You may wish to call up the website of your local NWS Office. Go to http://www.wrh.noaa.gov/wrh/forecastoffice_tab.php () to find your nearest NWS Office. Click on the station. From the regional map showing each office’s area of responsibility, you can click at your location to obtain the specific forecast for you. Take some time to explore the types of forecast products available.