A Guide to Weather Products from the National Weather Service

School has dominated a huge chunk of my time this summer, as a result the blog has become stagnant.  As I catch up between semesters, it is appropriate to post this assignment I wrote on using National Weather Service products offered at www.weather.gov.  The following goes beyond the basic TV or simplified online weather forecasts in explaining how to paint a comprehensive weather picture.  It is written generically, not specific for aviation or hang gliding.

Abstract

The ability to understand and interpret observed and forecast weather conditions is essential for the safety, enjoyment, and success of virtually any outdoor activity. The National Weather Service (NWS) provides a wealth of weather information through its website, www.weather.gov. Hazardous weather watches and warnings, observed conditions, radar images, forecast charts and textual discussions, and local conditions and forecasts are tools on the weather.gov website that a user can access to paint and complete and accurate weather picture. This author uses the technique of starting with broad conditions over a large geographical area, and then narrowing the focus to local conditions for a specific geographical area. This provides an understanding of not just what specific conditions are or will happen, but of the larger scale weather mechanisms that are driving them and trends of how they will change in the atmosphere’s continual evolution. The following is a summary of this author’s flow of the NWS products used in painting a complete weather picture:

1. NWS Home Page
 2. Preview Local Forecast (and current conditions)
3. Active Alerts Page
4. Forecast Maps Page - Surface Analysis Chart
5. Radar Page
6. Forecast Maps Page - Forecast Charts
7. Regional Forecast Discussion
8. Local Area Point Forecast

Colorado has one of the most physically active populations in America. Colorado also has some of the most dramatic weather in the country. The weather can play a dramatic role in the enjoyment, safety, and outcome of those pursuits. The Colorado Rocky Mountains lie at the intersection of three air masses; continental tropical from the southwest, continental polar from the northwest, and maritime tropical from the Gulf of Mexico (Fovell 2011). The lifting action of the Colorado Rockies, combined with the convergence of these air masses creates some of the most dramatic thunderstorms in the country. Lightning, gust fronts, microbursts, flash floods, and tornados are all hazards associated with summer storms. In the winter, snowstorms can be sudden and unpredictable. Being able to accurately predict the weather is essential to safe recreation in Colorado. The National Weather Service (NWS) is one of six governmental agencies that make up the National Oceanic and Atmospheric Administration (NOAA) (NWS 2011). The NWS offers many products through its website, www.weather.gov, that can be used to paint an accurate picture of the current weather and forecast conditions over North America. A thorough understanding of the products available through the National Weather Service and how to use them is key to maximizing the success, safety, and overall outcome of outdoor recreation. The first half of this guide will explore what is offered by the www.weather.gov website, the second half will be a more in depth examination of some of those products.

The National Weather Service website homepage is found at www.weather.gov (Fig 1). In order to gain a thorough weather understanding, it is best to paint a complete picture that encompasses weather alerts, current conditions, short term and long term national forecasts, and local forecasts. This author’s technique is to start with general information over a national scale and narrow to more detailed information related to the specific geographical area of concern. The weather.gov homepage facilitates this approach. On the left hand side is a text box where users can enter their location. This will load a page that shows the basic weather for the geographic location entered. For the broad to narrow technique, this provides a preview of the local conditions of the area to which the search narrows. On the new page (Fig 2) there is a picture showing the current cloud cover, precipitation (if it exists), and temperature. Next to that is a more specific observation from a weather station (usually an airport) that shows humidity, wind speed and direction, barometer, dew point, visibility, and precipitation (if it is occurring). Below is a simple five day forecast with pictures and basic textual descriptions showing cloud cover, precipitation type and potential, and the high and low temperatures. Underneath the pictorial forecast is a seven-day outlook with a basic textual forecast. With the “preview” complete go back to the weather.gov homepage.

Fig 1. NWS Homepage

Fig 2. Local Forecast for Boulder, CO

The main feature of the weather.gov home page is a map of the United States. The map shows state and county boundaries and overlays color-coded weather watches and warnings. Above that is a brief textual description of the weather threats affecting the country. Clicking on an area of the map will open the home page for the regional NWS Weather Forecast Office of that area (Fig 3). The counties within that region are outlined with a click on the nationwide map. For now, the active alerts are of the user’s main concern, the regional focus will come into play later. If there is an active alert for the location of interest, it may answer the forecast questions without having to delve any further. For example, if there is a severe thunderstorm warning over a lake where the user was planning on going boating, it might be best to plan something else.

Fig 3. Active Alerts Map with Northeastern Colorado Selected

 

The second tab at the top of the weather.gov homepage is labeled “Forecast”, found at www.weather.gov/forecastmaps (Fig 4). Clicking on this tab opens a new window with a wealth of forecast charts and maps. The same page can be then opened by clicking the “Forecast Maps” on the sub-header of the weather.gov homepage. Below the National Forecast Maps header is a menu of links to National Forecast Chart, National Temperature, Short Range Forecasts, Medium Range Forecasts, Precipitation Amounts, Surface Analysis, Temperature, Predominant Weather, Wind Speed and Direction, Chance of Precipitation, and Alaska, Hawaii, and Puerto Rico Forecasts.

Fig 4. National Forecast Maps Page

 

Moving in left to right descending order, here is a brief explanation of each. How the relevant items apply to painting a complete weather picture will be discussed in the second half of this paper.

• The National Forecast Chart is a basic forecast map issued for the current day that shows high and low pressure centers, fronts, and areas of forecasted significant weather. Mouse clicks on the chart will open the regional NWS Weather Forecast Office page, just like the Active Alerts map. It is best viewed by selecting the “High Resolution Version” hotlink below the chart. 
• The National Temperature shows forecast highs for the US. When the user clicks on an area, this map reveals a new window that opens showing several states, which is larger area than on the National Forecast Chart. This window has a menu on the left that shows a column of comprehensive forecast data for different times. Scrolling the mouse over the time of a specific forecast variable opens the corresponding forecast graphic. This is very useful for quickly seeing specific information such as temperature, winds, or precipitation potential.
• Short and Medium Range Forecasts are maps of forecasted conditions similar to the National Forecast Chart but with specific forecast times. They also show more detailed information including isobars, barometric pressure, troughs, and symbology for precipitation coverage and type. If the user clicks on either map a new page opens showing a large image of that map. At the top of both forecast maps are tabs for forecasts at specific valid times. The forecasted movement trends of pressure centers and fronts mentioned can be seen by selecting different valid times. Below the map are two hot links that show legends of fronts, and symbols used on the forecast.
• Precipitation Amounts shows a quantitative precipitation forecast for 1 to 3 days into the future. This is useful for both fire weather and agriculture.
• Surface Analysis is a chart of current surface conditions. It shows high and low pressure centers, frontal type and position, isobars, troughs, and station observations. This is an important component of painting an accurate weather picture because it shows what is currently happening and can be used as a basis for how the conditions will change through various forecast times in the future.
• Temperature, Predominant Weather, Wind Speed and Direction, Chance of Precipitation, Precipitation Amount, and Sky Cover each open to a window nearly identical to the National Temperature map. The format is exactly the same as the National Temperature map, except that a national map of the icon selected (i.e. temperature, sky cover, etc) opens in the new page. The menu options on the left that show forecast variables for different times are the same format as in the National Temperature map. Scrolling the mouse over a time for a specific variable opens that graphic. The user can narrow to several states by clicking an area on the map. These icons all open basically the same window; they are just laid out in an order that defaults to the icon initially selected. For example, Wind Speed and Direction opens to the Wind Speed and Direction map, but moving the mouse over any time to the left will open that forecast variable for the time scrolled over.

Before narrowing the weather focus regionally, there is one more national item that is important to look at. On the weather.gov home page, the third tab in the sub-header is labeled “Radar”, found at www.weather.gov/radar (Fig 5). Clicking on this will open a new map called the Doppler Radar National Mosaic. This map is a mosaic of radar sites across the US showing radar images of moisture over the country. The default image is a loop of the last six composite radar images. Clicking on the map opens the regional National Weather Service Forecast Office radar home page, similar to how the Active Alerts and National Forecast Chart pages are structured. Weather radar works by emitting electromagnetic energy in all directions from a station and reading how much of that energy is reflected back by moisture (NWS 2010). It’s important to note that radar is an observation of current moisture at a given time, not a forecast. A loop of different radar images can show the building, dissipating, and movement trends of cells. The density of the moisture is shown by colors indicating precipitation intensity level. A scale on the right shows the corresponding dBZ (decibels per Z, where Z is density) for each color (NWS 2010). This is useful for watching how precipitation or thunderstorms are moving. Combining this trend information with the forecast frontal movements in the forecast maps will complete the large scale picture of the current and forecast weather.

Fig 5. Doppler Radar National Mosaic

 

 With the large-scale picture complete, it’s time to narrow to the specific area of relevance to the user. Start at the “Active Alerts” tab on the weather.gov home page, found at www.weather.gov. Click on the area of interest and it will open the regional NWS Weather Forecasting Office homepage for that area (Fig 6). The main feature of the page is a detailed map of the area within that region. The map shows a zoomed-in picture of color-coded current weather warnings and watches. Below the map are four windows titled Weather Story, Radar, Satellite, and Weather Map. With the exception of the Weather Story these lead to zoomed in graphics of areas already discussed. On the left hand side is a menu of options showing links under the headers Current Hazards, Current Conditions, Forecasts, and other topics. These links offer a wealth of weather information for a variety of activities. It is best to start at the top and move down clicking on the relevant links for the activity being planned. For example, hang gliders would be interested in the Aviation Weather hotlink, while the Fire Weather hotlink is useful to fire fighters. The Forecast Discussion, the first hotlink under Forecasts, is arguably the most important link and is a useful tool in painting an accurate and complete weather picture. It will be discussed in detail in part two of this paper.

Fig 6. Denver / Boulder NWS Weather Forecast Office Homepage

 

 The last step in completing the weather picture is to click on the regional map for the actual area of interest. Zoom and arrows can be used to resize and scroll the map to a very specific geographical feature like a river, lake, town, or road intersection. This will open a page called the Point Forecast. The information shown is exactly the same as the page that was accessed by entering name of the town that was entered from the weather.gov home page. The difference is that the user can pinpoint an exact location, like four miles north of Boulder, instead of Boulder, Colorado (Fig 7). If the rough map isn’t accurate enough the user can use a more detailed Google Maps over lay to select a specific location. The website will interpolate the data from forecast stations and provide a best guess for that exact location.

Fig 7. Point Forecast for 4 Miles North of Boulder, CO

 

In order to paint the most accurate weather picture possible, a thorough understanding of the information given in various observations and forecast products is necessary. The following will be a detailed guide to the information available from the weather products analyzed in the preceding paragraphs. 

The Active Alerts map on the weather.gov homepage shows a comprehensive overlay of all the current weather watches and warnings in the country. Generally, a weather watch means the potential exists for the hazardous conditions to occur, while a weather warning means the phenomena is occurring or is very likely to occur (NWS 2011). The weather watches and warnings are shown by color coded to a key below the map. Forecasters analyze the severity of precipitation, thunderstorms, snow accumulation, visibility, and heat associated with weather systems and issue alerts based on the hazards they pose. While the forecast is a guess of what conditions might develop, the Active Alerts are the potential result on the ground if they do. A notable example of proactive weather alerting was the issuance of a tornado warning in April, 2012 a full day before tornados were expected to develop (Associated Press 2012). The Active Alerts Map in Figure 8 below shows a Hazardous Weather Outlook for most of the Western US, Flash Flood Watch for northwestern Arizona and southwestern Utah, a Heat Advisory for the central Southern US, and an Excessive Heat Warning centered in that area.

Fig 8. Active Alerts Map

 

The Surface Analysis Chart is tool showing observed conditions that meteorologists use to develop forecasts (NWS 2007). The surface analysis chart is accessed from the Forecast tab on the weather.gov homepage (Fig 9). Clicking the Surface Analysis icon opens the North American Surface Analysis chart. The North American scale is cluttered, but the user can zoom in by selecting a specific area on the drop down menu below the chart. The Cont US and specific region of the US are what that this author uses. A loop of past surface analysis charts (similar to radar loops) can be displayed showing the progression of surface conditions. The surface analysis chart shows a wealth of meteorological information in an easy to read graphic including isobars, high and low pressure centers, fronts, troughs, and observations from ground stations (NWS 2008).

Fig 9. Continental US Surface Analysis Chart

 

The ground stations show (moving clockwise from the top) high and middle cloud type, sea-level pressure, pressure gradient change in past three hours, pressure tendency, six hour precipitation, low cloud type, dew point, present weather (precipitation type), temperature, wind speed and direction, and total sky cover in the center of the station (Fig 10 & 11).

Fig 10. Surface Analysis Chart Ground Station Symbology and Weather Symbols

 

Fig 11. Surface Analysis Chart Ground Station Wind, Pressure, and Sky Symbology

 

Not all of this information will be given, only what is present. A guide to frontal and station symbology can be found in the hotlinks below the surface analysis chart graphic. Meteorologists keep a constant watch on observed conditions presented in the surface analysis chart. They provide forecasters the information needed to generate a forecast and update or amend the forecast as weather systems evolve.

Radar charts show Atmospheric precipitable moisture that surface analysis charts do not show. Rain, snow, thunderstorms, and hail are all forms of precipitation that can be seen in radar images. NexRad, or next generation radar is a nationwide network of Doppler radar with a high level of sensitivity and the capability to detect the velocity of radar echoes (NWS 2010). Radar echoes reflect the strength of the moisture in the form of dBZ or decibels of Z, where Z is density of water droplets in each cubic meter (NWS 2010). Practically dBZ is precipitation intensity. The intensity scale of -30 to +75 dBZ is shown through different colors. Figure 12, below, shows relationship of dBZ, color, and precipitation intensity.

Fig 12. dBZ and Precipitation Intensity

 

From the weather.gov homepage select the “Radar” tab from the sub-header above the Active Alerts map. The Doppler Radar National Mosaic map will open in a composite reflectivity loop format (Fig 5). Click on the area the user is interested in. This will open the NWS Weather Forecast Office radar home page (Fig 13). The regional map will default to base reflectivity. Base reflectivity is based on the tilt of the radar beam. It is an image of echo returns from the lowest beam angle based on radar station position and elevation (NWS 2010). Radar beams angle upward from the station. Imagine shining a flashlight at an angle into the sky. The width and altitude of the light beam will vary with distance away from the flashlight. A radar beam varies in the same way. As a result, storms closer to the station will reflect accurate bases, while ones farther away might not have their actual bases “painted” by the beam because they are below the beam angle. Composite reflectivity is an image of the highest echo return at any beam angle or station elevation. Composite reflectivity is useful for determining the size, elevation, and intensity of a storm, while base reflectivity is useful visualizing the location and movement of specific cell bases. This author’s technique for interpreting radar images is to start with the National Mosaic and observe the movement and development of moisture. These should correlate roughly with the fronts depicted on the surface analysis chart. With the big picture in mind, click on the regional area of interest and observe the direction and development trend of cells. Base reflectivity is more useful for where it is going to rain, hail, or lightning might be encountered at the surface. Rain, wet snow, and wet hail echoes reflect their precipitation intensity more accurately than dry snow and especially dry hail. This can be dangerous for aircraft relying on airborne radar flying near thunderstorms. Specific aviation products like the Radar Summary Chart add areas of echo’s containing hail and show speed and direction of large echoes and their tops.

Fig 13. Denver / Boulder, CO Radar Homepage

 

Forecast charts, also known as prognostic or prog charts, are the product of observations and forecasting models (NWS 2008). They can be found on the “Forecast” tab of the weather.gov home page. Forecast charts have two important times to note, issuance time and valid time. The issue time is when the forecast was issued and is useful to compare against current and past observations. Valid times are the period that the forecast is valid for. NWS Short Range Forecast charts are valid for six, twelve, eighteen, twenty-four, thirty-six, and forty-eight hour periods and are issued two times daily depending on the forecast. Medium Range Forecast charts are issued every twelve hours and are valid for a twenty-four hour period. Picking the correct chart is important to get an accurate forecast of the time period of interest. Short and Medium Range Forecast charts (Fig 14) show high and low pressure centers at the surface, isobars, troughs, frontal types and position, barometric pressure, and areas of forecast precipitation coverage and type. Precipitation type is indicated by standard symbology, while the coverage of an area is indicated by angled dashed lines (Fig 15). Legends for the frontal and precipitation symbology can be found by clicking the two hot links directly below the chart.

Fig 14 Short Range Forecast Chart

 

Fig 15. Short Range Forecast Chart Precipitation Symbology

 

High and low pressure centers, frontal movement, and isobars are important because they determine wind speed, potential for hazardous weather, and the generation and decay of fronts, troughs, and low pressure systems. Meteorologists use this data to issue weather alerts and issue marine, aviation, agricultural, river, and fire weather specific forecasts. This author’s tool for using forecasts is to start with the National Forecast Chart, then select the Short or Medium Range Forecast for the time of interest. Compare the forecast position of fronts, and high and low pressure centers to where they were observed on the surface analysis chart. Make a note of forecasted areas of precipitation associated with frontal positions. If the time is more distant into the future it is useful to observe the forecasts leading up to that time to determine trend information. It’s also important to keep in mind that the farther into the future a forecast is, the less accurate it will be. The Forecast Discussion, discussed next, is important in determining forecast accuracy. After observing the forecast charts, click on any of the Temperature, Predominant Weather, Wind Speed and Direction, Chance of Precipitation, Precipitation Amount, or Sky Cover icons (bottom of Fig 4). Wind speed and direction are usually of keen interest to this author so that is where he starts. The new page that opens is the Graphical Forecasts for the Continental US. It will default to a national map showing the item clicked, but scrolling the mouse over the time of the weather condition to the left will show the corresponding forecast for that condition at the time selected (Fig 16). This author moves from top to bottom of the column noting conditions of interest for the time of interest. Forecasters use this information to issue specific warnings, like a Red Flag Warning or Small Craft Advisory for high surface winds at a given time. For the user the forecast picture is almost complete, but the next area of focus will give it relevance and meaning far greater than a simple graphic can.

Fig 16. Graphical Forecasts for Continental US (with temp forecast shown)

 

The Forecast Discussion is a textual discussion of what the different forecast models are forecasting for the regional area (Fig 17). Although it is not a chart, the Forecast Discussion warrants a thorough explanation. In this author’s opinion the Forecast Discussion is the most important tool provided by the NWS. It gives relevance, background, and accuracy information to every other forecast product offered. The models used by the NWS are the NAM (North America Model), GFS (Global Forecast System), and SREF (Short Range Ensemble Forecast) (NWS 2012). The discussion gives insight into how the forecasters are interpreting the forecast models and how they issue various forecast products. It gives the user a look behind the scenes as to what they are thinking and provides clues to general trends and how accurate components of the forecast might be. If the discussion indicates the models are in disagreement then the user might be more skeptical of certain elements of the forecast. Advanced acronyms and terminology are used, but the NWS provides hotlinks to definitions of many terms. What makes the forecast discussion such a valuable tool is that it tells the comprehensive story of the forecast, not just snap shots of different elements. If the forecast discussion is too intimidating the Weather Story icon found on the regional NWS Weather Forecast Office home page is a simpler tool, but it is far less detailed.

Fig 17. Denver / Boulder, CO Forecast Discussion

 

The Point Forecast is the last item in the complete weather picture flow (Fig 18). This is the forecast for a very specific area found by clicking on the regional map of a NWS Weather Forecast Office homepage. For example if the user wants to see what the specific forecast items will be at a trail head, mountain peak, lake, or sports arena they can find its approximate location on the regional map and click on that area. The NWS website interpolates the data provided by local weather stations and their forecasts to give sky conditions, temperature, wind speed, and precipitation potential for that exact area. This item completes the weather picture by showing specific conditions for a specific area. Armed with a thorough knowledge of national observed and forecast conditions before this step, the user should know how likely it is that the local forecast will materialize and be able to correlate the greater mechanisms driving the phenomena predicted in the local forecast.

Fig 18. Point Forecast for 5 Miles North of Boulder, CO

 

The weather plays a role in the safety, enjoyment, and success of almost any outdoor activity. With a little knowledge and understanding of the tools available from the National Weather Service anyone can develop a weather picture for a specific location and activity. In an area like Colorado, with dynamic weather and rugged terrain, a thorough weather understanding is essential for an active population. In 2005, this author witnessed the death of a teenager due to heat exhaustion that would have been completely avoidable with just a little weather understanding and forecasting ability. Too many people rely on dumbed-down TV weather forecasts and neglect the wealth of in-depth tools available through the National Weather Service. Following the broad to narrow technique is just one example of how a user can paint an accurate and complete weather picture. One of the best ways to learn what is available and how to use it is to simply explore and experiment with the products available at the National Weather Service weather.gov website.

References

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Fovell, Robert G. (2011). Lecture 13 The Global Atmospheric Circulation. Retrieved from: http://www.youtube.com/watch?v=YQsevVEQ-jc.

National Weather Service. (2012). NCEP Central Operations. Retrieved from: http://mag.ncep.noaa.gov/NCOMAGWEB/appcontroller?prevpage=index&MainPage=index&cat=MODEL+GUIDANCE&page=MODEL+GUIDANCE.

National Weather Service. (2011). About NOAA’s National Weather Service. Retrieved from: http://www.weather.gov/about.

National Weather Service. (2011). Weather Watches and Warnings. Retrieved from: http://www.erh.noaa.gov/box/watchvswarn.html.

National Weather Service. (2010). Jetstream – Online School for Weather. What do the colors mean in the reflectivity images? Retrieved from: http://forecast.weather.gov/jetstream/doppler/radarfaq.htm#radar.

National Weather Service. (2010). Jetstream – What are the different types of radar images? Retrieved from: http://forecast.weather.gov/jetstream/doppler/radarfaq.htm#diff.

National Weather Service. (2010). Jetstream – How does the radar work? Retrieved from: http://forecast.weather.gov/jetstream/doppler/radarfaq.htm#radar.

National Weather Service. (2008). Hydrometeorological Prediction Center – About the Short Range Forecast. Retrieved from: http://www.hpc.ncep.noaa.gov/basicwx/about_bawx.shtml.

National Weather Service. (2008). Hydrometeorological Prediction Center – (Surface Analysis Chart) Sample Station Plot. Retreived from: http://www.hpc.ncep.noaa.gov/html/stationplot.shtml#weather.

National Weather Service. (2007). Hydrometeorological Prediction Center – About the Surface Analysis. Retrieved from: http://www.hpc.ncep.noaa.gov/html/about_sfc.shtml.