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AFD FAQ

Frequently Asked Questions (FAQs)

1. What is 18z, 00z, etc., or "why do meteorologists frequently use Z time?"
2.  What do GFS, NAM, ECMWF stand for?  Or, "what are the various models that are used in forecasting?"
3.  What is 500 mb, 700 mb, etc.?  Why do we use them?
3a. What are "dam?" Or, "what are the units being used for the various meteorological parameters?"
4.  What are Time-height sections?
4a. How do you interpret relative humidity and wind on time-heights?
5. How do we show fronts, trough axes, and ridge axes on our weather charts?
6. What is the difference between a shortwave and a longwave? Or, what is the difference between a short-wave trough and a long-wave trough?
7. What are ensemble forecasts and how do forecasters use them?
8. How are the "point-and-click" forecasts created?

For definitions of more words and phrases, you can check the NWS glossary.

1. What is 18z, 00z, etc., or "why do meteorologists frequently use Z time?"

"Z" in this context is short for Zulu time, also commonly called "UTC" (Coordinated Universal Time) or "GMT" (Greenwich Mean Time). Using a 24-hour clock similar to military time, this is a time that is equivalent to the time in Greenwich, London during winter in the UK. Using Z time is convenient for occupations such as aviation and meteorology, where information is shared across large areas, and worrying about time zones becomes more cumbersome. For example, if we say in our discussion that "thunderstorms are expected to form between 21z and 23z," and the time is currently 20z, the reader can know immediately that thunderstorms are expected to begin in one hour, rather than worrying about what time zone the reader or the author is in.

In practice, since much of the world runs on local time, meteorologists do have to know how to convert between Z time and local time. In Utah, we are 6 hours behind Z time in the summer (MDT), and 7 hours behind Z time in the winter (MST). So, 18z is the equivalent of noon (12 pm) MDT.

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2.  What do GFS, NAM, ECMWF stand for?  Or, "what are the various models that are used in forecasting?"

GFS, NAM, ECMWF, WRF-GFS, MM5-NAM, RUC, etc. are all acronyms for the complex computer models that forecasters may use to aid them in preparing a weather forecast.  The models are based on mathematical equations that describe and try to reproduce the physics that control how the atmosphere changes over time. These models are run by different organizations all over the world, frequently on powerful super-computers, and the data is shared with the various academic and meteorological organizations. Many of these models, including the ones run by the National Weather Service (NWS), are available to the general public via the internet. 

  • The GFS stands for Global Forecast System and is one of the two main models run by the National Weather Service.  The GFS is run 4 times per day based on the synoptic data times (00Z, 06Z, 12Z, 18Z) and the output is in two different resolutions, 90 kilometers (GFS90) and 40 km (GFS40). 
  • The other main NWS model is the NAM which stands for North American Mesoscale (model).   The NAM is a mesoscale regional model, meaning that it has higher resolution than a hemispheric model like the GFS.  There are trade-offs between using higher resolution mesoscale models and lower resolution hemispheric models.  The NAM is also run 4 times per day with output coming in two resolutions, 40 km (NAM40) and 12 km (NAM12). 
  • The ECMWF is the model that the European Center for Medium Range Weather Forecasting uses, and is comparable to the GFS. 
  • The Canadian is the model that Canada's weather service, "Environment Canada" uses and is also comparable to the GFS. 
  • The RUC is short for "Rapid Update Cycle" and is a model that is run every hour.  The main use of the RUC is in forecasting rapidly changing conditions.  An example of this would be thunderstorm outbreaks that can occur in the midwest.
  • More localized organizations, such as the University of Washington or our forecast office in Salt Lake City, run their own computer models, usually high resolution models that help account for detailed local effects such as terrain or bodies of water. These high resolution models include the MM5 model and the WRF model..  These computer models are 'initialized', or start with the initial conditions from either the GFS or NAM model, then the models continue on with their own forecasts.  The models are frequently run at resolutions as small as 4 km, and forecasters often look at and refer to them to get some idea of what kind of impact the small scale effects may have.
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3.  What is 500 mb, 700 mb?  Why do we use them?

"mb" is short for millibars, which is a unit of measure that meteorologists use for atmospheric pressure. You may also see pressure mentioned in terms of "hPa," which stands for hectoPascals, and are equivalent to millibars (i.e. 1020 mb = 1020 hPa). These are preferred by meteorologists over the units you may be used to seeing on television broadcasts, which is commonly inches mercury (e.g. 30.12 inHg, or sometimes just 30.12 in.).

Rather than use constant levels above the ground in feet or meters (for example, plotting pressure at 1500 feet above mean sea level), meteorologists prefer to make plots on constant pressure surfaces, such as at 500 mb. There are several different levels in the atmosphere that forecasters commonly look at.  The 500 millibar (mb) level is the bread-and-butter of meteorologists because it is about the middle of the atmosphere, if you think in terms of pressure.  The mean sea level pressure is around 1013 mb.  In terms of height, 500 mb will be near 18,000 feet in elevation.  This is a good level to look at since transient waves moving through the atmosphere have a fair chance of being detected at the 500 mb level, even if the strongest part of the wave is below 500 mb or above 500 mb.

Commonly viewed levels in the atmosphere:

Level     Feet Above Sea Level    Description
200 mb    about 35,000 Feet       Good view of the jet stream
300 mb    about 30,000 Feet       Good view of the jet stream
500 mb   about  18,000 Feet       Good for detecting significant waves (ridges and troughs) in the atmosphere
700 mb   about 10,000 Feet        Good for viewing moisture, temperature, temperature advection, and lower level winds.
850 mb   about  5,000 Feet        Near the surface in many parts of the Great Basin, including many valleys in Utah.
1000 mb  near sea level            This level is not commonly used in the Great Basin, as it is usually underground.
MSL  (mean Sea Level)            This level is commonly used for identifying fronts, surface lows and highs, and forecasting surface winds.
                                                        Also called MSLP (mean sea level pressure) in graphics.

In Utah, many of the valleys are in the 4,000 to 5,000 foot range, so the 1000 mb level is never used, and the 850 mb level is rarely used, as it falls below much of the higher terrain. All of the rest of the levels that are higher in the atmosphere are still frequently used. We will frequently refer to 700 mb, particularly the temperatures at 700 mb, as this is a good indicator of where the freezing levels and snow levels will be, and can be also be used to help determine what temperatures will be at the surface, particularly maximum temperatures during the summer months.

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3a.  What are "dam?"  Or what are the units being used for the various meteorological parameters?

 "dam" is shorthand for decameters, or tens of meters.  For example, 576 dam is the same as 5760 meters.  We often refer to dam instead of meters because meteorologists use charts, and it saves a tiny bit of space and clutter by labeling the contours 576 or 564 rather than 5760 or 5640.  You will most frequently encounter DAM when a forecaster is relating 500 millibar heights. 

Meteorologists will also commonly use Celsius for temperature and knots for wind speeds, although the forecasts themselves are still mostly made in degrees Fahrenheit and miles per hour, since those are the units that the customers are most familiar with. (Knots are nautical miles per hour; 100 knots = 115 mph (approximately), so they are close to equivalent in many cases.)

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4.  What are Time-height sections?

Time-height sections allow one to see how meteorological parameters change in the atmosphere above a given location as time goes by.  So if you are used to looking at graphs, in this case the X-axis or horizontal axis is time, and the Y-axis or vertical axis is height or elevation.  Take a look at the example below from Salt Lake City, UT on Friday, October 23, 2009 and Saturday, October 24, 2009.

time height 

The vertical axis is height, but the units shown on each side of the image  are in pressure or millibars (mb).   You will notice that there is no data for 850 mb or 1000 mb, as these levels are below the surface. Unlike many graphs you may be used to looking at, time begins on the right side and advances going to the left.  Why this local convention?  Because weather frequently moves over the United States from the west, or in physical coordinates (if you think of north as up) from left to right.  By plotting time from right to left, one can also pretty much view the image as kind of a physical cross-section of the weather. 

Let's look carefully at the times and date labels on the horizontal axis shown in the image below.  We will decode the time block
on the far right.
                                        23 . 18         23 is day 23 of the month, 18 means 18Z  and is the time this model was run
                                          0HR               0HR means this is the 'zero-th' hour of the forecast, the next time is 3HR which is the 3rd hour.
                                       18Z Fri        18Z Friday is the time the the values above that point are valid for.  18Z is 12 PM MDT.

Reading the time stamps from right to left, you can easily see that time advances in that direction.
time height zoom

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4a.How to interpret - Relative Humidity.  The time-height section above plots two different meteorological parameters, relative humidity and wind.  The relative humidity is plotted with both the blue contours and the image colors.  The color scale that labels each color with a relative humidity value is at the top left of the image.    You can quickly see that the low-to-mid level air mass is quite moist, with relative humidity 70% or more from about 700 to 500 mb on Saturday morning.  Above that, the air mass dries out, with green and red colors showing RH to be 50% and below.  These relative humidity time-height sections can be very useful for forecasting cloud ceiling heights for aviation. 

How to interpret - Wind.  The wind barbs show the wind direction and strength.  Interpret the direction as if you were looking at a flat map.  A wind barb pointing down (i.e. vertical with the 'tail-feathers' at the top) would mean a wind from the north. A wind barb pointing to the right (horizontal with the 'tail-feathers' on the left) would mean a wind from the west.  The 'tail-feathers' tell you how strong the wind is, one long slash is 10 knots, 2 long slashes = 20 knots, a short or half slash = 5 knots.  So three long and one short slash = 35 knots.   A triangle = 50 knots.  If you look at the main image again, in the dry air aloft from about 500 mb up to 350 mb, and from about 12Z (6 AM) Saturday onward, the winds aloft are from the northwest from 30 to 55 knots.

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5.  How do we show fronts, trough axes, and ridge axes on our weather charts?

In the northern hemisphere, upper level troughs appear from the side generally as a "U" shaped feature, or like the low spot or 'trough' in-between ocean waves.  In the diagram below left, the trough describes a "U" shape.  An upper level ridge would be like the crests of a waves in the same image, describing an upside down "U".

wave       map

The image above right shows an upper level trough on a 500 millibar (MB) weather chart.  Note the general "U" shape described by the yellow lines.  The red line in the image marks the center, or axis, of the upper level trough.    Below are two more examples of upper level troughs, each with the axis marked by a red line.

map  map

The image on the right shows just how small an upper level trough can be, but still has a faint "U" shape.  This small upper level trough just happens to be moving over the top of a big upper level ridge.  Upper level troughs can be found at all levels of the troposphere and can be found on all standard weather charts like 300 MB, 500 MB, 700 MB, and 850 MB charts.

Examples of upper level ridges can be seen below, along with a zig-zag light blue line that indicates the axis of the upper level ridge.  The left hand image shows a big upper level ridge.  The right hand image shows a somewhat smaller upper level ridge.

map  map

The image below left shows a very small upper level ridge.  Note that it still retains a faint upside down "U" shape even though it appears almost flat.  The image below right shows a larger upper level ridge, but this one is tilted to the right with the axis lying on a SW to NE line across the Pacific Northwest.

map  map

 

Upper Level Highs and Lows.

The only real difference between upper level troughs and and upper level lows is that the lowest value contour(s) in a low form a circle or some form of oval, or in weather-speak they have a 'closed contour'.  The only real difference between upper level ridges and and upper level highs is that the highest value contour(s) in a high form a circle or some form of oval, or in weather-speak they have a 'closed contour'.  Upper level lows may look like the image below left, and upper level highs like the image below right.

map  map

Meteorologists can sometimes use the trough vs. low and ridge vs. high terms interchangeably.  Especially with the large features the weather effects can be the same,  e.g. large lows often have the same effects as large troughs.
 

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6.  What is the difference between a shortwave and a longwave?  Or, what is the difference between a short-wave trough and a long-wave trough?

Waves that occur in the atmosphere are similar in many ways to waves that you can see at the ocean.  Both can be described by length and amplitude (or height).  Wavelength is the distance between similar parts of a wave, either crest-to-crest, or trough-to-trough.  You can see wavelength measured crest-to-crest in the image below.
wave

Short waves differ from long waves merely in the length of the respective waves.  The image below compares waves with shorter wavelengths to waves with longer wavelengths.  Note that both of the examples below have the SAME amplitude or height.

wavelength

The diagram below shows waves having different amplitudes.  The top waves have the smallest amplitude and the bottom waves have the greatest amplitude.  Note that all the waves, both top, middle and bottom all have the SAME wavelength.

wave

In the atmosphere, like the ocean, waves are all mixed together, long-waves, short-waves, waves with medium length and everything in-between.  There are several key points to remember when discussing long-waves and short-waves in the atmosphere,
         1) Long-waves are often both slow moving and changing
         2) Long-waves at mid-latitudes arranged around the hemisphere define a path or storm track
               that the short-waves follow.
         3) Short-waves are faster moving than long-waves and travel around the hemisphere by
              generally following the storm track that is defined by the long-waves (see below).
         4) Besides creating weather on their own, many short-wave troughs are associated with
              cold fronts, warm fronts, and occluded fronts. In fact, there will always be a short-wave
              trough driving any cold front, warm front, or occluded front that forms.
         5)  Not all short-wave troughs have fronts associated with them.  Most of the weaker
              short-wave troughs do not have fronts, but they still can create a lot of unsettled weather.

The animated image below demonstrates the third point.  Two short-wave troughs (denoted by the dashed red lines) are tracked as they move across North America.  Short-wave #1 actually splits into two separate troughs, 1a and 1b.  Note how the big upper level ridge near the west coast  (denoted by the zig-zag blue line) changes rather slowly.  Short-wave trough #2 ends up moving up over the top of the upper level ridge.

large map

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7. What are ensemble forecasts and how do forecasters use them?

Ensemble forecasting is when meteorologists run computer models multiple times at the same time, obtaining different results by slightly changing the initial conditions or observations that the model begins with. (Other ways to create an ensemble is to use different ways of calculating the physics that the model uses, or even using a few different models.) One reason this is done is because the sensors that obtain the initial measurements have a certain amount of uncertainty associated with them, so the ensemble creates a range of plausible solutions for the actual current state of the atmosphere. A meteorologist can then compare all the different model runs to help determine what the range of possible forecasts is, as well as help determine how confident they are about certain parts of the forecast. For example, part of an AFD might read, "Ensembles are in good agreement for high pressure persisting across the area for days 4 through 7...so confidence is high that our hot and dry pattern will continue."

There are multiple ways that meteorologists can look at these ensemble forecasts. One common way is to simply look at the ensemble mean, or average of all the model runs. Another common way is what is often referred to as a "spaghetti plot," where many model solutions are plotted on the same map. When these lines are close together, there is good agreement among the models and confidence is high. When there is poor agreement among the models, the plot becomes a lot more messy, and begins to resemble a bowl of spaghetti.

Below are two examples of ensemble forecasts for 500 mb heights, the first one being a 6 hour forecast, and the second one being a 288 hour forecast. Notice how the first one has great agreement among the models, whereas the second one has poorer agreement among the models, creating a more chaotic and uncertain look to the chart.

naefp run
naefs run

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8. How are the "point-and-click" forecasts created?

Each NWS forecast office creates and edits graphical forecasts for their area on a 2.5 x 2.5 km grid, with each grid box being assigned a unique value for each forecast element. (In the case of the Salt Lake City forecast area, this equates to more than 30,000 grid points.) These grids are created for many forecast elements, with the general public most frequently using elements such as temperature, wind direction and speed, probability of precipitation (POP), etc. When you click on our maps to get the forecast for your specific neighborhood, the program pulls the forecast from that particular grid box, and then decodes those values into a text forecast. An example of one of these graphical forecasts can be seen below.

grid map

Forecasters have a wide variety of tools for editing these grids, such as the ability to only edit certain areas at a time, tweak values at individual cities, copy in and edit grids directly from computer models, or nudge the forecast toward a particular computer model. These models are used extensively across the entire country, but even moreso in areas like Utah; in Utah, the complex terrain leads to wide variations in weather over very small distances, and the low population density in most areas leads to a decreased number of observations on the ground, so the models help forecasters deal with that complexity and fill in holes. So, whenever an AFD says that the meteorologist "lowered temperatures for Tuesday afternoon" or "increased POPs for tomorrow morning," they are indicating what changes they made to the graphical gridded forecast.

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For definitions of more words and phrases, you can check the NWS glossary.

If you have any questions or comments, you can email the webmaster.


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