Frequently Asked Questions (FAQs)

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

GFS, GFS90, GFS40, NAM, NAM40, NAM12, ECMWF, Canadian, WRF-GFS, MM5-NAM, RUC, etc.  Obviously you are dealing with NOAA, which could really mean the "National Organization for the Advancement of Acronyms rather than the "National Oceanic and Atmospheric Administration."  All of these acronyms are  names of complex computer models that forecasters may use to aid them in preparing a weather forecast.  These models are run by different organizations but the data is shared.  The GFS stands for Global Forecast System and is one of the two main models run by the National Weather Service (NWS).  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 Model.   The NAM is a mesoscale regional model, meaning that it has higher resolution that 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.

For many years, the University of Washington has run high resolution computer model.  They started with the MM5 model and are now using the WRF model as well.  The computer models they run are 'initialized', or start with the initial conditions from either the GFS or NAM model, then their models continue on with their own forecasts.  The WRF-GFS and MM5-NAM run in 36 km, 12 km, and 4 km resolutions. 

2.  What is 500 MB, 700 MB?  Why do we use them?

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:
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 and vertical motion in the atmosphere that may impact surface weather
850 MB   about  5,000 Feet         Good for viewing moisture, temperature, temperature advection, and lower level winds.
1000 MB  near the surface            The closest common pressure level that is near the surface
MSL  (mean Sea Level)                This level is commonly used for identifying fronts, surface lows and highs, forecasting surface winds
                                                         Also called MSLP (mean sea level pressure) in graphics.

2a.  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.  Why use some oddball unit like DAM?  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. 

Other units used are millibars (MB or mb), which is a measure of pressure and celcius (C) which is a measure of temperature.   Meteorology is a science (despite the jokes about tea leaves, dice, chicken bones, trick knees, random number generators, etc. when discussing weather forecasting), and as such uses the base 10  MKS (or Meters, Kilograms, seconds) unit system rather than english units.   Weather observations still have the aircraft altimeter setting in inches of mercury and the temperature in Fahrenheit in deference to our customers, but computer model forecast charts all use MB, C, and DAM. 
 

3.  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 that was used in an Enhanced AFD on May 6th, 2008.

 

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 1000 mb is the lowest level shown and is nominally used as the surface.  850 mb is approximately 5000 feet, 700 mb ~ 10,000 feet, 500 mb ~ 18,000 feet, and 300 mb ~ 30,000 feet.   Here is a curve-ball you wouldn't normally expect, time begins on the right side and advances going to the left.  Why this local convention?  Because weather frequently moves over western Washington 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.
                                        06 . 18         06 is day 6 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 Tue       18Z Tue is the time the the values above that point are valid for.  18Z is 11 AM PDT.

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



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 green 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 air mass is quite moist, with relative humidity 80% or more from the surface up to a little above 850 mb.  In fact if you interpolate this you could say that in the first 24 hours of the forecast the moist air (> 70% RH) is below about 7000 feet to 8000 feet.  Above that, the air mass dries out, with green and red colors showing RH to be 50% and below.  Looking at the text (in yellow) annotated onto the main image, the yellow arrow points to the Wednesday afternoon period where there is a small drying trend near the surface.  These relative humidity time-height sections can be very useful for forecasting cloud ceiling heights for aviation.  The interpretation of the minor decrease in RH Wednesday afternoon would be that ceilings rise to maybe 3500 feet or 4000 feet in the afternoon.

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 700 mb up to 400 mb, and from about 18Z (11 AM) Wednesday onward, the winds aloft are from the northwest 50 knots and higher.

4.  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".

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.

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.

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.

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.

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.
 

5.  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.
                                      

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.

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.

                                 

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.

The End

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