Forecast Model Details - Eta Model
 
 

Surface Forecast

2 meter above ground temperature field - The temperature field shows the location of warm and cold air near the surface of the earth and can be used to locate surface fronts or estimate high and low temperatures.  It should be noted, these are rough temperatures and won't reflect exact surface temperatures that would be reported at station locations.  Temperatures are plotted (Celsius) as color contours.

10 m above ground convergence field - The convergence field shows where low level wind conditions are favorable for thunderstorm development.  Positive areas (shaded) represent converging winds and result in forced upward movement of air.  Thunderstorms can develop in areas where convergence is strongly positive (>2).  Negative areas (non-shaded) represent diverging air which is often a result of descending air and indicates areas of clearing weather.

2 m above ground dewpoint field - The dewpoint field shows the amount of moisture in the atmosphere.  The higher the dewpoint, the higher the moisture content.  These are plotted as colored lines at 5 degree Celsius intervals,

     - thick gray                -30 C
     - thin gray            -25 and -20 C
     - thick magenta          -15 C
     - thick red                     0 C
     - thin red                   5 and 10 C
     - thick white                15 C
     - thick orange              20 C
     - dashed thick orange  25 C

Dewpoint temperatures are decent indicators of potential low temperatures.   The morning low temperature will rarely drop below the dewpoint.  Where moisture is significant enough to fuel thunderstorm development, 59 F (or 15 C) is a cutoff for strong thunderstorm development.  Areas of dewpoints greater than 20 C can generate air mass thunderstorms which often aren't reliant on low level convergence for initial development.

10 m above ground wind vectors - These are the estimated surface winds plotted as vectors.  This shows whether the low level flow is from the south where warmer more moist air would advect into the region or whether the flow is from the north where cooler drier air would advect in.
 
 

850 mb Forecast

850 mb temperature field - The temperature field shows where warm and cold air are located at the 5,000 ft. (msl) level.  Temperatures at this level do not show the diurnal temperature changes from morning low to afternoon high we see at the earth's surface so warm and cold air advection can be more easily traced.  You can estimate potential highs from these temperatures by adding:

     15 C in the summer
     12 C in the spring
     9 C in the winter

The 850 mb temperature is also a decent determiner of the type of precipitation.  Since most precipitation forms at 5,000 ft. or above, a temperature of freezing (0 C) or below would indicate snow whereas a temperature above freezing would indicate rain.

850 mb height field - The height field works very similar to the sea level pressure field.  Lows and highs can be found and compared to sea level locations.  Strength of winds are again related to the packing of the height contours.

850 mb wind vector field - The vector field shows wind direction and speed.  Often this can be used to qualitatively show areas of convergence and divergence.  In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from convergence) or downward (from divergence) air motion.  Upward motion is often linked to precipitation and downward to clear skies.
 
 

700 mb Forecast

700 mb vertical velocity field - The vertical velocity field shows the vertical motion in -mb/sec through the 700 mb level.  Positive values (greens, yellows and reds) are upward motion and negative values (blues and magentas) are downward motion.  The unit of mb/sec is roughly equal to a cm/sec so it is easy to see that vertical motion roughly two orders of magnitude smaller than horizontal motion.  This comes from the fact that our atmosphere is generally stable and that upward motion is generally an artifact of precipitation releasing heat energy.  In other words, moderate to strong positive areas (>5) reflect areas where there is precipitation or soon will be.  Strongly positive areas (>10) generally indicate areas of potential thunderstorms.  Negative areas show descending air which are generally associated with areas of fair weather and often clear skies.  If precipitation is occurring in these areas it will end soon.  Sometimes descending air will trap cloudiness below it and result in a low overcast that can linger for days. (Note that 1 cm/sec is approximately 2 ft/min.)

700 mb height field - The height field works very similar to the sea level pressure field.  Lows and highs can be found and compared to sea level locations.  Strength of winds are again related to the packing of the height contours.

700 mb wind vector field - The vector field shows wind direction and speed.  Often this can be used to qualitatively show areas of convergence and divergence.  In the middle and upper levels of the troposphere, this can be an indicator of existing upward and downward air motion.

Ref. Unisys

    http://weather.unisys.com/model/details.html

NAM Model Sounding and Metogram Data:

     http://www.arl.noaa.gov/ready/cmet.html#modelgraphics
 

Weather Models

While not overly scientific, the following descriptions are fairly accurate and should prove helpful:

Effective 1/25/05, the Eta is now known as the NAM (North American Mesoscale) and the AVN/MRF is known as the GFS model (Global Forecast System)

GFS model stands for the Global Forecast System model...the newest model version of what used to be called the AVN/MRF model (AVN=aviation, MRF= medium range forecast model). The GFS is a global spectral model and predicts for the many regions of the world. It's a relatively coarse model, not as fine a resolution as the NAM described below. Another subset of the model, the GFSX (X for extended) also predicts over longer ranges, as much as 144 hours to 384 hours into the future. It's run 4 times a day. While not as 'fine' a resolution as the NAM model, the GFS model has wonderful features and is the one I usually bet on.

NAM stands for the North American Mesoscale model. Previously known as the Eta model, it was renamed on 1/25/05. Eta wasn't an acronym...it's the name of the model's vertical axis of its mathematical step function. The NAM has different physics than the GFS and is a higher resolution, model. It attempts to predict in areas as small as several miles wide. In meteorology, that's referred to as a "mesoscale" area. (Thunderstorms are mesoscale phenomena.) It only predicts for North America. It's run 4x daily and the usual form predicts 84 hours into the future. Experimental extended versions are being developed.

NGM model is the Nested Grid Model, an outdated model, but still run for comparisons. Development on NGM was ceased in 1991. The NGM was developed from the first real weather model of the 70s and 80s...the LFM model (Limited Fine Mesh). The NGM is run twice daily and is used as a comparison model. It likely will be discontinued soon. 

NOGAPS model is the model developed and used by the US NAVY. Stands for the Navy Operation Global Atmospheric Prediction System. A global model, somewhat similar in coverage as the GFS, but different physics. Run twice a day and extends out to 144 hours, similar to the GFS. I find it very useful for hurricane predictions 

GFS MOS and the NAM (Eta) MOS: MOS stands for Model Output Statistics. MOS data are forecasts for up to 84 hours (NAM and GFS) or 144 hours (GFSX). Gives temperature, humidity and precipitation probabilities every 3, 6 and 12 hours, depending upon the specific MOS. MOS forecasts use historical information and reinterpret the raw data for specific locations. MOS forecasts do not correct for model biases. 

RUC model is a short term model and stands for the Rapid Update Cycle. The standard RUC model is based on the NAM data system and is updated every hour for periods up to 12 hours. It basically reiterates a new forecast based on the current conditions, taking into account what was predicted and what's actually happening. It uses its own physics, different than the NAM. Other versions extending 24 hours and longer are being developed. I find the RUC not very useful, except for predicting mixed precipitation and changeover of snow to sleet.

Other models include the Canadian GEMS, MM5, UKMET, ECMWF and Christie Brinkley :-).
 

All models have their specific biases, inaccuracies, etc. New forms of each model are constantly being developed and tested.
 

The Acronyms and other meteorological concepts

QPF is "quantity of precipitation falling". Most of the models predict the amount of precip that will fall in a given period of time, based on the total amount of available moisture that can precipitate (PWAT = precipitable water) and the physical conditions (lift, convection, etc.) that will cause it to precipitate. The amount of snow (snow:water ratio) is usually calculated by multiplying the QPF in inches by a factor of 12-20, depending upon the temperature.

Atmospheric Thickness Levels: Heights in the atmosphere are often measured based on where the pressure is a constant value. The 'thickness' of the atmosphere is the three dimensional depth of the atmosphere between two pressure values. A useful thickness value is the thickness (or depth) of the atmosphere between the pressure of 500 millibars (mb) (about 18,000 feet) and 1000 mb (millibars) (about the earth surface near sea level). The thickness values become higher when the upper atmosphere is warmer and become lower when the upper atmosphere is colder. Thickness values are useful in predicting rain/snow or sleet. They correlate with temperatures at specific heights that are correlated with snow or sleet or rain. 

Useful Temperature Levels: 800 millibars and 900 millibars- Both of these levels must be at or below 0 degrees C (freezing) for snow to form.
 

Email questions to:    glenn@theweatherguy.net