Information for Understanding Forecast Discussions

The Weather Models

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

Effective 6/20/06, the NAM model (North American Mesoscale) (formerly using the Eta physics)now uses WRF (Weather Research Forecast) physics. Improved accuracy in predicting thunderstorms and other convective weather. Still poor performance for tropical storms and other features, best handled by the GFS.

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. On 6/20/06, the NAM was converted from using Eta physics to WRF (Weather Research Forecast) model physics.

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NGM model is the Nested Grid Model, an outdated model, but still run for comparisons.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 DIGEX, Canadian GEMS, MM5, UKMET, ECMWF.

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.

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