GFS (Global Forecast System) Global Model from the "National Centers for Environmental Prediction" (NCEP)
4 times per day, from 3:30, 09:30, 15:30 and 21:30 UTC
Greenwich Mean Time:
12:00 UTC = 13:00 BST
0.5° x 0.5° for forecast time <= 180 hrs
2.5° x 2.5° for forecast time > 180 hrs
CAPE and vertical velocity at 700 hPa
The Convectively Available Potential Energy (CAPE) map - updated every 6 hours - shows the modelled convectively available
potential energy. CAPE represents the amount of buoyant energy (J/kg) available to accelerate a parcel vertically, or the amount of work
a parcel does on the environment. The higher the CAPE value, the more energy available to foster storm growth. The
potential energy can be converted to kinetic energy reflected in upward motion.
It should be remembered that CAPE represents potential energy, and will only be used should a parcel be lifted to the level of free convection.
When values are above 3500 j/kg and storms do develop, they may build rapidly and quickly become severe.
Often these storms are referred to as "explosive storms" by chasers and professionals. In a high CAPE environment
storms that develop can usually be seen by the human eye as rising rapidly.
Higher CAPE typically involves stronger storms with a higher chance of large hail and other severe weather. Note that
CAPE is usually of lesser importance than the vertical shear environment for tornadoes. The probability of large hail increases
with CAPE, given at least moderate shear(values around 500-1000 J/kg are sufficient).
CAPE is very sensitive to small differences in the moisture and temperature profiles. While the maps indicate
1000 J/kg CAPE at some location, a skew-T thermodynamic diagram
at that location may indicate 500-1500 J/kg.
(Source: The Lightning Wizard
Table 1: Characteristic values for CAPE
| 3500 +
|| Extremely Unstable
The Global Forecast System (GFS
) is a global numerical weather prediction computer model run by NOAA. This mathematical model is run four times a day and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time it is widely accepted that beyond 7 days the forecast is very general and not very accurate.
The model is run in two parts: the first part has a higher resolution and goes out to 180 hours (7 days) in the future, the second part runs from 180 to 384 hours (16 days) at a lower resolution. The resolution of the model varies in each part of the model: horizontally, it divides the surface of the earth into 35 or 70 kilometre grid squares; vertically, it divides the atmosphere into 64 layers and temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour.
Numerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. Although the first efforts to accomplish this were done in the 1920s, it wasn't until the advent of the computer and computer simulation that it was feasible to do in real-time. Manipulating the huge datasets and performing the complex calculations necessary to do this on a resolution fine enough to make the results useful requires the use of some of the most powerful supercomputers in the world. A number of forecast models, both global and regional in scale, are run to help create forecasts for nations worldwide. Use of model ensemble forecasts helps to define the forecast uncertainty and extend weather forecasting farther into the future than would otherwise be possible.
Wikipedia, Numerical weather prediction, http://en.wikipedia.org/wiki/Numerical_weather_prediction
(as of Feb. 9, 2010, 20:50 UTC).