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Notes on GFS forecast maps
The Global Forecast System (GFS) is a National Centers for Environmental Prediction (NCEP)
weather forecast model that generates data for dozens of atmospheric and land-soil variables,
including temperatures, winds, precipitation, soil moisture, and atmospheric ozone concentration.
The system couples four separate models (atmosphere, ocean model, land/soil model, and sea ice) that work
together to accurately depict weather conditions.
GFS is a global model with a base horizontal resolution of 0.25° (28 kilometers) between grid points. The GFS is run 4 times a day, the temporal resolution covers analysis and forecasts out to 16 days.
GFS Italy: Relative Humidity [%] and Vertical Velocity [Pa/s] at 850 hPa
VERSIONE ITALIANA: CLICCA QUI
These maps show the relative humidity (in %) and the vertical baric velocities (in Pa/s) at a 850 hPa pressure level,
corresponding to an altitude of about 1500 m (see the geopotential maps). The formation of low clouds in an area
occurs with high relative humidity values and negative vertical (omega) velocities. Humidity close to 100% is not necessary if at the same time there
are sufficient ascending motions (negative vertical speeds) which, cooling by expansion, can bring the air to saturation. Here are 3 conditions for
which an overcast sky is expected: 1) RH=60÷80%, Ω <-0.4 Pa/s;
2) RH>80%, Ω<-0.3 Pa/s; 3) RH>90%, Ω=-0.3÷0 Pa/s.
On the top left is shown the UTC initialization time. UTC time is indicated with Z letter (Zulu Time). On the top right is shown the UTC forecast time.
The maps at 00Z, 06Z, 12Z and 18Z are updated respectively starting from 04.30 AM, 10:30 AM, 4:30 PM, 10.30 PM (CEST TIime).
Forecast model: Global Forecast System (GFS) with 0.25°x0.25° (lat x lon) spatial resolution.
NOTE: in central European countries, when solar time is in effect, the CET time is used, which is equal to UTC +1 hour, while when daylight saving time is in effect, the CEST time is used, which is equal to UTC +2 hours. Right now it is 22:59 UTC.