GFA – Clouds and Weather

General

The graphic area forecast (GFA) clouds and weather chart provides a forecast of cloud layers and/or surface-based phenomena, visibility, weather and obstructions to vision at the valid time indicated. Relevant synoptic features that are responsible for the portrayed weather are also depicted, with an indication of their speed and direction of movement at the valid time.

Synoptic Features

The motion of synoptic features when the speed of movement is forecast to be 5 kt or more will be indicated by an arrow and a speed value. For speeds less than 5 kt, the letters QS (quasi-stationary) are used. The image below depicts the various synoptic features that may be found on any given chart.

Organized and Unorganized Cloud Coverage

The bases and tops of clouds between the surface and 24,000 ft ASL are indicated on the GFA clouds and weather chart. Only the tops of convective clouds are indicated above 24,000 feet ASL and cirrus clouds are never depicted. A scalloped border encloses organized areas of clouds where the sky condition is either broken (BKN) or overcast (OVC)

Frequently Used Abbreviations

Convective and non-convective cloud and precipitation are depicted using the following abbreviations:

Precipitation

Areas of precipitation are shown in one of two ways. Showery or intermittent precipitation is depicted as a hatched area enclosed by a dashed green line while continuous precipitation is shown as a dotted area enclosed by a solid green line. The prevailing visibility, obstruction to vision and intensity of precipitation are indicated within the shaded area as shown below.

Obstruction to Vision

Areas of obstruction to vision not associated with precipitation, where visibility is 6 SM or less, are enclosed by a dashed orange line.

Freezing Precipitation

Areas of freezing precipitation are depicted in red and enclosed by a solid red line.

Surface Based Layers

The vertical visibility within surface-based layers is measured in hundreds of feet AGL. For example, local obscured ceilings with a vertical visibility of between 400 and 600 ft AGL would be indicated as follows:

LCL CIGS 4–6 AGL

Visibility

The forecast visibility is measured in statute miles. When the visibility is expected to be greater than 6 SM, it is indicated as P6SM.

Winds

The speed and direction of forecast surface winds with a sustained speed of at least 20 kt are indicated by wind barbs. When accompanied by strong gusts, mean sustained winds of less than 20 kt may also be included, at the forecaster’s discretion, if moderate mechanical turbulence is expected to occur as a result of the wind gusts. Wind gusts are indicated by the letter “G,” followed by the peak gust speed in knots. In the following example, the surface wind is forecast to be from the west (270°T) with a speed of 25 kt and a peak gust speed of 35 kt.