About the models

The Alaska Experimental Forecast Facility (AEFF) produces forecast plots of the ETA and MRF model data. The MRF plots are produced once a day using NCEP global hemispheric data. MRF forecast data extends out to 10 days. The resolution of the MRF is relatively coarse, therefor synoptic features influenced by topography may be washed out. The ETA plots have better spatial resolution and topography. The ETA forecasts are produced four times daily at approximately 4 hrs. after the model initialization time with forecasts out to 60 hrs. The 0600 z and 1800 z runs are not run from new sounding data they are initialized on new surface dat where available and the previous 6 hr forecast from the preceding run.

Surface

The surface plot displays 4mb contours of sea-level pressure, a vector representation of surface winds, the surface temperatures are indicated by shading. The model output winds are representative of winds above the lowest levels of the boundary layer, real local surface winds may be influenced by local topography and local circulations that may not be captured by the model. The sea-level pressures are adjusted sea-level pressures. i.e. If the surface of the earth was at sea-level over the entire grid, this is the expected pressure field. The sea-level pressure field becomes noisy and unreliable near complex terrain. For this reason the 850 mb pressure field often gives a clearer depiction of low pressure systems over complex terrain. The surface temperatures are predicted based on model topography and surface type. Since the resolution of the model terrain is coarse, local temperature fields are likely to vary considerably from model output. In general the surface plots need to be viewed as a smoothed and general representation of surface conditions. Considerable caution should be used in predicting fine-scale phenomena and conditions from the represented fields.

1000-850 mb thickness plot

The 850 mb plot indicates the height of the 850 mb pressure surface as contours in meters. Mean winds for the 1000-850 mb layer are shown using wind vectors. The shading of the plot indicates the thickness of the 1000-850 mb layer. The thickness value is indicative of the average temperature of the layer. Warm layers are associated with greater thickness values. Strong temperature gradients associated with fronts are often visible as gradients in thickness in the lowest 5000' (1500m) of the atmosphere. The thickness field can also be used with the wind field to diagnose areas of warm and cold advection. Wind vectors crossing thickness contours from low to high (cold to warm) indicate areas where cool air is moving towards an area of relative warmth (cold advection) or vice versa (warm advection).

700 mb plot

The 700 mb plot uses a grayscale shading to represent relative humidities of 50, 70 and greater than 90 percent on the 700 mb pressure surface. Winds at 700 mb or 10,000' (3000m), the ridge-top-level, are indicated by vector symbols. Contouring indicates the magnitude of omega or vertical velocity in units of mbars/sec. Negative values of omega are associated with upward motion. Regions of upward motion and relative humidity greater than 70% is commonly associated with precipitation and cloudiness at 700 mb.

850-500 mb thickness plot

This plot is contoured with the 700 mb heights. The shading indicates the thickness of the 850-500 mb layer. The thickness value is indicative of the average temperature of the layer. Warm layers are associated with greater thickness values. The wind vectors represent the winds on the 700 mb surface, or a good approximation of the mean wind for the 850-500 mb layer. The thickness field can also be used with the wind field to diagnose areas of warm and cold advection. Wind vectors crossing thickness contours from low to high (cold to warm) indicate areas where cool air is moving towards an area of relative warmth (cold advection) or vice versa (warm advection). This depiction allows easy recoginition of upper level fronts that are likely to be areas of increased turbulence due to wind shear along the thermal boundaries.

500 mb plot

The 500 mb plot indicates heights of the 500 mb pressure surface as contours in meters. Winds are indicated by wind vectors. Shading indicates values of absolute vorticity. The 500 mb level is approximately half-way between the surface and the top of the atmosphere. It is a level generally considered to be unaffected by surface frictional effects. Vorticity, a measure of the rotation of a fluid, is most often calculated on this level. Positive vorticity at 500 mb is associated with upper-level diffluence. Therefore the presence of larger values of absolute vorticity is indicative of areas of upward motion, cloudiness an precipitation.

500 mb temperature plot

The 500m\ mb temperature plot is useful for recognizing cold air advection at 500 mb. Cold air advection at the 500 mb level is commonly associated with favorable conditions for development and deepening of storm systems at the surface.

Upper level fronts are easily recognized on these plots.

300 mb plot

The 300 mb level is near the height of the polar jet. Wind direction is indicated by vectors symbols pointing downstream. The shading indicates contours of windspeed. Jet entrance and exit regions are easily recognized in this representation. The right entrance (air entering the wind maximum) and left exit (air leaving the velocity maximum) regions have been shown to be characterized by increased divergence due to the ageostrophic circulations associated with accelerating and decelerating air parcels along the jet. This upper level divergence is often found during in regions where fronts and low level circulations form.