Forecast Model Details

This document discusses the various types of products available in the NGM, ETA, Aviation and MRF model plots.

500 mb/SLP Plots

Sample MRF 500 mb Plot

The 500mb/SLP plots show two parameters: 500 mb heights (in color contours) and sea level pressure (in black line contours).

Sea level pressure
The sea level pressure field can be used to find low and high pressure systems as well as the location of cold fronts. The highs and lows can be located by the white H and L symbols on the map. Cold fronts will generally start from areas of low pressure and follow the trough of low pressure south and to the west of the low. Rain and/or snow is likely in the regions directly around the low and somewhat less likely along the cold front.

500 mb heights
The 500 mb heights are often referred to as the sterring winds of the atmosphere.   Following a specific color will show where the winds and thus pressure systems (cyclones and anticyclones) will move.  If the flow is from north to south, the weather will be genreally collder than normal.  If the flow is from south to north, the weather will be warmer than normal.

The heights can also be used to estimate surface temperatures since height of a pressure surface is related to the mean temperature of the air below it. Here is a rough estimate of temperatures based on color. The variability is on the order of +-10 degrees F:

Modify these by:

Sea Level Pressure Forecast

Sample Surface Plot

The pressure forecast chart includes three parameters: sea level pressure (cyan lines), 1000-500 mb thickness (brown dotted lines) and quantitative precipitation (color contours).

Sea level pressure field
The sea level pressure field will detail the location and strength of high and low pressure systems as well as locate warm, cold and stationary fronts. The pressure contours or isobars are drawn every 4 millibars. Cold fronts generally follow the pressure trough flowing south and west from the low pressure system. Warm fronts can be located at times as pressure troughs going east out of a low but are generally hard to find. Surface winds are related to the packing of the pressure contours (isobars). The tighter the packing, the stronger the winds are.

Quantitative precpitation field
The quantitative precpitation field shows estimated 12 hour precipitation (liquid equivalent) for the 12 hour period prior to the valid time. For example a 12 hour precipitation field valid 12Z THU 11 NOV 93 would show 12 hour accumulated precipitation from 00Z THU to 12Z THU. The contours follow this scale:

In addition, the precipitation forecast can be used to estimate snow fall. A general 10 to 1 ratio can be used for snowfall based on the liquid equivalent shown in this chart. Using the 850 chart freezing line to determine whether the precipitation is snow or not, you take the precipitation total at a location and multiply it by 10 to get potential snowfall.

1000-500 mb thickness field
Thickness lines are drawn in dotted brown at a 60 m interval. The 5100, 5400, and 5700 m lines are solid. This represents the thickness in meters between the 500 mb pressure surface and the 1000 mb surface. This is directly related to mean layer temperature and is often used to determine precipitation type. The 5400 thickness line is often referred to as the rain/snow line. Also, it is also known that surface pressure systems tend to move along thickness lines.

Surface Forecast (ETA only)

Sample surface Plot

The surface chart details near surface weather conditions and is intended to aid in estimation of surface conditions. The parameters plotted are temperature in Celsius (in color contours), convergence (black lines, interval=2, shaded > 0), dewpoints in Celsius (plotted in various colors) and winds plotted as vectors.

2 m above ground temperature field

The temperature field shows the location of warm and cold air near the surface of the earth and can be used to locate surface fronts or estimate high and low temperatures. It should be noted, these are rough temperatures are won't reflect exact surface tempreatures that would be reported at station locations.

10 m above ground convergence field
The convergence field shows where low level wind conditions are favorable for thunderstorm development. Positive areas (shaded) represent converging winds and result in forced upward movement of air. Thunderstorms can develop in areas where convergence is strongly positive (>2). Negative areas (non-shaded) represent diverging air which is often a result of descending air and indicates areas of clearing weather.

2 m above ground dewpoint field
The dewpoint field shows the amount of moisture in the atmosphere.  The higher the dewpoint, the higher the moisture content.  These are plotted as colored lines at 5 degree Celsius intervals:

Dewpoint temperatures are decent indicators of potential low temperatures. The morning temperatures will rarely drop below the dewpoint.  where moisture is significant enough to fuel thunderstorm development. 59 F or 15 Celsius is a cutoff for strong thunderstorm development. Areas of dewpoints greater than 20 C can generate air mass thunderstorms which often aren't reliant on low level convergence for initial development.

10 m above ground wind vectors
These are the estimate surface winds plotted as vectors.  This shows whether the low level flow is from the south where warmer more moist air would advect into the region or whether the flow is from the north where cooler drier air would advect in.

1000 mb Forecast

Sample 1000 mb Plot

The 1000 mb chart details near surface weather conditions and is plotted since most models don't report surface conditions. The parameters plotted are the same as the surface plot except for the level being 1000 mb. These include temperature in Celsius (in color contours), convergence (black lines, interval=2, shaded > 0), dewpoints in Celsius (colored lines, interval=5) and winds plotted as vectors.

1000 mb temperature field

The temperature field shows the location of warm and cold air near the surface of the earth and can be used to locate surface fronts or predict high and low temperatures. Low level instability can estimated by taking the 1000-850 mb temperature differences. A difference of 12 or greater indicates potential instability.

1000 mb convergence field
The convergence field shows where low level wind conditions are favorable for thunderstorm development. Positive areas (shaded) represent converging winds and result in forced upward movement of air. Thunderstorms can develop in areas where convergence is strongly positive (>2). Negative areas (non-shaded) represent diverging air which is often a result of descending air and indicates areas of clearing weather.

1000 mb dewpoint field
The dewpoint field shows where moisture is significant enough to fuel thunderstorm development. 55 F or 13 Celsius is a cutoff for strong thunderstorm development. Areas of dewpoints greater than 19 C can generate air mass thunderstorms which often aren't reliant on low level convergence for initial development.

1000 mb wind vectors
These are the estimated 1000 mb winds plotted as vectors.  This shows whether the low level flow is from the south where warmer more moist air would advect into the region or whether the flow is from the north where cooler drier air would advect in.

NOTE: Most of the western third of the country have surface pressures well below 1000 mb and these charts don't apply to those regions.

850 mb Forecast

Sample 850 mb Plot

The 850 mb chart details weather conditions at the 850 mb level or around 5000 feet above sea level. The parameters plotted are temperatures in Celsius (in color contours), heights in white lines and winds plotted as vectors.

850 mb temperature field

The temperature field shows where warm and cold air are located at the 5000 ft level. Temperatures at this level do not show the diurnal temperature changes from morning low to afternoon high we see at the earth's surface so warm and cold air advection can be more easily traced. You can estimate potential afternoon highs from these temperatures by adding: 15C in the summer, 12C in spring and fall, and 9C in the winter and converting to Fahrenheit. For example, if the 850 temp is 4C in summer, the afternoon temperature could reach 4+15=19C=67F. In the winter, 4+9=13C=55F. This algorithm does not work in the western third of the country due to its high altitude.

The 850 temperature is also a decent determiner of the type of precipitation. Since most precipitation forms at 5000 feet or above, a temperature of freezing (0 Celsius) or below would indicate snow whereas a temperature above freezing would indicate rain.

850 mb height field
The height field works very similar to the sea level pressure field. Lows and highs can found and compared to sea level locations. Strength of winds are again related to the packing of the height contours.

850 mb wind vector field
The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion. Upward motion is often linked to precipitation and downward to clear skies.

700 mb Forecast

Sample 700 mb Plot

The 700 mb chart shows the forecasted vertical velocities (colors), heights (solid white lines) and wind vectors.

700 mb vertical velocity field

The vertical velocity field shows the vertical motion in -mb/sec through the 700 mb surface. Positive values (greens, yellows and reds) are upward motion and negative values (blues and magentas) are downward motion. The unit of mb/sec is roughly equal to a cm/sec so it is easy to see that vertical motion is roughly two orders of magnitude smaller than horizontal motion. This comes from the fact that our atmosphere is generally stable and that upward motion is generally an artifact of precipitation releasing heat energy. In other words, moderate to strong positive areas (>5) reflect areas where there is precipitation or soon will be. Strongly positive areas (>10) generally indicate areas of potential thunderstorms. Negative areas show descending air which are generatlly associated to areas of fair weather and often clear skies. If precipitation is occurring in these areas it will end soon. Sometimes descending air will trap cloudiness below it and result in a low overcast that can linger for days.

700 mb height field
The height field works very similar to the sea level pressure field. Lows and highs can found and compared to sea level locations. Strength of winds are again related to the packing of the height contours.

700 mb wind vector field
The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion.

500 mb Forecast

Sample 500 mb Plot

The 500 mb chart shows the absolute vorticity (in color contours), the height field (in white lines) and wind vectors.

500 height field
The 500 mb level is often refered to as the steering level as most weather systems and precipitation follow the winds at this level. The winds follow the height contours and generally run from 30 to 100 knots. The speeds can be roughly estimated from the 300 mb winds as roughly 2/3rds their magnitude. This level averages around 18,000 feet above sea level and is roughly half-way up through the weather producing part of the atmosphere called the troposphere.

500 mb vorticity field

The vorticity field shows small eddies in the atmosphere that generally are not detectable on the standard height field. Values of 14 (green) or higher highlight those eddies. The higher the vorticity, the stronger the eddy. These eddies can help strengthen surface low pressure systems and induce precipitation and are often used as a predictive tool.

500 mb wind vector field
The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion.

300 mb Forecast

Sample 300 mb Plot

The 300 mb chart shows the winds at the 300 mb level. The wind speeds are shown in color contours, the heights in solid white lines and the vectors show wind direction and speed.

300 mb wind speed field

The 300 mb chart investigates the upper portion of the troposphere where most of the weather producing phenomena occur. This level lies around 30,000 feet up and is often referred to as the jet stream level. Areas of strong winds at this level (winds > 100 knots or 115 mph), commonly referred to as jets, indicate areas of high atmospheric energy. These are created by strong temperature contrasts in the lower and middle tropopause and reflect areas of potential storm development. A strong jet (winds > 130 knots) can indicate the potential for the development of a strong low pressure system especially when if moves over the Rocky mountains and into the Plains states. A strong jet hitting the west coast can indicate the existence of a strong precipitation producing system.

300 mb wind vector field
The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion. Upward motion is often linked to precipitation and downward to clear skies.

Relative Humdity/Lifted Index Forecast

Sample RH-LI Plot

The Relative Humidity/Lifted Index chart depicts two fields: integrated relative humdity from 850 to 500 mb (in color contours) and lifted index (in white line contours).

1000-500 integrated relative humidity field

The RH field is a good predictor of cloud location and thickness. Areas of RH <60% generally are clear or have partly cloud skies. Areas of 60-80% are generally overcast or mostly cloudy. Areas greater than 80% are overcast with a high likelihood of precipitation as RH approaches 100%.

Lifted index field
The LI field shows instability in the atmosphere. Where LIs are <0 (also highlighted in a gray shading), thunderstorms are possible. The lower the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms could become. Values of -4 or lower indicate areas where severe thunderstorms are possible. Values > 10 indicate areas of stable weather where skies are generally clear.

NOTE: For the aviation and MRF models, the lifted index field is not broadcast so the showalter index (a rough equivalent) is plotted instead.

CAPE Plots (ETA only)

CAPE represents the amount of energy a parcel might have if it were lifted. Often this reflects the strength of updrafts within a thunderstorm. CAPE values of greater than 2000 represent enough energy to produce thunderstorms. A value greater than 3000 represents enough energy to produce strong thunderstorms. Values < 1000 denote a reletively stable atmosphere.

Helicity Plots (ETA only)

Helicity is used to indicate where rotation/shear is high enough to allow thunderstorms to organize into severe or supercell storms. In the lack of helicity, storms develop vertically and the precipitation will snuff out the updraft killing the thunderstorm. Severe storms need helicity to maintain an organized structure allowing hte storm to develop to severe limits. A value of 400-500 is often needed to produce severe storms.   Often this is used in conjunction with CAPE to determine severe storm location.

Energy Helicity Index (EHI) Plots (ETA only)

The Energy Helicity Index (EHI) is a combination of CAPE and Helicity to show where there is a potential for strong and possibly tornadic thunderstorms may form.  CAPE shows the ability of the atmosphere to develop a thunderstorm.  Strong thunderstorms need shear or rotation which is reflected in the Helicity value.  The combination of the two not only shows where thunderstorms will develop but also where shear/rotation in the atmopshere is strong enough for tornado formation.  Typical thunderstorm producing values are around 1-2.  A value greater than 3 indicated the potential for strong thunderstorms.  A value greater than 4 indicates the potential for tornado formation within those storms.

4 Panel Plots

Sample 4 panel Plot

This is a combination of the following plots:

850 mb plot in the upper left
300 mb plot in the upper right
Sea level pressure plot in the lower left
Relative humidity/lifted index plot in the lower right