--------------------
500mb Geopotential Heights, Height Change and Vorticity(
Source)
* Black contours indicate the geopotential height of the 500 millibar surface, in tens of meters.
o Low geopotential height (compared to other locations at the same latitude) indicates the
presence of a storm or trough at mid-troposphere levels.
o Relatively high geopotential height indicates a ridge, and quiescent weather.
* In the forecast panels, the colored contours indicate the change in geopotential height during
the 12 hours leading up to the valid time.
o Decreasing geopotential height usually indicates an approaching or intensifying storm.
o Increasing heights usually indicate clearing weather for the period.
* The color shading indicates vorticity at 500 millibars: Red for positive vorticity, blue for negative.
o Positive vorticity indicates counterclockwise rotation of the winds, and/or lateral shear of
the wind with stronger flow to the right of the direction of flow.
o Negative vorticity indicates clockwise rotation of the winds, and/or lateral shear of the wind
with stronger flow to the left of the direction of flow.
o Positive (or negative in the Southern Hemisphere) vorticity at 500 millibars is associated
with cyclones or storms at upper levels, and will tend to coincide with troughs in the
geopotential height field.
o Negative (positive in SH) vorticity is associated with calm weather, and will tend to coincide
with ridges in the geopotential height field.
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Sea Level Pressure and 1000-500mb Thickness (
Source)
* The colored contours indicate sea level pressure in millibars. High pressure is red, low pressure
in green or blue. Only the last 2 digits shown -- sea level pressure is usually around 1000
millibars, so add 1000 to values in the range of 00-50, and add 900 to values in the range of
50-98.
o Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea
level pressure indicates calm weather.
* The black contours indicate the vertical distance, or thickness, between the 1000 millibar
surface and the 500 millibar surface, measured in tens of meters.
o Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume
over a specified surface area, the thickness between two pressure levels is proportional to
the mean temperature of the air between those levels. Thus, low values of thickness mean
relatively cold air.
o The 540 line is highlighted, since this line is often used as a rule of thumb to indicate the
division between rain and snow for low terrain. When there is precipitation where the
thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is
usually rain (or sleet if the air next to the surface is below freezing).
--------------------
Vertical Velocity and Precipitation (
Source)
* The colored contours indicate vertical velocity of the wind at the 700 millibar level, in millibars
per hour (since pressure decreases with height, negative values indicate ascending air, and
positive values denote sinking).
o Ascending motion is associated with cloudiness and rain. Large negative values of vertical
velocity correspond to areas of heavy rainfall if moisture is available (see description of
panel 4). These areas tend to correspond with the storms in the first two panels.
* The green shading in the forecasts indicate 12 or 24 hour accumulated precipitation, measured
in millimeters.
o The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding
the verification time in the lower lefthand corner of the map.
o Comparison with the 540 thickness line in panel 2, and the 0ºC isotherm in panel 4 can give
a good indication of the dividing line between snow and rain.
--------------------
850mb Temperature, Humidity and Winds (
Source)
* Colored contours indicate the air temperature at the 850 millibar level, in degrees Celsius. The
0ºC contour is highlighted, as this is also often used as a divider between rain and snow.
* The green shading indicates the relative humidity percentage at the 850 millibar level. High
values indicate the availability of moisture. When large rates of ascent (in panel 3) are located
with high moisture availability, heavy rainfall will likely occur.
* The barbs indicate the direction and speed of the wind, in meters per second. Each full barb
indicates 10 m/s, and each half barb 5 m/s. The direction of the wind is parallel to the shaft
with the barbs pointing into the wind.
o Advection of moisture by the wind can be inferred by noticing the direction and rate at
which moist areas appear to be blown. Similarly, temperature advection can be inferred by
noticing whether the wind is blowing cold air toward a warm region, or warm air toward a
cold region.
--------------------
200mb Winds (
Source)
* Purple shading indicates the speed of the winds at the 200 millibar level, in meters per second.
This altitude is near the level of the core of the jet stream. So the tracks of the jet streams
can be seen very clearly.
* The streamlines indicate the direction of flow of the wind, which is generally from west to east
throughout most of the subtropics, mid- and high-latitudes.
* The color of the streamlines indicates a relative measure of divergence of the flow in the upper
troposphere. Orange and red indicates strong divergence at upper levels, usually associated with
strong vertical velocities in the middle troposphere, and severe weather/heavy rainfall.
--------------------
Precipitable Water and Convective Available Potential Energy (CAPE) or Total-Totals Index (TTI) (
Source)
* The colored contours indicate total precipitable water in the atmosphere. Precipitable water is
the total depth of liquid water that would result if all water vapor contained in a vertical column
of air could be "wrung out", leaving the air completely dry. It indicates the total humidity of the
air above a location, and is a good indicator of the amount of moisture potentially available to
supply rainfall.
* In the analysis and forecasts for the ETA model, the yellow-brown shading indicates the amount
of CAPE in the atmosphere, which is a good indicator of the potential for strong thunderstorms
and severe weather. High values of CAPE indicate that most (but not necessarily all) conditions
exist for strong thunderstorms.
* For the other models the yellow-brown shading indicates the value of the TTI, which is a
measure of the vertical stability of the atmosphere, and over central and eastern North America
is also a good empirical indicator of the potential for severe weather.
o TTI = TD[850]-T[500] + (T[850]-T[500])
where T is temperature, TD is dew point (both in Celsius), and the [number] is the pressure
level.
o Values of TTI of around 40-45 indicate the potential for thunderstorms. Around 50, severe
thunderstorms are possible. Around 55, storms producing tornados are possible. This
rule-of-thumb does not hold over western North America and other areas where there is
a lot of high terrain.
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