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Variable Info



Absolute Vorticity

Absolute vorticity is the strength of rotation present at a given moment. Positive values represent counterclockwise rotation, and negative values represent clockwise rotation. Highly positive values often represent storm systems. On our weather maps, absolute vorticity is given in units of .00001 hertz.


CAPE

CAPE, or Convective Available Potential Energy, is a measurement of the instability of an air mass at a given moment. Large CAPE values are a good indication that severe thunderstorms containing large hail, strong winds, and tornadoes are likely. On our weather maps, CAPE is given in joules per kilogram.


Cloud Cover

Cloud cover is the amount of the sky in a given area that is covered by clouds at a given moment. On our weather maps, cloud cover is given as a percentage.


Convective Precipitation

Convective precipitation is the amount of precipitation caused by unstable air in a given period. Convective precipitation is commonly associated with thunderstorms. Convective precipitation rate is the rate at which convective precipitation is falling at a given moment. On our weather maps, convective precipitation is given in millimeters or inches, and convective precipitation rate is given in millimeter per hour or inches per hour.


Dew Point

Dew Point is the temperature at which a given parcel of air will produce condensation at a given moment. Dew point is a measure of the absolute humidity of the air. On our weather maps, dew point is given in degrees Celsius or degrees Fahrenheit.


Geopotential Height

Geopotential height is the altitude at which the air is at a given pressure at a given moment. When meteorologists mention "upper-level low pressure systems", they are referring to low geopotential heights, usually at the 500mb level. Low geopotential heights are often associated with storm systems and cold air. Geopotential height anomaly is the departure of the geopotential height from average at a given level. On our weather maps, geopotential height and geopotential height anomaly are given in decameters.


Lifted Index

Lifted index is a measurement of the stability of an air mass at a given moment. Positive values denote stable air masses, and negative values denote unstable air masses. Negative values are an indication that strong thunderstorms may occur in the area. On our weather maps, lifted index is given in kelvins.


Maximum/Minimum Temperature

Maximum temperature and minimum temperature are the maximum and minimum temperatures reported during a period of time. Maximum temperature can also refer to the maximum temperature from 2m above the surface through the 500mb level at a given moment. This definition of maximum temperature is valuable for determining a rough snow/non-snow line. On our weather maps, maximum temperature and minimum temperature are given in degrees Celsius or degrees Fahrenheit.


Mean Sea Level Pressure

Mean sea level pressure is the atmospheric pressure adjusted for altitude at a given moment. When weather reports mention surface pressure or barometric pressure, they are usually referring to mean sea level pressure. On our weather maps, mean sea level pressure is given in millibars or inches of mercury.


Perceived Temperature

Perceived temperature is the temperature at a given moment as perceived by a human being (not the actual air temperature). The perceived temperature map is a combination of two non-overlapping factors: heat index and wind chill. In hot weather, heat index, which is based on temperature and humidity, is used. In cold weather, wind chill, which is based on temperature and wind speed, is used. On our weather maps, perceived temperature, heat index, and wind chill are given in degrees Fahrenheit. Celsius maps for these variables are not available on this site since these variables are based on formulas only used in the United States.


Precipitable Water

Precipitable water is the amount of precipitation that would fall if all of the water vapor in the troposphere at a given moment were to condense and precipitate. High precipitable water values often indicate that flooding may occur if rain is forecast. On our weather maps, precipitable water is given in millimeters or inches.


Precipitation

Precipitation is the total liquid content of rain, snow, sleet (ice pellets), and freezing rain which falls out of the atmosphere in a given time period. Precipitation rate is the rate at which precipitation is falling at a given moment. On our weather maps, precipitation is given in millimeters or inches, and precipitation rate is given in millimeters per hour or inches per hour.


Precipitation Type

Precipitation type is the type of precipitation to be expected at a given moment. On our weather maps, precipitation type is given as a categorical measurement, with green for rain, blue for snow, orange for ice pellets, and pink for freezing rain. Individual weather maps for categorical rain, categorical snow, categorical ice pellets, and categorical freezing rain are also available.


Relative Humidity

Relative humidity is the amount of water vapor in the air at a given moment as compared to the maximum amount of water vapor that air at the pressure and temperature that it is at can typically hold. On our weather maps, relative humidity is given as a percentage.


Simulated Reflectivity

Simulated reflectivity is the radar image at a given moment. On our weather maps, simulated reflectivity given in decibels.


Snow Cover

Snow cover is the area on which there is at least 1 centimeter of snow.


Snow Depth

Snow depth is the amount of snow on the ground at a given moment. Snow depth change is the change in snow depth in a given period. Snowfall increases snow depth, causing snow depth change to be positive; and melting and compaction decrease snow depth, causing snow depth change to be negative. On our weather maps, snow depth and snow depth change are given in centimeters or inches.


Snow Depth Water Equivalent

Snow depth water equivalent is the amount of water that would result from melting all snow that is on the ground at a given moment. On our weather maps, snow depth water equivalent is given in millimeters or inches.


Snowfall

Snowfall is the amount of snow which falls to the ground in a given time period. Snowfall rate is the rate at which snow is falling at a given moment. On our weather maps, snowfall is given in centimeters or inches, and snowfall rate is given in centimeters per hour or inches per hour.


Temperature

Temperature is the temperature at a given level of the atmosphere at a given moment. Temperature change is the change in temperature over a given time period. On our weather maps, temperature is given in degrees Celsius or degrees Fahrenheit.


Thickness

Thickness is the difference between the geopotential heights at two levels of the atmosphere at a given moment. The black line on each map denotes roughly where the freezing line is for that section of the atmosphere. Thickness anomaly is the departure of the thickness from average. On our weather maps, thickness and thickness anomaly are given in decameters.


Vertical Velocity

Vertical velocity, also called omega, is the speed of air motion in the vertical direction. Negative values represent rising air, and positive values represent falling air. On our weather maps, vertical velocity is given in decipascals per second.


Wind

Wind is the sustained wind speed and direction at a given level of the atmosphere at a given moment. Wind gust is the speed of wind gusts at a given moment. On our weather maps, wind speed and wind gust are given in knots or miles per hour.

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Map Status


Model Run Hour
GFS 12Z 384
NAM 12Z 084
RAP 18Z 021
ECMWF 12Z 240
GEFS 12Z 384
CMC 12Z 240
ECMEPS 00Z 240

Latest News

2017-01-02: Extended RAP maps to forecast hour 21.