Surface Temperature - Air temperature at 10-meter height or 30 feet above ground-level, in degrees Fahrenheit.

Surface Dew point - Dew point temperature at 10-meter height or 30 feet above ground-level, in degrees Fahrenheit. The dew point is a relationship between relative humidity (see below) and the air temperature (see above). It is the temperature at which condensation (dew or frost) occurs. The dew point temperature is useful to predict overnight low temperature and temperature changes during precipitation. A large difference between the dew point and air temperature indicates low humidity. If the dew point is equal to the air temperature the humidity is 100%. The dew point temperature can never be greater than the air temperature and it does not vary with temperature, as does relative humidity. Since dew point is a direct measure of the actual moisture content in the air, it is the preferred unit of moisture measurement in meteorology.

Surface Relative Humidity and Winds - Relative humidity (RH), expresses a measure of the amount of water in the air compared with the amount of water the air can hold at the current temperature. RH is depicted in percent (%) at 10-meter height or 30 feet above ground-level. Since RH changes with the temperature, it's difficult to compare over a period of time. For example, as temperature rises during the day, RH falls, and as temperature falls at night, RH rises. For this reason, meteorologists typically use dew point to provide a better measure of atmospheric moisture. Winds (at 10-meter level, 30 feet) are depicted with red 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15.

Surface Wind / Pressure - Winds (at 10-meter height or 30 feet above ground-level) are depicted with cyan 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15. Pressure is the weight of the atmospheric at mean sea level (either directly measured by stations at sea level or derived from the station pressure and temperature for stations not at sea level). Mean sea level pressure is used as a common reference for analyses of surface pressure patterns, and is measured in millibars (mb). To convert mb to inches of mercury, multiple by 0.0295.

Steering Wind & Mean RH - Steering wind is the average wind occurring between the 850 and 650 mb levels of the atmosphere (generally, 5000 to 12,000 feet above ground-level) and is often used to estimate the motion of deep convective cells. Winds are depicted with red 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15. Mean RH is the average relative humidity  occurring between the 850 and 650 mb levels of the atmosphere (generally, 5000 to 12,000 feet above ground-level), and is measured in percent (%). Mean RH can help depict regions where precipitation is occurring or becoming more favorable to occur (generally areas greater than 80%).

1-km Relative Humidity & Winds - Relative humidity (%) and winds (knots) at the 1-kilometer height (approximately 3300 feet above ground-level). Under certain daytime conditions, these quantities depicted at the 1-km height can mix vertically downward to ground level. Therefore, these values can sometimes be used as a forecast estimate for surface RH and winds, and can be especially useful to fire weather and aviation forecast purposes. Winds are depicted with blue 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15.

0-1 km Helicity - This is a derived parameter which quantifies the tendency for airflow in the layer from ground-level to a height of 1-kilometer (approximately 3300 feet above ground-level) to acquire rotation, and thus increase the potential for existing or future convective storms to develop mesoscale circulation's, possibly leading to tornado development. An assumption inherent in the helicity calculation is that any storms which form will move 30 degrees to the right of, and 75% of the magnitude of, the mean 0-6 kilometer wind flow (ground-level to approximately 20,000 feet above ground-level). This deviant storm motion is common with severe storms, and approximates "storm-relative" helicity. Helicity has units of energy and can therefore be interpreted as a measure of wind shear energy (directional wind shear). Helicity is depicted in units of m**2/s**2 (meters squared per second squared). If there is no directional wind shear, helicity will be zero. If the wind backs (turns counter-clockwise) with height then helicity will be negative, if wind veers (turns clockwise) with height then helicity will be positive. Helicity values are conditional - i.e. high values can occur without the necessary conditions to produce deep convection. Therefore, if helicity values are high, favorable atmospheric instability and lift must also be present in order to produce a threat for rotating storms. If convection occurs or is forecast (especially "shallow convection" or cells with small vertical extent; common along or ahead of Winter cold fronts, or within tropical cyclone outer rainbands), high values of 0-1 km helicity can be indicative of potential severe weather.

0-3 km Helicity - Same as 0-1 km Helicity (above), except for the 0-3 kilometer layer (ground-level to approximately 10,000 feet above ground-level). If convection occurs or is forecast (especially "deep convection" or cells with large vertical extent; common along or ahead of Spring cold fronts and some Winter cold fronts), high values of 0-3 km (storm-relative) helicity can be indicative of potential severe weather. A Study correlating the occurrence of tornadoes with 0-3 km (storm-relative) helicity in the Midwest found a value of 150 is the approximate threshold for supercell (persistent, rotating storms) development, especially when the 0-3 km (storm relative) inflow is 20 knots or greater. The following helicity versus tornado strength correlation's were also found:
Helicity = 150 to 299 > weak tornadoes (F0 and F1 on the Fujita scale)
Helicity = 300 to 499 > strong tornadoes (F2 and F3 on the Fujita scale)
Helicity = 450+ > violent tornadoes (F4 and F5 on the Fujita scale)

Convective Parameters: Numerical values used to depict the degree of atmospheric instability; i.e. the potential for (strong/severe) convection.

Lifted Index - The lifted index (LI) is a common measure of atmospheric instability, which takes into account low level moisture. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (normally 500 mb, or 18,000 feet) and comparing that temperature to the actual temperature at that level. A negative LI usually indicates the possibility of convection. A lifted index less than -6 suggests the possibility of strong/severe convection. To convert Celsius to Fahrenheit, multiply by 1.8, then add 32. This graphic can be especially useful for convection initiation and severe weather forecast purposes. In general, the following relative LI classifications can be useful:
LI = positive > stable conditions, but convection possible for LI = 1 to 3 if strong lifting is present
LI = 0 to -3 > marginally unstable
LI = -3 to -6 > moderately unstable
LI = -6 to -9 > very unstable
LI = -9 or below > extremely unstable

CAPE - CAPE (or Convective Available Potential Energy) represents the potential energy available to an air parcel to be converted to kinetic energy in a buoyant updraft (i.e. a measure of the amount of positive (upward) buoyancy present or forecast). CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. The units of CAPE are Joules per kilogram (J/kg; units of energy). This graphic can be especially useful for convection initiation and severe weather forecast purposes. In general, the following relative CAPE classifications can be useful:
CAPE = 0 to 1000 > marginally unstable
CAPE = 1000 to 2500 > moderately unstable
CAPE = 2500 to 3500 > very unstable
CAPE = 3500 or greater > extremely unstable

Convective Inhibition - Convective INhibition (or CIN) provides a measure of the amount of energy needed in order to initiate convection. Values of CIN typically reflect the strength of capping inversions (layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms). Whereas, CAPE (see above) is indicative of positive (upward) buoyancy, CIN reflects negative (downward) buoyancy. CIN must be overcome and replaced with sufficient CAPE in order for convection to form. The units of CIN are Joules per kilogram (J/kg; units of energy). This graphic can be especially useful for convection initiation and severe weather forecast purposes. A general guide for CIN values is shown below:
CIN = 15 or below > fair weather cumulus field (CIN overcome early)
CIN = 15 to 50 > a few strong thunderstorms may form (if CIN is overcome)
CIN = 50 to 150 > strong thunderstorms may form (if CIN is overcome)
CIN = 200 or greater  > strong capping inversion present and thunderstorm development unlikely (CIN usually difficult to overcome)

Estimated MDPI - Microburst-Day Potential Index (MDPI) utilizes vertical atmospheric profiles (measured or forecast) to ascertain the difference between moist, warm, low-level air and dry, cool, midlevel air. The greater the spread between these values, the greater the effect that relatively cool, dry environmental air will have on a developing a downburst (a strong downdraft resulting in an outward burst of potentially damaging winds on or near the ground. Downburst winds can produce damage similar to tornadoes. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder). MDPI index values of 1 or higher represent a high risk of microburst. Since the MDPI does not account for the probability of convection, use of MDPI requires the assumption that convection will form on a given day. This graphic can be especially useful for severe weather forecast purposes.

Cloud Parameters: Values which depict the amount and height of clouds. These graphics can be especially useful for aviation forecast purposes.

Total Cloud Cover - The percentage of cloud cover; 0-30% = Mostly Clear skies, 30-60% = Partly Cloudy skies, 60-70% = Partly Cloudy to Mostly Cloudy skies; 70-90% = Mostly Cloudy skies, 90%+ = Overcast skies.

Cloud Top Heights - The altitude of cloud tops in feet. The amount of cloud cover is not represented in this graphic. 'Cloud top heights' should be used in conjunction with 'total cloud cover'.

Ceiling Heights - Cloud ceiling represents the altitude of the cloud base when the amount of cloud cover is 60% or greater. Therefore, this graphic takes into account both the altitude and amount of cloud cover (when skies are partly cloudy to overcast).

Surface Moisture Flux Divergence and Winds - Moisture divergence occurs when the distribution of winds within a given area results in a net horizontal outflow of moisture from the region. Negative values of moisture divergence signify areas where the distribution of winds within a given area results in a net horizontal inflow of moisture to the region. Higher positive/negative values indicate a greater amount of divergence/convergence. Divergence near the surface leads to sinking motion from aloft and therefore locally increased stability (suppression). Conversely, convergence near the surface leads to rising motion and therefore locally increased instability (lift). Surface moisture flux divergence is expressed as a unit-less quantity (x 104), where a value shown as +/- 10 on the graphic actually represents +/- .0010. Winds (at 10-meter level, 30 feet) are depicted with blue 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15. Under certain conditions, localized convergence can help promote development of convection, or may even lead to fog formation.

950-700 mb Theta-e Lapse Rates - Theta-e (or 'equivalent potential temperature') is the temperature a parcel of air would have if it was lifted until it became saturated, all water vapor was condensed out, and it was returned (without transfer of heat or mass) to a pressure of 1000 mb (approximately 350 feet above ground-level). Since theta-e is directly related to the amount of heat present in an air parcel, it can be useful for diagnosing atmospheric instability. The 950-700 mb theta-e lapse rates represents the change of  theta-e with height at lower levels, from 950 mb (approximately 1800 feet above ground-level) to 700 mb (approximately 10,000 feet above ground-level). A steep lapse rate (large negative number) implies a rapid decrease with height (a sign of instability) and a decreasing lapse rate with time implies that destabilization is occurring. The theta-e lapse rate is expressed in degrees Kelvin per kilometer (K/km), which is equivalent to degrees Celsius per approximately 3300 feet. This graphic can be especially useful for convection initiation and severe weather forecast purposes.

0-1000 foot Shear and Shear Vector - The difference in wind speed (in knots) between ground-level and a height 1000 feet above ground-level is depicted with contouring and shading. The 'shear vector' (difference of wind direction and speed between the two heights) is shown with blue 'flags', where one full barb on a flag staff indicates a 10 knot (12 mph) wind, one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates the direction the wind is coming from, for example, an east wind of 15 knots (17 mph) would be indicated by:   A calm wind is indicated by a circle. To convert knots to mph, multiply by 1.15. This graphic can be especially useful for aviation and severe weather forecast purposes.


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.