National Weather Service United States Department of Commerce

Flooding Threat from Western Monsoon; Hot in the South; Severe Storms Possible in the southern Mid-Atlantic

Heavy rainfall from monsoonal activity will continue the potential for flash flooding, especially in complex terrain, burn scars, and urban areas from the northern Great Basin to the southern Rockies today. Dangerous heat and humidity continues across the Deep South today. Strong to severe storms with damaging wind and perhaps a tornado may impact southern portions of the Mid Atlantic today. Read More >

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Mesoscale Analysis Parameter Decsriptions

A measure of the warmth of the ambient air measured by a suitable instrument such as a thermometer.

Dew Point (Dew-Point Temperature)
A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.

Sustained Wind Speed Descriptive Term
40 mph or greater Strong, dangerous, or damaging
30-40 mph Very Windy
20-30 mph Windy
15-25 mph Breezy, Brisk, or Blustery
5-15 mph or 10-20 mph None
0-5 mph Light or light and variable wind
The horizontal motion of the air past a given point. Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force."

High and low pressure are relative. There's no set number that divides high and low pressure. Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots. The following table gives descriptions of winds used in National Weather Service forecasts.

The greatest distance an observer can see and identify prominent objects.

Relative Humidity
A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.

Apparent Temperature - Heat Index
The Heat Index is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. To find the Heat Index (HI), look at the Heat Index (HI) Chart (right).

As an example, if the air temperature is 90°F (found at the left side of the table) and the Relative Humidity (RH) is 70% (found at the top of the table), the Heat Index (HI)--or how hot it actually feels--is 106°F. This is at the intersection of the row 90°F and the 70% column.

This index was devised for shady, light wind conditions. Exposure to full sunshine can increase Heat Index (HI) values by up to 15°F. Also strong winds, particularly with very hot, dry air, can be extremely dangerous.

Any value Heat Index (HI) greater than 105°F is in the Danger Category. When the Heat Index is between 105-115°F for 3 hours or more, a Heat Advisory will be issued by the local National Weather Service Forecast Office.

Category Classification Heat Index/Apparent Temperature (°F) General Affect on People in High Risk Groups
IV Extremely Hot 130°F or Higher Heat/Sunstroke HIGHLY LIKELY with continued exposure
III Very Hot 105°F - 130°F Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity
II Hot 90°F - 105°F Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity
I Very Warm 80°F - 90°F Fatigue POSSIBLE with prolonged exposure and/or physical activity

Apparent Temperature - Wind Chill
The wind chill is the effect of the wind on people and animals. The wind chill temperature is based on the rate of heat loss from exposed skin caused by wind and cold and is to give you an approximation of how cold the air feels on your body.

As the wind increases, it removes heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0°F and the wind is blowing at 15 mph, the wind chill temperature is -19°F. At this level, exposed skin can freeze in just a few minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. The inanimate object will not cool below the actual air temperature. For example, if the temperature outside is -5°F and the wind chill temperature is -31°F, then your car's radiator temperature will be no lower than the air temperature of -5°F.

Convective Available Potential Energy (CAPE)
It defines the vertically integrated positive buoyancy of an adiabatically rising air parcel on a sounding. This is proportional to the amount kinetic energy that the air parcel gains while it is warmer that its surrounding environment. As a result, CAPE provides the best measure of the potential instability available in the atmosphere. Increasing values of CAPE generally lead to progressively vigorous convection. However, severe thunderstorms can form in environments showing weak to moderate CAPE, especially if the Storm Relative Helicity values are high.

CAPE = 0 to 1000 > marginally unstable
CAPE = 1000 to 2500 > moderately unstable
CAPE = 2500 to 3500 > very unstable
CAPE = 3500 or greater > extremely unstable

Lifted Index (LI)
Lifted Index (LI) Thunderstorm Indication
< -5 Very Unstable, Heavy/strong thunderstorm potential
-3 to -5 Unstable, Thunderstorms probable
0 to -2 Marginally Unstable, Thunderstorms possible
It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet) denotes the LI. Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the Showalter Index (SWI). The greater negative values of LI indicate energy available for parcel ascent.

Showalter Index (SWI)
Showalter Index Thunderstorm Consideration
< -6 Extremely unstable - good strong thunderstorm potential
-4 to -6 Very unstable - good heavy thunderstorm potential
0 to -3 Unstable - thunderstorms probable
3 to 1 Thunderstorm possible - strong trigger needed
It is a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.