• What SPC products does AccuWeather have?
• What DIFAX products help forecast severe weather?
• What indices does AccuWeather produce for severe weather?
• What is the National Excessive Rainfall Outlook?
• What is a SKEW-T?

What SPC products does AccuWeather have?

These technical products are issued by the Storm Prediction Center (SPC) in Norman, Oklahoma.

Severe Weather Today & Tomorrow - The Day-1 Convective Outlook, and Day-2 Severe Weather Outlook are guidance products issued by the Operational Guidance Branch (OGB) unit of the Storm Prediction Center. Both outlooks outline areas in the continental United States where severe thunderstorms may develop during the time period.

Mesoscale Discussion - When conditions actually begin to shape up for severe weather, SPC sometimes issues a Mesoscale Discussion (MCD) statement. If there is no current MCD statement the last issued one is shown.

To learn more about the SPC products, see the SPC FAQs.

What DIFAX products help forecast severe weather?

These black and white fax maps come from the National Weather Service. Because of the way they are sent, some of the maps may be oriented incorrectly. DIFAX maps and charts are very large and detailed images, so you will have to use the scroll bars at the right side and bottom of your browser window to view the entire image. You can print each map from your browser, but remember to reduce the size to fit your paper.

U.S. Radar Composite Chart - Depicts shaded areas of precipitation over the continental U.S. Precipitation cells and expected movement is shown. Weather Watch areas. Administrative messages from National Meteorlogical Center. Coverage Area: Continental U.S.

U.S. Surface Plot- Chart shows surface high and low pressure centers and frontal systems. Each reporting station shows wind direction and speed, cloud cover, visibility, temperature, dew point, precipitation type and amounts, and barometric pressure. Outflow boundaries, the boundaries of thunderstorm outflow air, which may travel far from the original storms and could contribute to new storm development, are sometimes noted.

NGM 0 to 36 -hour Lifted Index - Four-Layer Lifted Index. Four panel chart shows current and NGM forecast 12-hr, 24-hr and 36-hr maximum/minimum lifted indexes. See the section below for detailed information on Lifted Indices.

What indices does AccuWeather produce for severe weather?

Many indices can be useful for predicting severe weather. The importance of one particular index varies from situation to situation. The following are explanations of US maps produced; these and other indices can be shown for each individual Upper Air Reporting Station from across the world by viewing the SKEW-T's.

For detailed information on SKEW-T's, see below.

Current Total Totals Index- This US map is produced every morning and evening from upper air data. This index determines the potential for thunderstorms and is very helpful in diagnosing the difference between weak and strong thunderstorms. The relationship between the TTI and Thunderstorm Acitivity is as follows:
44 - Spotty, usually light thunderstorm.
46 - Spotty, usually moderate thunderstorm.
48 - Usually heavy thunderstorm.
50 - Usually heavy thunderstorm with hail possible.
52 - Usually heavy thunderstorm with a tornado possible.
56 - Heavy thunderstorm with tornadoes likely.

Current/Forecast Lifted Index- The Current US map is produced every morning and evening from upper air data. The forecast maps are produced twice a day from the ETA and NGM forecast models. This index helps predict overall atmospheric stability. The lower the number, the more unstable the atmosphere. Guidlines for interpreting the numbers are as follows:
>+3: Stable atmosphere.
+3 to +1: Slightly unstable, supports showers
+1 to -1: Unstable, supports showers and isolated T-storms
-1 to -3: Moderately unstable, supports widespread T-storms
-3 to -6: Very unstable, supports severe T-storms
<-6: Extremely unstable, supports widespread severe weather.

Current K Index- This map is produced every morning and evening from upper air data. This stability index is useful in predicting non-frontal thunderstorm situations. Relationship of K-Index to probability of thunderstorms:
15-20: < 19%
21-25: 20-39%
26-30: 40-59%
31-35: 60-9%
36-40: 80-89%
>40: >90%

Explanations of other indices shown on individual SKEW-T's can be found below

What is the National Excessive Rainfall Outlook?

This product is issued by the Hydrometeorological Prediction Center (HPC). It discusses the potential of Flash Flooding in the United States based on current and forecast conditions. It is usually in plain language, but standard US Postal Service abbreviations and directional abbreviations are sometimes used (SEWARD = Southeastward, ERN = Eastern, etc).

What is a SKEW-T?

A Sounding (SKEW-T) comprises a vertical picture of the atmosphere which is a helpful tool in determining the stability of the atmosphere, type of precipitation, and maximum temperature. Pressure levels are shown on the left side of the sounding corresponding to horizontal lines across the diagram. Temperature (in degrees Celsius) is shown at the bottom of the diagram, with temperatures lines running diagonally (or skewed) across the diagram. Also provided is mixing ratio information, plotted wind barbs, and many thermodynamic variables (see below for a list).

LCL	Lifting Condensation Level - Height at which a parcel of air becomes saturated when it is lifted dry adiabatically.
CCL	Convective Condensation Level - Height at which a parcel of air, if heated sufficiently from below, will rise adiabatically until condensation occurs. It is the height of the base of cumuliform clouds which are or would be produced by thermal convection solely from surface heating. Severe weather usually occurs when the CCL is at or below 770 millibars.
LFC	Level of Free Convection - The height at which a parcel of air lifted dry adiabatically until saturated, and saturated adiabatically thereafter would first become warmer than the surrounding air, and then continues to rise until it becomes colder than the surrounding air. Severe weather usually occurs when the LFC is at or below 650 millibars.
FL	Freezing Level - Height at which temperature is at freezing.
WBO	Wet Bulb Zero Height - Height where wet bulb reaches zero. Generally, if the wet bulb zero height ranges between 7,000-9,000 feet, large hail, damaging winds, and tornadoes are possible. At the Gulf Coast, the range must be between 9,000-10,500 feet for damaging winds and hail to occur.
EL	Equilibrium Level - Height where the temperature of a buoyantly rising parcel of air again becomes equal to the temperature of the surrounding air. Usually, if thunderstorm echo heights are above this equilibrium level, it is referred to as 'Over-Shooting Tops' and is a precursor to severe weather.
KI	K-Index - Stability index useful in predicting non-frontal thunderstorm situations. Relationship of K-Index to probability of thunderstorms: K 15-20: < 19%, K 21-25: 20-39%, K 26-30: 40-59%, K 31-35: 60-9%, K 36-40: 80-89%, K >40: >90%.
LI	Lifted Index - This index helps predict overall atmospheric stability. Lifted Index guidelines: +3 to +1: slightly unstable, supports showers, +1 to -1: unstable, supports showers and isolated T-storms, -1 to -3: moderately unstable, supports widespread T-storms, -3 to -6: very unstable, supports severe T-storms, <-6: extremely unstable, supports widespread severe weather.
SSI	Showalter Index - A stability index similar to the Lifted Index, but values are typically not as low for severe weather. Generally, an index of +3 to +1 supports T-storms and <-3 supports severe weather.
SI	Sweat Index - Stability index indicating severe T-storms and tornado potential.
CTI	Cross Totals Index - This index determines the horizontal transport of unstable air. A value in excess of 25 is needed for strong T-storms.
VTI	Vertical Totals Index - This index determines the vertical transport of unstable air. Values in excess of 25 are needed for strong T-storms.
TTI	Total Totals Index - This index determines the potential for T-storms and is very helpful in diagnosing the difference between weak and strong T-storms. The relationship between the TTI and Thunderstorm Acitivity is as follows: 44 - Spotty, usually light thunderstorm. 46 - Spotty, usually moderate thunderstorm. 48 - Usually heavy thunderstorm. 50 - Usually heavy thunderstorm with hail possible. 52 - Usually heavy thunderstorm with a tornado possible. 56 - Heavy thunderstorm with tornadoes likely.
EPA	Positive Area - Positive area denotes the buoyancy of the atmosphere. The higher the positive area, the more buoyant energy available. Values in excess of 500J/KG support T-storms, values between 1000 and 3000 J/KG support T-storm development with moderate shear.
PW	Precipitable Water - The amount of water that would be found in a column of air above the surface. Comparing sounding values indicates the amount of water available for rain or snowfall potential.
TH	Tropopause Height - Height where the tropopause begins, or the temperature curve warms. The location of a tropopause height helps determine severe T-storm potential. If T-storm echo tops exceed tropopause height, severe weather is likely.
STC	Convective Temperature - The surface temperature that must be reached to begin formation of convective clouds by solar heating of the surface air layer.
HSZ	Hail Size - The size that hail may reach given the state of atmospheric updrafts. This is based on the calculation of critical updraft speed which determines the size of hailstones that can stay aloft for a given updraft speed.
STC	Richardson Shear Index - This value indicates how turbulent the flow is. Low RI numbers are associated with fast moving systems in an environment that has strong shear. Small RI numbers support greater shear.