Frequently Asked Questions about Severe Weather Products

Answers

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).