The Day 1, 2 and 3 Convective Outlooks consist of a narrative and graphic depicting severe thunderstorm threats across the continental United States. The Day 1 and 2 Outlooks also include a forecast where thunderstorms are expected. Beginning September 25, 2012, the SPC will add general thunderstorm information to the Day 3 Convective Outlook.
The outlook text are written in technical language and intended for sophisticated weather users. A synopsis is included, especially in the early morning outlooks, and provides a general overview of the synoptic pattern with an emphasis on those systems directly related to the convective threat. The header for each severe thunderstorm or thunderstorm text area is preceded by a headline listing the geographical area for the potential threat. Within the individual geographic threat areas, the narratives provides the meteorological reasoning for the risk area as well as explicit information regarding the timing, the greatest severe weather threat and the expected severity of the event. Three risk categories (SLGT, MDT, and HIGH) are used to symbolize the coverage and intensity of organized severe weather such as supercells, squall lines, and multicell thunderstorm complexes. Pulse type thunderstorms, consisting primarily of solitary brief severe updrafts (often found in weakly sheared environments) are not considered organized. Isolated severe storms with marginal intensities or short durations of severe weather will likely not be included in one of the three severe risk areas. The unlabeled (brown) thunderstorm line on the outlook graphic depicts, to the right of the line, a 10% or higher probability of thunderstorms during the valid period. A SLGT risk implies well-organized severe thunderstorms are expected, but in small numbers and/or low coverage. A MDT risk indicates a potential for a greater concentration of severe thunderstorms than the slight risk, and in most situations, the storms and associated severe weather are more intense. A HIGH risk area suggests a major severe weather outbreak is expected, with the expectation of either a concentration of strong tornadoes or an enhanced likelihood of a long-lived wind event (derecho) with the potential of higher end wind gusts (80+ mph) and structural damage.
On days when the severe storm coverage or intensity is not expected to be sufficient for a SLGT risk, but some threat exists, a SEE TEXT label will be placed on the graphic where a 5% severe probability exists.
The SLGT, MDT and HIGH categories are dependent on the probabilities used to determine the severe threat. The Tables below show the conversion from probabilities to the appropriate categorical outlook for the Day 1, 2 and 3 Outlooks. The Day 1 Outlooks contains individual severe probabilities for wind, hail and tornadoes, while the Day 2 and 3 Outlooks only forecast the combined probability of all three types of severe weather. Whether an combined or individual probability, the values represent the chance of severe weather occurring within 25 miles of any point, which is about the size of a major metropolitan area.
As an example, if you were located within an area that had a 15% probability for tornadoes, this means you have a 15% chance of a tornado occurring within 25 miles of your location. This may seem low, but severe weather is uncommon at any one location, so your chances of getting hit by a tornado or other severe weather are small. However, a 15% chance of a tornado within 25 miles of a point means a tornado occurring nearby is probably at least 15 times greater than climatology and should be taken seriously. The probabilities also have the same meaning as they do for tornadoes, but they typically will be higher than the tornado probabilities since severe hail and wind occur much more frequently. The probabilities for hail and wind are likely to be higher than tornadoes in most forecasts, since hail and wind occur much more frequently than tornadoes.
Sometimes, a blue hatched area will be overlaid with the severe probabilities. This blue hatched area indicates a 10% or greater probability for significant events occurring within 25 miles of any location in the area. A significant event includes any of the following:
For the combined Day 2 and Day 3 Outlooks, the blue hatched area means there is a 10% or greater chance for significant tornadoes, and/or damaging winds and/or hail. On the Day 1 Outlook, the hatched area will be on the specific hazard graphic (tornado, wind or hail) which the enhanced threat exists.
Page last modified: July 19, 2012