Severe Thunderstorms – Warning Alert Notifications


The severe thunderstorm notifications issued by the National Weather Service’s (NWS) Storm Prediction Center (SPC) in Norman, Oklahoma are in the order of severity:[1]

Storm cell - Sun Prairie, WI[2]

The National Weather Service issues a Significant Weather Advisory for strong thunderstorms (and occasionally, other weather situations such as downbursts) that may not pose any threat or pose limited threat to life and property; it is typically issued if the hail size in a thunderstorm is less than 1 inch in diameter, and/or strong winds of 39–57 miles per hour (63–92 km/h). The product is mainly used by forecast offices located in the National Weather Service's Southern Region Headquarters. These advisories are issued on a county by county basis.

Many NWS offices do not use the significant weather advisory title and instead just use no warning text, though. This product is not an official product, instead it is issued within a special weather statement product. Special weather statements are also used for any type of weather approaching watch/warning/advisory level. Many offices do not use significant weather advisories, instead issue normal special weather statements mentioning much of the same text used in a significant weather alert.

A Severe Thunderstorm Watch is issued when weather conditions are favorable for the development of severe thunderstorms. A severe thunderstorm contains large damaging hail of 1 inch (2.7 cm) diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater.

If thunderstorms are expected to be of sufficient strength such that there is a significant risk that they may produce tornadoes, then a Tornado Watch (which also automatically implies that severe thunderstorms are possible) is issued. A Severe Thunderstorm Watch can also be upgraded to a Tornado Watch as conditions warrant (in which case the existing Severe Thunderstorm Watch, or a portion of it, would be replaced).

A watch does not necessarily mean that severe weather is actually occurring, only that atmospheric conditions have created a significant risk for severe weather to occur. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours.

In the event that a Severe Thunderstorm Watch is likely to lead to very destructive winds or hail (usually from a major derecho event), enhanced wording with the words Particularly Dangerous Situation (PDS) can be added to the watch.

NWS meteorologists were monitoring the developing dangerous weather conditions on Thursday, July 22nd, 2010, and by Friday morning began to include that threat in the Hazardous Weather Outlook with thunderstorms that "may produce strong gusty winds." That threat continued in the outlook through the weekend. As NWS meteorologists became even more confident on during the morning and midday hours on Sunday July 25th, 2010, a Severe Thunderstorm Watch was first issued for an areas including northern MD, northwestern VA, and northeast WV. That was followed by another Severe Thunderstorm Watch for an area just south of the initial one, including Washington D.C.[3]














PDS (Particularly Dangerous Siutation) Severe Thunderstorm Watches are issued when there is a higher than normal risk of severe thunderstorm winds capable of major structural damage (in addition to large hail and perhaps a few isolated tornadoes), usually due to a strong and persistent derecho. These watches are very rare (an average of only two each year), as the risk for tornadoes must remain low enough to not warrant a tornado watch (a normal tornado watch would be issued if the tornado risk is significant alongside the extreme wind threat). An expected severe wind event (derecho) is the mostly likely reason for a PDS Severe Thunderstorm Watch to be issued, with widespread winds greater than 90 mph (150 km/h or 80 knots) possible. These watches are usually issued for a larger area by the Storm Prediction Center in Norman, Oklahoma than a regular Severe Thunderstorm Watch, and are usually valid for a longer period of time than a regular Severe Thunderstorm Watch. This type of watch is usually only reserved for forecast "high-end" severe weather events.

In this example, there is a powerful line of severe thunderstorms with confirmed reports of widespread wind damage and Doppler radar indications of very high wind speeds. Above is a radar image with an example warning on the left, with a Doppler velocity image showing very high wind speeds on the right.[4]
In this example, there is a severe thunderstorm with confirmed reports of 2" in diameter hail moving into a highly populated area. Above is a radar image of the storm on the left, with a cross section showing a core of large hail aloft approaching the populated area on the right.[5]



















A Severe Thunderstorm Warning is issued when trained storm spotters or a Doppler weather radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. A severe thunderstorm contains large damaging hail of 1 inch in diameter or larger, and/or damaging winds of 58 mph (93 km/h) or greater.

A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning. While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado (and can actually cover a much wider area than a tornado). If a tornado is detected on radar or sighted, a tornado warning will be issued either in replacement of or concurrently to the existing severe thunderstorm warning. Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.

The local NWS forecast offices in the Great Plains or Southeastern U.S., sometimes include the wording "Severe thunderstorms can produce tornadoes with no advance warning..." or a similar reference in their severe thunderstorm warning products, usually when there is a tornado threat or especially when a tornado watch is in effect, though sometimes the wording is included even if the threat of severe weather does not include a threat of tornadic activity.

Some severe thunderstorms, especially in the Great Plains, may produce massive hailstones the size of baseballs (2.75 inches (7.0 cm)) or larger, falling fast enough to potentially kill a person by repeated blunt trauma.

In this example, there is a radar-indicated severe thunderstorm with 60 mph winds and quarter size hail expected. Above is a radar image of the storm on the left, with a cross section of the storm on the right.[6]











Convective Outlook Risks

The Storm Prediction Center (SPC), located in Norman, Oklahoma, issues convective outlooks (AC) consisting of categorical and probability forecasts describing the general threat of severe convective storms over the contiguous United States for the next 6 – 192 hours (Day 1 – Day 8). They are labeled and issued by the day and are issued up to five times a day. These outlooks are:[7]

A slight risk day typically will mean the threat exists for scattered severe weather, including scattered wind damage or severe hail and possibly some isolated tornadoes. During the peak severe weather season, most days will have a slight risk somewhere in the US. Isolated significant severe events are possible in some circumstances, but are generally not widespread.

A moderate risk day indicates that more widespread and/or more dangerous severe weather is possible (sometimes with major hurricanes), with significant severe weather often more likely. Numerous tornadoes (including some strong tornadoes), more widespread or severe wind damage and very large/destructive hail could occur. Major events, such as large tornado outbreaks, are sometimes also possible on moderate risk days, but with greater uncertainty. Moderate risk days are not uncommon and typically occur several times a month, especially during peak season.

A high risk day indicates a considerable likelihood of a major tornado outbreak or (much less often) an extreme derecho event. On these days, the potential exists for extremely severe and life-threatening weather, including widespread strong or violent tornadoes and/or very destructive straight-line winds (Hail cannot verify or produce a high risk on its own, although such a day usually involves a threat for widespread very large hail as well). Many of the most prolific severe weather days were high risk days. Such days are quite rare; a high risk is typically issued only a few times each year.



  1. National Weather Service’s (NWS) Storm Prediction Center (SPC) Website –
  2. Image Source: [Accessed August 17, 2013]
  3. Image Source: [Accessed August 18, 2013]
  4. Image Source: [Accessed August 17, 2013]
  5. Image Source: [Accessed August 17, 2013]
  6. Image Source: [Accessed August 17, 2013]
  7. National Weather Service’s (NWS) Storm Prediction Center (SPC) Website –