National Weather Service Tampa Bay Area
 Severe Thunderstorm Forecast

 Explanation Page

 

NOTE: ALL graphics below represent product examples from the past, and DO NOT represent current or forecast weather conditions. 

The NWS definition of a "severe" thunderstorm states any storm that produces one or more of the following elements:

1) A Tornado

2) Damaging straight-line winds, or winds measured
   
50 knots (approx. 58 MPH) or more.

 
 3) Hail 1 inch in diameter or larger.  

The severe thunderstorm forecast process begins with the Convective Outlook issued by the Storm Prediction Center (SPC) in Norman OK, which is a forecast of where both severe and non-severe thunderstorms are expected to occur around the country. Areas of possible severe thunderstorms are labeled "SLGT" (slight risk), "MDT" (moderate risk), or "HIGH" (high risk), depending upon the coverage and intensity of expected severe thunderstorms in a region. A detailed meteorological discussion concerning the severe threat is also produced.

NWS offices use this Outlook and discussion to make advanced emergency staffing decisions before severe weather begins. SPC Outlook forecasts can be found at http://www.spc.noaa.gov/products/outlook/

Three separate Convective Outlooks are issed for day 1 through day 3 along with severe weather probabilities for the potential threat.

The Day 4-8 Severe Weather Outlook graphic depicts those days where a 30% or higher probability for severe storms is expected.

So, Just what does a SLIGHT (SLGT) risk of severe weather mean to you:

 

A SLGT risk implies that well-organized severe thunderstorms ARE expected but in relatively small numbers and/or small areal coverage, or a small chance of a more significant severe event. Not all severe storm events will be covered with a SLGT risk, especially during the summer when short-lived, "pulse-type" severe storms are relatively common during the afternoon.
Slight Risk Explantion Slight Risk Map Example

How about a MODERATE (MDT) risk:


A MDT risk implies a greater concentration of severe thunderstorms, and in most situations, greater magnitude of severe weather and greater forecaster confidence compared to a SLGT risk. A MDT risk is usually reserved for days with substantial severe storm coverage, or an enhanced chance for a significant severe storm outbreak. Typical MDT risk days include multiple tornadic supercells with very large hail, or intense squall lines with widespread damaging winds.
Moderate Risk Explantion Moderate Risk Map Example

Finally, a HIGH risk:


The HIGH risk implies that a major severe weather outbreak is expected, with large coverage of severe weather and the likelihood of extreme severe (i.e., violent tornadoes or very damaging convective wind events). The HIGH risk category is reserved for the most extreme events with the least forecast uncertainty, and is only used a few times each year.
High Risk Explantion High Risk Map Example

Convective Outlooks can also be expressed in terms of Convective Probabilities. Click the following link for detailed explanation of how convective probabilities are calculated and how to use these numbers to express severe weather risk:

http://www.spc.noaa.gov/products/outlook/probinfo.html

Probability of Severe Hail Probability of Severe Wind Probability of Tornado

It is important to understand these probabilities when making severe weather preparedness decisions. Many people confuse the generally lower probabilities as being a low threat. Keep in mind that one should NOT compare the threat from these probabilities to the typical probabilities related to the chance of rain or snow. The climatological potential for precipitation is far higher at any location than it is for a severe weather event. Therefore, seeminly low probabilities can represent a significant threat for severe weather when compared to climatology.

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Ok, now imagine we have reached the day of a potential severe weather event, and the details of the potential severe weather are becoming more clear. At this point, the SPC will issue a Mesoscale Discussion (MD) that focuses on the most likely areas to see severe weather and the potential for a Severe Weather "Watch".


When conditions appear favorable for severe storms development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch. SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain.
Mesoscale Disscusion Explanation Mesoscale Discussion Example Graphic

When confidence in severe thunderstorms becomes high enough and conditions becoming favorable for imminent thunderstorm development, the SPC will collaborate with local NWS offices and issue a Severe Thunderstorms or Tornado Watch for the area of greatest threat. http://www.spc.noaa.gov/products/watch/


When conditions become favorable for organized severe thunderstorms and tornadoes to develop, the SPC issues a severe thunderstorm or tornado watch. A tornado can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for either multiple and or strong tornadoes. Watches encourage the general public to stay alert for changing weather conditions and possible warnings. For emergency managers, storm spotters, and the broadcast media, watches provide valuable lead time to gear up operations and increase staffing
 Convective Watch Explanation  Tornado Watch Graphic Example

Severe thunderstorm and Tornado Warnings are issued by meteorologists at your local National Weather Service Office. Warning decisions are based on data from a combination of sources, including doppler radar data and real-time reports from storm spotters, emergency management, and the general public.  

Severe Thunderstorm Warning- Issued when an ongoing thunderstorm is expected to have wind gusts of 58 mph or greater an/or hail of 1 inch or more in diameter.

 

Tornado Warning- Issued when there is likelihood of a tornado within the given area from an ongoing thunderstorm based on radar data or actual sighting. It is often accompanied by conditions indicated for Severe Thunderstorm Warning.

 Convective Warning Explanation  Tornado Warning Graphic Example

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