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SPC Probabilistic Outlook Information

Note: For probability to categorical outlook conversion, please see table in HTML.

The SPC produces probabilistic Convective Outlooks in conjunction with the traditional categorical Convective Outlooks. These outlooks are done for all Day 1, Day 2, and Day 3 periods.

Categorical Convective Outlooks

The traditional Convective Outlook is a categorical forecast that specifies the perceived level of threat via the descriptive wording: Slight Risk, Moderate Risk, and High Risk. This structure does not give the forecaster's expectations of the threats of individual severe weather hazards (large hail, damaging winds, and tornadoes). While the accompanying discussion for the outlook may describe the forecaster's thoughts about the individual hazards, the accompanying graphic does not explicitly depict these expectations.

Example Day 1 categorical Convective Outlook

Day 1 categorical Convective Outlook graphic

The graphic clearly defines the geographic threat areas. A large Slight Risk is forecast for portions of the Mississippi and Ohio valleys. The graphic does not provide much information concerning forecaster expectations of the individual severe weather hazards which the forecaster may have. This outlook will be discussed later in conjunction with the probabilistic forecasts for this event.

Probabilistic Convective Outlooks

Forecasting rare events such as tornadoes and the occurrence of large hail and damaging wind gusts is a very difficult process and one that contains a large amount of uncertainty. In the traditional Convective Outlooks this uncertainty is conveyed via the Slight/Moderate/High Risk terminology. A more direct method of expressing the forecaster's uncertainty is to use probabilities. Probabilities directly express a level of confidence that an event will or won't occur. While probabilities may seem somewhat difficult to understand at first, once you have a grasp of how to interpret them you will quickly gain an appreciation for how much more information they provide. (A great introduction to why probabilistic forecasting is so useful may be found in an online essay by Chuck Doswell and Harold Brooks.)

Definition of the probabilities

The probabilities used in the SPC Convective Outlooks are what are known as subjective probabilities. The forecasters make their best estimate of the probability of an event occurring. They may have guidance concerning what probabilities are appropriate but the values they choose are not created automatically by a computer or via statistics.

The probabilities that you see on the images represent the probability of one or more events occurring within 25 miles of any point during the outlook period. This is done because the probability of severe weather at an given point is quite small and because the Convective Outlook is not a small-scale, short-term forecast but one that covers the entire U.S. for periods up to 24 hours in length. There is a large amount of uncertainty in forecasting severe weather on these scales. How many times have you experienced a tornado in your neighborhood? Not very many times, if ever! Now think of how many times you can think of severe weather occurring within 25 miles of your location. It's probably safe to say that you can think of some examples. How large of an area is a circle with a 25 mile radius? Below you'll see an example of the Oklahoma City metro area where the large blue circle represents such an area.

Map of OKC Metro area

You should be able to imagine that the probability of having severe weather occur within such an area is much larger than the probability of having it occur specifically within your neighborhood. Keep this in mind as we head into the discussion of the probability values that you may see expressed in the outlooks.

So, how do you interpret the forecast values? For example, a 15% contour on the hail forecast outlines an area within which the probability of 1 or more reports of large (3/4" or greater) hail occurring within 25 miles of any point in that area during the forecast period is 15%. Larger values imply greater risk. For example, if you find that the probability for large hail in your area is 30% on a given day and it was 15% the day before, you can tell that the SPC believes there is a higher threat of large hail for your area on that day.

You may be thinking that 30% is not a very large probability of having 1 or more of these severe weather hazards occur near you on a given day, but 30% is roughly a 1 in 3 chance. To better understand this, let's put the probability values into perspective in terms of climatology.

The climatology of severe weather is very different than the climatology of precipitation. Compare the number of days that you have experienced rain at your home to the number of days that you have had large hail at your location, or even a tornado. The number of times a given location experiences severe weather in a year is much less frequent than the amount of time it experiences rainfall. Because rainfall occurs so frequently (on average) the daily climatological values approach 20% on any given day in many locations (i.e., a 1 in 5 chance, or it rains 1 day out of 5 on average). Suppose you hear a forecast calling for a 40% chance of rain. Given your knowledge of climatology, you can immediately say that the forecaster believes the chance of rain is twice as high as normal (40%/20%). This does not mean that rain will definitely occur but does mean that the forecaster believes that there is a higher than normal risk of precipitation occurring on that day. A knowledge of the climatology of the event being forecast is useful, even necessary, in order to interpret the probabilities being forecast. In the case of probability of precipitation forecasts, these values typically run from 0% (certainty that it will not rain) to 100% (which implies certainty that it will).

As stated previously, the probabilities of severe weather occurring at any given location are much lower than those for precipitation. How much lower? The following image shows the probability of 1 or more tornadoes occurring within 25 miles of a point for the week of April 29 - May 6.

Example probabilities

The image shows that the probabilities for this week range from 0 to 1.5%. These are very small values! Climatological values of rare events such as severe weather are much, much smaller than the climatology of cloudy days, or the probability of precipitation occurring.

As a part of the probabilistic forecasting program at the SPC, a representative severe weather climatology has been developed by members of the National Severe Storms Laboratory (NSSL) and the SPC for use by the SPC, the emergency management community, and the general public. This project is available on the NSSL web site. You can find a tremendous amount of information there to assist yourself in understanding the severe weather climatology for your area.

Because severe weather occurs relatively infrequently, there is a large amount of uncertainty as to precisely where it will occur. Accurate Yes/no forecasts of whether or not you will experience a tornado in your neighborhood in the next 24 hours are simply not possible many hours ahead of time. Further, the role of the Convective Outlook is not to do such pinpoint forecasts. The product is a national-scale forecast that highlights areas where severe weather is possible over the lower 48 states. The product is simply not meant to be a localized, short-term forecast of severe weather for your neighborhood. Because of this and the fact that the climatological probabilities of severe weather are so small, the probabilities that you will see used in the forecasts will generally be much smaller than you might expect.

The following table shows the probabilities that you may see in the various probabilistic outlooks:

Day 1
Tornadoes 2%, 5%, 10, 15%, 30%, 45%, 60%
Large Hail 5%, 10%, 15%, 30%, 45%, 60%
Damaging Wind 5%, 10%, 15%, 30%, 45%,60%
Day 2
Any severe weather 5%, 15%, 30%, 45%, 60%
Day 3
Any severe weather 5%, 15%, 30%, 45%

These values represent the only probabilities that you will see in the SPC forecasts. How should you interpret them? Very simply put, the smallest values represent areas where the most uncertainty exists and correspondingly where the smallest expected coverage of storm reports exists. The higher the probabilities, the greater the perceived threat and the greater the expected coverage of that hazard being forecast. The highest probabilities are generally reserved for the more significant severe weather events and may be used very infrequently, if at all, during the year.

Another way of thinking of the values is related to climatology. Consider our earlier discussion of tornado probabilities for the first week of May where the peak values were approximately 1.5%. Let's assume that the SPC forecaster drew a 30% area which included northwest Texas and southwestern Oklahoma. The ratio of the forecast to climatology (30%/1.5%) yields a value of approximately 20. The SPC forecaster is stating that they believe the risk of tornadoes in that region is 20 times larger than climatology. By comparing the forecast probability to climatology, you can more easily understand the magnitude of the risk on a given day.

Description of the probabilistic outlooks

Day 1

The most specific Convective Outlooks are those issued during the Day 1 period. Accordingly, the SPC forecasters have the most information available to them to differentiate the threats of the individual severe weather hazards. During this period, the SPC produces probabilistic outlooks for each primary severe weather hazard (tornadoes, damaging wind, and large hail) separately. By producing separate forecasts for tornadoes, damaging wind, and large hail, the user is given substantially more information upon which to make decisions than in the traditional outlook. In addition to the probabilities for separate types of severe weather occurring, areas are shown where there is a 10% or greater chance of significant severe weather occurring. Significant severe weather is defined as F2 or greater tornadoes, damaging winds with speeds greater than 65 knots, or large hail 2" or greater in diameter. If the forecaster believes that there is less than a 10% chance of significant severe weather occurring in the outlook area, then that threat will not appear on the graphics.

Day 2

Probabilistic Outlooks are issued for the Day 2 period as well. Because many of the specific details of severe weather forecasting can only be known hours ahead of time, rather than several days, the severe weather probabilities for the Day 2 Outlooks represent the probability of any severe weather hazard (large hail, damaging wind, or tornadoes) occurring (rather than producing individual forecasts for each hazard). Areas where there is a 10% or greater probability of significant severe weather events (again, defined as 2" or larger hail, 65 knot or stronger winds, and F2 or stronger tornadoes) are also indicated on the graphics.

Day 3

On November 7, 2001, the SPC began issuing Convective Outlooks for the Day 3 period to the public. These outlooks will be similar to the Day 2 outlooks in that the probabilities represent the probabilities of any type of severe weather hazard (tornadoes, large hail, damaging wind) within 25 miles of any point.

Example Day 1 probabilistic Convective Outlook along with the corresponding Categorical Outlook

categorical outlook
Conventional Categorical Outlook

probabilistic hail outlook
Probabilistic Hail Outlook

probabilistic wind outlook
Probabilistic Wind Outlook

probabilistic torn outlook
Probabilistic Tornado Outlook

These images show the categorical Convective Outlook issued at 1630 UTC on February 16, 2006 as well as the corresponding probabilistic forecasts valid for the same time period. The conventional categorical outlook which depicts a large Slight Risk area (shown in green) for portions of the Mississippi and Ohio Valleys. The unlabeled brown line represents regions where general thunderstorms are forecast. An emergency manager, or storm spotter, or member of the general public, may use this graphic to determine the relative level of threat for their area. However, forecaster expectations of locations of tornadoes, large hail, and damaging winds are not provided.
The remaining 3 figures show the forecast probabilities of hail, damaging wind, and tornadoes. Probabilities shown are 2% (green), 5% (light brown), 10% (dark brown), 15% (blue), and 30%(red). Light gray hatched areas represent areas where a 10% or greater chance of significant severe weather (as defined above) is forecast. As you can see, the forecaster has identified different areas for large hail, damaging wind, and tornadoes.

Benefits of the probabilistic Convective Outlooks

We believe the new Probabilistic Convective Outlooks issued by the SPC do a better job of expressing uncertainty, as well as detail, compared to the traditional Convective Outlooks. These outlooks directly express forecaster uncertainty through the use of probabilities. Further, in the Day 1 period, forecaster expectations of large hail, damaging winds, and tornadoes are explicitly conveyed through individual forecasts. By producing forecasts of each hazard individually, users who are sensitive to one particular threat (e.g., car dealers and large hail) can make more informed decisions.

Even without a complete understanding of what the probabilities mean, you can directly assess from the graphics:

  • Geographic areas where the various severe weather hazards are expected. These areas may or may not overlap with one another.
  • The perceived levels of threat for the severe weather hazards. The higher the probabilities are, the increased threat of that hazard occurring. Refer to the discussion above concerning the probabilities used in the outlooks and especially the range of probabilities used.
  • Areas where significant severe weather is expected.

-Richard Thompson

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