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Severe Weather Forecast Jargon Explained

By: Courtesy of The National Weather Service
By: Courtesy of The National Weather Service

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

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 issued 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, Moderate, and High Risk. This graphical outlook, however does not display the forecaster's expectations of the individual severe weather hazards (large hail, damaging winds, and tornadoes). While the accompanying discussion for the outlook usually describes the forecaster's thoughts about the individual hazards, the accompanying categorical graphic does not.

Example Day 1 categorical Convective Outlook

The outlook graphic defines the geographic threat areas. In the example above, a large Slight Risk is forecast for portions of the Mississippi and Ohio Valleys. The graphic does not indicate the forecaster expectations of the individual severe weather hazards. 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 than slight, moderate and high risk by themselves. (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 known as subjective probabilities. The forecasters make their best estimate of the probability of an event occurring. The probability values forecast are not created automatically by a computer or via statistics, but by the SPC outlook forecaster.

The probabilities that you see on the graphics represent the probability of one or more events occurring within 25 miles of any point during the outlook period. This definition is used as the probability of severe weather at an given point is quite small. Also 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. 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? For most people, the answer is never. Now think of how many times severe weather has occurred within 25 miles of your location. It's probably safe to say that you can think of some close by severe weather events. How large of an area is a circle with a 25 mile radius? Below you'll see the Oklahoma City metro area where the large blue circle represents such an 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 any one neighborhood. Keep this in mind as we further discuss the probability values expressed in the outlooks.

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

Although 30% is not a very large probability for having 1 or more of these severe weather hazards occur near you on a given day, it represents 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 experience 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 east of the Rockies (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. You can immediately say that the forecaster believes the chance of rain is twice as high as normal. 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 climatological knowledge of the event being forecast is useful, even necessary for interpreting the probabilities being forecast. In the case for probability of precipitation forecasts, these values typically run from 0% (certainty that it will not rain) to 100% (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.

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 was developed by the National Severe Storms Laboratory (NSSL) for use by the National Weather Service, 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 determining the severe weather climatology for your area.

Since 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 pinpoint the specific location for severe weather. The product is a national-scale forecast that highlights areas where severe weather is possible over the lower 48 states. Since 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 range of probabilities used in the various probabilistic outlooks:

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

How should you interpret these values? 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 extreme severe weather events and are used 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 they believe the risk of tornadoes in that region is 20 times larger than climatology. By comparing the forecast probability to climatology, you can better determine 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 categorical (slight, moderate, high) 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 extreme severe weather. Extreme 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 extreme severe weather occurring in the outlook area, then the hatched area will not appear on the graphics.

Day 2/Day 3
Probabilistic Outlooks are issued for the Day 2/3 period as well. Since many of the specific details of severe weather forecasting can only be determined hours ahead of time, rather than several days, the severe weather probabilities for the Day 2 and Day 3 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 extreme 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, when forecast.

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

Conventional Categorical Outlook

Probabilistic Hail Outlook

Probabilistic Wind 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 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). The forecaster for this event identified different areas for large hail, damaging wind, and tornadoes.

Benefits of the probabilistic Convective Outlooks
We believe the new Probabilistic Convective Outlooks 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 extreme severe weather is expected.


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