Note: For
probability to categorical outlook conversion, please see table in HTML.
**CHANGE LINK to http://www.spc.noaa.gov/misc/SPC_Prob_Conv_Otlk_Change_20060214.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
The graphic clearly defines the geographic threat areas. A large Slighty 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 any given point is quite small
and because the Convective Outlook is not a smallscale, shortterm
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.
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 29May6.
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 nationalscale
forecast that highlights areas where severe weather is possible over the
lower 48 states. The product is
simply not meant to be a localized, shortterm 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 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
click to view the full image
This image shows 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 bottom right panel
shows the conventional 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 panels of the figure show the forecast probabilities of
hail (upper right), damaging wind (lower left), and tornadoes (lower
right). 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 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.
Comments/suggestions?
Richard Thompson
