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
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 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 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 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 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
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
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.
Comments/suggestions?
Richard Thompson
