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 smallscale, shortterm 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 nationalscale 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 more
signiifcant 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 significant severe weather.
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 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 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, 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 significant severe weather is expected.
Comments/suggestions, please send to
spc.feedback@noaa.gov.
