Outlook Product Information
Knowing how to interpret severe weather outlooks is key to better understanding of
what is expected in your area, when, and what elements to look for. This in turn,
helps everyone plan their daily activities accordingly, and be watchful for such
conditions later in the day. In recent years, our outlooks have slowly been evolving
with greater details of what is forecast, where, and approximate time frame of
occurrance. Such "explicit" details will help foster an even greater
understanding of what specific elements are forecast, streamlining an even better
and quicker way to get the message to everyone. This is the primary reason
for every outlook product issued by our "Storm Center". Our office issues a suite
of forecast products for everything from public to marine interests on two of our
Great Lakes. We have redesigned this page since there are ongoing changes with our
outlooks. You can learn more about the upcoming changes to our outlook products by
This is a listing of all current "operational" products
online now. We also have new products we are testing, and will be placing online
soon as experimental products. To view the latest products being tested at this
time, head here. This page will be
updated with new information as new products are released.
- CATEGORICAL: The "categorical" method of forecasting
severe local storms began in earnest with the formation of a specialized component
to the USWB that's "United States Weather Bureau", in 1950. When that office began
to release what was then referred to as a "severe weather forecast" for the
day, it contained three individual categorical levels of risk for severe storms.
Those risk levels are illustrated below:
SLIGHT RISK: This indicates widely isolated severe storms are possible.
MODERATE RISK: This indicates scattered to numerous severe storms are possible.
HIGH RISK: This indicates widespread severe storms are possible.
Over the years since it's inception with the U.S. Weather Bureau in 1950, SELS or
"Severe Local Storm Warning Center", has extensively modified the definitions to
each existing categorical level, to better correspond with the latest ongoing trends
with storm development. Today, both the "Storm Prediction Center" and our "Storm Center"
still actively use this legacy method of predicting such severe local storms,
although some new changes have recently been made to the existing categories, with two
new categorical levels added. The categorical outlook also comes with an accompaning
"probablistic" section, which can more accurately expresses the degree of threat
in terms of percentage contours. Each categorical risk has pre calculated percentage
contour levels. Those levels reflect the likelihood (degree), of severe weather expected.
Thus, the higher the probability, the more likely those severe weather elements
(i.e. tornadoes, damaging wind, or hail), are. As mentioned above, two new categorical
levels were added to the existing levels shown above. Those being "limited" and
"elevated". These additions, together with the probabilities, will foster even
greater understanding of the severe weather potential. So, the new categorical levels
now used are as shown below.
New risk categories
Limited risk implies widely isolated severe thunderstorm coverage (5 percent).
This means that if severe storms do develop, they will be quite scattered and low in
- Thunderstorms (no label)
Elevated risk implies isolated to scattered severe thunderstorm coverage
(between 25 and 30 percent). If severe storms develop, they will be scattered in
- THUNDERSTORM INITIATION: This is basically a timing issue of
when we believe severe thunderstorms will form. Since thunderstorms are fairly small
on the mesoscale, and many factors can play a role in where or when they develop,
it can be very difficult to determine exactly when a storm will form. For these
reasons, we use "estimations" of the most likely time frames a storm may form.
Keep in mind here these estimations are not always cold-cut either, and in some
cases, storms may not develop at all, or outside the time frame specified.
- THUNDERSTORM MODE: This section will make reference to the type of
thunderstorms which are expected. Modes of thunderstorms can and are also expressed
as "classifications" of such storm types based on certain parameters which breed them.
There are many different and unique storm classifications or "types", but a few of
the more common types are shown below:
LINEAR, or the more common "squall line". Squall lines can also be broken
line segments as well.
SINGLECELL, which also is known as a "renegade" or "hit or miss" storm. This
storm type is basically one single storm.
MULTICELL, which consists of clusters, broken segments, or groups of loosly
organized storms. Intensity is modulated by organization.
SUPERCELL, like the singlecell above, supercells too, are separated from other
storms which allows them to feed off the moist environment for miles around, and
rapidly mature into a violent and very destructive type of thunderstorm.
MESOSCALE CONVECTIVE SYSTEM, is basically a well organized large
thunderstorm complex, which often forms along or near warm fronts, or perhaps a
stationary front. Very heavy rainfall, hail, and high winds are common with these
Given the current limitations in the prediction of such severe local storms, you need
to understand that forecasting the specific type of storm which may occur, is quite
challenging and difficult at best. For this reason, we try to select the most likely
storm mode which is possible, and mention that in our current severe weather outlook.
- SEVERE THUNDERSTORM FORECAST DISCUSSION: This is the multi-
part text portion of each outlook (days 1-3). Here, our forecasters attempt to
summerize the situation expected and explain the meteorological reasoning for the
forecast. Until 2010, much of this section of the outlook was written in contracted
format using technical lingo which only those with a good solid background in
meteorology could understand. In May of 2013, a move was made to eliminate all or
parts of the contractions used, and technical wording. This made it possible for
all our clients and the public to understand better.
Here is a breakdown of the existing section:
HIGHLIGHTS - This section will provide a brief "summary" of any expected
convection across the coverage region both severe and non severe. If severe
weather is expected, this section will highlight major city locations which may be
effected. This information is useful with the media and other groups.
SYNOPSIS - Here, the forecaster will discuss the meteorological reasoning
for the forecast. While much of the contractions and technical wording has been
omitted, there are occasions when some may still appear. Even using plain language
format, this subsection can be difficult to understand without a good background
DISCUSSION - This subsection will outline those areas under risk for
severe thunderstorm development, and provide additional details of what is to
come, and other relavant information. This is explained in plain language format,
so most people can usually understand it without much difficulty.
PRIMARY AREAS OF CONCERN - This section will specify the specific areas we
are watching for a more concentrated or robust thunderstorm development. It will
also be used for honing in on significant tornado development as well.
SIGNIFICANT WEATHER - Thia aubsection will highlight the potential for any
significant or widespread major severe weather event. This would include any major
tornado or Derecho outbreaks. However, it should be noted that such major and/or
significant severe outbreaks are rare in our coverage region. This is explained
in plain language format as well.
- NEXT OUTLOOK UPDATE: This basically informs people of when the
next update to the outlook is scheduled. The day 1 outlooks are routinely updated
several times during the period. This is to ensure the latest information is posted
online for our clients and public. The day 2 outlooks are routinely updated at
least twice per day for the following day period. The day 3 outlooks are routinely
updated once per day for the day 3 period.
- 4-8 DAY SEVERE OUTLOOKS: This product is an "extended" version
of our shorter range (1-3 day) outlooks. This outlook product will discuss the
potential for any severe weather for the outlook period, which is based on current
and future surface and upper air data from the medium range forecast guidance. The
discussion is a mix of technical and plain language formats, but is suitable for
the average person to understand. This outlook covers a five (5) day period
beginning with the end of the day 3 period. The valid period of each day during
this outlook is from 1200 UTC (700 am), to 1200 UTC (700 am) of the next day of
the outlook period. These outlooks are routinely issued once per day.
- GREAT LAKES SEVERE OUTLOOK: This outlook contains a discussion
of the potential for severe thunderstorms over two of the main lakes - Michigan
and Superior. Each lake has it's own separate forecast details. These discussions
are broken down into subsections for easier interpretation and understanding:
WATERSPOUT PROBABILITY: Provides the probabilities for waterspout
development. This is expressed in percentages from the available guidance.
DAMAGING WIND PROBABILITY: Provides the probabilities for thunderstorm
wind gusts exceeding 40 knots inclusive. This too, is expressed in percentages.
LARGE HAIL PROBABILITY: Provides the probabilities for large diameter hail
of 1.00" or larger. Again, this is expressed in percentages from the guidance.
TOTAL SEVERE PROBABILITIES: This provides a combined total of all hazard
probabilities, which like the above elements is expressed as a percentage.
SEVERE THUNDERSTORM POTENTIAL: This section expresses both the categorical
and probablistic chances for severe thunderstorm development. The probabilities
noted in this section are not the same as for inland severe weather
outlooks. We use a different calculation for marine severe probabilities. Also
included in this section is a brief discussion in plain language explaining what
is expected and other factors which may trigger the storms.
TIMING OF STORM DEVELOPMENT: Here we attempt to approximate when such
severe weather may form. Thunderstorms are fairly small scale features on the
mesoscale, and many factors may be involved in their formation and where they do
develop. This makes it difficult at best to predict where or when they will form.
Therefore, we will use estimated time frames of when we believe they may occur.
PRIMARY STORM MODE: This provides all mariners with the type(s) of
thunderstorms expected. Thunderstorms can occur in one or more modes.
PRIMARY STORM MOTION VECTOR: This provides critical information as to
storm movement (direction of travel), and speed of such movement. This is
expressed as a numerical value, and is based largely on wind patterns at differing
heights into the atmosphere. For example "27030" would mean the storms will be
moving from west to east at 30 knots. The 270 part of this figure represents the
degrees on a compass, and the 30 denotes the speed in knots. All this is very
important to larger shipping interests and mariners alike.
PRIMARY HAZARDS EXPECTED: This summerizes what severe elements are expected
with the storms. Remember, severe thunderstorms produce large hail, destructive
winds, dangerous lightning, and waterspouts. When out on the open water, be
watchful for signs of developing storms.
WIND AND WAVES: This provides critical information about current and
forecast surface wind patterns, along with existing and expected wave heights.
Some of this information is based on marine data, while the rest is based on
the latest surface observational and computer generated data. This information
is a must have for all mariners and shipping interests alike.
CURRENT WEATHER WATCH AREAS: This is a summary of any current severe
weather watches issued by SPC which affects all or portions of lakes Michigan or
Superior. All affected nearshore and open water zones will be highlighted on the
graphics, and mentioned in this subsection, as well as the valid time period of
the watch. If a watch is extended, replaced, or cancelled, this information will
- HEAVY RAINFALL OUTLOOK: This product is a forecast of the
expected heavy or significant rainfall accumulation, and potentials for flood or
flash floods. For more information, please go here.
- OFF SEASON OUTLOOK: This product is used during the "off"
season months for severe weather (1-October to 31-March). This is primarily a
current-day 1 type of outlook, and is updated several times as needed, to provide
the latest information. The overall format of this outlook product is similar to
our day 1 in season outlook. These outlooks are only issued when a threat
for severe weather is expected on days during the off season months. It is not
very common for severe weather during this off period, but has occasionally
If you have questions, just e-mail us!