Determining Severe Weather Potential
Dark skies, the bright dazzling flash of lightning and the
ominous roar of thunder. It can terrify
some, while fascinating others. What
causes violent severe storms and how can we know in advance that they may
strike? First, let’s define some
terms. The National Weather Service
classifies “severe” storms as those containing winds in excess of 58 mph, hail larger than 1” in diameter or producing
tornadoes. A 58 mph wind is strong
enough to knock a tree branch down or topple a small, weak tree. One-inch diameter hail is roughly the size
of a quarter. So even if you see dark
clouds and lightning and hear a lot of thunder, the storm may not be considered
“severe”, it may just scare you out of your wits.
Meteorologists study current weather patterns and the output
from various computer models to determine if the “right” conditions exist for
severe weather to occur. This analysis
is based on whether proper amounts of wind shear, temperature, moisture
differences and atmospheric energy are present in certain a orientation and
strength. However, it’s not an exact
science; even the most seasoned forecaster will admit that. In many ways, its pattern recognition – if
enough ingredients have combined in the past to produce severe weather, odds
are they will likely do so again.
It’s the duty of the meteorologists at the Storm Prediction Center
(SPC) in Norman, Oklahoma to assess storm potential and identify parts of the
U.S. that may be impacted by severe thunderstorms and tornadoes. They prepare an outlook for risk areas three
days in advance, categorizing risk areas in order of ascending danger as
follows: thunderstorm, slight, moderate and high.
Thunderstorm means that storms are expected
in this area, but they may not become severe.
Of course all thunderstorms are inherently dangerous because they
contain lightning, but they may not be classified as “severe” according to the
definition above. Slight risk typically means scattered severe storms are possible, producing
scattered wind damage or severe hail and possibly some isolated tornadoes. Moderate
risk means more a widespread outbreak containing numerous tornadoes severe
wind damage and very large and destructive hail could occur. High
risk indicates the likelihood of a major tornado outbreak or an
extreme downburst wind (derecho) event.
This risk suggests widespread strong or violent tornadoes and destructive
straight-line winds accompanied by damaging large hail.
Typically, these risk areas
are drawn broadly, covering several states because it is just not possible to
forecast with precision the exact location of expected severe weather. I liken it to baking a cake – if all the
ingredients are mixed properly and the pan is set in the oven at the right
temperature for the right duration, you will likely get a cake.
When severe weather is expected, SPC forecasters usually post a watch anywhere from six to eight
hours before the onset of any storm activity.
This is a “heads-up” to people in the affected areas where
either severe thunderstorms or tornadoes are expected. These watches may cover several counties in
one state or multiple counties across several states. A watch is your notice that it is time to plan
and prepare. It also signals local
forecast offices in the watch area to be ready to monitor developing
storms.
Once a storm reaches severe
limits, forecasters at local National
Weather Service offices issue a warning. This means that one or more of the severe
criteria mentioned above has been reached and a real danger exists to the
public. This is the time to take
protective action. Warnings typically can last anywhere from 15
to 30 minutes. The warning area shape approximates
where the storm is expected to move during the warning period. It may cover a part of a whole county, or
segments of several counties, depending on their orientation. If the storm continues moving, a new warning
may be issued covering the new target locations.
Depending on the movement
of the storm threat, SPC may decide to remove some counties from the watch or
add new ones. So even if you are not
under a watch now, it is important to stay aware of changing weather conditions
in the event that the threat moves toward you.
Severe weather is most likely in the spring and fall, but is possible
any time of the year, depending on weather conditions.
You can read more on the difference between a watch and a warning here. To learn more about severe
weather safety awareness, click here.
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