Thunderstorms
Chapter 13 review
• Abepersistence
forecast is a prediction that future weather will
the same as the present weather, whereas a climatological
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forecast is based on the climatology of a particular region. A
steady-state forecast assumes the weather systems will
continue to move in the same direction and at the same speed as
they have been moving so far.
Weather forecasts for up to a few hours are called very shortrange forecasts; those that range from about 6 hours to a few
days are called short-range forecasts; medium-range forecasts
extend from about 3-5 days into the future, whereas long-range
forecasts extend beyond, to about 8.5 days.
Seasonal outlooks provide an overview of how temperature and
precipitation patterns may compare with normal conditions.
For a forecast to show skill, it must be better than a
persistence forecast or a climatological forecast.
Predicting the movement of weather systems
♦ Steady-state method
♦ Lows (Highs): towards the greatest pressure drop (rise)
♦ In the direction of the winds aloft (5500 m)
Examples
Thunderstorms
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• Favorable conditions:
Storm with lightning and thunder. Also heavy
precipitation, sometimes hail, gusty surface winds.
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♦ Unstable atmosphere
♦ Strong convection (convective storm)
Buoyant force – the force on a less-dense
object immersed in a denser environment
Trigger for the convection:
♦ Weather fronts - fast uplift of warm air
♦ Unequal surface heating
♦ Surface convergence and divergence aloft
♦ Topographic barriers
♦ Arrival of cold air aloft
Eureka!
Ordinary (Cell) Thunderstorms
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• winds have no significant change of either strength or
Also: Air mass thunderstorms, pop-up thunderstorms.
Typically they form where the surface converging
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direction with height (limited wind shear). What goes
up comes back down in the same place.
Typical summer afternoon thunderstorms
Last no more than an hour
Extend no more than a kilometer
Rarely produce strong winds or large hail
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Stages of Development - I
Cumulus (growth) stage:
♦ Updraft: warm and moist air is
uplifted, expands, condenses
and forms towering clouds.
♦ Latent heat is released -the
temperature in the cloud is
higher than outside the cloud,
air keeps rising.
♦ The updrafts are strong so that
the cloud droplets remain
suspended in the cloud.
♦ Usually no precipitation and
lightning during this stage.
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Stages of Development - II
Mature stage:
♦ Precipitation starts
♦ Entrainment: dry air from
around the cloud is drawn into
the cloud. Some of the cloud
droplets evaporate and chill
the air.
♦ Downdraft: the cold and heavy
air is sinking
♦ The updraft and the
downdraft form a storm cell.
♦ Gust front: the boundary
between the cold and the
warm air at the surface
♦ Lightning and thunder
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Stages of Development - III
Dissipating stage:
♦ the updraft weakens
♦ the gust fronts move away
♦ the precipitation is light
♦ the downdrafts dominate:
cut off the fuel supply of
the storm
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Multicell Storms
The downdraft from the dissipating storm fuels the
formation of next storm cell.
Multicell storm complex: example
Severe Thunderstorms
• At least one of the following:
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♦ Large hail precipitation
♦ strong wind gusts
♦ tornadoes..
The longer a storm lasts the grater the chance for it to become a
severe storm. Often forms along a weather front.
They typically form if strong vertical wind shear (large changes of the
wind with height) is present. The warm air updraft is not suppressed
by the downdraft and the precipitation.
Shelf cloud
Roll Cloud
Supercell thunderstorms
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rotating updraft
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Supercell storm: consists
of a single violently
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Favorable conditions:
when the speed and
direction of the winds
aloft change with height
♦ formation of a rotating
updraft
Updraft and downdraft
do not cross -> the storm
lives on for hours
Often produce large hail,
damaging surface winds,
tornadoes
A supercell thunderstorm
with a tornado sweeps over Texas
Skip the discussion around
Figs. 14.6 and 14.7
Squall Lines and MCCs
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• Mesoscale convective systems: a
Squall line: A line system of
thunderstorms. It shows as a line
of storms on the radar images.
It forms along or in front of an
advancing cold front.
Pre-frontal squall lines may be due
to gravity waves
large circular cluster of storms
Thunderstorm Movement
• winds aloft (middle troposphere).
The storms typically move in the direction of the
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Some move at 30 deg to the right of the winds aloft.
(Effect of surface friction).
In general the multicell storms move in the direction
where the humid and unstable conditions in the
atmosphere prevail.
The squall line storms move in the direction of the
front movement.
Flooding
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• the amount of water that can drain away from the region.
Flash floods: floods which rise rapidly with little or no
advance warning
Conditions: the influx of water into an area is more than
♦ The ground is saturated from a prolonged rainy season
♦ Very strong precipitation over a short time period
♦ Not so strong precipitation but for an extended timestorms reoccur at the same location (training).
♦ Stationary weather fronts can result in a series of
thunderstorms over the same region.
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Thunderstorm Climatology
Most thunderstorms occur in Florida
♦ Unstable atmospheric conditions (warm and moist
surface air) prevail throughout the year.
♦ Summertime afternoon air mass thunderstorms.
Most severe thunderstorms occur in the Great Planes.
♦ The warm (unstable) air is dry and shallow and the ice
crystals do not have time to melt.
Stormy days / year
Days with hail / year