Download Low pressure trough

Midlatitude cyclones
Air mass
• Large body of air with uniform temperature
and moisture
• Air acquires characteristics from underlying
surface
• Two dominant air masses influence eastern
US
– Maritime tropical (mT) from Gulf of Mexico/Atlantic
• Warm and moist; potentially unstable
– Continental polar (cP) from interior of Canada
• Cool and dry; stable
Our single-digit temperatures
are caused by Arctic air
masses instead of Canadian
ones.
Fronts
• Boundary between two air masses
• Type of front is designated by the air
mass that is moving
– Cold front-cold advances on warm
– Warm front-warm advances on cold
– Stationary front-neither warm nor cold air
mass is moving
Cold front-cold advances on warm
Warm front-warm advances on cold
Midlatitude cyclones
Midlatitude cyclones
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Large low pressure storm system
consisting of a warm and cold front
Dominant weather system for US in
winter when polar jet stream further
south
Develops along the polar jet stream
Migrate W to E
Also called wave cyclones
Can create sharp contrasts in
weather conditions over a couple of
days
Since they occur in conjunction with
the polar jet stream, they are mainly
active at our latitude in Lexington
during the fall through spring
Cyclogenesis – formation of a
MLC
• Requires interaction of surface
and upper atmospheric
processes
• MLCs are a deep atmospheric
phenomena
• Surface conditions needed:
– Strong baroclinity, i.e., a sharp
contrast in temperatures over a
short distance. This enhances
surface low pressure and
instability
• Upper atm conditions:
– Trough in polar jet stream, i.e., an
upper level low pressure
Polar front is location where
both criteria met
High pressure ridge
Open high (circulation
is not closed)
Cold air
Warm air
Geostrophic winds on a map of
geopotential heights
Surface winds on a surface isobaric map
Low pressure trough
Open low (circulation is not
closed). The trough creates
downstream speed and
directional divergence that
enhances instability
Early stage
Open wave stage
Zone of strong surface baroclinity
Dissipating stage
Occluded stage
Occluded front
formation marks a
decaying MLC.
MLCs lose organization
7-10 days after
redistributing the heat
that powers them
MLC’s act to balance the heat budget
1. Zonal polar jet stream flow
2. Azonal polar jet stream flow
(waves develop in the polar jet
stream)
3. Back to zonal flow
MLC circulation creates lifting and
potential for precipitation
Conveyers belts
are areas in a MLC
where lifting occurs
and thus where
there is the potential
for precipitation
Nor’easter
• Strong midlatitude cyclone that moves up the
US East coast
• Requires strong polar jet stream support
(troughing with speed and directional
divergence)
• Intensity fueled by strong baroclinic zone
along land-sea border (large temperature
contrasts between cold land and warm sea)
• Bombing: extreme drop in central barometric
pressure of a nor’easter
Nor’easters
• Produce heavy snowfall events for the
southeast, mid-Atlantic, and New
England
• Can produce winds as strong as a
hurricane
• May last for several days and impact a
large coastal area (hurricane damage
often restricted to a much smaller area)
Famous nor’easters
• Storm of the
Century (1993)
• The Perfect
Storm (1991)
• Ash Wednesday
Storm (1962).
(3/1993) Storm of the Century
Hurricane-strength
winds on west
coast of Florida
Ash Wednesday Storm (also called the Five High storm because
it lasted five days)
Superstorm Sandy
Superstorm Sandy
• Began as a
hurricane
• Tropical storm
characteristics
merged with
nor’easter
(extratropical)
conditions
• Hybrid storm
• Created large wind
field
• Identify and describe the North American air masses
that influence the weather patterns for Lexington
• Differentiate between frontal types and recognize
their map symbols
• Explain how precipitation patterns may vary between
a warm and a cold front
• Describe the stages of cyclogenesis
• Detail the weather conditions associated with the
passage of a midlatitude cyclone
• Describe how a midlatitude cyclone forms from the
interaction of upper atmosphere and surface features
• Define and describe directional divergence and
speed divergence
• Distinguish between a noreaster and a hurricane