Midlatitude cyclones

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

Air mass Large body of air with uniform temperature and moisture Air acquires characteristics from underlying surface Two air masses important to SE 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

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 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

MLC’s redistribute heat by mixing air masses.

Cyclogenesis Interaction of surface and upper atmospheric processes Ideal surface conditions: –Strong baroclinity: contrast in temperatures over a short distance: creates surface low pressure Ideal upper atm conditions: –Trough in polar jet stream –Creates upper level low pressure Polar front is location where both criteria met

High pressure ridge Open high (no closed circulation) Warm air Cold air Low pressure trough Open low (no closed circulation) Creates downstream speed and directional divergence that enhances instability Geostrophic winds on a map of geopotential heights Surface winds on a surface isobaric map

Early stageOpen wave stage Occluded stage Occluded front formation marks a decaying MLC. Dissipating stage Lose organization 7-10 days after redistributing the heat that powers them Zone of strong surface baroclinity

Nor’easter Strong fall through spring midlatitude cyclone that moves up the US East coast. Intensity enhanced by: –Strong contrasts in temperature between land (cold) and water (warm) at surface –Strong upper level (jet stream) support 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

Began as a hurricane Tropical storm characteristics merged with nor’easter (extratropical) conditions Hybrid storm Created large wind field