MID-LATITUDE CYCLONES Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

Ahrens, Ch. 8

Mid-Latitude Cyclones

Mid-Latitude Cyclone (MLC) A cyclonic storm that most often forms along a frontal boundary in the middle latitudes 30 – 60 °N/S Region of low pressure Counterclockwise in the Northern Hemisphere Synoptic scale: 1,000 – 5,000 miles across Period: 2 – 10 days Produces large areas of clouds and precipitation

Why are MLCs important? Bring a variety of significant weather Can produce snow storms, tornadoes, hail, strong winds, rain storms, ice storms, etc. Transport warm air from subtropical regions Transport cold air from the polar regions Transport warm air from the surface to the upper atmosphere Redistribute moisture

Cyclogenesis

Cyclogenesis Means origin or intensification of a cyclone Part of the life cycle of a midlatitude cyclone For MLCs, this involves a linkage between fronts and low pressure areas near the Earth’s surface, and the Polar Jet near the tropopause.

Divergence occurs downwind of an upper-level trough and upwind of an upper-level ridge ALOFT H

UPWARD VERTICAL MOTION DIVERGENCE ALOFT POLAR JET UPWARD VERTICAL MOTION (CHIMNEY) L LOW-LEVEL CONVERGENCE SOUTHWEST NORTHEAST

Chimney SURFACE

Cyclogenesis The MLC’s surface low forms directly below the area of upper-level divergence For the surface low to continue intensifying the upper-level trough and low must continue to position themselves such that the low is right under the area of upper-level divergence, and, The upper divergence must be stronger than the low-level convergence

Cyclogenesis Surface low will try to remain beneath area of upper-level divergence East of the upper-level trough Follows flow of Polar Jet Stream The MLC begins dying when the upper-level low or trough moves directly overhead the surface low Surface low no longer positioned below the area of upper-level divergence

Life Cycle

Polar Front Theory Theory to describe life cycle of a midlatitude cyclone formation and development For cyclogenesis of an MLC to occur, it needs Boundary between warm and cold airmasses at the surface THE POLAR FRONT POLAR FRONT CAUSES POLAR JET STREAM Curvature of the Polar Jet Stream AN UPPER-LEVEL TROUGH PROVIDES DIVERGENCE ALOFT STAGES

(a) Initial Stage Front separates cP and mT air masses Manifested as stationary front Wind flow is parallel to front in opposite directions Low-level convergence at the front Beginning of cyclonic wind flow

(b) Frontal Wave Wavelike kink develops With sufficient divergence aloft, surface low develops Boundary between airmasses is deformed by rotation: Cold front and warm fronts develop Overunning creates precipitation mostly north of warm front

(c) Growing Wave System moves with mid-level wind 500 mb Steers the storm Central pressure drops Isobars encircle low and wind speeds increase Precipitation forms along cold front Warm sector develops

(d) Mature Wave System moves eastward and strengthens Warm sector decreases in size as cold front begins overtaking warm front Widespread precipitation Comma cloud can be identified on satellite images

(e) Occlusion Cold front overtakes warm front Storm is at its most intense Can’t intensify anymore Clouds and precipitation cover large area Comma cloud develops dry slot

(f) Dissipation (The End) Low separates from fronts Cut off from divergence aloft Low-level convergence begins filling low Cold air encircles the surface low No supply of energy storm dies

(a) Initial Stage (b) Frontal Wave (c) Growing Wave (d) Mature Wave (e) Occlusion (f) The End

North American MLCs

North American MLCs Midlatitude Cyclones affecting the CONUS have distinct formation regions Some more common in Winter when Polar Jet is “low zonal” – Large waves; flow has large north-south excursions Systems are strong and slow moving Some more common in Summer when Polar Jet is “high zonal” – Small waves; jet is mostly straight from west to east Systems are weaker and relatively fast moving

L L L L L L NORTH AMERICAN MLCs GULF OF ALASKA LOW ALBERTA CLIPPER GREAT LAKES LOW L L HATTERAS LOW COLORADO/ WEST TEXAS LOW L GULF OF MEXICO LOW

ALBERTA CLIPPER L L L L L L

GREAT LAKES LOW L L L L L L

COLORADO/WEST TEXAS LOW

GULF OF MEXICO LOW L L L L L L

CAPE HATTERAS LOW L L L L L L

GULF OF ALASKA LOW L L L L L L

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter – often follows Hatteras Low by about 24 hrs Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

North American MLCs Alberta Clipper Great Lakes Low Move out of Canadian Rockies very fast in the middle of the winter Great Lakes Low Often forms from same upper-air feature as Gulf of Alaska Low Colorado/West Texas Low Form on the lee side of the Rockies in CO, OK, WY, or TX Gulf of Mexico Low Form over the Gulf of Mexico Affect the East Coast Cape Hatteras Low Affect East Coast Produce Nor’easters in New England Alaska Low Form over the Gulf of Alaska Affect Pacific Northwest GENERALLY DO NOT REACH NEW ENGLAND

Nor’easters Known by meteorologists as “coastal” storms Intense MLC offshore of the northeastern US Produce strong NE winds along the coast Normally develop off the coast of North Carolina and move northward along the coast Hatteras low moving northward along coast

Nor’easters Frequently very intense in the Gulf of Maine Associated with Heavy snows in the NE, heavy rains further south, strong winds, high waves, etc.

Context

Are cyclonic storms unique to the Earth? Context Are cyclonic storms unique to the Earth?

Mars

Are cyclonic storms unique to the Earth? Context Are cyclonic storms unique to the Earth? No.

Review

Overview of MLC Cyclogenesis Definition Scale and period Importance to midlatitude weather Cyclogenesis Role of Polar Front, Polar Jet, Divergence aloft and low-level convergence Meteorological “bomb” Conditions necessary for continued intensification

Stages of Life Cycle North American MLC’s Initial Frontal Wave Growing Wave Mature Wave Occlusion The End North American MLC’s Alberta Clipper Great Lakes Low Colorado/West Texas Low Gulf of Mexico Low Cape Hatteras Low Alaska Low

Additional Graphics Sources http://www.capelinkscapecod.com/images/nor_easter_1.jpg http://www.brandi.org/photos/snowstorm-16-17feb2003/page2.html