How Do the Blizzards Form

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How Do the Blizzards Form?
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The magic of water on earth
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Review of last lecture

• Tropical cyclone genesis 6 necessary conditions,
  4 stages
• Tropical cyclone tracks
• Tropical cyclone structure 3 major components,
  rotation direction of inflow and outflow, 3
  feedbacks
• Tropical cyclone destruction 4 reasons? Which
  side has the most intense destruction? Why?
• Tropical cyclone forecast track and intensity
  Currently which skill is better?

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Weather-related Disasters Winter Storms

U.S. Annual mean fatalities 57,
annual mean loss 329 million
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The Streets
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What is a mid-latitude cyclone?

• The mid-latitude cyclone is a synoptic scale low
  pressure system that has cyclonic
  (counter-clockwise in northern hemisphere) flow
  that is found in the middle latitudes (30N-55N,
  30S-55S).
• It has a larger size than a tropical cyclone

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Midlatitude cyclones often form near the jet
streams

• Caused by steep temperature gradients between
  cold and warm air masses
• Polar front - marks area of contact, steep
  pressure gradient ? polar jet stream
• Low latitudes ? subtropical jet stream
• Stronger in winter, affect daily weather patterns
  

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How does a mid-latitude cyclone form?
In mid-latitude there is a boundary between
northern cold air and southern warm air
In the boundary a initial cyclone can advect warm
air northward and cold air southward
If the upper level low is to the west of surface
low, the cyclone will amplify and precipitation
will form.
Mature stage. Cold air begins to catch up with
warm air (occluded).
Cold air cools down the cyclone. Dissipation.
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Why do some frontal waves develop into huge
cyclonic storms, but others dont?

• Complex challenge to forecasting
• Atmospheric conditions at the surface and aloft
  affect cyclogenesis.
• Key is to look at the upper level winds
  (longwaves, shortwaves).

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Longwave disturbances (Rossby waves)
Earth's poles are encircled by 3 to 6 longwaves,
or Rosby waves, directing upper level winds
around lows at the 500 mb surface. Small
disturbances in these waves can trigger storms.
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Shortwave Disturbances
Shortwave ripples within the Rossby waves move
faster, and propagate downwind into the Rossby
troughs and cause them to deepen. Barotropic
conditions, where isobars and isotherms are
parallel, then degenerate into a baroclinic state
where the lines cross and cold or warm air is
advected downwind.
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Regions of cyclogenesis and typical tracks

• Gulf of Mexico, east coast
• Alberta Clipper from eastern side of Canadian
  Rockies
• Colorado Low from eastern slope of American
  Rockies
• Lee-side lows, lee cyclogenesis

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The life cycle happens as the cyclone moves
eastward
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Summary

• The developmental stages and vertical structure
  of middle latitude cyclones (boundary between
  northern cold air and southern warm air, upper
  level low to the west of surface low)
• How upper level longwaves and shortwaves may
  enhance cyclonic development at the surface
  (upper level low to the west of surface low)
• The three regions of cyclogenesis and typical
  tracks