MC

Weather Patterns and Fronts Flashcards

Early Weather Setup

  • The early weather setup might reflect a Rossby wave from the jet stream.
  • The jet stream divides cold and warm air.
  • Bends in the jet stream can lead to cold air outbreaks and warm air moving north.

Mid-Cycle Weather Patterns

  • The mid-cycle looks familiar, with a cold front catching up.
  • Warm air at the surface is rapidly lifted along the area of occlusion.

Wind Dynamics

  • Wind direction is driven by the pressure gradient over a local area.
  • Low pressure at the center implies higher pressure all around.
  • Air rushes in from all directions to converge on the low pressure zone.

Air Sectors

  • Three sectors:
    • Warm
    • Cool
    • Cold
  • Wind direction shifts as warm and cold fronts approach and pass.

Wind Direction and Warm Air Zones

  • In the Northern Hemisphere, winds need a southerly component to maintain a warm air zone.
  • Behind a cold front, winds tend to have a west-northwest component, bringing in colder air.
  • In the cool sector, air comes from the north to northeast, bringing cooler air.

Reading Weather Maps

  • You can read isobars on a weather map to determine wind direction more precisely.
  • Conceptually, at location one (warm front approaching), the wind direction would likely have an east-northeast component as air rushes towards the low.
  • Variations occur based on high-pressure zones. A strong high-pressure zone to the east might cause more easterly winds; a high-pressure zone to the north might lead to a more northern component.

Basic Principles of Wind

  • Air must converge on the low from high pressure zones.
  • Wind direction shifts as the warm front passes, exposing the location to southeasterly winds.
  • After the warm front passes, winds shift to a southerly direction.

Occlusion Process

  • As the occlusion progresses, warm air at the surface is lost, causing the system to break down.

Resetting After Occlusion

  • After occlusion, cooler air remains, requiring a new setup with blocks of cold and warm air for another low to form.
  • Global surface maps often show multiple such systems stacked up off the coast, especially in winter.
  • Typical winter conditions involve storms every three to five days, with cold fronts and warm fronts passing through.

Southern Hemisphere

  • In the Southern Hemisphere, weather patterns occur in reverse.

Wind Progression

  • Wind progression goes from northeasterly to southerly winds, then westerlies after the cold front passes.

Reading Isobars

  • Isobars are lines connecting equal air pressure.
  • High pressure moves towards low pressure, dictating wind direction.
  • By reading isobars, we can discuss wind direction even without knowing about mid-latitude cyclones.

Examples of Isobar Interpretation

  • At location D, with high pressure to the north, the wind direction is northeasterly.
  • At location B, with high pressure off the coast, there's a more easterly component, but it's still warm air coming from the south onshore.
  • Near the cold front, there's a westerly flow due to a ring of high pressures.

Lab Task

  • Write a forecast for letter E (ahead of the warm front, cold front, and behind the cold front).
  • Use locations B and A as proxies.

Forecasting Tips

  • To determine precipitation at letter D after the cold front passes, consider what's happening at letter A right now.
  • Sketch out the progression of uncertainty.

Exam Style Questions

  • Simple questions focusing on relative conditions at different locations.
  • Example: "Temperatures at location one are blank compared to two" (warmer).
  • Example: "Precipitation at two is blank compared to number three" (heavier).
  • Recognize a cold front and associated weather patterns, temperature, etc.

Front Speeds

  • Warm fronts move at about 10 miles per hour.
  • Cold fronts move at 20 to 40 miles per hour.
  • Significant temperature differences can cause even faster movement.

Northwest Weather Patterns

  • Constant parade of fronts moving through.
  • Even if a cold front is approaching, it is still technically a cold front if colder temperatures are behind it.
  • Shoulder seasons (fall and spring) offer the best opportunities for robust thunderstorms due to contrasting air masses.

Temperature Differences

  • Surface heating ramps up, leading to warm air, while colder air slides in.
  • Large temperature differences are possible in the shoulder seasons.
  • Example: 86 degrees in Central Washington followed by 60 degrees.

Thunderstorms

  • Eastern Washington has numerous thunderstorms in the spring due to converging warm and cool air.
  • Two main drivers of thunderstorms:
    • Convective lifting (warm air rising).
    • Cold fronts.

Thunderstorm Timing

  • Thunderstorms most common in the afternoon due to convection.
  • Thunderstorms at two in the morning usually indicate a cold front situation.
  • Timing and seasonality offer clues about the mechanism driving the storm.

Rare Phenomenon: Thundersnow

  • Thundersnow: A thunderstorm happening while it's snowing.
  • Requires a huge temperature gradient.

Thundersnow Explanation

  • Ocean temperature remains relatively constant (40–50 degrees).
  • Cold Arctic air outbreak leads to below-freezing surface temperatures and extremely cold temperatures high in the atmosphere (-20 to -40 degrees).
  • Warm air rises from the ocean into extremely cold air, rapidly cooling and producing thunderstorm conditions.
  • Storm produces snow falling through cold air.

Analogous Situation in Eastern Washington

  • Surface temperatures of 80 to 90 degrees combine with freezing air aloft.