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.