Severe Weather Lecture Notes

Overview of Severe Weather Topics

• Main topics: thunderstorms, lightning, tornadoes, hurricanes.
• Initial focus: thunderstorms
  • Types: air mass thunderstorms, frontal thunderstorms, orographic thunderstorms.

Thunderstorms

Air Mass Thunderstorms

• Definition: Form within a single air mass.
• Characteristics:
  • Convectional lifting: warm, humid air rises within the air mass.
  • Common in maritime tropical (Mt) systems (warm temperatures, high moisture).
  • Stages:
    1. Cumulus stage: Formation of cumulus clouds with convectional uplift.
    2. Mature stage: Development of cumulonimbus clouds, leading to precipitation.
    3. Dissipating stage: Descending air cools and halts moisture supply.
  • Duration: Typically 30-60 minutes.
• Locations:
  • Common in South and East Texas, Gulf Coast.

Frontal and Orographic Thunderstorms

• Frontal Thunderstorms:
  • Caused by colliding air masses along a frontal boundary.
  • Severe along cold fronts due to rapid uplift.
  • Can create cumulonimbus clouds that overshoot the tropopause.
  • Potential for intense thunderstorms with hail.
• Orographic Thunderstorms:
  • Uplift caused by topographic effects (mountain ranges).
  • Clouds will tilt, creating long-lived storms with updrafts and downdrafts.
• Gust Fronts and Downbursts:
  • Gust Front: Result of descending air hitting the ground and creating strong winds.
  • Microburst and Macroburst:
    • Microburst: Smaller, intense downburst.
    • Macroburst: Larger downburst.
  • Downbursts can be wet or dry.

Severe Thunderstorm Features

• Wall Clouds
  • Indicate severe thunderstorms, potential rotation.
• Mammatus Clouds
  • Indicate atmospheric instability.
• Shelf Clouds
  • Pronounced boundary layer at frontal slopes.
• Roll Clouds
  • Indicates horizontal rotation, less severe.
• Squall Lines
  • Long lines of thunderstorms (hundreds of miles long).
  • Form at cold front boundaries, with intense updrafts and downdrafts.
  • Leads to supercell thunderstorms with severe weather potential.

Wind Patterns in Severe Thunderstorms

• Straight-line Winds: Indicate deratio events.
  • Long-lived storms with straight-line wind damage.
  • Most common in warm seasons with strong pressure gradients.
• Deratio: Long-lived storms with damaging winds:
  • Types: Serial and progressive.
  • Bow Echo: Distinct radar signature indicating strong winds.
  • Common in central US, especially in warm seasons.

Tornadoes and Severe Thunderstorms

• Formation:
  • Interaction between different air layers and jet streams.
  • Wind shear and inversion layers play key roles.
  • Violent uplift can tilt clouds, potentially leading to tornadoes.
• Key Indicators:
  • Two jet streams crossing directions.
  • Warm, wet air below cold, dry air, causing inversion.
  • Rapid cloud growth and vertical tilting of clouds.

Damage Patterns

• Downbursts:
  • Air descends and spreads out from a center point.
• Deratio:
  • Straight-line wind damage: trees and structures knocked down in the same direction.
• Tornadoes:
  • Spiral damage patterns due to the rotational nature of tornadoes.

Summary

• Differentiating between various severe weather phenomena is crucial for accurate forecasting and damage assessment. Understanding the characteristics and patterns of thunderstorms, deratios, and tornadoes can help in mitigating their impacts.