Weather Forecasting Basics Introduction

Weather

• Meteorology: Unit 4

Air Masses & Fronts

• Air Mass: A large body of air with similar temperature and moisture at a given altitude.
• Source Region: The area where an air mass originates (often polar or tropical). Mid-latitudes are where air masses clash, producing weather activity.

Air Mass Classifications

• Humidity:
• Maritime (moist)
• Continental (dry)
• Temperature:
• Polar (cold)
• Tropical (warm)

Air Masses of North America

• cP and cA: Produce extreme cold winter weather.
• Lake Effect Snow: Occurs when extremely cold air moves across a warm body of water (like the Great Lakes), creating snowstorms.
• mP: Responsible for cold, damp weather along the NE coast of N. America and for cold, rainy winter weather along the west coast of N. America.
• mT: Responsible for hot, muggy weather found in the eastern US in summer.
• cT: Responsible for heat waves in the western half of the US in summer.

Fronts

• Front: A transition zone between two air masses of different densities. Often associated with temperature and humidity differences.
• 4 Types:
• Cold Front
• Warm Front
• Stationary Front
• Occluded Front

Locating Fronts on a Surface Map

• Sharp temp. change: Over relatively short distances.
• Changes in moisture: Content (shown by dew point).
• Shifts in wind direction.
• Pressure & pressure changes.
• Clouds & precipitation patterns.

Cold Front

• Stable air replaces unstable air.
• Drawn with a solid blue line with blue triangles showing the direction of movement.
• Often has rain and strong thunderstorms along the leading edge.
• Air behind cools quickly.
• Tend to move toward S, SE, or E.
• Occasionally, special conditions create a 'back door' cold front, moving from the east.
• Cold air damming can occur when a cold front stalls at the Appalachian Mts.

Warm Front

• Warm, unstable air replaces colder air.
• Drawn with a solid red line with red half-circles (semi-circles) along the front showing the direction of movement.
• Moves much slower than cold fronts (~½ speed).
• Associated with light rain and more gradual changes than a cold front.

Stationary Front

• Essentially no movement.
• Red semi-circles point towards colder air.
• Blue triangles point towards warmer air.
• The line marks the boundary where cold, dense air meets warmer air.
• Often the transition to either a warm front or cold front.

Occluded Front

• Forms when a cold front catches up to and overtakes a warm front.
• Shown by a purple line with alternating cold front triangles and warm front half circles.
• On the same side of the line.
• Pointing in the direction the front is traveling.
• Air behind the front is colder and drier than air ahead of it.
• Can have heavy precipitation where the cold front overtakes the warm front.

Mid-Latitude Cyclones

• Fluid boundary where warm tropical air masses meet cold polar air masses. Known as the polar front.
• Ridges and troughs move and change ("stretchy") with different air masses.
• Meeting of contrasting air masses (polar and tropical) leads to mid-latitude weather systems.

Cyclogenesis

• Generation of depressions and formation of cyclones.
• The Norwegian Model (1910s-1920s): Depressions form from waves that develop along a polar front.
• Stages:
  1. A segment of the polar front is a stationary front. High pressure on both sides. Warm air below, blowing from the west and cold air above, blowing from the east. Wind shear creates cyclonic wind.
  2. A low-pressure system forms at the point of rotation (central pressure), the junction of the two fronts. The stationary front begins moving, with the warm front to the east and the cold front to the west. Precipitation begins to form.
  3. Cold air displaces warm air upward, creating precipitation ahead of the warm front and a "kink" in the pressure system. The pressure difference is greater as the faster moving front (cold) moves toward the warm front (slower).
  4. The cold front moves faster than the warm front, the warm sector shrinks in size and central pressure drops. The system becomes a mature cyclone and has an increased likelihood of intense thunderstorms and widespread rain.
  5. The cold and warm fronts merge, forming an occluded front at the point of central pressure (the "triple point"). The system begins to dissipate as frontal air moves away.

Convergence and Divergence

• Convergence: The piling up of air in a region when it slows down.
• Divergence: The spreading out of air in a region when it speeds up.
• Both occur all along the polar front.
• On surface maps, convergence is depicted with converging arrows, and divergence with diverging arrows.

Conditions for Cyclogenesis

• Ventilation/air flow
• Sharp contrast between temperatures of opposing air masses
• Jet stream plays a role (and can shift or change patterns).
• Sharp contrast between opposing air masses is crucial for cyclogenesis. Areas like the Hatteras and Colorado lows offer these contrasting conditions.
• Hatteras Low: Warm gulf water creates warm, moist air below the stationary front. This often creates Nor'easters.
• Colorado Low: Forms on the leeward side of the Rockies. Provides mP air and continental air (P or T).

Special Cyclonic Storms

• Bomb Cyclone: When the central pressure falls by more than 24 mb in 24 hours.
• March 26, 2014: Example of a bomb cyclone where rapid cloud development and heavy rain occurred.

Weather Forecasting Methods

• Numerical Weather Prediction (NWP): Routine daily forecasting using computers and mathematical equations. Using atmospheric conditions from current measurements.
• Models: Used because atmospheric conditions are always changing and based on temperature information.
• Forecast Types:
• Short range (12-72 hours, 3 days). Doppler radar and surface weather maps & recognition.
• Medium range (3-8 days). Statistical forecasts and charts
• Long range (8+ days). Computer modeling and "outlooks," overview of expected conditions.
• Climate prediction (forecasting for seasons.)

Weather Observations

• Data from land-based stations (temperature, dew point, wind, pressure) with over 10,000 stations.
• Ships and buoys
• Airports gather data every hour.
• Weather balloons.
• Satellites (visible and infrared images)
• Doppler radar (information on rain, snow, sleet, & hail)
• Various warnings.
• Computer models used to create analyses (and forecasts)
• Large to small patterns and analyses.

Forecasting Techniques

• Persistence: Predicting weather will be the same as current conditions.
• Trend: Predicting the continuation of existing conditions.
• Pattern recognition: Using similarities in past events to anticipate future outcomes.
• Statistical: Making predictions based on numerical data analysis.
• Probability: Based on prior data for the likelihood of a specific event (like a white Christmas).
• Weather type: Grouping patterns (such as when a jet stream dips down).
• Climatological forecasts: Made months in advance and based on typical patterns for an area.

What is a Chance of Rain?

• Percentage of area coverage that will experience rain.
• Percentage of time rain falls in a given period.
• Confidence level of the forecaster's prediction.
• (Random) number a dog might pick based on its food bowl

Time Range of Forecasts

• Very short range: Up to 6 hours. Data from Doppler radar, pattern recognition, experience. Forecasters distinguish between Watch (conditions favor hazardous weather) and Warning (hazard is occurring).
• Short Range: 12 to 72 hours (3 days). Uses Doppler radar & surface maps.
• Medium range: 3 to 8 days into the future. Uses statistical forecasts.
• Long range: Beyond 8 days, up to 16 days. More of an overview, using computer models to project conditions.
• Climate: Seasonal forecasts, often from teleconnections like El Niño and La Niña.

Accuracy and Skill in Weather Forecasting

• A bias for fair weather exists, making a simple forecast for fair weather half the time already statistically average.
• Skilled forecasters improve predictions beyond persistence and climatology.

Determining Movement of Weather Systems

• Mid-latitude cyclones and fronts move in the same direction and speed as the past six hours.
• Low-pressure areas move parallel to isobars of warm air, toward the greatest pressure drop.
• High-pressure areas move toward the greatest pressure rise.