Chapter 14, Part 2
Weather Forecasting Using Surface Charts
Overview
1. Predict motion of warm fronts, cold fronts, highs, and lows. 2. Based on these predictions, forecast the weather at different locations.
Current weather map.
Surface Weather Map 6AM, Tues.
• Note: Cold front, warm front, H, L, snow and rain
1 Determining Movement of Weather Systems
1. Short time intervals (6 hours) direction and speed same. 2. Lows tend to move parallel to isobars in warm air ahead of cold front. 3. Lows tend to move to the region of greatest pressure drop. 4. Surface pressure systems tend to move in same direction as wind at 500mb level (~18000 ft).
Surface Weather Map 6AM, Tues.
• Rules 1 and 2 indicate movement to the NE.
Isallobars
• Lines connecting points of equal pressure change. • Low intensifies as it moves forward, and H off East coast weakens, while H in west remains strong.
2 500-mb Chart
• Upper level winds also indicate the storm will move to the NE.
12 and 24 Hour Movement
Movement over Cities
• Augusta, GA – H to east, warm front approach • Washington, DC – warm front approaching • Chicago, IL – cold front passes through • Memphis, TN – warm front, then cold front • Dallas, TX – cold front passing through • Denver, CO – cP high
3 Typical Airflow at a Cold Front
• Cold dense air forces warm air upwards. • Warm moist air rises and condenses into clouds, producing rain showers at the front. • Leading edge of front is steep (1:50 for a 25 knot front).
Typical Weather at a Cold Front
Before During After Wind S or SW Shifting W or NW Temperature Warm Dropping Cooler Pressure Falling Minimum Rising Precipitation Short Heavy Clearing showers showers Visibility Fair to poor Poor Good Dew point High Sharp drop Lowering
Typical Airflow at a Warm Front
• Warm air rises over cold air, • creating clouds and rain showers ahead of the front. • Leading edge of front has a gentle slope (1:300). • There is a temperature inversion (frontal inversion) just ahead of the front.
4 Typical Weather at a Warm Front
Before During After Wind S or SE Shifting S or SW Temperature Cool Rising Warmer Pressure Falling Minimum Rising Precipitation Light to Drizzle Usually moderate none Visibility Poor Poor, improving Fair Dew point Steady rise Steady Rise
Augusta, GA
“Clear and cold this morning with moderating temperatures by afternoon. Increasing high clouds with skies becoming overcast by evening. Cloudy and not nearly as cold tonight and tomorrow morning. Winds will be light and out of the south or south east. Barometric pressure will fall slowly.”
• High (anticyclone) moving out to east • Warm front moving in
Washington, DC
“Increasing clouds today and continued cold. Snow beginning by early Wednesday morning, possibly changing to rain. Winds will be out of the southeast. Pressures will fall.”
• Warm front moving in
5 Chicago, IL
“Cloudy and cold with light snow beginning by noon becoming heavy by evening and ending by Wednesday morning. Total accumulations will range between 6 and 10 inches. Winds will be strong and gusty out of the east or northeast today becoming northerly tonight and northwesterly by Wednesday morning. Barometric pressure will fall sharply today and rise tomorrow.” • Cold front passes through
Memphis, TN
“Cloudy and cool with light rain, low clouds, and fog early today, becoming partly cloudy and warmer by late this afternoon. Clouds increasing with possible showers and thunderstorms later tonight and early Wednesday morning and turning colder. Winds southeasterly this morning, becoming southerly and southwesterly this evening and shifting to northwesterly by Wednesday morning. Pressures falling this morning, leveling off this evening, then falling again tonight and rising by Wed. morning.”
• Warm front passes through • Cold front passes through
Dallas, TX
“Increasing cloudiness and mild this morning with the possibility of showers and thunderstorms this afternoon. Clearing and turning much colder tonight and tomorrow. Winds will be southwesterly today, becoming gusty north and northwesterly this afternoon and tonight. Pressures falling this morning, then rising later today.”
• Cold front passes through
6 Denver, CO
“Clear and cold through tomorrow. Northerly winds today becoming light and variable by tonight. Low temperatures tomorrow morning will be below zero. Barometric pressure will continue to rise.”
• High of cP air moves through
Summary
• We discussed four methods for predicting the movement of weather systems. • For an example, based on the movement of fronts, highs, and lows, we forecast the weather in six different cities in the US. • Current weather map.
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