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
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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.
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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
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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
Heavy
showers
Poor
Clearing
Visibility
Short
showers
Fair to poor
Dew point
High
Sharp drop
Lowering
Good
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.
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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
Drizzle
Visibility
Light to
moderate
Poor
Dew point
Steady rise
Steady
Usually
none
Poor, improving Fair
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
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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
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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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