Chapter 9 -- Weather Patterns & Fronts


The primary weather producer in the middle latitudes (for our purposes, the region between southern Florida and Alaska, essentially the area of the westerlies) is the middle-latitude or midlatitude cyclone. Middle-latitude cyclones are large low pressure systems with diameters often exceeding 1000 kilometers (600 miles) that generally travel from west to east. They last a few days to more than a week, have a counterclockwise circulation pattern with a flow inward toward their centers, and have a cold front and frequently a warm front extending from the central area of low pressure. In the polar front theory, (also called the Norwegian cyclone model), wave cyclones develop in conjunction with the polar front.

Power Point Presentations

Fronts Clouds and Precipitation 4 Ways Air Raises Chapter 9 Fronts

Fronts are boundary surfaces that separate air masses of different densities, one usually warmer and more moist than the other. As one air mass moves into another, the warmer, less dense air mass is forced aloft in a process referred to as overrunning. The five types of fronts are
(1) warm front, which occurs when the surface (ground) position of a front moves so that warm air occupies territory formerly covered by cooler air,
(2) cold front, where cold continental polar air actively advances into a region occupied by warm air,
(3) stationary front, which occurs when the air flow on both sides of a front is neither toward the cold air mass nor toward the warm air mass,
(4) occluded front, which develops when an active cold front overtakes a warm front and wedges the warm front upward,
(5) a dryline, a boundary between dry, dense air and less dense humid air often associated with severe thunderstorms during the spring and summer. The two types of occluded fronts are the cold-type occluded front, where the air behind the cold front is colder than the cool air it is overtaking, and the warm-type occluded front, where the air behind the advancing cold front is warmer than the cold air it overtakes.

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Tonight’s Low
24hr Low Temperatures
48hr Low Temperatures

Surface Station Model

Temp (F)
Dewpoint (F)
Pressure (mb)
Sky Cover
Wind (kts)
Data at Surface Station
Temp 45 °F, dewpoint 29 °F,
overcast, wind from SE at 15 knots,
weather light rain, pressure 1004.5 mb

Station Plots - NE Surface - USA Surface Plot Map
How to read 'Surface' weather maps
How to Read Station Plots

Sky Cover Wind
Shaft is direction wind is coming from
Fronts and Radar Selected
Weather Symbols
clear   Calm cold front 


1/8   1-2 knots (1-2 mph) warm front 

Rain Shower

scattered   3-7 knots (3-8 mph) stationary front 


3/8   8-12 knots (9-14 mph) occluded front 


4/8   13-17 knots (15-20 mph) trough 


5/8   18-22 knots (21-25 mph) squall line 

Snow Shower

broken   23-27 knots (26-31 mph) dryline  Freezing Rain
7/8   48-52 knots (55-60 mph) Radar Intensities
{radar intensities}
Freezing Drizzle
overcast   73-77 knots (84-89 mph) Fog
obscured 103-107 knots (119-123 mph) Haze


Dust or Sand
Blowing Snow

Wind Direction

Middle Latitude Cyclone
primary weather producer
large Low pressure system, travels west to east
lasts a few days to more than a week
counter-clockwise rotation, net flow inward
cold front and sometimes warm front extend from center
cloud development, precipitation

Middle Latitude Cyclone - Weather Conditions

A Falling pressure, high cirrus clouds. Clouds lower and thicken as warm front approaches.
B As front nears, precipitation increases, temperature rises, and winds change from E to SE.
C S winds, warm temperatures, clearing skies or fair weather cumulus.
D Gusty winds, precipitation along cold front. Possibly severe weather, thunderstorms. Winds change from SW to NW, N.
E Rising pressure, cooler dry air, clearing skies as high pressure builds in.
F-G Temps remain cool, continual drop in pressure, increasingly overcast. This area often generates snow storms in winter.

Cold Fronts

    Before Passing   While Passing   After Passing
Winds   south-southwest   gusty; shifting   west-northwest
Temperature   warm   sudden drop   steadily dropping
Pressure   falling steadily   minimum, then sharp rise   rising steadily
Clouds   increasing: Ci, Cs and Cb   Cb   Cu
Precipitation   short period of showers   heavy rains, sometimes with hail, thunder and lightning   showers then clearing
Visibility   fair to poor in haze   poor, followed by improving   good, except in showers
Dew Point   high; remains steady   sharp drop   lowering

Warm Front

    Before Passing   While Passing   After Passing
Winds   south-southeast   variable   south-southwest
Temperature   cool-cold, slow warming   steady rise   warmer, then steady
Pressure   usually falling   leveling off   slight rise, followed by fall
Clouds   in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer   stratus-type   clearing with scattered Sc; occasionally Cb in summer
Precipitation   light-to-moderate rain, snow, sleet, or drizzle   drizzle or none   usually none, sometimes light rain or showers
Visibility   poor   poor, but improving   fair in haze
Dew Point   steady rise   steady   rise, then steady

According to the polar front model, cyclones form along fronts and proceed through a generally predictable life cycle. Along the polar front, where two air masses of different densities are moving parallel to the front and in opposite directions, cyclogenesis (cyclone formation) occurs and the frontal surface takes on a wave shape that is usually several hundred kilometers long. Once a wave forms, warm air advances poleward invading the area formerly occupied by colder air. This change in the direction of the surface flow causes a readjustment in the pressure that results in almost circular isobars, with the low pressure centered at the apex of the wave. Usually, the position of the cold front advances faster than the warm front and gradually closes the warm sector and lifts the warm front. This process, known as occlusion, creates an occluded front. Eventually, all the warm sector is forced aloft, and cold air surrounds the cyclone at low levels. At this point, the cyclone has exhausted its source of energy, and the once highly organized counterclockwise flow ceases to exist.

Guided by the westerlies aloft, cyclones generally move eastward across the United States. As an idealized midlatitude cyclone moves over a region, the passage of a warm front places the area under the influence of a maritime tropical air mass and its generally warm temperatures, southerly winds, and clear skies. The passage of a cold front is easily detected by a wind shift, the replacement of a south or southwesterly airflow with winds from the west or northwest. There is also a pronounced drop in temperature. A passing occluded front is often associated with cool, overcast conditions, and snow or glaze during the cool months.

Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In a cyclone, divergence aloft does not involve the outward flow of air in all directions. Instead, the winds flow generally from west to east, along sweeping curves. Also, at high altitudes, speed variations within the jet stream cause air to converge in areas where the velocity slows, and to diverge where air is accelerating. In addition to speed divergence, directional divergence (the horizontal spreading of an air stream) and vorticity (the amount of rotation exhibited by a mass of moving air) also contribute to divergence (or convergence) aloft.


Intensifying Surface Cyclone:
For example, if a region of diverging winds at upper levels is stronger than the converging winds of a suface low pressure center below it, the low will deepen (intensify). This is because more air is being removed from the vertical column of air above the low than flowing into it, causing the pressure at the surface to decrease. A drop in pressure means an intensification of the low pressure center.

Weakening Surface Cyclone:
In contrast, if a region of diverging winds at upper levels is weaker than the converging winds of a suface low pressure center below it, the low begins to fill (weaken). This is because more air is flowing into the vertical column of air above the low than flowing out of it, causing the pressure at the surface to increase. An increase in pressure means a weakening of the low pressure center.

During the colder months, when the temperature gradients are steepest, cyclonic storms advance at their greatest rate. Furthermore, the westerly airflow aloft tends to steer these developing pressure systems in a general west-to-east direction. Cyclones that influence western North America originate over the Pacific Ocean. Although most Pacific storms do not cross the Rockies intact, many redevelop on the lee (eastern) side of these mountains. Some cyclones that affect the United States form over the Great Plains and are associated with an influx of maritime tropical air from the Gulf of Mexico. Another area where cyclogenesis occurs is east of the southern Appalachians. These cyclones tend to migrate toward the northeast, impacting the eastern seaboard.

Due to the gradual subsidence within them, anticyclones generally produce clear skies and calm conditions. One to three times each winter, large highs, called blocking highs, persist over the middle latitudes and deflect the nearly zonal west-to-east flow poleward. These stagnant anticyclones block the eastward migration of cyclones, keeping one section of the nation dry for a week or more while another region experiences one cyclonic storm after another. Also due to subsidence, large stagnant anticyclones can produce a temperature inversion that contributes to air pollution episodes.

In the spring, Earth's pronounced north-south temperature gradient can generate intense cyclonic storms. At a midlatitude location, as a spring cyclone with its associated fronts passes, temperatures can change quickly from unseasonably warm to unseasonably cold, and thunderstorms with hail can be followed by snow showers.

Sample Station Plot 
[Print version]

station plot sample
For more information about an item marked with a (*), click on the appropriate link:
[Weather]  [Wind]  [Sea-Level pressure]  [Pressure trend]  [Sky cover]

Click here if you are interested in sample ship or buoy observations.


A weather symbol is plotted if at the time of observation, there is either precipitation occurring or a condition causing reduced visibility.
Below is a list of the most common weather symbols:

weather symbols

Wind is plotted in increments of 5 knots (kts), with the outer end of the symbol pointing toward the direction from which the wind is blowing. The wind speed is determined by adding up the total of flags, lines, and half-lines, each of which have the following individual values:

Flag: 50 kts
Line: 10 kts
Half-Line: 5 kts

If there is only a circle depicted over the station with no wind symbol present, the wind is calm. Below are some sample wind symbols:

examples of wind symbol

Sea-level pressure is plotted in tenths of millibars (mb), with the leading 10 or 9 omitted. For reference, 1013 mb is equivalent to 29.92 inches of mercury. Below are some sample conversions between plotted and complete sea-level pressure values:

410: 1041.0 mb
103: 1010.3 mb
987: 998.7 mb
872: 987.2 mb


The pressure trend has two components, a number and symbol, to indicate how the sea-level pressure has changed during the past three hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred. Below are the meanings of the pressure trend symbols:

example of pressure tendency

The amount that the circle at the center of the station plot is filled in reflects the approximate amount that the sky is covered with clouds. Below are the common cloud cover depictions:

example of sky cover


Product Legends
Surface Fronts and Boundaries Precipitation Areas and Symbols

Surface Fronts and Boundaries
In addition to High and Low centers, you may see one or more of the following eight features on a surface analysis or forecast.  The definitions provided below are derived from the National Weather Service Glossary.

Cold Front - a zone separating two air masses, of which the cooler, denser mass is advancing and replacing the warmer.
Warm Front - a transition zone between a mass of warm air and the cold air it is replacing.
Stationary Front - a front between warm and cold air masses that is moving very slowly or not at all.
Occluded Front - a composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front.  Two types of occlusions can form depending on the relative coldness of the air behind the cold front to the air ahead of the warm or stationary front.  A cold occlusion results when the coldest air is behind the cold front and a warm occlusion results when the coldest air is ahead of the warm front.
Trough - an elongated area of relatively low atmospheric pressure; the opposite of a ridge.  On WPC's surface analyses, this feature is also used to depict outflow boundaries.
Squall Line - a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.
Dry Line - a boundary separating moist and dry air masses.  It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).
Tropical Wave - a trough or cyclonic curvature maximum in the trade wind easterlies.
Depiction of frontogenesis and frontolysis
Frontogenesis refers to the initial formation of a surface front or frontal zone, while frontolysis is the dissipation or weakening of a front.   Frontogenesis is depicted on WPC's surface analysis and forecast charts as a dashed line with the graphical representation of the developing frontal type (the blue triangle for cold fronts, the red semicircle for warm fronts, etc...) drawn on each segment.   For example, the image below shows a forming cold front.

Frontolysis is depicted as a dashed line with the graphical representation of the weakening frontal type drawn on every other segment.  Below is an example of a dissipating warm front.

Precipitation Areas and Symbols
Areas of precipitation expected at the valid time of the forecast are outlined in green. Shading within these lines, or lack of shading, indicates the expected coverage (not intensity) of precipitation. Examples:

Note that on the full color forecast graphics with the terrain background, the above dashed line is blue. (View an example)

Below are symbols found on our short range forecasts that represent categories (and in some cases intensities) of precipitation. In forecast areas where the form of the precipitation is expected to vary, two symbols will be depicted and separated by a slash (/). For instance, rain showers and thundershowers are often combined in regions where convection is forecast.

24 Hour Precipitation Total - Day 1

Day 1 QPF
Day 1 QPF
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6 Hourly Precipitation Amounts - Day 1

Update (00-06 hr QPF) (00-06 hr)
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06-12 hr QPF 06-12 hr.
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12-18 hr QPF 12-18 hr.
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18-24 hr QPF
18-24 hr.
[contours only]
24-30 hr QPF
24-30 hr.
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0-6 hr snowfall forecast 6-12 hr snowfall forecast 12-18 hr snowfall forecast
0-6 hr Snowfall Forecast
06-12 hr Snowfall Forecast
12-18 hr Snowfall Forecast
18-24 hr snowfall forecast 24-30 hr snowfall forecast 30-36 hr snowfall forecast
18-24 hr Snowfall Forecast
24-30 hr Snowfall Forecast
30-36 hr Snowfall Forecast