Deciphering Surface Weather Maps (Part Two)

In part one (click here), I introduced surface weather maps, meteorological time-keeping systems, the difference between Issued and Valid, and barometric pressure. In part two, we’ll look at the symbols and meteorological shorthand used on surface weather maps.

Pressure Patterns
The solid black lines winding across the country on the forecast map valid at 00Z on Thursday, February 23, 2017 (figure 1) are isobars, which are contours of constant sea level barometric pressure measured in millibars (mb). Isobars are typically drawn at 4 mb intervals on NWS maps and are labeled somewhere along the contour. The 1012 mb isobar, for example, starts northeast of the Bahamas and crosses the US coast near the border of Georgia and South Carolina (click here for annotated image). After heading west for a bit, the 1012 mb isobar makes a right turn and heads northeast parallel to the Appalachian Mountains. At any point along this isobar, sea-level pressure is forecast to be 1012 mb. Isobars allow forecasters to understand the overall pressure pattern and quickly identify areas of low and high pressure, along with other surface features such as troughs and ridges.

Figure 1: Surface Forecast from the Weather Prediction Center.

Highs & Lows
Areas of high and low pressure, marked by blue Hs and red Ls, are easy to spot. Near each High or Low is a number indicating its sea-level pressure. For example, the High off the coast of Virginia is 1019 mb, while the Low centered over western Oklahoma is 1000 mb. The average annual sea-level pressure across the United States is approximately 1013 mb, although the threshold separating high and low pressure designations is dependent on the prevailing weather pattern. In other words, sometimes a pressure reading, such as 1010 mb, indicates an area of high pressure and at other times it is low pressure – it’s all relative to the overall weather pattern.

Air Masses
Many of the symbols on surface weather maps identify boundaries, or fronts, between air masses. An air mass is a body of air that extends across a large area (hundreds of square miles or more) and is nearly uniform with respect to temperature and moisture. The nature of an air mass is influenced by the region in which it originates. For example, an air mass surging south from Canada is typically cooler and drier than an air mass moving north from the Gulf of Mexico. The boundaries where these air masses meet are important to weather forecasting because they are often associated with rapid changes in temperature, abrupt wind shifts, precipitation, and thunderstorms. In other words, weather happens near frontal boundaries.

Figure 2: Air Masses

Frontal Boundaries
Frontal boundaries – more commonly referred to as fronts — are associated with low pressure systems. However, not not all Lows have fronts, particularly small-scale Lows that form in response to local conditions. While Lows may or may not have frontal boundaries, Highs never have fronts.

Figure 3: Surface Boundaries And Features

The designation of a boundary as a stationary, cold, or warm front is determined by the movement of the colder air mass. Warmer air always yields to the flow of the colder and denser air mass. If the boundary between a cold and warm air mass is stalled or moving at 5 knots or less, it is called a stationary front. Stationary fronts are drawn as alternating blue triangles and red half-circles on opposing sides of the line (click here for an example).

Cold fronts mark the boundary between colder and warmer air when the colder air is advancing towards the warmer air. Cold fronts typically extend from a Low and are drawn as a blue line with blue triangles (click here for an example). The blue triangles are placed on the warm side of the boundary and indicate the direction the front is moving. On average, cold fronts move forward at about 20 knots.

When the cold air along a boundary is retreating instead of advancing, the boundary is identified as a warm front. Warm fronts are drawn as a red line with red half circles (click here for an example). The red half circles, which appear on the cold side of the line, show the direction the front is moving . The nature of warm fronts results in an average forward speed of 10 knots – about half the speed of cold fronts.

As a low pressure system matures, its faster moving cold front often overtakes the warm front. This boundary is an occluded front and appears as a series of alternating purple triangles and half circles along the same side of a purple line (click here for an example). An occluded front marks the boundary between the advancing cold air behind the cold front, and retreating cold air ahead of the warm front. The triangles and half circles signal the direction the front is moving.

Although often included in descriptions of surface boundaries, troughs are not a boundary between air masses. Instead, troughs — shown as a yellow dashed line — are elongated areas of low pressure within an air mass. (A lengthy trough extends from southwestern Canada to southeastern Wyoming on Figure 1.) Although troughs don’t usually produce severe weather or dramatic temperatures changes, their passage often results in significant wind shifts. While areas of high pressure never have frontal boundaries, they do have ridges which are elongated areas of high pressure extending from the high’s center (click here for an example).

A Final Word About Frontal Boundaries
Since the type of the front is determined by whether the colder air mass is advancing or retreating, fronts are often reclassified from one type to another. For example, if an advancing cold air mass stalls (to less than 5 knots), the boundary transitions from a cold front to a stationary front. An advancing cold air mass that stalls and then retreats, transitions from a cold front, to a stationary front, and then to a warm front. It’s part of what makes watching the weather so interesting.

Forecasting Precipitation & Thunderstorms
The WPC’s surface weather forecasts extend seven days into to the future. In addition to showing the overall barometric pressure pattern (isobars) and frontal boundaries, the maps covering the initial 2 ½ days display the probability of precipitation and thunderstorms.

Figure 4: Forecasting Precipitation Type

The likelihood of precipitation, precipitation (rain or snow) or thunderstorms occurring in a particular region is shown through the use of color shading and hatching (closely spaced parallel lines). The WPC separates the precipitation type and probability into the ten categories listed in Figure 5.

Figure 5: WPC Precipitation Type Categories

A more detailed description of the ten categories and how to identify them on WPC forecast maps appears below:

  • NDFD Rain (Chance): A 10% chance of at least 0.01 inches of rain is shown in light green. Click here for sample image
  • NDFD Rain (Likely): Greater than a 10% chance that at least 0.01 inches of rain will fall is shown in dark green. Click here for sample image.
  • NDFD Snow (Chance): A 10% chance of measurable snow (0.01” of liquid water if melted) is show in light blue. Click here for a sample image.
  • NDFD Snow (Likely): Greater than a 10% chance of measurable snow (0.01” of liquid water if melted) is shown in dark blue. Click here for sample image.
  • NDFD Mix (Chance): A 10% chance there will be a precipitation mix, such as a combination of rain and snow, rain and sleet, or snow and sleet. Click here for sample image.
  • NDFD Mix (Likely): Greater than a 10% chance there will be a precipitation mix, such as a combination of rain and snow, rain and sleet, or snow and sleet. Click here for sample image.
  • NDFD Ice (Chance): A 10% chance of at least 0.01” of freezing rain. Click here for sample image.
  • NDFD Ice (Likely): Greater than a 10% chance of at least 0.01” of freezing rain. Click here for sample image.
  • NDFD Thunderstorm (Chance): A 10% chance of thunderstorms. The areas where a chance of thunderstorms exist are overlaid on areas where rain is expected, and are shown with light red parallel diagonal lines surrounded by a light red border. Click here for sample.
  • NDFD Thunderstorms (Likely and/or Severe): A greater than 10% of thunderstorms or a potential for severe thunderstorms. A severe thunderstorm is expected to produce hail greater than 1” in diameter, a wind gust greater than 50 knots, or a tornado. Areas are shown in dark red. Click here for a sample image.

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