HPC Analysis 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 HPC’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.|
A hash mark denotes a change in frontal type, as in the example below.
Note: The hash mark will always be drawn perpendicular to the boundaries. They are not drawn at “triple points” (the intersection of an occluded, cold and warm or stationary front) and where a low pressure center separates the different frontal types.
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 HPC’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.
HPC Surface Analysis Sample Station Plot
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:
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:
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:
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: