Surface Analysis Chart


  • Surface Analysis Charts are computer-generated charts with frontal and pressure analysis issued from the Hydro-meteorological Prediction Center (HPC)
  • Found at
Surface Analysis Chart
Figure 1: Surface Analysis Chart

Issuance & Validity:

  • Issued every 3 hours
  • Valid Time (VT) of the chart on the lower left corresponds to the time of the observations
  • Valid time is given in UTC
  • Information is about 2-3 hours old
  • Information is all observed data


  • Isobars are drawn as solid lines in order to represent pressure
  • Each interval is spaced at 4 millibars (mb)
    • hectoPascals (hPa) are the metric equivalent of millibars
  • Gradient, or pressure gradient force, is measured by how far apart the isobars are from one another
  • When the pressure gradient is very shallow, intermediate isobars (short dashed lines) are sometimes drawn at one-half the standard interval

Pressure Systems:

  • The letter "L" denotes low pressure
  • The letter "H" denotes high pressure
  • The pressure center of each is indicated by a three or four digit number that is the central pressure in mb (hPa)
Dry Line
Figure 3: Dry Line
Figure 2: Trough
Tropical Wave
Figure 5: Tropical Wave
Squall Line
Figure 4: Squall Line
Frontal Change
Figure 6: Frontal Change
Figure 7: Frontogensis
Figure 8: Frontolysis


  • Shows positions and types of fronts
  • A three-digit number near a front classifies it as to type, intensity, and character enclosed in brackets ([ or ])

Troughs and Ridges:

  • A trough of low pressure with significant weather will be depicted as a thick, dashed line running through the center of the trough and identified with the word "TROF"
  • The symbol for a ridge of high pressure is very rarely, if at all, depicted


  • The observations from various stations are plotted
    • These are referred to as station models
  • Round station symbols indicate observations taken by an observer
  • Square station symbols indicate the sky cover was determined by an automated machine
  • Models appearing over water are data from ships, buoys, and offshore oil platforms
  • An outflow boundary will be depicted as a thick, dashed line with the word "OUTBNDY"
  • A dry line will be depicted as a line with unshaded pips or a through symbol identified with the words "DRY LINE"
  • Pressure is plotted in tenths of millibars, with the leading 10 or 9 omitted
  • A legend is printed on each chart stating is name, valid date, and valid time

Frontal and Pressure Markings:

  • 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
  • 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)
  • Squall Line:
    • a line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms
  • Tropical Wave:
    • A trough or cyclonic curvature maximum in the trade wind easterlies
  • Frontal Change:
    • A hash mark denotes a change in frontal type
    • 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
  • Frontogensis:
    • Refers to the initial formation of a surface front or frontal zone
    • 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
  • Frontolysis:
    • the dissipation or weakening of a front
    • depicted as a dashed line with the graphical representation of the weakening frontal type drawn on every other segment