The Surface Analysis Chart (SAC) provides a snapshot of current atmospheric conditions at the Earth’s surface. These charts visualize the state of the weather at a specific moment, making them indispensable for forecasters, pilots, and marine operators. Understanding the frequency of issuance is essential for effectively tracking weather changes and patterns. The schedule for these charts depends on the geographic area, the issuing agency, and the level of detail required.
Defining the Surface Analysis Chart
A Surface Analysis Chart summarizes conditions at ground level, compiled from observations taken at a specific time. Its primary function is to identify and locate major weather-producing systems, which is essential for understanding current weather and anticipating short-term changes. These charts are observational tools, reflecting the weather occurring at the chart’s valid time, not a future forecast.
The chart depicts isobars, which are lines connecting points of equal atmospheric pressure. Closely spaced isobars suggest a strong pressure gradient and strong winds, while widely spaced lines denote calmer conditions. High-pressure centers (blue ‘H’) generally signify stable and clear weather, while low-pressure centers (red ‘L’) are often associated with cloudiness and precipitation.
Fronts are also depicted, representing boundaries between air masses with different temperature and moisture characteristics. The four main types—cold, warm, stationary, and occluded—are shown with distinct colored lines and symbols. Many charts include station plots, which are small symbols representing observations from individual ground stations. These plots provide localized details on temperature, dew point, wind, sky cover, and significant weather conditions.
Standard Schedule of Issuance
Major national meteorological centers follow a routine synoptic schedule based on Coordinated Universal Time (UTC). In the United States, the Weather Prediction Center (WPC) issues national-scale charts eight times daily, following a 3-hourly cycle. This cycle ensures consistent tracking of surface weather systems across the country.
The core observation times are the main synoptic hours: 00Z, 06Z, 12Z, and 18Z. These times are the internationally recognized standard for comprehensive weather analysis and align with the global launching of weather balloons. The WPC also issues charts for the intermediate times of 03Z, 09Z, 15Z, and 21Z to provide greater temporal resolution.
This 3-hourly frequency allows for the tracking of features that develop and move quickly over land. Due to the time lag between data collection and publication, the information is typically 90 minutes to two hours old when viewed. The WPC’s analysis products cover North America, serving as a standardized resource for weather professionals.
Regional Variations and Specialized Charts
The 3-hourly cycle is standard for continental analyses, but issuance frequency varies based on the specialized focus or geographic scope of the issuing center.
Oceanic and International Schedules
Centers with an oceanic focus, such as the Ocean Prediction Center (OPC) or the National Hurricane Center (NHC), often adhere to a less frequent schedule. This is because observations over the vast ocean are more limited, typically supporting a 6-hourly analysis cycle (00Z, 06Z, 12Z, 18Z). These specialized centers focus on large synoptic-scale systems like pressure systems and tropical cyclones, which are adequately tracked with the 6-hour interval. International meteorological services may also follow different standards; for instance, some European services issue their analysis charts every 12 hours, reflecting varying data availability and analytical focus.
High-Resolution and Unified Charts
Regional forecast offices or specialized aviation weather centers may produce charts with higher resolution or issue them more frequently during severe weather. The Aviation Weather Center (AWC) provides products tailored for pilots, sometimes integrating WPC analysis with specialized information. The Unified Surface Analysis Chart is a collaborative effort issued four times daily at the main synoptic hours, demonstrating a tailored approach for a broad, multi-agency product. The agencies involved are:
- The Weather Prediction Center (WPC)
- The Ocean Prediction Center (OPC)
- The National Hurricane Center (NHC)
Accessing Current and Archived Charts
Finding Surface Analysis Charts is straightforward, as government meteorological services maintain public portals for current and historical data. The primary source for the national chart in the United States is the Weather Prediction Center (WPC), part of the National Weather Service (NWS). These charts are typically available on NWS and National Oceanic and Atmospheric Administration (NOAA) websites in accessible image formats.
University and academic departments often host archives valuable for research or educational purposes. The National Centers for Environmental Information (NCEI) is the official source for certified copies of past products, maintaining a long-term archive. Users must confirm the chart’s valid time, displayed in UTC, to ensure they are referencing the correct weather snapshot.
The WPC provides interactive pages allowing users to zoom, pan, and view animations of recent charts. The online availability of these charts, often with a short-term archive of several days or weeks, is a powerful resource for tracking surface weather conditions. Specialized charts, such as those from the Ocean Prediction Center, also maintain short-term archives, often available for the past 14 days.