4-10 is always 1. The second and third digits describe the wave period (in seconds), and the final two digits give the wave height (in half meters).
10603:
1: A group identifier. The first digit will always be 1 06: The wave period is 6 seconds 03: The wave height is 3 half meters 10515:
1: A group identifier 05: The wave period is 5 seconds 15: The wave height is 15 half meters In some charts by the OPC, only the wave height (in feet) is plotted.
4.1.1.4 Analyses.
Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts.
4.1.1.4.1 Isobars.
An isobar is a line of equal or constant pressure commonly used in the analysis of pressure patterns.
On a surface analysis chart, isobars are solid lines usually spaced at intervals of 4 mb. Each isobar is labeled. For example, 1032 signifies 1,032.0 mb and 992 signifies 992.0 mb.
4.1.1.4.2 Pressure Systems.
On a surface analysis chart, a High (H) is a maximum of atmospheric pressure, while a Low (L) is a minimum of atmospheric pressure. Central pressure is the atmospheric pressure located at the center of a High or Low. In general, the central pressure is the highest pressure in the center of a High and the lowest pressure at the center of a Low. The central pressure is denoted near each pressure center.
A trough or an elongated area of low atmospheric pressure is denoted by dashed lines and identified with the word TROF. A ridge or an elongated area of high atmospheric pressure is denoted by saw-toothed symbols. Ridges are rarely denoted on charts produced by the WPC.
Tropical storms, hurricanes, and typhoons are low-pressure systems with their names and central pressures denoted.
4-11
4.1.1.4.3 Fronts.
NWS surface analysis charts positions and types of fronts are shown by symbols in Figure 4-3. The symbols on the front indicate the type of front and point in the direction toward which the front is moving. Two short lines across a front indicate a change in front type.
4.1.2 Unified Surface Analysis Chart.
The NWS Unified Surface Analysis Chart is a surface analysis product produced collectively and collaboratively by NWS’s WPC, the OPC, the National Hurricane Center (NHC), and Weather Forecast Office (WFO) Honolulu. The chart contains an analysis of isobars, pressure systems, and fronts.
This chart is available on the OPC website as well as the AWC. Users can zoom in by clicking an area on the map (see Figure 4-11, Unified Surface Analysis Chart Example) to enlarge (see Figure 4-12, Unified Surface Analysis Chart Example) and show station plot models (see paragraph 4.1.1.3).
4.1.2.1 Issuance.
The Unified Surface Analysis Chart is issued four times daily for valid times 00, 06, 12, and 18 UTC.
Figure 4-11. Unified Surface Analysis Chart Example
4-12 Figure 4-12. Unified Surface Analysis Chart Example
4.1.2.2 Analysis Symbols.
Unified Surface Analysis Charts use the symbols shown in Figure 4-3.
4.1.3 AAWU Surface Chart.
The NWS’s Unified Surface Analysis chart covers the Alaskan area. The AAWU also provides a fixed area image of the Unified Surface Analysis Chart centered over Alaska (see Figure 4-13, Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska). This chart is available fro m AAWU’s website.
4-13 Figure 4-13. Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska
4.2 C eiling and Visibility.
4.2.1 Ceiling and Visibility Analysis (CVA).
The CVA product provides a real-time analysis of current observed and estimated ceiling and visibility conditions across the continental United States (CONUS). The product is primarily intended to help the general aviation pilot (particularly the Visual Flight Rules (VFR)-only pilot) avoid instrument flight rules (IFR) conditions. However, CVA’s overview of ceiling and visibility conditions can be useful to others involved in flight planning or weather briefing.
4.2.1.1 Issuance.
CVA is issued every 5 minutes and is available through https://www.aviationweather.gov.
4.2.1.2 Content.
CVA presents information via full-CONUS graphic and 18 regional graphics.
Each graphic is rendered on a horizontal grid of 5 km resolution and shows viewer-selectable representations of ceiling height (AGL), surface visibility in statute miles (sm) and flight category designation.
Each regional display includes an overlay of station plots showing the current ceiling and visibility observations reported at selected Aviation Routine
4-14 Weather Report (METAR) stations. To avoid overcrowding, the overlay shows only a subset of the total number of stations available at the CONUS scale view. Additional stations are displayed when zooming in.
4.2.1.3 CVA Ceiling Analysis.
The CVA Ceiling Analysis uses observed and estimated ceiling heights as follows:
• At display points corresponding to METAR locations, CVA uses the ceiling values observed by the nearest METAR.
• At display points between METAR locations (where there are no direct observations), CVA uses estimated ceiling values derived by adjusting the ceiling observation from the nearest METAR to take into account intervening changes in terrain height. When corresponding Geostationary Operational Environmental Satellite (GOES) observations yield an unambiguous indication that no cloud is present, adjusted ceiling observations are reset to indicate clear conditions.
• At display points where observed or estimated ceiling height is less than 200 feet (ft) AGL, CVA reports “Possible Terrain Obstruction.”
The CVA Ceiling Analysis graphic is rendered as follows:
• Ceiling less than 1,000 ft AGL: pale yellow.
• Ceiling less than 200 ft. AGL and Possible Terrain Obstruction: orange.
Fi gure 4-14, CONUS Display of CVA Ceiling Analysis, shows a CONUS-scale CVA Ceiling Analysis. Figure 4-15, Regional Display of CVA Ceiling Analysis, shows a regional-scal e Ceiling Analysis.
Figure 4-14. CONUS Display of CVA Ceiling Analysis Ceiling and Visibility Display at 1340 UTC 19 Jun 2015
52.118.-128.075 Map: Plot Options: Data Options: Overlays:
• Light • Hover 1340 UTC 19 Jun • Time • Highways
• Dark FitCat - Plottype • Top Jetroutes
• Simple •ARTCC/FIR Bounds FItCat Possible Terrain Obscuration IFR FIt Cat: • VFROMVFR OIFR OLIFR 4-15
Figure 4-15. Regional Display of CVA Ceiling Analysis Ceiling and Visibility Display at 1325 UTC 19 Jun 2015
41.794-101.713 Map: Plot Options: Data Options: Overlays:
• Light • Hover 1325 UTC 19 Jun • Time • Highways
• Dark Visib - Plottype • Top Jetroutes
• Simple •ARTCC/FIR Bounds Vis FIt Cat: •VFR •MVFR • IFR OLIFR Less than 3 miles
4.2.1.4 CVA Visibility Analysis.
The CVA Visibility Analysis (shown as a regional view in Figure 4-16, Regional Display of CVA Visibility Analysis) reports the observed visibility at the nearest METAR. The graphic is rendered as follows:
• Surface visibility less than 3 sm: pale yellow.
4-16
Figure 4-16. Regional Display of CVA Visibility Analysis Ceiling and Visibility Display at 1325 UTC 19 Jun 2015
41.794-101.713 Map: Plot Options: Data Options: Overlays:
• Light • Hover 1325 UTC 19 Jun • Time • Highways © Dark Visib • Plottype • Top Jetroutes
• Simple •ARTCC/FIR Bounds VIS FIt Cat: • VFR •MVFR • IFR OLIFR Less than 3 miles
4.2.1.5 CVA Flight Category Analysis.
The CVA Flight Category Analysis (shown as a regional view in Figure 4-17, Regional Display of CVA Flight Category Analysis) uses observed and estimated ceiling and visibility values to report the flight category at each display point. The graphic is rendered as follows:
• IFR-pale yellow: Ceiling less than 1,000 ft AGL and/or visibility less than 3 sm.
• LIFR-orange: Ceiling less than 200 ft AGL and Possible Terrain Obstruction.
4-17
4-18 Figure 4-17. Regional Display of CVA Flight Category Analysis
Areas of possible terrain obscuration (indicated by estimated ceiling less than 200 ft AGL) are also shown.
4.2.1.6 Strengths and Limitations.
4.2.1.6.1 CVA Strengths.
• The CVA’s area-wide graphics highlight observed and estimated IFR, VFR, and possible ground obscuration conditions, enabling a user’s quick recognition of ceiling and visibility hazards, as represented by existing observational tools.
• The CVA is issued every 5 minutes using the most current METAR and GOES observations available. These rapid updates enhance recognition of trends when conditions are changing. The CVA incorporates “special” METAR observations as they are issued.
4-19
4.2.1.6.2 CVA Limitations.
• The CVA is an analysis of estimated real-time conditions only. It is not a forecast and cannot be used in place of a forecast.
• The CVA must only be used with products such as METARs, Terminal Aerodrome Forecasts (TAF), and Airmen’s Meteorological Information (AIRMET).
• The CVA’s representation of ceiling and visibility conditions in regions between METARS (where direct observations are unavailable) can be significantly in error. As distance from the nearest METAR increases, the uncertainty in represented conditions increases.
• Impacted ceiling and visibility conditions can be highly localized and smaller in scale than the 5 km grid used to convey CVA information.
Thus, small-scale variations in ceiling and visibility conditions may not be represented by CVA, even in regions close to observing stations.
4.2.1.7 Use.
General aviation pilots, weather briefers, and others involved in flight planning can use CVA as a companion to METARS, TAFs, and AIRMETs to help them recognize and avoid IFR conditions. Along with the foregoing weather information sources, the CVA can aid IFR avoidance and escape for the en route pilot who inadvertently encounters IFR conditions.
Pilots and flight planners must exercise judgment when using CVA information, as it is based in recent weather information and calculated estimations. Degrading conditions are more likely as the distance and time increase from the station’s reported weather.
The CVA is one of the data fields included in the part of the helicopter emergency medical services (HEMS) Tool (see paragraph 6.1).
4.2.2 Weather Depiction Chart.
The Weather Depiction Chart (see Figure 4-18, Weather Depiction Chart—Example) is being phased out by the NWS, in favor of newer ceiling and visibility products, like the CVA pr oduct.
4-20 Figure 4-18. Weather Depiction Chart—Example
4.3 Upper-Air Analyses.
4.3.1 Issuance.
The NWS National Centers for Environmental Prediction (NCEP) Central Operations (NCO) produces and provides upper-air analyses and forecast products. Their website, Model Analyses and Guidance (see Figure 4-19, NWS NCEP Central Operations Model Analyses and Guidance Website), contains a user’s guide that provides descriptions, details, a nd examples of the various products. A select subset of these products is available on https://www.aviationweather.gov.
Figure 4-19. NWS NCEP Central Operations Model Analyses and Guidance Website DOC NOAA NWS NCEP Centers: AWC CPC EMC NCO NHC OPC SPC SWPC WPC NCEP Home ≥ NCO Home ≥ Systems Integration Branch ≥ Model Analyses and Guidance Model Analyses and Guidance Information + Website Check here for the Latest News Select Model Guidance, Observations and Analyses, or Tropical Guidance
MODEL GUIDANCE OBSERVATIONS AND ANALYSES TROPICAL GUIDANCE Upcoming Changes T User's Guide I Frequently Asked Questions I Product Description Document MAG V3.0.0 NOAA/ National Weather Service Disclaimer Privacy Policy National Centers for Environmental Prediction Credits About Us 5830 University Research Court Glossary Career Opportunities College Park, MD 20740 NCEP Internet Services Team Page last modified: May 20 2013 17:08 PM UTC.
A user's guide, located within Website Information (upper right corner), provides descriptions, details, and examples of the various products.
4.3.1.1 Constant Pressure Analyses.
Constant Pressure Charts (see Figure 4-20, 500 MB Analysis Chart-Example) are being phased out by the NWS in favor of other products (see Figure 4-21, 500 MB Height, Temperature, and Wind, 00-hour Forecast
(i.e., Model's Analyses from the NWS's North American Model [NAM] Example)) available on NCO's Model Analyses and Guidance website.
See paragraph 5.17.1 for additional information.
4-21