4-1 CHAPTER 4. ANALYSIS The second of three distinct types of weather (meteorological) information are analyses.
Analyses of weather information are an enhanced depiction and/or interpretation of observed weather data. Prior to the 1990s, most analysis charts were hand drawn by forecasters. Today’s analyses are automated, and depending on the weather information provider (i.e., NWS, commercial weather services, and flight planning services), the appearance and content of these analyses will vary.
Chapter 4 will only focus on those analyses produced by the NWS and made available on various websites, including the Aviation Weather Center (AWC), the Weather Prediction Center (WPC), the Ocean Prediction Center (OPC), and the Alaska Aviation Weather Unit (AAWU).
4.1 Surface Analysis Charts.
Surface analysis charts are analyzed charts of surface weather observations. The chart depicts the distribution of several items, including sea-level pressure; the positions of highs, lows, ridges, and troughs; the location and type of fronts; and the various boundaries such as drylines. Pressure is referred to in mean sea level (MSL) on the surface analysis chart while all other elements are presented as they occur at the surface point of observation.
Figure 4-1. NWS Surface Analysis Chart Example
4-2
4.1.1 WPC Surface Analysis Charts.
The WPC in College Park, MD, produces a variety of surface analysis charts for North America that are available on their website. Figure 4-1, NWS Surface Analysis Chart Example, is one example of several surface analysis products available on their website. WPC’ s surface analysis is also available at https://www.aviationweather.gov.
Some of WPC’s surface analysis charts are combined with radar or satellite imagery (Figure 4-2, NWS Surface Analysis and Radar Composite Example) as well as have different background features (e.g., terrain).
Figure 4-2. NWS Surface Analysis and Radar Composite Example
4.1.1.1 Issuance.
The WPC issues surface analysis charts for North America eight times daily, valid at 00, 03, 06, 09, 12, 15, 18, and 21 Coordinated Universal Time (UTC).
4.1.1.2 Anal ysis Symbols.
Figure 4-3, NWS Surface Analysis Chart Symbols, shows analysis symbols used on NWS surface analysis charts.
Figure 4-3. NWS Surface Analysis Chart Symbols
COLD FRONT COLD FRONTOLYSIS
WARM FRONT WARM FRONTOLYSIS
STATIONARY FRONT STATIONARY
FRONTOLYSIS
OCCLUDED FRONT
OCCLUDED FRONTOLYSIS
CHANGE OF FRONT TYPE
COLD FRONTOGENESIS HIGH PRESSURE CENTER
WARM FRONTOGENESIS LOW PRESSURE CENTER
STATIONARY TROPICAL (TRPL) WAVE
FRONTOGENESIS TROUGH (TROF) OR 1 XXX TROPICAL DEPRESSION
OUTFLOW BONDARY (OUTFLOW BNDRY)
TRPCL STORM TROPICAL STORM
DRYLINE XXX
RIDGE HURCN HURRICANE
SQUALL LINE XXX
4.1.1.3 Station Plot Models.
Land, ship, buoy, and Coastal-Marine Automated Network (C-MAN) stations are plotted on the chart to aid in analyzing and interpreting the surface weather features. These plotted observations are referred to as station models.
Some stations may not be plotted due to space limitations. However, all reporting stations are used in the analysis.
Figure 4-4, NWS Surface Analysis Chart Station Plot Model, and Figure 4-5, NWS Surface Analysis Chart Ship/Buoy Plot Model, contain the most commonly used station plot models used in surface analysis charts.
4-3
Figure 4-4. NWS Surface Analysis Chart Station Plot Model Wind Sea-level pressure Temperature 57 107 Weather -6 56 Pressure trend (mb) Dew Point(F) Sky cover Figure 4-5. NWS Surface Analysis Chart Ship/Buoy Plot Model Wind Sea-level Temperature (F) * Pressure 57 107 Current _Pressure Weather -6 Trend (mb) Station > 41010 56 10605< Wave Identification Information 62 080801 Water Dew Sky Cover Swell Temperature (F) Information Point (F) WPC also produces surface analysis charts specifically for the aviation community. Figure 4-6, NWS Surface Analysis Chart for Aviation Interests Station Plot Model, contains the station plot model for these charts.
4-4
4-5 Figure 4-6. NWS Surface Analysis Chart for Aviation Interests Station Plot Model
4.1.1.3.1 Station Identifier.
The format of the station identifier depends on the observing platform.
• Ship: Typically four or five characters. If five characters, then the fifth will usually be a digit.
• Buoy: Whether drifting or stationary, a buoy will have a five-digit identifier. The first digit will always be a 4.
• C-MAN: Usually located close to coastal areas. Their identifier will appear like a five-character ship identifier; however, the fourth character will identify off which state the platform is located.
• Land: Land stations will always be three characters, making them easily distinguishable from ship, buoy, and C-MAN observations.
4.1.1.3.2 Temperature.
The air temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.3 Dewpoint.
The dewpoint temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.4 Weather.
A weather symbol is plotted if, at the time of observation, precipitation is either occurring or a condition exists causing reduced visibility.
Figure 4-7, NWS Surface Analysis Chart Common Weather Symbols, contains a list of the most common weather symbols.
4-6 Figure 4-7. NWS Surface Analysis Chart Common Weather Symbols
4.1.1.3.5 Wind.
Wind is plotted in increments of 5 knots (kts). The wind direction is referenced to true north and is depicted by a stem (line) pointed in the direction from which the wind is blowing. Wind speed is determined by adding the values of the flags (50 kts), barbs (10 kts) and half-barbs (5 kts) found on the stem.
If the wind is calm at the time of observation, only a single circle over the station is depicted.
Figur e 4-8, NWS Surface Analysis Chart Wind Plotting Model, includes some sample wind symbols.
4-7 Figure 4-8. NWS Surface Analysis Chart Wind Plotting Model
4.1.1.3.6 Ceiling.
Ceiling is plotted in hundreds of feet above ground level (AGL).
4.1.1.3.7 Visibility.
Surface visibility is plotted in whole statute miles (sm).
4.1.1.3.8 Pressure.
Sea-level pressure is plotted in tenths of millibars (mb), with the first two digits (generally 10 or 9) omitted. For reference, 1013 mb is equivalent to
29.92 inches (in) 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
4.1.1.3.9 Pressure Trend.
The pressure trend has two components, a number and a symbol, to indicate how the sea-level pressure has changed during the past 3 hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred.
4-8 Figure 4-9, NWS Surface Analysis Chart Pressure Trends, contains the meanings of the pressure trend symbols.
Figure 4-9. NWS Surface Analysis Chart Pressure Trends
4.1.1.3.10 Sky Cover.
The approximate amount of sky cover can be determined by the circle at the center of the station plot. The amount that the circle is filled reflects the amount of sky covered by clouds. Figure 4-10, NWS Surface Analysis Chart Sky Cover Symbols, contains the common cloud cover depictions.
4-9 Figure 4-10. NWS Surface Analysis Chart Sky Cover Symbols
4.1.1.3.11 Water Temperature.
Water temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.12 Swell Information.
Swell direction, period, and height are represented in the surface observations by a six-digit code. The first two digits represent the swell direction, the middle digits describe the swell period (in seconds), and the last two digits are the swell’s height (in half meters).
090703:
09: The swell direction is from 90 degrees (i.e., it is coming from due east) 07: The period of the swell is 7 seconds 03: The height of the swell is 3 half meters (m) 271006:
27: The swell direction is from 270 degrees (due west) 10: The period is 10 seconds 06: The height of the swell is 6 half meters
4.1.1.3.13 Wave Information.
Period and height of waves are represented by a five-digit code. The first digit
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.