7.4 Aviation Weather Charts & Graphic Products

Aviation Weather Charts & Graphic Products

Quick Answer: The Surface Analysis Chart displays synoptic pressure centers, frontal boundaries, and isobars spaced at 4-millibar (mb) intervals. The Low-Level Significant Weather Prog Chart (Surface to FL240) provides 12- and 24-hour forecasts of IFR (solid line) and MVFR (scalloped line) flight categories, turbulence, and freezing levels. In Winds Aloft (FB) forecasts, winds $>100\text{ kt}$ are coded by subtracting 50 from direction and adding 100 to speed (e.g., 731945 = $230^\circ$ at $119\text{ kt}$, $-45^\circ\text{C}$); 9900 means light and variable ($<5\text{ kt}$). Cockpit datalink NEXRAD radar has a 5–20 minute latency and is strictly for strategic avoidance, never tactical penetration.

Instrument flight planning requires synthesizing text observations with graphic analysis and prognostic charts. These charts provide a broad spatial overview of air masses, frontal systems, freezing levels, turbulence, and upper-level wind fields.

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Surface Analysis Chart

The Surface Analysis Chart is an analyzed synoptic chart depicting surface weather conditions observed across North America, updated every 3 hours.

+---------------------------------------------------------------------------------------------+
|                         SURFACE ANALYSIS CHART FEATURES & SYMBOLS                           |
|                                                                                             |
|  1. Isobars: Solid lines connecting points of equal Mean Sea Level (MSL) barometric         |
|     pressure, drawn at 4-MILLIBAR (mb / hPa) INTERVALS (e.g., 1008, 1012, 1016, 1020 mb).   |
|     - Closely spaced isobars = Steep pressure gradient = Strong surface winds.             |
|     - Widely spaced isobars = Weak pressure gradient = Light surface winds.                |
|                                                                                             |
|  2. Pressure Centers:                                                                       |
|     - 'H' (Blue) = High-pressure center (anticyclone, sinking air, diverging winds).        |
|     - 'L' (Red) = Low-pressure center (cyclone, rising air, converging winds).              |
|                                                                                             |
|  3. Frontal Boundaries:                                                                     |
|     - Cold Front: Blue line with solid triangles pointing toward warmer air / direction.   |
|     - Warm Front: Red line with solid semicircles pointing toward colder air / direction.   |
|     - Stationary Front: Alternating blue triangles and red semicircles on opposite sides.   |
|     - Occluded Front: Purple line with alternating triangles and semicircles on same side. |
|     - Trough (TROF): Dashed brown or orange line indicating elongated low-pressure trough. |
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The Surface Station Model

Plotted at individual reporting airports, the station model encodes surface observations:

• Circle: Represents cloud cover (Open = Clear, Quarter-filled = Scat, Half-filled = Broken, Solid = Overcast).
• Wind Barb: Arrow shaft points into the wind; feathers indicate speed (Half feather = $5\text{ kt}$, Full feather = $10\text{ kt}$, Pennant = $50\text{ kt}$).
• Temperature / Dew Point: Plotted in whole degrees Fahrenheit on the left (upper = Temp, lower = Dew Point).
• Sea Level Pressure: Three-digit code plotted in upper right (e.g., 142 = $1014.2\text{ mb}$; 982 = $998.2\text{ mb}$).

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Ceiling and Visibility Analysis (CVA)

The Ceiling and Visibility Analysis (CVA) is the graphic companion to the Surface Analysis Chart and is named explicitly in the Instrument Rating ACS alongside it. The two products answer different questions: the Surface Analysis Chart is a pressure analysis, while the CVA is a flight-category analysis of what the ceiling and visibility are doing right now.

• What it depicts: A color-shaded national picture of ceiling and visibility, generated by blending METAR/SPECI surface observations with satellite and model data so the field is continuous rather than limited to the scattered points where reporting stations happen to sit.
• Color convention: The standard flight categories — green = VFR, blue = MVFR, red = IFR, magenta = LIFR — identical to the shading on the GFA and on the Low-Level Prog Chart.
• Update cycle: Regenerated hourly on aviationweather.gov. It is an analysis of current conditions, not a forecast; pair it with the TAF and the GFA Clouds and Surface tabs to establish the trend.
• Why an instrument pilot cares: The CVA answers the two questions that decide an IFR go/no-go faster than any text product can — how large is the low-ceiling area and where is the nearest legal air? If the red shading covers three states, the "legal" alternate 40 NM down the road is not a real alternate.

Convective Outlook (AC)

The Convective Outlook, bulletin identifier AC, is issued by the NOAA Storm Prediction Center (SPC) and is the only convective product that reaches beyond the 2-hour horizon of a Convective SIGMET. It is required ACS knowledge and is the backbone of a thunderstorm go/no-go decision made the night before, not thirty minutes before engine start.

• General Thunderstorms: An area labeled TSTM (light green) forecasts non-severe thunderstorms. This is not a benign label for an instrument flight — non-severe cells still produce embedded convection inside a stratiform deck, moderate-to-severe turbulence, hail, and low-level wind shear on approach.
• Severe Risk Categories: The escalating labeled areas are MRGL (Marginal), SLGT (Slight), ENH (Enhanced), MDT (Moderate), and HIGH, supported by separate probability contours for tornado, wind, and hail.
• How it fits the advisory hierarchy: Convective Outlook (hours-to-days ahead) → Convective SIGMET (2-hour validity, issued at H+55) → airborne radar or a lightning detector (real time). Each layer is shorter-range and higher-confidence than the one before it.
• Planning Rule: The Convective Outlook is a risk forecast, not an avoidance tool. Use it to decide whether to launch at all, how much extra fuel to carry, and which alternate is upstream of the activity — then use airborne radar or a Stormscope, never datalink NEXRAD, to miss individual cells once airborne.

Low-Level Significant Weather Prognostic Charts

The Low-Level Significant Weather Prog Chart covers the airspace from the Surface up to FL240 (400 mb / 24,000 ft MSL) across the 48 contiguous United States. Issued 4 times daily (00Z, 06Z, 12Z, 18Z), it is divided into a standard 4-panel display displaying 12-hour and 24-hour forecasts.

+---------------------------------------------------------------------------------------------+
|                        4-PANEL LOW-LEVEL PROG CHART LAYOUT (SFC - FL240)                    |
|                                                                                             |
|   +------------------------------------+------------------------------------+                |
|   |    12-HOUR SIGNIFICANT WEATHER     |    24-HOUR SIGNIFICANT WEATHER     |                |
|   |  - IFR / MVFR flight categories    |  - IFR / MVFR flight categories    |  <- Upper      |
|   |  - Turbulence areas & bases/tops   |  - Turbulence areas & bases/tops   |     Panels     |
|   |  - Freezing level contours         |  - Freezing level contours         |                |
|   +------------------------------------+------------------------------------+                |
|   |        12-HOUR SURFACE PROG        |        24-HOUR SURFACE PROG        |                |
|   |  - High/Low pressure centers       |  - High/Low pressure centers       |  <- Lower      |
|   |  - Fronts & trough lines           |  - Fronts & trough lines           |     Panels     |
|   |  - Precipitation types & areas     |  - Precipitation types & areas     |                |
|   +------------------------------------+------------------------------------+                |
+---------------------------------------------------------------------------------------------+

Significant Weather En Route Symbols (Upper Panels)

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Winds and Temperatures Aloft Forecasts (FB / DATA)

Winds and Temperatures Aloft Forecasts (FB) provide computer-generated forecasts of true wind direction, wind speed in knots, and temperature in degrees Celsius for specific flight levels across reporting stations.

$\text{Format: } \mathbf{ddff\pm tt} \quad (\text{Direction, Speed, Temperature})$

FT  3000   6000     9000     12000    18000    24000    30000    34000    39000
ORD        2418+05  2524-02  2632-08  2745-18  2858-28  731945   752852   283558
DEN 9900   2812+08  2720+01  2735-06  2848-16  2960-26  781544   792451   283056

Universal Rules for FB Decoding

1. Elevation-Based Omissions:
   • No Winds Forecast: Winds are omitted for levels within 1,500 feet of station elevation.
   • No Temperatures Forecast: Temperatures are omitted for levels within 2,500 feet of station elevation, and never forecast at the 3,000-foot level.
2. Negative Temperatures Aloft: Above 24,000 feet MSL (FL240), all temperatures are negative ($<0^\circ\text{C}$). Therefore, the minus sign is omitted from the data group (e.g., 30000 level 731945 has temperature $-45^\circ\text{C}$, not $+45^\circ\text{C}$).
3. Light and Variable Winds (9900): When wind speed is less than 5 knots, it is coded as 9900 followed by temperature (e.g., 9900+08 = Wind light and variable, temperature $+8^\circ\text{C}$).
4. High-Speed Winds (100 to 199 Knots): When forecast wind speed is between 100 and 199 knots:
   • Subtract 50 from the two-digit wind direction.
   • Add 100 to the two-digit wind speed.
   • Example: 731945 at 34,000 ft: $\text{Direction} = (73 - 50) \times 10^\circ = \mathbf{230^\circ \text{ True}}$ $\text{Speed} = 19 + 100 = \mathbf{119 \text{ knots}}$ $\text{Temperature} = \mathbf{-45^\circ\text{C}}$
5. Extreme Winds ($\ge 200\text{ Knots}$): Wind speeds of 200 knots or more are coded as speed 99 with direction modified by +50 (e.g., 779955 = $270^\circ$ at $\ge 199\text{ knots}$, $-55^\circ\text{C}$).

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Weather Radar Graphics & Cockpit Datalink Latency

Modern glass cockpits receive datalink weather via FIS-B (Flight Information Service - Broadcast via 978 MHz UAT) or commercial satellite providers (SiriusXM Aviation).

Base Reflectivity vs. Composite Reflectivity

• Base Reflectivity: Displays returns from a single lowest tilt angle scan ($0.5^\circ$) of the NEXRAD antenna. It shows precipitation closest to the surface but may overshoot mid-level convective cores at extended distances.
• Composite Reflectivity: Combines the maximum radar reflectivity (dBZ) across all elevation tilt angles for each geographic grid. It reveals the full vertical structure and severity of thunderstorm updrafts, anvil hail, and aloft precipitation.

The Datalink Radar Trap: Critical Age Latency

+-----------------------------------------------------------------------------------+
|                    COCKPIT DATALINK RADAR LATENCY REALITY                         |
|                                                                                   |
|   Ground NEXRAD Volume Scan:     5 to 6 minutes to complete                        |
|   Ground Mosaic Processing:      1 to 2 minutes                                   |
|   Uplink Transmission Queue:     2 to 5 minutes                                   |
|   TOTAL COCKPIT DISPLAY AGE:     5 TO 15+ MINUTES OLD                             |
|                                                                                   |
|   *DANGER:* A severe supercell storm moving at 40 knots will travel 7 to 13 MILES |
|   from the position shown on your cockpit display!                                |
+-----------------------------------------------------------------------------------+

Mandatory Operational Rule: Cockpit datalink weather is exclusively a strategic decision-making tool for wide-area routing around convective clusters (e.g., remaining 20+ NM away). It must NEVER be used for tactical navigation or attempting to penetrate between closely spaced thunderstorm cells ("hole-hunting").