Two of the most practically useful—yet frequently misunderstood—weather analysis products available to pilots and dispatchers are the Graphical Turbulence Guidance (GTG) Analysis and the Real-Time Mesoscale Analysis (RTMA). Both are produced by the National Weather Service (NWS), both are objective (computer-generated without forecaster edits), and both serve a specific gap-filling role in the pre-flight and in-flight weather picture. Understanding what each product actually shows, how it is generated, and—critically—where its limitations lie is essential for safe and legal use in flight operations.
These products are covered in Chapter 25 of the FAA Aviation Weather Handbook (FAA-H-8083-28B) under the broader category of weather analysis products. Unlike forecasts, analysis products attempt to characterize current or near-current atmospheric conditions using the best available data at the time of issuance. That distinction matters enormously when you are comparing analysis output against actual PIREPs or planning a departure.
Graphical Turbulence Guidance Analysis: How It Works
The GTG is produced by the NWS Aviation Weather Center (AWC) and is derived entirely from NWS numerical weather model data. No human forecaster modifies the output—what you see is the raw result of automated processing. This makes the GTG fast, consistent, and geographically comprehensive, but it also means the product cannot incorporate the kind of situational awareness a meteorologist might apply during a rapidly evolving event.
Rather than relying on a single turbulence-detection algorithm, the GTG computes results from more than ten separate turbulence algorithms simultaneously. Each algorithm approaches the problem differently—some analyze vertical wind shear, others examine upper-level jet stream dynamics, mountain wave indicators, or convectively induced turbulence signatures. Once all algorithms have run, the GTG then compares each algorithm's output against real-world turbulence observations drawn from two sources: pilot reports (PIREPs) and Aircraft Meteorological Data Relay (AMDAR) data. AMDAR is an automated system in which commercial airline aircraft continuously transmit meteorological data—including turbulence measurements—during flight. This continuous feedback loop allows the GTG to weight each algorithm according to how well it matched actual reported conditions, producing a blended, statistically validated turbulence potential field.
The result is a gridded depiction of turbulence potential that covers a wide range of altitudes and geographic areas. The GTG is available for clear-air turbulence (CAT) at upper levels (as high as FL390 and above) and for mountain wave turbulence. It is presented on color-coded charts where the intensity scale generally ranges from none through light, moderate, and severe, consistent with standard turbulence intensity definitions.
GTG Analysis vs. GTG Forecast
The GTG Analysis is essentially a 0-hour GTG forecast—it is labeled as such on the product. This means it represents the model's best estimate of current turbulence potential at the valid time shown, not a projection into the future. Because it is based on model data, there is an inherent processing and dissemination delay; by the time the product reaches a pilot, it may reflect conditions from up to an hour or more in the past. The GTG Analysis overlays turbulence PIREPs that correspond to its valid time, which allows users to visually cross-check whether reported turbulence aligns with the modeled output—a valuable sanity check. Separate GTG Forecast products (covered in Chapter 27 of the handbook) extend this guidance forward in time.
The AWC continues to refine the GTG product over time. Users can expect improvements in model resolution, the addition of horizontal layers, and algorithm enhancements as more observational data and computational resources become available. The AWC turbulence web page is the authoritative source for the latest product updates.
Real-Time Mesoscale Analysis: How It Works
The RTMA is an hourly surface analysis system operated by the NWS Environmental Modeling Center. It uses a sophisticated data assimilation process to blend available surface observations with high-resolution model background fields, producing analyses of surface weather elements on a fine geographic grid. For aviation purposes, the FAA has specifically determined that RTMA-derived temperature and altimeter setting data are a suitable substitute for missing surface observations at certain airports.
The primary use case is straightforward: when an airport lacks a working Automated Surface Observing System (ASOS), Automated Weather Observing System (AWOS), or human weather observer—due to equipment outage, sensor failure, or remote location—RTMA fills that gap. Without a valid temperature or altimeter setting, certain instrument approach procedures cannot be legally executed or require conservatively adjusted minimums. RTMA provides a computer-derived value to support continued operations.
RTMA coverage for aviation purposes includes airports in the contiguous United States (CONUS), Alaska, Hawaii, Guam, and Puerto Rico. The product is issued every hour, 24 hours a day. Each report lists the airport identifier, latitude, longitude, 2-meter temperature in degrees Celsius, and altimeter setting in inches of mercury (inHg). When a value cannot be computed for a given station—such as altimeter setting at some locations—the report shows N/A for that field.
RTMA Adjustments: The Critical Step Pilots Must Not Skip
The RTMA is not a drop-in replacement for a live surface observation without adjustment. The FAA specifies that RTMA values are 95 percent accurate across the United States for both temperature and altimeter setting—but only when the following mandatory adjustment is applied:
- Altimeter setting: Increase the Minimum Descent Altitude (MDA) or Decision Height (DH) on the applicable approach chart by 100 feet, and increase the required flight visibility minimums by ½ statute mile. This adjustment compensates for the uncertainty inherent in a model-derived altimeter setting versus a real-time barometric reading from a calibrated surface sensor.
This adjustment is not optional guidance—it is an FAA-specified mitigation required to achieve the stated 95% accuracy threshold. Flying an approach to published minimums using an unadjusted RTMA altimeter setting negates the safety margin the FAA built into this product's use criteria.
Why These Products Matter Operationally
The GTG and RTMA solve real operational problems. Turbulence remains one of the leading causes of in-flight injuries, and the GTG gives flight crews and dispatchers an objective, multi-algorithm picture of turbulence potential across a wide swath of airspace—far more systematic than a single PIREP or a hand-drawn SIGMET boundary. Used alongside AIRMETs for turbulence (AIRMET Sierra and Tango), SIGMETs, and current PIREPs, the GTG helps build a three-dimensional understanding of where rough air is likely to be encountered.
RTMA addresses a different but equally important problem: instrument approach safety at airports where the surface observation network has gaps. Remote Alaskan strips, small island airports, and temporarily inoperative ASOS sites all benefit from RTMA coverage. Pilots and dispatchers can legally and safely use the product to support operations that would otherwise be impossible or require significant deviation, provided the required adjustment is applied.
Key Numbers and Rules
- GTG uses results from more than 10 turbulence algorithms, validated against PIREPs and AMDAR data.
- GTG Analysis is a 0-hour forecast; the valid time may reflect conditions up to an hour or more old by the time the product is received.
- RTMA is issued every hour, 24/7, covering CONUS, Alaska, Hawaii, Guam, and Puerto Rico.
- RTMA temperatures are reported in degrees Celsius; altimeter settings in inches of mercury.
- Altimeter adjustment: raise MDA/DH by 100 ft and increase visibility minimums by ½ statute mile.
- RTMA accuracy is stated as 95 percent when the adjustment is properly applied.
- GTG turbulence PIREPs are overlaid when they correspond to the product's valid time.
Common Test Traps
- Treating GTG Analysis as a real-time snapshot: The GTG Analysis is labeled as a 0-hour forecast, not a live observation. By the time you view it, conditions may have changed. It is not equivalent to a current PIREP.
- Skipping RTMA adjustments: The 100 ft / ½ sm approach minimums adjustment is mandatory, not optional. Exam questions may ask what adjustment is required for altimeter setting substitution—know it.
- Confusing RTMA with a primary source: RTMA is a substitute for missing surface observations, not a preferred source. If a functioning ASOS or AWOS is available, that observation takes precedence.
- Assuming GTG is forecaster-modified: Unlike an AIRMET or SIGMET, the GTG involves no human forecaster edits. It is a purely automated, model-derived product.
- Missing the AMDAR connection: The GTG validation process uses both PIREPs and AMDAR data. AMDAR (automated airline turbulence reports) is an important but often overlooked data source; knowing it contributes to GTG accuracy is a common exam point.