Forecast grids & meteorological services

What is stored between the lines on a forecast map?

A model evolves fields over horizontal and vertical coordinates, with time as another dimension.

Grid structure and prediction

Numerical weather prediction represents the atmosphere on a three-dimensional grid or equivalent numerical mesh: horizontal latitude/longitude or projected coordinates, and vertical height, pressure or terrain-following/hybrid levels. At each location/level it carries state variables such as wind components, temperature, pressure and moisture. An initial analysis combines observations with a previous model state through data assimilation; physical equations and parameterisations advance it through forecast time.

A grid cell is not a physical box with uniform real weather inside it. Values describe the model’s resolved state and processed quantities at its resolution. Smaller processes—some turbulence, convection, cloud microphysics and surface exchange—need parameterisation or diagnostics. A 10 km grid spacing does not imply that every 10 km-wide cell is placed or predicted exactly, and a display with many pixels cannot create independent information absent from its source.

The forecast has a model origin/run, lead time and valid time. Vertical interpolation and horizontal interpolation make route values usable, but can smooth a sharp gradient or small cell. Terrain-following lower levels differ from pressure levels and may be below terrain elsewhere. Before comparing two layers, confirm coordinate, units, resolution, update cycle and whether an element is instantaneous, maximum or averaged over a period.

What do the world area forecast centres provide?

Global planning fields become useful only with their metadata and product definitions.

Global fields for flight planning

The two World Area Forecast Centres—WAFC London and WAFC Washington—produce globally coordinated upper-air forecasts within the World Area Forecast System. Flight-planning data include upper wind, temperature and humidity, along with defined significant-weather and hazard products. Winds may be supplied as vector components and must be rendered with the correct direction, units and vertical reference. Digital GRIB products are gridded fields; IWXXM/other object products and rendered charts have different structures.

The WAFS product family also provides tropopause level and temperature, maximum-wind direction/speed and its level, geopotential height and fields describing CB, icing and turbulence. The available levels, horizontal spacing, forecast times, thresholds and encoding depend on the particular dataset/version. A maximum-wind field is not simply wind at every requested cruise level; a tropopause field is not the terrain height. Geopotential height associates pressure surfaces with their height structure.

Retrieve the data through an authorised current service such as SADIS or WIFS, directly or through a suitable briefing provider. Read its product documentation and service notices: 2025 introduced higher-resolution and multi-time products, while some older formats remain during transition. Exact timetable and resolution tables can change and should not be treated as unchanging physical laws or inferred from a textbook’s older chart.

Can one smooth report cancel a forecast turbulence area?

The model and the aircraft sample have different footprints and uncertainties.

CB and turbulence visualisation

A CB diagnostic can be visualised as occurrence/extent and vertical structure, while a turbulence field can be plotted by level and a defined severity-related quantity such as eddy dissipation rate (EDR). Read whether the value is a mean, maximum, percentile, probability or categorical threshold. The colour scale and processing determine what a shaded pixel establishes; CB occurrence is not identical to radar precipitation intensity, and EDR is not identical to the load experienced by every aircraft.

A flight-planning or awareness system can merge gridded guidance with automated aircraft observations and pilot reports. Displaying an aircraft track with observed turbulence, level and UTC time beside the forecast can reveal agreement, a displaced hazard or a developing area. Preserve observation time, position, level/range, aircraft/report source, latency and any quality flag; plotting a report at today’s aircraft position rather than its encounter position would mislead.

An isolated smooth report samples a particular aircraft path and time; it cannot clear an entire forecast volume for all levels and times. A moderate report also does not mean every aircraft will have the same response. Conversely, a credible worse-than-forecast report calls for reassessment even outside a coloured area. Blending is a defined information-processing method, not permission to average incompatible measures blindly or overwrite the official warning with the newest-looking icon.

Who supplies the TAF, the METAR and the FIR warning?

Several products can appear in one briefing while coming from different functions.

Local observations and forecasts

An aeronautical meteorological station makes observations and prepares aerodrome reports such as METAR/SPECI and local MET REPORT/SPECIAL as applicable. Sensors and accredited observers supply the assessed state at the observation time; a station report is not a forecast of the next hours.

An aerodrome meteorological office prepares and maintains the aerodrome forecasts such as TAF, and provides briefing, consultation and other required flight documentation or local services according to the designated arrangements. It monitors relevant forecasts and issues amendments/corrections when required. Several aerodromes may be served by one office; the forecaster need not physically sit in the aerodrome tower.

A meteorological watch office maintains watch over conditions affecting flight operations within its area of responsibility, normally linked to FIR/CTA arrangements, and issues SIGMET and, where provided, AIRMET. It coordinates relevant information with ATS and neighbouring offices. A FIR warning is not issued by the control tower simply because the tower relays it. A briefing provider can collect products, but that collection does not change who issued them or their validity.

Does a VAAC advisory replace the FIR SIGMET?

The specialist centre describes the event; the warning and aerodrome products retain their roles.

VAAC and TCAC responsibilities

A Volcanic Ash Advisory Centre monitors and assesses volcanic ash within its designated responsibility region using satellite data, eruption information, observations and dispersion guidance. It issues volcanic ash advisories, including observed/estimated ash and forecast horizontal/vertical extent and movement, and supplies information to the relevant meteorological watch offices, area control centres and WAFCs. A volcano observatory’s eruption notice and an ash advisory are related but different products.

A Tropical Cyclone Advisory Centre monitors tropical cyclones in its responsibility region and provides advisories on centre position, movement, intensity and forecast evolution, including graphical forms where provided. The relevant meteorological watch office uses the advisory context when issuing the appropriate SIGMET. A cyclone-centre symbol or advisory track does not delimit all rain, outflow, turbulence or swell effects, and a forecast track has uncertainty.

For a route threatened by ash or a tropical cyclone, obtain the current advisory, relevant SIGMETs, aerodrome forecasts/reports and operational information including NOTAMs and operator guidance. Compare valid times, altitude ranges and geographical coverage. A specialist advisory does not itself authorise penetration, set the aircraft’s limit or replace a FIR warning. If a product is cancelled or superseded, verify the current successor and reason rather than treating a missing icon as disappearance of the hazard.

Why do both ICAO and WMO appear in meteorology references?

A shared international framework supports nationally designated services.

Complementary international roles

ICAO establishes international aviation requirements, Standards and Recommended Practices and associated procedures for meteorological service to international air navigation, including the information users need and its operational provision. WMO coordinates meteorological observations, exchange, methods and scientific/technical standards and supports the competence and quality of meteorological services. They cooperate so the aviation requirements can be met with consistent meteorological capability.

States designate and oversee their service arrangements; ICAO and WMO do not personally issue every airport’s METAR or TAF. In the European system, applicable EU regulations and EASA implementing guidance define the relevant obligations, alongside published national arrangements. An international standard, European rule, acceptable means of compliance, local AIP provision and manufacturer procedure are different kinds of authority; identify which one answers the question.

Meteorological procedures are updated. The 2025 framework introduced ICAO PANS-MET, Doc 10157, alongside the revised Annex 3 structure; older references can put detailed procedures in different places. Use the current examination reference list and operational editions for their respective purposes. A course can explain a historical code or product for a labelled reference exercise without presenting it as the current operational method.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.