General Weather

TAFs and METAR Trends

The first forecast cross-check for most flights is the combination of a TAF and recent METAR observations. A TAF gives the official terminal forecast for wind, visibility, weather, and ceiling at supported airports, while METARs show what is actually happening now. Used together, they answer both the current-state question and the trend question.

Operationally, pilots should not read a TAF in isolation. Compare the forecast to the last several METARs and ask whether conditions are evolving toward or away from the forecast. If the TAF says improving ceilings by arrival time but the observations are not moving in that direction, confidence in the forecast should drop and alternate planning should get more conservative.

GFA and Prog Charts

The Graphical Forecasts for Aviation (GFA) are the broad-area planning tool that helps a pilot see how weather fits together over route, not just at airports. Within GFA, the prog chart view is especially useful for fronts, pressure systems, precipitation areas, and the larger synoptic picture driving the flight.

This matters because airport forecasts can hide the reason behind the weather. A TAF may show lowering ceilings, but the GFA or prog chart reveals whether that is tied to a warm front, coastal low, convective line, or broad overrunning pattern. That context is what lets a pilot judge whether deterioration is localized, widespread, temporary, or likely to worsen along the entire route.

For ceilings and cloud layers, the GFA clouds layer helps visualize sky cover, IFR areas, mountain obscuration, freezing levels, and related fields in one graphical workflow. It is particularly helpful when deciding whether a route will remain above, below, or inside an extensive cloud deck.

Hazards and Advisories

Hazard products tell the pilot where the forecast becomes operationally unacceptable rather than merely inconvenient. For en route planning, the AWC GFA hazard layers show icing, turbulence, IFR conditions, mountain obscuration, and freezing level information in map form. In the contiguous U.S., G-AIRMETs have replaced the older text AIRMET format, giving more precise time-and-space depiction of advisory hazards.

For higher-severity events, a SIGMET warns of significant hazards such as severe icing, severe or extreme turbulence, dust storms, volcanic ash, or organized convective activity. Near busy terminal areas, Center Weather Advisories (CWAs) and related center-issued statements can add short-fuse regional hazard context that is directly relevant to IFR operations.

The practical rule is that advisories should change the planning posture, not just be noted. A broad icing G-AIRMET, an active SIGMET, or a CWA near the destination should trigger specific questions about altitude changes, reroutes, escape options, and whether the planned aircraft and pilot margin are still adequate.

Soundings and Model Guidance

For vertical structure, forecast sounding and model-based tools available through AWC-linked products are often more useful than surface forecasts alone. They help answer questions a TAF cannot answer well: where is the freezing level relative to cruise altitude, how deep is the cloud layer, is there a strong inversion, and is the atmosphere supportive of convection or wave activity?

Forecast soundings are especially useful when the go/no-go question depends on layers rather than surface conditions. A destination may be forecast VFR at the surface, but the sounding may reveal a shallow but strong inversion, a freezing layer in the descent path, or a deep moist layer that suggests a much less forgiving IFR picture than the surface forecast alone implies.

Radar and satellite are also part of this model cross-check process. The radar layer helps confirm precipitation intensity, coverage, and movement, while cloud and satellite views help validate whether forecast cloud decks and frontal structures are forming where expected. These are not just tactical tools in flight; they are valuable preflight confirmation that the forecast is or is not verifying.

For an IFR-focused walkthrough of how to read a skew-T, how to avoid common radar traps, how to forecast turbulence layers, and how to judge ceiling and visibility trends before they become a missed-approach problem, continue to Advanced IFR Weather Interpretation.

MOS Forecasts

MOS stands for Model Output Statistics. It is an automated forecast product that takes raw numerical weather model output and statistically refines it using long-term local climate and observation data. For pilots, MOS can provide airport-specific guidance for weather elements such as ceiling, visibility, surface wind, cloud cover, precipitation type, and thunderstorm probability.

The main operational value of MOS is coverage. Many airports do not have a TAF, but MOS guidance exists for far more locations, so it can help fill in the forecast picture for a departure, destination, or alternate that otherwise has no official terminal forecast. This is especially useful when a pilot wants a more airport-specific look at expected IFR versus VFR conditions.

MOS is helpful, but it is not the same thing as a TAF. A TAF is a forecaster-issued product with human judgment behind it. MOS is automated guidance. If a TAF exists for the airport, the TAF remains the controlling forecast product for legal IFR planning questions such as whether an alternate is required under 14 CFR § 91.169(b). MOS should be treated as additional decision support, not as a replacement for an official TAF.

MOS also has limits. It does not capture all the nuance that may appear in a TAF, such as multiple cloud layers, some vicinity weather details, precipitation intensity, or variable wind in the same way a human forecaster can communicate those conditions. The right way to use MOS is to compare it with METAR trends, nearby TAFs, area forecasts, radar, satellite, and the broader weather setup rather than relying on it by itself.

In practical flight planning, MOS is most valuable when it answers a focused question: what is the likely ceiling, visibility, or wind at an airport that does not have a TAF? It is also useful as a trend check. If MOS, nearby TAFs, and the synoptic picture all point toward deteriorating ceilings or visibility, that adds confidence that the risk is real and not just a single-product anomaly.

For a pilot-focused explanation of how MOS works, where it helps most, and why it should not replace a TAF, see FLYING: What Is a MOS Forecast?.

Basic Concepts (Parcel Theory)

Parcel theory is a simple way to judge stability: imagine a small "parcel" of air lifted upward adiabatically (no heat exchange with its surroundings). Compare the parcel's temperature to the surrounding environment:

• Stable: The lifted parcel becomes cooler than the environment and sinks back — vertical motion is suppressed (stratus clouds, fog, poor visibility).
• Unstable: The parcel remains warmer than the environment and continues to rise — favors cumulus development, showers, and turbulence.
• Neutral: The parcel and environment cool at the same rate — parcels neither accelerate away nor return.

Cloud Structure in Rising Air

When air is forced to rise by a front, terrain, convergence, or surface heating, the resulting cloud structure depends mainly on stability, but moisture content and the strength of the lifting mechanism also matter.

• Stable air: Forced ascent remains shallow and organized, producing layered clouds such as stratus, altostratus, stratocumulus, and nimbostratus.
• Unstable air: Once lifted, the parcel continues rising on its own, favoring vertically developed clouds such as cumulus, towering cumulus, and cumulonimbus.
• Moisture supply: Greater moisture lowers cloud bases, thickens cloud layers, and increases the chance that rising air will reach saturation quickly.
• Strength and depth of lift: Gentle widespread lift tends to produce stratiform clouds; stronger or more abrupt lifting supports deeper convective development.
• Lifting mechanism: Warm-front overrunning usually favors layered cloud decks, while cold fronts, strong daytime heating, and vigorous upslope flow can support towering clouds if instability is present.

In practical terms, the atmosphere determines whether forced ascent produces a smooth sheet of cloud or a vertically growing convective cloud by controlling how far the air continues to rise after the initial lift begins.

Four Families of Clouds

Clouds are commonly grouped into four broad families by altitude and vertical extent. This classification helps pilots connect cloud appearance with likely stability, moisture structure, and flight conditions.

• High clouds: Cirrus, cirrostratus, and cirrocumulus. These form at high altitudes where temperatures are very cold and are often composed largely of ice crystals. Cirrostratus commonly appears as a thin, milky veil and may produce halos around the sun or moon, often signaling moisture spreading ahead of an approaching frontal system.
• Middle clouds: Altostratus and altocumulus. These often mark deeper moisture aloft and can signal approaching frontal weather or developing instability. Altostratus frequently follows cirrostratus in a warm-front sequence as the cloud deck thickens and lowers.
• Low clouds: Stratus, stratocumulus, and nimbostratus. These are commonly associated with stable air, reduced ceilings, haze, drizzle, or steady precipitation. Nimbostratus often represents the thicker, precipitating part of a layered frontal cloud system.
• Clouds of vertical development: Cumulus, towering cumulus, and cumulonimbus. These indicate rising air and are most closely associated with instability, turbulence, showers, and thunderstorms.

A useful shortcut is that cirro- usually means high, alto- usually means middle, stratus suggests layered structure, and cumulus suggests vertical buildup.

A common layered progression ahead of a warm front is cirrus/cirrostratus to altostratus to nimbostratus, reflecting deepening moisture and steadily lowering cloud bases as the frontal system approaches.

Lapse Rates and Practical Rules

The environmental lapse rate (ELR) is the actual temperature decrease with height. Compare the ELR to standard parcel lapse rates:

• Dry adiabatic lapse rate (DALR): ~9.8°C/km — applies to unsaturated parcels.
• Moist adiabatic lapse rate (MALR): ~4–7°C/km — applies to saturated parcels, variable with moisture.

Rules of thumb:

• If ELR > DALR → strongly unstable (rapid vertical motion and strong convection possible).
• If MALR < ELR < DALR → conditionally unstable (saturation can trigger instability).
• If ELR < MALR → stable (limited vertical motion).

Common Situations and Operational Impacts

• Surface heating on clear days: Produces a steep near-surface lapse rate and afternoon instability — expect thermals, cumulus, and turbulence on climb and in cruise at low levels.
• Cold front passage: Cold, dense air undercuts warm air and forces rapid lift — strong instability, convective showers, and possible severe turbulence or thunderstorms.
• Warm front / widespread ascent: More gentle lifting produces stratiform clouds (stratus, nimbostratus) and extended periods of low ceilings and reduced visibility.
• Temperature inversions: A layer where temperature increases with height caps vertical mixing — surface-based fog and very stable conditions near the ground.
• Moisture and conditional instability: Even when the ELR is moderate, added moisture can lower the parcel lapse rate once saturated, turning a stable column into conditionally unstable.

In operational terms, steady precipitation usually points to stable, layered lift ahead of a warm front or in broad overrunning conditions, while showers usually point to unstable, convective lift more typical of cold fronts or daytime heating.

How to Assess Stability

• Soundings/RAOB/METAR/TAF: Look at forecast soundings (CAP, ELR, lifted indices) for instability, and watch for predicted cloud bases and convective parameters.
• Surface observations: Rapid climbing temperatures after sunrise indicate strong surface heating.
• PIREPs: Reports of turbulence, cloud tops, or convective activity are direct evidence of instability.
• Visual cues: Towering cumulus and rapidly growing clouds indicate unstable air; widespread low stratus and shallow fog point to stable conditions.

Implications for Flight Planning

• Unstable air increases the risk of convective clouds, turbulence, and wind shear — avoid convective areas and increase spacing on approaches and climbs.
• Stable air often produces low ceilings and reduced visibility — plan alternates and be prepared for instrument conditions.
• Inversions near the surface can hide stronger winds aloft — be cautious on takeoff/landing for unexpected shear and gusts.
• When conditional instability exists, small triggers (fronts, terrain lift) can rapidly initiate convection — monitor convective outlooks closely.