L14 Weather Forecasts for Flight Planning

Introduction

  • Lesson focuses on pre-flight weather information available to pilots and its use during flight planning.

  • Weather forecasting relies on weather reports from a global network of reporting stations measuring current weather conditions.

  • National meteorological services use complex models to generate forecasts and disseminate them as synoptic charts, AIRMET/GAMET, F214/F215, F414/F415, TAFS, SIGMETS, and SPECIS.

  • The lesson progresses from macro-level synoptic charts to detailed Terminal Area Forecasts (TAFs).

Synoptic Charts

  • Synoptic charts, also known as surface pressure or MSL pressure charts, provide a brief overview of the general weather situation, including pressure and frontal systems.

  • The validity of the information is crucial due to constantly changing weather; the UK Met Office issues synoptic charts valid at UTC midday and midnight.

  • Example weather situation: high pressure over eastern Europe, low pressure off Spain, low-pressure trough between Iceland and Greenland, and a stationary occluded front across Scotland.

Synoptic Chart Symbols

  • Synoptic charts use various symbols to represent weather elements.

  • MSLP: Mean Sea Level Pressure

  • H: Centre of a High

  • L: Centre of a Low

  • Cold Front: standard cold front symbol

  • Warm Front: standard warm front symbol

  • Occluded Front: standard occluded front symbol

  • Trough: standard trough symbol

AIRMET

  • AIRMETs are semi-plain language regional low-level forecasts, replaced by GAMETs in the UK.

    • Met Situation: Summary of forecast conditions.

    • Strong Wind Warnings: Warnings for winds ≥ 15 KT or gusts ≥ 28 KT.

    • Freezing Level: Summary of freezing level with deviations.

    • Weather Conditions: Forecast visibility, cloud, weather, and warnings.

    • Wind and Temperatures: Spot values at 1000, 3000, and 6000 ft for selected locations.

    • Regional Outlook: Forecast for 6 hours following the validity period.

  • Issued up to 4 times per day for different regions, valid for 6-9 hours.

GAMET

  • GAMETs (General Aviation Meteorological Forecasts) replace AIRMETs in the UK and include:

    • UK Extended Outlook: Forecast for 24 hours after the regional outlook.

  • The UK Met Office issues GAMETs up to three pages long.

Significant Weather Charts (SIGWX)

  • SIGWX charts display observed or forecast significant weather phenomena affecting aircraft operations.

  • Higher-level SIGWX charts typically show:

    • Cumulonimbus cloud activity.

    • Areas of moderate or severe icing.

    • Areas of moderate or severe turbulence.

    • Jet stream positions and associated clear air turbulence (CAT).

    • Tropopause flight level.

    • Volcanic ash warnings.

  • Common SIGWX symbols:

    • Boundary of significant weather area.

    • Boundary of Clear Air Turbulence (CAT).

    • Moderate and Severe Icing/Turbulence.

    • Jetstream with Flight Level (solid triangle = 50kts, each feather = 10kts).

    • Flight Level of Significant Weather/Tropopause.

    • Flight Level above chart upper limit.

  • F215 UK low-level forecast covers surface to 10,000 ft; F415 extends to Norway, Pyrenees, and central Europe.

  • METFORMS include forecast weather chart for a fixed time, visibility, weather, cloud details, and freezing level for each area.

  • F215s are issued every 6 hours with an outlook for the following 7 hours.

F215 Example

  • Chart validity must cover the entire planned flight; amended if weather significantly worse than forecast.

  • Areas separated by scalloped edges, lettered A, B, C, and D with sub-areas.

  • It is important to consider both forecast and actual weather reports together.

Forecasting Areas - Example Decode (Area B)

  • Surface Visibility and Weather:

    • Visibility 25 km where there is no weather.

    • Occasionally 7 km in moderate rain.

    • Occasionally 3000 m in mist and drizzle in sub-area B1.

    • Isolated areas of 200 m visibility in fog over the sea and windward coasts; occasional in B1.

    • Occasional moderate turbulence and isolated severe turbulence in the far northwest.

    • Occasional hill fog.

Forecasting Areas - Cloud

  • Cloud Forecast: Widespread broken or overcast altocumulus (AC) and altostratus (AS) with base 7000 ft AMSL and tops above 10,000 ft AMSL; moderate icing expected in the northeast.

  • Cloud Forecast: Broken or overcast cumulus and stratocumulus (SC) with base 1500-3000 ft AMSL and tops 7000 ft AMSL.

  • Cloud Forecast: Occasional scattered or broken stratus (ST) with base 300-1000 ft AMSL and tops 1500 ft AMSL; base 0 ft AMSL where there is fog; widespread in sub-area B1.

  • 0° Isotherm:

    • Temperature falls to 0°C between 7000-9000 ft AMSL in the northeast and above 10,000 ft AMSL in the southwest.

Forecasting Areas - Outlook

  • The outlook is often overlooked; it indicates expected conditions beyond the validity period.

  • Example: Outlook until midnight on the 7th September (070000 Z) is 'SIMILAR'.

Notes (F215)

  • A single numerical value represents the mean of an expected range (+25% or ±500 m visibility/ ±500 ft cloud heights, whichever is larger).

  • Thunderstorm (TS) and/or cumulonimbus (CB) imply hail, severe turbulence, and icing.

  • Hill fog warning when cloud base is at or below the highest ground; fog refers to visibility < 1 km, but hill fog often < 180 m.

  • Compass points indicate direction or area.

Abbreviation (F215)

  • SLOW: Front movement of less than 5 knots

  • ISOL: Isolated (<25% of area)

  • OCNL: Occasional (25-50% of area)

  • LCA: Local (<25% of area)

  • FRQ: Frequent (>50% of area)

  • GEN: >75% of area (Convective types)

  • WDSPR: (>50% of area)

  • N, NE, E, ENE: Compass points for directions or zone parts

  • DZ: Drizzle

  • BR: Mist

  • FM 13 Z: 'From' 1300 hours Zulu

  • GR: Hail

  • SN: Snow

  • MTW: Mountain waves

  • MAX: Maximum

  • RA: Rain

  • FZ: Freezing

  • WKN: Weakening

  • FPM: Feet per minute

  • SH: Showers

  • TS: Thunderstorm

  • VSP: Vertical speed

  • Hill FG: Hill fog (<200m)

  • FG: Fog

  • GR: Hail

  • FRONTS: Near fronts

  • ✓: Or

  • HZ: Haze

  • GS: Small hail

  • COT: Coasts

  • SEA: Over the sea

  • +TSRA: Heavy precipitation

  • AC: Altocumulus

  • CU: Cumulus

  • CB: Cumulonimbus

  • SC: Stratocumulus

  • FEW: 1-2 Octas

  • BKN: 5-7 Octas

  • ST: Stratus

  • AS: Altostratus

  • SCT: 3-4 Octas

  • OVC: 8 Octas

  • XXX: Cloud tops higher than 10,000 Ft AMSL

  • CC: Cirrocumulus

  • CI: Cirrus

  • MOD/SEV I: MOD/SEV icing

  • MOD/SEV TURB: MOD/SEV Turbulence

  • 25 KM NIL: 25 km visibility weather expected to be fine

  • (…): Cloud becoming

Terms (F215)

  • Widespread: Conditions affecting many places (>50% area), difficult to avoid (non-convective and convective types).

  • Frequent: Little separation between phenomena within an area, spatial coverage >50% (convective types only).

  • Occasional: Well-separated features affecting 25-50% area; can be avoided.

  • Isolated: Individual features affecting <25% area; easily avoided.

  • Cloud Amount:

    • FEW: 1-2 oktas

    • SCT: 3-4 oktas

    • BKN: 5-7 oktas

    • OVC: 8 oktas