SIGWX

What a SIGWX chart is

A fixed-time forecast of significant en-route weather produced by the two World Area Forecast Centres (WAFC London and WAFC Washington). It is a snapshot for the validity time printed on the chart, not a forecast for a period. Legacy T+24 charts are issued for 00, 06, 12 and 18 UTC. ICAO: the forecast may be used from 3 h before to 3 h after the validity time.

Chart	Layer	Main content
SWH High level	FL250 – FL600 (FL630 before Jan 2025)	Jet streams, CB (OCNL/FRQ), MOD/SEV turbulence, tropopause, volcanoes, radioactive release, tropical cyclones
SWM Medium level	FL100 – FL450	As SWH plus MOD/SEV icing areas; regional versions may add fronts, freezing level, sandstorm etc.

Heights are flight levels (hundreds of feet), speeds in knots. XXX = the level is outside the chart's layer (above the top or below the base). Only moderate and severe phenomena are shown — light turbulence or icing never appears.

Reading method — where to look

Tap a number on the chart or a step below — the matching area is highlighted. Practice SWH chart, invented data.

Header box. Check the chart type (SWH/SWM), the layer (e.g. FL250–600), the validity date/time and the issuing WAFC. A chart may be used from 3 h before to 3 h after its validity time — use the chart whose validity is closest to the time you are over each part of the route.

Standing notes. Read the fixed text: units, and the note that “CB implies TS, GR (hail), moderate or severe turbulence and icing” — those hazards are therefore not drawn separately around CB areas. Also “CHECK SIGMET, ADVISORIES FOR TC AND VA, AND ASHTAM AND NOTAM FOR VA” and, when a radioactive release is shown, “CHECK SIGMET AND NOTAM FOR RDOACT CLD”.

Jet streams. Thick arrowed lines where wind ≥ 80 kt. At the wind flag read the maximum speed and its FL; if ≥ 120 kt a pair of levels gives the depth of the ≥ 80 kt layer. Double bars across the jet (change bars) mark a speed change of 20 kt or a core-height change of 3,000 ft. Read the direction from the arrowhead — jets are not always westerly. Flags sit on the left of the core looking downwind in the NH, on the right in the SH. Think headwind/tailwind and CAT.

Cumulonimbus. Scalloped outlines labelled with amount (OCNL / FRQ) and top over base (e.g. 470 over XXX). Compare the top with your cruise level — an aircraft at FL450 does not overfly a CB area topping FL470.

Turbulence. Heavy dashed outlines, usually numbered, with the symbol (single hat = MOD, double hat = SEV) and top/base. Since 2025 this covers CAT, in-cloud and orographic turbulence (EDR-based).

Icing (SWM). (not on this SWH chart) MOD/SEV icing areas with top/base — relevant for climb and descent through FL100–FL250.

Tropopause. Thin dashed contours labelled FL (e.g. FL300). Low tropopause = colder air mass, often with jets and CAT nearby; tropopause breaks and steep slopes are CAT areas.

Other hazards. Volcano (name + position), radioactive release, tropical cyclone (name), sandstorm/duststorm. SIGWX only shows the location — get details from VA advisories, SIGMET, NOTAM and TC advisories.

Sources

ICAO Annex 3, Appendix 1 (Model SN — symbols and notations; model SWH/SWM charts) · Met Office “Guidelines for interpreting WAFC SIGWX forecasts” · UK CAA AIC P 168/2024 “Changes to ICAO upper-air SIGWX forecasts” (replaces P 131/2024) · ICAO WAFS transition briefings 2025. The practice chart is invented for training and is not a real forecast. Line colours in this app are for study only — identify features by line style and label.

For study only — not for operational use. Always use the official forecasts, SIGMETs and advisories issued for your flight. © LR Aviation · lraviation.com.br

Invented practice data — not a real forecast. Tap any feature on the chart.

Tap a feature

Tap a line, area or symbol on the chart, or pick one from the list below.

Features

Chart questions

Jet stream patterns — hemisphere, season, latitude

Schematic globe — jets drawn on the visible hemisphere. Line thickness = relative strength; dashed = weak / intermittent.

Pole-to-pole cross-section: tropopause (dashed purple) and jet cores just below the tropopause breaks.

Polar front jetSubtropical jetTropical easterly jetTropopause

The main jets

Jet	Typical latitude	Typical level	Direction	Character
Polar front jet (PFJ)	~40–60° (30–70°)	FL300–350	Westerly	Strongly meandering, broken into segments; tied to mid-latitude fronts and lows
Subtropical jet (STJ)	~25–35°	FL350–400	Westerly	More continuous and zonal; poleward edge of the Hadley cell
Tropical easterly jet (TEJ)	~5–20°N	FL450–500	Easterly	Northern summer only (Jun–Sep), Asian monsoon: India → Arabia → Africa

Both westerly jets sit just below the tropopause breaks: the PFJ between the low polar and the mid-latitude tropopause, the STJ between the mid-latitude and the high tropical tropopause — so the STJ is higher than the PFJ.

Season — both hemispheres

Winter: larger pole–equator temperature contrast → jets stronger (PFJ often 150–200 kt+) and displaced towards the equator (PFJ ~35–45°).
Summer: jets weaker and shifted poleward (PFJ ~50–60°); in the NH the STJ can almost disappear in places.
Autumn / spring: transition — strengthening and moving equatorward in autumn, weakening and moving poleward in spring.
Rule of thumb: the winter hemisphere has the strongest jets, closest to the equator.

Northern vs southern hemisphere

NH: more land and mountains (Rockies, Himalaya) → jets more wavy and variable. Strongest jets off the east coasts: Japan / W Pacific (STJ can exceed 200 kt in winter) and eastern USA / N Atlantic.
SH: mostly ocean → jets more zonal and more constant through the year; persistent STJ over Australia, southern Africa and southern Brazil / northern Argentina in winter (~25–35°S).
In both hemispheres the cold (poleward) side is the low-pressure side: that is where the flags are drawn (left in NH, right in SH, looking downwind) and where CAT is usually strongest, just below the core.

Other patterns

Jet merging: where PFJ and STJ come together (Japan, eastern USA) speeds add up — CAT zone.
Troughs and ridges: the jet follows the waves; in deep troughs it can blow northward, southward or even westward. Sharp curvature = CAT.
Entrance / exit regions: acceleration and deceleration along the axis (change bars) also produce turbulence.
Not on SWH: African easterly jet (~FL100–150, northern summer, W Africa), polar night jet (stratosphere, winter, far above FL600), low-level jets.

N Atlantic ops: in winter the N Atlantic PFJ is strong and further south — large tailwind eastbound, strong headwind westbound, NAT tracks move the most. In summer it weakens and moves north; the STJ lies over N Africa and the Mediterranean.

Figures are typical values for study — the actual jet on any day is the one on the chart.

WAFS SIGWX changes — effective 23 Jan 2025

Originally planned for 26 Nov 2024, delayed for technical reasons (UK AIC P 168/2024).

Item	Before	From 23 Jan 2025
SWH layer	FL250 – FL630	FL250 – FL600
SWM layer	FL100 – FL450	Unchanged
Turbulence	CAT areas only	“Turbulence” areas: CAT + in-cloud + orographic, based on EDR — MOD ≥ 0.20 and < 0.35, SEV ≥ 0.35
CB	ISOL EMBD, OCNL EMBD, FRQ EMBD, OCNL, FRQ	Only OCNL and FRQ CB (> 50 % coverage). Isolated/embedded CB → WAFS gridded CB extent data
SWM icing	Combined in-cloud icing & turbulence areas	MOD and SEV icing only; associated in-cloud turbulence no longer on SWM
Tropopause	Spot heights in boxes (plus H / L boxes)	Thin dashed contours labelled with FL (e.g. FL300)
WAFC PNG charts	Black and white	Colour; icing drawn with a different line style from CB
Unchanged	Jet streams, tropopause (as a feature), tropical cyclones, volcanic eruptions, release of radioactive material
Issue time	—	T+24 charts for 00/06/12/18 UTC, available about 18 h before validity (≈ 1 h earlier than before)

New IWXXM SIGWX data

One global dataset FL100 – FL600 (no separate SWH/SWM).
Validity T+6 to T+48 every 3 hours — used by flight planning / EFB systems to draw charts for any of these times.
CB base is not included (it was usually “XXX” anyway).

Timeline

23 Jan 2025	Content changes above; IWXXM SIGWX T+6…T+48 available
Nov 2026	BUFR SIGWX discontinued — IWXXM becomes the only digital format
Nov 2028	Centrally produced WAFC PNG briefing charts discontinued

Charts from your flight-planning provider may be drawn by the provider from WAFS data and can look different (colours, fonts) from WAFC charts. The symbols and labels follow ICAO Annex 3.