Thunderstorms and cumulonimbus

Cumulonimbus is the only cloud type a weather report is required to name explicitly. That is no accident: a CB combines almost every hazard that can reach a light aircraft into one single structure.

Applicability: European Union (EASA framework) for the reporting format; the meteorological description itself is not laid down in regulation
Rulemaking authority: Commission Implementing Regulation (EU) 2017/373, Annex V (Part-MET)
Reviewed by NorthTrack on: 2026-09-13

Why CB is singled out in the reports

In the cloud group the Regulation requires the amount and the base to be given – but the cloud type only for cumulonimbus and towering cumulus (TCU, cumulus congestus). No other type is named. A BKN018CB is therefore an entirely different statement from a BKN018.

The abbreviations around thunderstorms

Building blocks

A thunderstorm code in a METAR is rarely a single abbreviation. It is almost always built from a descriptor plus a precipitation type.

TS — Thunderstorm
With precipitation as TSRA; alone when thunder has been heard or lightning seen but no precipitation is falling at the aerodrome.
SH — Shower
Always with a type of precipitation, for example SHRA — rain showers.
CB — Cumulonimbus
One of only two cloud types that are reported at all.
TCU — Towering cumulus
The precursor to cumulonimbus; the second reportable type.
GR — Hail
Also appears in SIGMET phenomena, for example EMBD TSGR.
RA — Rain
A type of precipitation.

How a thunderstorm code is assembled

The order is fixed: intensity or proximity, then descriptor, then precipitation. Once you know the pattern you can also read combinations you have never seen before.

TSRA = TS (Thunderstorm) + RA (Rain)
A thunderstorm with rain at the aerodrome.

TSGR = TS (Thunderstorm) + GR (Hail)
A thunderstorm with hail — for a light aircraft the most dangerous variant.

SHRA = SH (Shower) + RA (Rain)
A rain shower without a thunderstorm. SH never stands alone.

+TSRA = + (Heavy) + TS (Thunderstorm) + RA (Rain)
A heavy thunderstorm with rain. The intensity sign always comes first.

VCTS = VC (Vicinity of the aerodrome) + TS (Thunderstorm)
A thunderstorm in the vicinity of the aerodrome but not over it.

Thunderstorms in SIGMET messages

In a SIGMET, TS is always preceded by a qualifier describing how the thunderstorms are arranged and whether they can be seen at all.

OBSC — Obscured *
A thunderstorm hidden in other cloud and therefore not identifiable under visual flight rules.
EMBD — Embedded *
A thunderstorm embedded in layer cloud — likewise not visible.
FRQ — Frequent *
Thunderstorms with little separation between them, barely possible to fly around.
SQL — Squall line *
Thunderstorms arranged in a line.

* The English expansion of these codes is defined only in ICAO Doc 8400 (PANS-ABC). That document is not freely available, so the expansion has not been verified against the primary text for this edition.

The four SIGMET thunderstorm forms

Each of these may additionally carry hail, and is then written … TSGR.

OBSC TS = OBSC (Obscured) + TS (Thunderstorm)
An obscured thunderstorm: hidden in other cloud and therefore not reliably identifiable under visual flight rules.

EMBD TS = EMBD (Embedded) + TS (Thunderstorm)
An embedded thunderstorm: buried in layer cloud — likewise not visible.

FRQ TS = FRQ (Frequent) + TS (Thunderstorm)
Frequent thunderstorms, with so little separation between them that flying around is barely possible.

SQL TS = SQL (Squall line) + TS (Thunderstorm)
A squall line: cells arranged in a line, moving as a front.

OBSC and EMBD are the dangerous variants
A visible isolated thunderstorm can be flown around. An obscured or embedded thunderstorm cannot be reliably identified under visual flight rules – which is precisely why the SIGMET system singles these two cases out.

What the reporting format does not say

The Regulation expressly provides that a SIGMET for thunderstorms does not include separate information on associated turbulence and icing (MET.TR.250). The reason is not that they are absent – they are taken to be inherently included. So reading a thunderstorm SIGMET and finding no turbulence entry is not an all-clear.

The hazards of a CB, in brief

Vertical gusts far beyond what a light aircraft can take structurally and in terms of controllability
Hail, which can occur several kilometres to the side of the cell as well
Icing in the updraught region, from the high content of supercooled water
Gust front and wind shear at the surface, often ahead of the visible cell
Downburst, with sudden loss of height on final approach
Lightning strike and, following it, failures of electrical and navigation systems
Loss of visibility in heavy precipitation, within seconds

The practical consequence for VFR

For flights under visual flight rules the relevant question is seldom "how strong is the cell" but "where can I still go". A thunderstorm situation changes not only the planned route but the available outs: alternates can be affected at the same time, and the way back can close behind the cell.

No substitute for a briefing
This page explains terms and reporting formats. The assessment of the situation for an actual flight is made from the current briefing, the forecast charts and, where appropriate, advice from the aviation weather service.

The NorthTrack weather decision modules practise exactly this judgement on worked examples. Open the NorthTrack app

Official sources

European Commission / EASA: Commission Implementing Regulation (EU) 2017/373 (Annex V "Part-MET", Annex VI "Part-AIS") – Consolidated version 02017R0373-20250501. Open source (retrieved 2026-09-13)
Deutscher Wetterdienst (DWD), the German national meteorological service: METAR / TREND / TAF – aviation weather reports and forecasts (bilingual code tables) – DWD 134, edition 01/2024. Open source (retrieved 2026-09-13)
ICAO: Doc 8400 – Procedures for Air Navigation Services: Abbreviations and Codes (PANS-ABC). Named by Regulation (EU) 2017/373 as the reference for the meaning of the weather abbreviations. – —. Open source (retrieved 2026-09-13)
Deutscher Wetterdienst (DWD), the German national meteorological service: The DWD aviation weather service – advice and self-briefing (German site) – continuously updated. Open source (retrieved 2026-09-13)