Trough of Warm Air Aloft (TROWAL)

Trough of Warm Air Aloft (TROWAL)


The position of an occluded front on a surface weather analysis is often offset from the associated weather, which occurs at the TROWAL (short for TROugh of Warm Air ALoft), the projection on the Earth's surface of the wedge of warm air above the surface.

The weather associated with Occluded fronts

A dissipating extra-tropical cyclone with an occlusion wrapping around the Low.

A term favoured by Canadian forecasters during winter weather, TROWAL is used to describe a "tongue" of relatively warm/moist air aloft that wraps around to the north and west of a mature cyclone. Areas of intense lift and frontogenesis are commonly associated with TROWALs, and they are favoured regions for heavy and/or prolonged precipitation. During a winter storm, the heaviest snowfall amounts frequently occur along and north of the TROWAL axis.

Warm Occlusion showing position of the TROWAL ahead of the occluded front

Depiction on weather charts

A trowal is indicated by junction of blue and red lines like the junction of cold and warm fronts aloft.

TROWAL symbol with alternating blue and red lines similar to a cold/warm front junction

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