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A medium-size moth (3.1-3.4 cm wingspan) with dark grey forewings and yellow hindwings. The forewings are dark brownish-grey except for the median area below the sigma, which is a contrasting large red-black block. Stigma is a thin, silver shallow U-shape, usually with a distal extension or satellite spot. Hindwings are bright yellow with a black terminal band. The large thoracic hair tuft is mostly red-brown. Antennae are simple and both the sexes are similar. Borea is very similar to several other Syngrapha species, in particular S. diasema and S. orophila. Diasema has pale buff, not yellow, hindwings. Orophila is usually a bit larger, with lighter grey forewings, and a grey instead of red-brown thoracic hair tuft. The stigma shape in orophila is a deeper U with a hook or sharp angle in the lower distal corner. Questionable specimens can be identified by examining the genitalia. Male borea have a single cornutus on the vesica (2 in orophila), and females have a smooth or wrinkled (not grooved or notched) plate on the ostium bursae. The other Syngrapha species with yellow hindwings (ignea, microgamma, montana and alticola) are much smaller or have different color forewings and/or stigma shapes.

Borea occurs along the south coast of Greenland, and in western North America (disjunct ?). In western North America, it has been found from central Alaska, east to the Northwest Territories, south of Great Bear Lake and southward in the mountains to southern Alberta and British Columbia.