Reading the section
−5σ−20+2+5σ
Red: the polar cap is higher and warmer than usual, the vortex is weak. Blue: colder and lower, the vortex is strong
Dashed line: start of the forecast. Left of it analysis, right of it the GFS run
Black line: lower edge of the +2σ warmth, the front that sinks towards the ground
Strip below: zonal wind at 60° N and 10 hPa. Below zero the vortex has reversed
60–90° N mean geopotential height · ERA5 1990–2020 climatology · 1000–1 hPa