Why A geomagnetic storm is a disturbance of Earth's magnetic field caused by solar wind and coronal mass ejections. Charged particles funnel down the field lines near the magnetic poles, disrupting and depleting the F layer and creating an absorbing auroral zone. The result is that HF paths crossing high latitudes, such as polar circuits to Europe or Asia, get weak, fluttery, or disappear entirely, and the K index climbs to 5 or higher.
Watch out Ground wave travels along the Earth's surface below the ionosphere, so it is unaffected by geomagnetic activity either way; only the sky wave paths suffer.
Storm hits the poles first: high K index means high-latitude HF goes down, not up.