The gasket on the underside of a weatherhead may have fine cracks that will keep out moisture from the top and sides, but yield to water under pressure from a hose stream.
Watch out for damaged weatherheads. They are designed to resist wind, rain and snow, but they can still be damaged by impact from solid objects.
Be careful around service masts. If you slip and fall against a service mast, you could pull wires loose and risk electrical shock.