The picture becomes snowy when you lose signal strength. The snow is static that you see. Usual troubles are breaks in the lead in wire. The break can occur at the antenna head connection anywhere along the lead in wires length, at TV input connection and on rare occasions on the piece of lead in wire that connects the antenna TV terminals and the tuner.
The job, find the break and splice it. Or if the wire is old and worn replace the entire length.
The picture will become distorted in this fashion if your antenna is not pointed correctly at the transmitter. This can happen if it blows around a bit or even if a new building is erected between you and the transmitter. To correct you must reorient your antenna. The best procedure for orientation is one person on the roof and one by the TV.
The one on the roof slowly rotates the antenna. The person by the set yells up the quality of the picture. At best picture setting the antenna should be tightened up permanently.
Flashing: Your picture will flash and break up into all forms of wild disturbances if you develop a loose length of lead in wire and the wind causes it to fray or break. Then as the wind continues to buffet the wire the screen symptoms will appear. The cure for this trouble is, replace or splice the lead in wire and snug it down so it won’t move in the wind.