31.   The leading edge of polar air will be marked by a front stretching from Michigan to the central Rockies.

32.   The leading edge of polar air will stretch from the Ohio Valley west into the central Missouri Valley.

33.   The polar air will recede some on Friday, but there is no relief in sight for the frozen Dakotas.

34.   The storms that derive energy from these contrasts will gradually strengthen enough to draw cold polar air south for the winter.

35.   The storms which derive energy from these contrasts will gradually strengthen enough to draw cold polar air south for the winter.

36.   Sometimes these depressions deepen, as was the case this week, as they meet with cold polar air pushing across Turkey from the North Pole and Siberia.