11.   An upper-level low pressure system centered over western Wyoming will augment the lifting of moist air.

12.   An expansive low pressure system whirling through the Ohio Valley will bring unsettled weather to much of the Northeast Wednesday.

13.   As a result of these large temperature contrasts, high and low pressure systems are born.

14.   Anticyclogenesis, the process by which high pressure systems develop, usually gets less attention, given that highs are associated with fair weather.

15.   As a high pressure system drifts east from Louisiana, humid air will stream northward from the Gulf of Mexico.

16.   Arctic air will ooze eastward into the western Great Lakes region in the wake of a low pressure system that dove southeast from Alberta a couple of days ago.

17.   As a result of hot air and its concomitant low density, a weak low pressure system will continue to hold steady near Yuma, Ariz.

18.   As the jet stream begins its inexorable southward migration for winter, it sometimes steers Canadian high pressure systems into the Rockies.

19.   Bands of heavy showers and a few thunderstorms will circulate through the Middle Atlantic States as humid air intercepts a sluggish low pressure system.

20.   Before June, the jet stream repeatedly steered low pressure systems and fronts into Texas.