Rain Around the Ridge

You may have noticed on this week's national CoCoRaHS precipitation maps that the rainfall pattern was generally in a large arc extending from the Rockies across the Dakotas, the upper Midwest, through the southern Great Lakes and then to the mid-Atlantic coast. South of that arc was a large area of from Nebraska and Iowa southward with no rain.   The arc of precipitation marked the northern edge of the upper level ridge of high pressure anchored over the Central Plains and Midwest. The pattern did not change much during the first four days of this week as can be seen from this animation:

National CoCoRaHs map July 21 through July 25

The 500 millibar upper air chart for July 22, 2012.

The subsiding air underneath the ridge kept the weather hot and dry. An upper level low pressure wave moving through the northern U.S. (seen just on the British Columbia coast on the map above) weakened the ridge and allowed a cold front to drop through the central U.S. today. It brought some rain to the parched Plains and Midwest last night and this morning. At the time of this post a line of storms extended from Texas to New Jersey, and severe thunderstorm watches were in effect from the mid-Mississippi Valley to the Atlantic coast.