Something happened today that hasn't happened all month - we had "normal" temperatures. The morning low Monday was 46° and the afternoon high was 65° and the mercury could climb higher later this week. So far, December has followed a pattern of alternating cool and warm weeks. The first week was warmer than normal, last week was cooler than normal and this week, we warm up again. If that pattern holds, the week leading up to Christmas could be cool again.
The reason for the changes are variations of the jet stream pattern. A rise in the jet stream over us means warmer weather and a dip delivers cool days. Every week or so, that pattern has undulated. Prior to the dip, there's usually been rain and this should be the case later this week. A slow-moving west coast low pressure system will eventaully move across Texas, dragging a cool front with it. However, unlike past fronts, this one will likely stall towards week's end so rain that begins late Wednesday could linger into the weekend.