Published: 2007-05-12 12:39:04
Updated: 2007-05-12 12:39:04
Posted May 12, 2007
MIKE MOSS SAYS: Al, April does indeed have the lowest monthly precipitation average for our area, and the reason it is known for "showers" is probably because (a) it rhymes nicely with "May flowers," and (b) it is true that precipitation becomes more "showery" in nature (cellular, often scattered in coverage and characterized by rapid and or sudden changes in intensity) as you head from March through April and into May.
The meteorological reasons for this are that April is about the middle of the transition season between frequent passage of large scale traveling low pressure systems, upper level troughs and active frontal boundaries during the cool season, which often lead to widespread and sometimes persistent precipitation, into the warm season which is characterized by hit and miss, but often quite intense, precipitation associated with summertime showers and thunderstorms. In April, we have already started to see less in the way of large scale precipitation-producers affecting the region, but we also haven't reached a point where warming ocean, lake and river temperatures have loaded up the atmosphere with the kinds of humidity that resides here in the summer. The added "precipitable water" available to summer showers and storms, combined with the occasional passage of tropical cyclones or their very wet remnants in the June to September time frame, more than makes up for the loss of winter's large scale low pressure systems and fronts on average. Since April, again over the long term, thends to fall in between these other heavy precipitation production mechanisms, it averages lower amounts of rain than the preceeding or succeeding months.
If you look at monthly, or a graph of daily average, rainfall amounts for RDU, you'll notice a similar minimum in rainfall for the mid-October to mid-November time frame. This is due to the opposite trend, as these months fall into an "in-between" state similar to April as the atmosphere cools and dries a bit, tropical systems diminish, and yet the wintertime lows and fronts have not yet fully established themselves.