A dry stretch
Posted May 3, 2010
After a welcome stretch of significant precipitation from around November into early February, which nicely refilled reservoirs and recharged groundwater across much of the area, we've quietly slipped into a rather dry period over the past couple of months. And that trend made an appearance on the drought monitor map last week in the form of an area dubbed "abnormally dry" across the southwestern half or so of the Triangle, much of the Sandhills and the southern Coastal Plain (areas shaded yellow in the map). Such a designation doesn't necessarily mean we're about to drift back into drought conditions, but does flag the trend as something to watch. We can look at a number of markers to see how the trend have run up to this point.
First, a plot of accumulated rainfall versus normal at RDU for the past year (second image) shows a rather dry period from July-October 2009, but a very moist stretch after that until early February of this year, but we've seen the actual rain taper off relative to normal since then. Over the past 90 days, we're about 3.23 inches below normal at the airport, and a map of estimated rainfall amounts across the region over that time frame shows most of the area with deficits in that general range, which translate to about 70 to 80 percent of normal rainfall (third image). There are a couple of streaks across the area where 90-day rain is close to or slightly above normal, mainly across northern Caswell and Person County, and also an area from easternmost Wake County across Nash and into Edgecombe and southern Halifax.
The reduced rainfall is leading to much below normal streamflows across most of central NC, and this has begun to impact lake levels noticeably, as you can see in the fourth image, a graph of Falls lake levels over the past year. After spiking to between 4 and 8 feet above the normal, or "guide" level during December to February, the level has trended down since and was near the normal level at the end of last week. A similar trend at Jordan lake saw levels climb as much as 4-10 feet above normal for while, but now hovering just a few inches above normal. Although it is a little slower to react, and with more gradual adjustments, groundwater is showing a fairly similar trend, as seen in the next to images of levels reported so far this year from Chapel Hill and Grantham. At Chapel Hill, levels were climbing early this year, and were above the long-term median, but trended flat more recently to just above that level, while at Grantham, in southern Wayne County, a sharper fall in the level has taken the groundwater to about the lowest 10% of historical levels for April.
So where do we go from here? In the very short-term, there is a chance of decent rainfall this afternoon and tonight as a slow-moving cold front drifts through from the west. While amounts may vary a good bit from place to place due to the convective nature of some of the rain, something in the range of one-quarter to three-quarters of an inch looks to be a good bet for many of us, while it wouldn't be out of the question to exceed an inch in some localized areas.
On a longer scale, we are not projected for drought development at this time, with no particular precipitation trends discernable by the Climate Prediction Center, which shows us having an equal chance of above, below or near normal rain through the next few months. There is still an El Nino pattern evident in the Pacific, but it has relatively little dependable influence on our precipitation totals during the warmer half of the year, and on top of that has shown a recent weakening trend and may fade to neutral by June or July, and perhaps flip over to a La Nina pattern by the September-October time frame. If this occurs, it will be supportive of the forecasts of well-above normal numbers of tropical cyclones in the Atlantic this year, which could in turn result in notable rains from tropical systems or their remnants during the late summer and fall months. Stay tuned!