Published: 2011-02-07 08:29:25
Updated: 2011-02-07 08:29:25
Posted February 7, 2011
By Mike Moss
While we've had a number of episodes of rain and snow through the course of this winter, so far the pre-season outlooks suggesting below normal precipitation have held up, even though the corresponding La Nina-related projections for warmer than normal temperatures have not. After a stretch of time with much of the state simply considered abnormally dry, the U.S. drought monitor has trended central sections of the state up to moderate drought in January and then last week upgraded a band covering much of our viewing area west of I-95 or so to the severe drought category (first image above) .
This was based on a combination of increasing rainfall deficits, declining groundwater and stream flow levels, low soil moisture values and very low inflows to reservoirs. Because of the cold weather and limited agricultural activity this time of year, the impacts of the drought conditions are minimized, but raise a lot of concern in terms of where we go as we head into the spring and summer.
A look at a few additional images provides a snapshot of some of the conditions around the area. First, we'll note that we did have a helpful rainfall event that ended on Saturday, leaving many of us with anywhere from .5-1.5 inches, with a few higher amounts just to our southeast. A National Weather Service map of radar and rain gauge-estimated rainfall totals over the past 7-days (2nd image) shows this nicely, and also highlights that there was a general north to south gradient in amounts that favored southern NC as compared to the north. That means that some of the streams and reservoirs across the northern half of the state got less help than areas farther south. When we look at a similar map (3rd image) over the past 60 days and show contours of the percentage of normal rain, which ran mainly in the 40 to 60% range for much of our area, with somewhat higher levels in the south and east.
This rainfall deficit is also apparent when we look at rainfall versus normal at the RDU airport (4th image) showing that over the last three months, we've had 5.86 inches of rain compared to a normal of 10 for the period, or just under 60% of the normal range, and of course this would have been notably lower without the inch and a half or so that we just received. This is having some impact on groundwater levels, as seen in the 5th image that shows groundwater levels measured at Chapel Hill over the past year (blue line) compared to the longer-term median (brown line). As you can see, the level has fallen noticeably below the median in recent months, a time of year when groundwater levels and reservoirs are typically steady or beginning to be recharged somewhat. The reading at the end of the graph is close to 3 feet below the median and falls within the lowest 20% of historical values. The final image shows the lake level at Falls lake plotted over the past year, and includes the "guide level," a sort of normal for the lake, along with the fact that it has fallen several feet below that level over the past several months.
We have a chance at some rain later today and tonight, but probably on the order of a few hundredths to a couple of tenths, and for the time being the next round of precipitation (that could come in the form of light snow and drizzle) later this week looks rather light as well, and longer-term outlooks from the Climate Prediction Center favor normal to below normal precipitation for the next couple of weeks and in projections through the spring and into the summer, resulting in a drought projection that indicates a good chance the current one may persist for some time.