Some helpful rain
Posted March 6, 2012
Updated March 7, 2012
A pair of frontal systems and a nearby Alberta Clipper all brought some precipitation to parts of NC over the past week, and while some severe weather occurred in western parts of the state, along with some snow in the mountains, most other parts of the state got a solid dose of needed rainfall, considering that during February the bulk of central NC was placed back into a moderate drought category, while the southeast corner of the state saw an expansion of severe drought that started in January.
It wasn't enough to bring us back to normal in the short-term, but as seen on the map in the first image above, we had decent rainfall across the state in the past week, with one-half to one and a half inches for many areas, including across Wake County, along with some areas that topped two inches (yellow contour) over southern and eastern parts of the state. That amount of rain was fairly close (within a half-inch) of normal for a seven day period, as seen by all the gray colors on the second map.
At the Raleigh-Durham airport, these rains leave us with about 74% of our normal amount over the past 30 days, and 69% of normal for the past 90 days. Looking back across an entire year, we're actually a touch above normal, with 103%, but the trend has been on the dry side for the latter 4-5 months of that time span.
Lake level response has varied some around the area, with Jordan Lake rising in the wake of the recent rains to about 1.5 feet above normal, compared to about six-tenths of a foot above normal in mid-February. At Falls Lake, the level was around 2.7 feet below normal at that time, but rose after this weekend's rains to about 1 foot below normal. It remains to be seen if this is a spike that will also subside quickly.
Another issue with the timing of drier weather late last year and into the winter is groundwater. The nearest long-term, well maintained real-time monitoring well is the one in Chapel Hill, and during February to early March, it has reached a level that may be the lowest ever recorded this time of year. As you can see in the third image by comparing the blue (observed) line to the green line that denotes the long-term median, and the blue shading that outlines the highest and lowest readings in the past, the trend has been for the water table to be quite a ways below normal since about the beginning of 2011. Note that the blue line end on October 26th, but more recent provisional data extending into early March (shown as red diamonds in the fourth image) illustrates that it falls below the lowest March (bottom of the red box) monthly value ever observed at the well, which has been in operation since the 1940s.
Our prospects for more substantial rain the week ahead are rather uncertain at this point, with what appears to be a decent chance of some showers accompanying a cold front on Friday or Friday night, but lots of question marks after that due to large differences in the patterns indicated by different computer models for Saturday through Monday, and large spreads of outcomes within ensembles of the same computer model for that time period. Ensembles are multiple runs of the same model for the same time period, but with small "perturbations" applied to the initial data that attempt to mimic the impact of errors in observations or errors in the physics approximations included in the models. When many of the "perturbed" ensemble results are quite similar to the operational model or to the "control" run that is based on the unchanged input data, confidence is higher in the projected weather conditions than when the various small perturbations cause the ensemble members to produce widely divergent outcomes. That latter case is what we are seeing for days 5-7 in the forecast today, so we'll have to be prepared for possible changes to the forecast on those days.