Abate? A Bit!
Posted June 16, 2008
Last week at this time I was writing about a string of record setting hot afternoons around the area in which the Raleigh-Durham airport reached the century mark on four straight days. Since then, temperatures have remained largely above normal, but have at least backed off by 8-10 degrees or so on average. It appears that we'll start the week this afternoon with one more notably hot day (thankfully with humidity that's not especially oppressive), before a cold front and a developing upper trough across the eastern U.S. gives us a nice break starting Tuesday and running through the end of the week, allowing the above normal temperatures to abate some more.
The pattern bringing normal to slightly below normal temps is something of a mirror of the one that toasted us the past couple of weeks. At that time, we had a broad ridge of high pressure aloft across the eastern third of the country, with a trough farther west. Now we're heading into a stretch with a notable ridge stalled over the southwestern U.S. and a trough across the northeast at around jet stream level (see the first image, from the Unisys web site, showing the pressure pattern about 30,000 feet up). This corresponds to a temperature pattern at about 5000 feet shown in the second image, where the reds and purples are particularly warm air while the blues and greens are on the cool side, and it's the southern fringes of that cooler air that we should be feeling for the better part of the week. On Wednesday and Thursday in particular, we may see some low to mid 80s highs combined with dry air (dew points in the 40s and 50s) for a very comfortable couple of days.
While we are getting a break from heat, it's less clear how much rainfall we might manage with the pattern chance. There were some helpful showers and storms around the area this weekend. I managed 7 tenths of an inch at my home, while RDU picked up a little over 4 tenths and in a general sense amounts were greater toward the south and east and lower toward the north and west. There were some small streaks with especially heavy rain, 2-4 inches in spots across the Sandhills and southern Coastal Plain. We do have another chance at some showers and storms tonight and tomorrow, though quite possibly on a widely scattered basis. After that, it appears dry through the week before some potential for rain sets back in over the weekend.
Through the late winter and early spring, enhanced rainfall helped a lot with reducing drought conditions across the region, especially in terms of surface water storage in area lakes and reservoirs. However, 30 and 90 day rainfall trends have started to fall below normal again after running above normal for a while, and we've seen low streamflows, continued low groundwater levels (mainly in the Piedmont - see the attached graph from a well in Chapel Hill) and more recently some decrease in lake levels beginning to set in - note that you can track how lakes have responded to the dry and wet conditions using the Lake Levels link in our Almanac section. I've attached a couple of maps of the state with the departure of rainfall from normal over the past 90 and 30 days. The areas of gray to green indicate normal to above normal rain, while the yellows, oranges and reds are below normal. The notable pattern on the 90-day map is good rainfall over that time along and a county or so either side of I-95, and also from about the Triad area up into central Virginia, with a drier band in between.
On a shorter time scale (that also corresponds to the recent heat that has additionally enhanced evaporation and prompted an increase in water use), the 30-day map shows a small area of above normal rain near Greensboro and near-normal rain from about Wilson and Goldsboro down toward Fayetteville and Raeford, but otherwise large areas with rainfall 1-3 inches shy of normal, which equates to about 25-60% of normal for much of the area. Drought monitoring agencies have responded to this by pushing the moderate drought designation east into central NC again, while eastern parts of the state have moved back into the "abnormally dry" range for now. Longer range precipitation prospects for the summer are simply too uncertain at this point to make a call as to whether drought will worsen, stay about the same or diminish.