Vicious summer cycle: Heat begets drought, drought begets heat

Sign up for our free weeky email weather newsletter, featuring weather journalist Kevin Myatt.

• The Daily Everything we publish, every weekday
• Cardinal Weather In-depth weather news and analysis on our region, sent Wednesdays

Lynchburg didn’t quite make it to 100 degrees officially on Saturday. Lynchburg’s official gauge also hasn’t quite had a twentieth of an inch of rainfall in a June that is less than a week from ending.

High marks on the thermometer — 99 in Lynchburg’s case — and low marks on the rain gauge — 0.04 inch for all of June through Tuesday in the Hill City — aren’t always directly correlated, but they are intertwined. The largely Southwest and Southside Virginia coverage area of Cardinal News has entered a cycle where hot weather has exacerbated dryness, while intensifying dryness is making hotter temperatures more likely.

A cold front moving in from the northwest will curb the latest spike of heat — mid 90s to near 100 highs are expected in the lower elevation parts of our region as this column posts Wednesday afternoon — and bring scattered to numerous thunderstorms Wednesday night and Thursday. This may bring downpours to some locations, but it doesn’t look likely to provide the widespread soaking rain our region is getting more in need of by the day.

What rain has fallen this month has come in a splotchy, spotty manner with thunderous downpours in storms a few miles away from sprinkles in the dust, if that.

Someone in a different part of Lynchburg will protest that it’s rained a lot more than the 0.04 inch officially reported there, with a downpour in some parts of the city just a week ago Monday on June 17 (the Lynchburg airport gauge collected nothing in that storm). And they would be right — just as some folks on the western side of Blacksburg would say that they got a lot more than the 0.16 inch measured that day at the National Weather Service office at the southeast edge of the town on the same day. Flipping the script, a lot of us who live in the Roanoke Valley didn’t get anything close to the almost 2 inches that fell in the gauge at the airport in the northwest part of the Star City.

South Boston, like Lynchburg, had just four-hundredths of an inch of rain for just over three weeks of June when a strengthening storm that had dropped nearly half an inch on Chatham on Sunday began moving from Pittsylvania County into Halifax County. But it scraped just north, and between that and a later shower, South Boston picked up just 0.04 — doubling its monthly total to 0.08.

Some other regional locations with less than an inch total for the month of June include Martinsville, 0.05; Galax, 0.13; Appomattox, 0.17; Pulaski, 0.43; Bedford, 0.47; Danville, 0.55; Abingdon, 0.56; Covington, 0.56; Rocky Mount, 0.58; and Blacksburg, 0.63.

Roanoke (2.48) and Christiansburg (2.30) appear to have had the most rain in the region, mainly based on fluke downpours of between 1.5 and 2 inches right over the official gauges (June 5 for Christiansburg, June 17 as previously mentioned for Roanoke.) But even these locations will need to get about an inch of rain in the remaining days of June to be near normal for the month.

The vicious cycle

The hotter it gets, and the longer it stays hot, the faster moisture evaporates from the ground, and the faster vegetation dries out without rain.

The less moisture there is at the surface, the less heat energy is used up evaporating it. Much of that heat then ends up getting absorbed by the ground itself, which radiates back to the air, enhancing temperatures.

The more dried up vegetation becomes, the less efficient it is at absorbing solar radiation. More of that heat energy also ends up being absorbed by the ground, which radiates back to the air.

The air gets hotter, and the hotter it gets, the more the ground dries out, which helps it get hotter, which helps the ground dry out … on and on it goes.

The caveat to this is that a hotter atmosphere can become more unstable if there is cooler air aloft, or if atmospheric boundaries are present that the hotter temperatures can help sharpen. In those cases, updrafts can develop that lead to stronger and sometimes more widespread thunderstorms, under certain conditions.

But scattered thunderstorms with localized torrential downpours are not as helpful for overall dryness as widespread light to moderate rain.

Think of two tic-tac-toe grids with nine spaces. In the first, put a 4 in one square, and a 3 in two other squares. In the second, put a 2 in all nine squares. The first grid has bigger numbers and adds up to a total of 10. The second grid has lower numbers, but more of them, and adds up to 18. Besides being more evenly spread, the second grid has a greater sum than the first.

Even if the heavier downpour might soak your spot on the map, it is better on the whole for easing regional dryness for you to get a little less and others to get a little more. That’s why the U.S. Drought Monitor maps might not respond as some would expect who just got a 3-inch downpour while others around got much less.

There is also the problem that torrential downpours tend to run off quickly rather than soak the ground. That might help raise low stream levels for a couple days, but the effects on overall dryness are much weaker once the hot sun pops out and starts heating and drying everything again.

Any hope of change?

The best way to put a dent in developing drought is for a low-pressure system to dig over the southern U.S. and lift copious Gulf of Mexico and western Atlantic moisture over our region over a period of two or three days. But systems like this are rare in summer, when the stronger upper atmospheric steering flow has typically retreated to the U.S.-Canada border or even farther north, and large stagnant “heat dome” highs rule the roost. These kind of lows were not frequent after January through this particular winter and spring, so there is little reason we should expect them now. It would take some sort of fluke blocking pattern that would deliver such a rain event now.

A shift to a pattern of frequent daily thunderstorms could help ease the dryness over a couple of weeks. A day or two of sporadic storms are not great for easing regional dryness, but if the stormier pattern continues for several days or a few weeks, with a tap of moisture from the subtropical and tropical regions of the Gulf of Mexico and Atlantic Ocean, sooner or later everyone gets pretty wet and dryness can be eased.

Other ways drought gets reversed in midsummer have possible high-risk side effects, as drought can teeter into flash flooding far easier than seems logical. Stalled frontal systems can provide an impetus for showers and storms to develop over several days and move over the same locations repeatedly, especially if there are waves of surface low pressure along the front. And, of course, the remnants of one or more tropical systems moving into our region can dump inches of rain on previously dry soil.

A hot, dry June does tilt the odds of having similar weather develop in July and August, but there is still time for the pattern to shift. There may be some indication of that with the center of “heat dome” high pressure settling into the south-central U.S. in days ahead, not oddly oozing toward us from New England as happened last week, leading to many 90s temperatures with 99 at Lynchburg and the John H. Kerr Dam on Saturday, June 22, as apparently the hottest official temperatures in our region.

That will be enough of a shift that clockwise flow around the high should bring occasional cold fronts, as happened Monday and is again occurring on Thursday, to ease sweltering heat temporarily and stir up storm chances about every three or four days into early July. Beyond that, the horizon gets fuzzy.

Having a hot, dry summer is not a done deal for our region, just yet.

Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally owned funeral home with locations throughout the Roanoke Valley.

The post Vicious summer cycle: Heat begets drought, drought begets heat appeared first on Cardinal News .