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When the digits roll over, it means something special: on your odometer, your age, or for a decade- or century-ending calendar year.
Such is true when 99 edges up to 100 on a thermometer measuring air temperature.
As a “heat-dome” high-pressure system builds over Virginia this week, most locations in our state except some of the higher elevations rimming the western side will likely be near or exceed 90 degrees Friday through Sunday, and many are expected to be in the mid to upper 90s. It is possible that a few locations in the state’s lower elevations in southern, central or eastern Virginia could reach the 100-degree mark, though a widespread run of many triple-digit temperatures does not appear likely at this time.
While some recent summers have been among the warmest on record at multiple locations in Southwest and Southside Virginia, seasonal average temperatures boosted by warmer low temperatures and frequent above normal but not historically extreme daytime heat, our region has had a relative lack of 100-degree high temperatures for the past dozen years since the extraordinary heat wave ending June and starting July that included the infamous derecho of June 29, 2012.
Danville, for instance, is now in its longest streak on record going back to 1917 without hitting 100 degrees. It last happened July 8, 2012, some 4,364 days ago, or almost 12 years ago. Danville’s previously longest sub-100 streak was seven years from 1969 to 1976. That one happened during one of the coolest periods for temperatures nationally, while this longer streak has happened when temperatures have been rising to new heights globally and nationally.
Likewise, to Danville’s north and west, neither Lynchburg nor Martinsville have hit 100 since July 8, 2012, after each doing so six times from 2010 to 2012.
Roanoke — which recorded the region’s hottest temperature so far this year on Monday at 95 — has reached 100 only once since July 8, 2012, that occurring on July 20, 2020, amid a record run of 29 straight days at or above 90 degrees. The Star City recorded 10 days at or above 100 degrees between 2007 and 2012.
About 30 miles east of Danville in Southside Virginia, South Boston in Halifax County, which had seven 100-degree days in 2012, went six years without a 100-degree temperature from 2013 to 2018 before having four total in 2019 and 2020, though none since. A little farther east, Clarksville in Mecklenburg County had 29 days of 100-degree temperatures from 2006 to 2012, but like South Boston went 2013 to 2018 without a single 100-degree high temperature. There were two triple-digit days in Clarksville in 2019, but none since.
South of Clarksville at the John H. Kerr Dam, which appears to be quite often the hottest official weather station in Cardinal News’ coverage area, the mercury crossed the 100-degree mark 49 days between 2005 and 2012 but has only done so on 20 days since 2012. Three of those were the only 100-degree readings officially reported in our region in 2023, peaking at 101 on Sept. 9.
There may be broader climatic reasons to consider over time about whether our region is becoming a little more tropical, with greater moisture advection off warmer oceans, trending toward more frequent sticky 80s-90s highs rather and somewhat less frequent mid 90s-lower 100s highs with drier air masses at summer’s peak.
But the chief reason it hasn’t hit 100 as much in the past 12 years appears to be the happenstance of year-to-year atmospheric patterns. Our region simply has not been squarely under stronger heat-dome high pressure systems for extended periods of time since the June 28-July 9 period of 2012. We’ve seen these blister other parts of the nation in the intervening years, but we’ve mostly missed out, or been left on the periphery, or only experienced weaker or shorter versions.
For two or three days later this week into the weekend we will be under a rather potent heat-dome high as it sinks southward from causing a bout of extreme heat in the Midwest and Northeast, but it doesn’t appear likely to become deeply rooted before a cold front pushes in late in the weekend and early next week to stir up some storminess amid the heat, and curb high temperatures a bit moving into next week. The high-pressure ridge might build back after that, however.
So, while a few locations in Virginia inside or outside the Cardinal News coverage area might possibly touch 100 on Friday through Sunday, it will be many days or even weeks later before we know if this summer can bring the kind of longer-lasting heat wave that could break through for widespread 100-degree highs in our region for the first time since 2012.
Why 100 Fahrenheit matters
Americans are often criticized, perhaps rightly so, for not widely embracing the metric system, as nearly the entire rest of the world has, with its inescapably simple and logical structure of 10 units equaling one of the next magnitude for length, weight, and many other types of measurements.
The metric system of temperature called Celsius (or sometimes centigrade) doesn’t quite work like that, but it does place 0 and 100 at logical spots based on physics, the freezing point and boiling point of water at mean sea level.
The Fahrenheit scale has endured, however, as the American system of measuring temperature, and it’s so ingrained into our mindset that most of the time, including in this space, we don’t even bother identifying it as Fahrenheit.
Virtually everyone reading this column with a vested interest in our regional weather would understand that a forecast high in the upper 30s signals a pretty cold day, not a very hot one.
Yet the 100-degree mark in Fahrenheit converts to 37.7778 degrees in the Celsius scale, a cluster of digits too inelegant to be used as a benchmark for anything.
For a temperate, four-season climate like most of the U.S. experiences, the Fahrenheit scale, for whatever other faults or inefficiencies it may have, somewhat inadvertently places the 0 and 100-degree levels very well to mark what many locations would consider extremely cold and extremely hot air temperatures. Having a 100-degree day suddenly become a 38-degree day might be hard sell for American weather geeks.
100-degree frequencies vary
Frequencies of experiencing 0 or 100 vary greatly across different parts of our nation and even across our region. Desert Southwest areas routinely blow past 100 in spring, summer and fall, while many northern latitudes of the U.S. drop below zero at least a few days even in milder winters.
Historically, at Roanoke, triple-digits have come up more than three times as often as zeroes or minus-signs in the Star City’s temperature record (106 triple-digit days to 30 that were 0 or below), but neither is frequent enough that it’s something that’s expected every year. While 106 days of 100+ temperatures would average out to almost one a year in 112 years of records, triple-digit heat isn’t spread evenly over time, but rather occurs in clusters. All of the days with 100-degree or higher temperature have occurred in just 29 of those 112 years at Roanoke, with a fifth of them in 1930 alone (21) and almost half (47) from 1930 to 1936, the “Dust Bowl” years nationally.
But travel west about 100 air miles west to Burke’s Garden in Tazewell County, perhaps the coolest official station in our region with a long period of record dating into the late 1890s, and the scene shifts considerably. Burke’s Garden has never recorded a 100-degree temperature, peaking at 97 in 1930, but has gone to 0 or below in 111 of its 129 years of weather records. The last time it was above 90 at Burke’s Garden was 12 years ago, 92 on July 1, 2012.
Wytheville, with records dating to the 1930s, and Wise, going back to the 1950s, have each never recorded an official 100-degree high temperature. Wytheville has maxed out at 98 three times, most recently in 2012, while Wise hasn’t been higher than 95 in — you guessed it — 2012.
Abingdon, with records starting in the 1970s, has hit 100 only once, in 1988. Blacksburg, with records going all the way back to the 1890s, has done so twice, in 1926 and 1952.
We’ll know it’s an extraordinary heat wave if any of the preceding locations puts three digits on the temperature scoreboard.
40 Celsius instead of 100 Fahrenheit?
Celsius-scale regions of the world sometimes mark 40 degrees, equivalent to 104 Fahrenheit, as a special rollover point marking extreme heat. If this became the case in the U.S., the pool of 40 Celsius days in our region would be few and far between even in the areas that more frequently reach 100.
There have just been 12 days at or above 40 C/104 F in 112 years at Roanoke — only once since 1983, that being derecho day, June 29, 2012. Lynchburg has had only had six such days since 1893, and not since 1936.
Danville has had 17 such days since since 1917. But these last 12 years — Danville also last reached 104 on June 29, 2012 — wouldn’t be the longest streak between 40 C/104 F days. That would be 27 years between 1955 and 1982.
Heat-breaking downpours
The line was thin between thunderous downpours and not even settling the dust on Monday, as thunderstorms formed in the afternoon and evening near and west of the Blue Ridge.
Roanoke officially measured 1.8 inches of rain at its airport gauge even as some spots nearby in the Roanoke Valley had minimal amounts. Flipping that, no rain was recorded officially at the airport on the south edge of Lynchburg, even though other parts of the Hill City had street-ponding torrents. At Blacksburg, there was only 0.16 in the gauge at the National Weather Service on the southeast edge of town while some were closer to 2 inches a few miles northwest.
The storms developed as moisture was swept in from the Atlantic Ocean by southeasterly flow around the developing heat-dome high pressure system to the north. Lifted against the higher terrain and interacting with an upper-air disturbance, thunderstorms with copious rainfall developed, but as is often the case in summerlike conditions, coverage was splotchy and there were only distant rumbles shaking the dust at some locations.
Any hot day in our region brings some risk of isolated thunderstorms developing, especially over the mountains, but sinking air beneath strong high pressure will limit the number and strength of updrafts that can develop into storms in coming days. By Sunday and early next week, a cold front approaching from the west will introduce more storminess back into our forecast.
We will also need to pay attention to the Atlantic, as there is at least some potential for a tropical system to develop and affect the Southeast U.S. coast, though prevailing wind patterns aloft would probably carry such a system well south of our region once it moves inland.
Iffy tropical systems, sporadic afternoon thunderstorms and triple-digit talk — summer weather has arrived.
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.
