Focus on Danville: Five factors that drive its weather

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A damaging tornado, the worst flooding in recent memory, the biggest snowstorm in a generation, and the most annual rainfall in recorded weather history.

That summed up 2018 for Danville, a year that succinctly displayed many of the weather dangers the Southside Virginia city of over 42,000 along the North Carolina line can suffer, though infrequently.

Throughout the months ahead, as daily and weekly weather tends to get into a much more slowly changing summer groove, we’re going to take an occasional look at various specific locations in the Cardinal News coverage area encompassing Southwest and Southside Virginia and examine what makes the weather for each of them at least a little bit different than the rest of the region.

Today, we start this intermittent series with Danville.

·        Almost every Cardinal Weather column will contain discussion of recent, current and upcoming regional weather, sometimes as the main subject, sometimes in sections farther down in the column. Read through or scroll down to see a recap of Tuesday’s severe storms and a look at temperature trends ahead.

Recent Cardinal News articles by Danville reporter Grace Mamon have highlighted the coming casino and what economic impact it will have (a temporary location having opened just this week), the economic potential generated by a new industrial megasite nearby in Pittsylvania County, and a recent spike in the demand for housing.

What any new industries or people moving into the former textile-and-tobacco center reinventing itself along the Dan River will find is mostly a four-season, temperate climate not quite as warm as most Southern cities and not nearly as cold as upper Mid-Atlantic and Northeastern cities.

In the Cardinal News region, Danville generally runs a touch warmer than locations more north and west, for the obvious reason of simply being more south and east, and also being lower in elevation than the localities nearer the Blue Ridge and westward. Danville ranges between 500 and 700 feet above sea level.

Danville’s normal high temperature, based on 1991-2020 data, peaks at 89 degrees much of July, and the normal low temperature bottoms out at 28 for a few days in mid to late January. This compares to 88 and 28 for Roanoke, 87 and 26 for Lynchburg, and 83 and 22 for Blacksburg.

Most residents elsewhere in Southwest and Southside Virginia probably associate Danville, Pittsylvania County and Southside in general with a greater occurrence of severe thunderstorms. There are geographical reasons this is the case, explained below.

Danville has – thankfully – yet to suffer a particularly large, devastating tornado, hopefully a trend that will continue. On April 15, 2018, an EF-1 tornado that moved north-northeast out of northern North Carolina raked the western edge of the city, causing significant roof damage to several homes and destroying outbuildings. This was the same day as the more destructive EF-3 Lynchburg and Amherst County tornado, spun off in extreme shear from a strong upper-level low just west of the region.

Other F1 tornadoes – the next to weakest of six categories, pre-dating the new enhanced Fujita scale that started in 2008 – touched down in Danville on June 13, 1953; May 15, 1976; and Aug. 12, 2004, the latter of which damaged three buildings and injured one person.

The landmark weather event of modern times – or perhaps that should be watermark – happened on October 10, 2018, when Tropical Storm Michael – a Category 5 hurricane when it made landfall in the Florida Panhandle – dumped an all-time daily record of 6 inches of rain on Danville, causing widespread flooding.

Two people were killed in Danville during the Michael flooding of 2018, causing over $5 million in damage. The Dan River crested at 30.01 feet, about 9 feet above flood stage, the highest crest on record (though the gauge’s official data on National Weather Service web site only goes back to the 1990s). The Danville Life Saving and First Aid Crew reported making 40 water response calls during Michael’s flooding.

Michael’s rainfall contributed to Danville’s record annual rainfall of 67.61 inches in 2018, beating out 62.78 inches in 2003 and 60.03 in 1996, the only other 60-plus-inch rainfall years on record.

Danville and Southside Virginia are also known for often getting more lightly brushed by snowfall events that affect most of the region farther north and west, as warmer air aloft has an easier time sweeping in from the south and east. For example, the 2009-10 winter, which produced 30-60 inches of snow along the U.S. 460 corridor and from the Blue Ridge and Roanoke Valley westward, dropped a comparatively modest 13.4 inches total on Danville, though that’s considerably more than the 7.6-inch norm.

But there are times when winter storms track more to the south and east, dumping on Danville while others to the west and north are left high and dry.  One of these storms happened on Feb. 26-27, 2004, when Danville, joining several North Carolina Piedmont locations, got 10 inches of snow. Martinsville, 30 miles west, got nothing and Chatham, almost 20 miles north in Pittsylvania County, only got an inch.

On Dec. 9-10, 2018, Danville collected 15.2 inches of snow in our region’s last widespread foot-plus snowstorm, apparently the city’s greatest total since topping 16 inches in the 1993 Superstorm.

So what drives Danville weather? Here five important geographical influences.

·        North Carolina. State boundaries are arbitrary, surveyed in Colonial times with no regard for weather patterns, so they really don’t have a tangible impact on weather. But Danville abuts the North Carolina border, and commonly has weather more similar to Greensboro or Raleigh than Lynchburg, Roanoke or points west. Thunderstorms tracking northeast out of North Carolina roll across Southside, and warm air aloft changing snow over Virginia to wintry mix or cold rain in North Carolina easily oozes across the border over Danville. Summer heat waves broiling the Carolinas are also often worse at Danville than farther north and west in Virginia.

·        East of the Blue Ridge. Danville is too far east for the principal effects of moist upslope easterly or southeasterly flow from the Atlantic Ocean against the Blue Ridge, but there is a gentle upglide from the ocean shore to Danville, so there is at least some upslope effect. When an upper-level low became stalled for five days in May 2020, focusing a constant firehose of Atlantic moisture northwestward, Danville topped 4 inches, while Roanoke with more intense Blue Ridge upslope effects topped 9. Danville sometimes gets stuck in the “wedge” of cool, damp air when high pressure to the north banks it against the Blue Ridge, but the city is just far enough south and east that it can end up on the edge of the wedge, a boundary of contrasts that can be a breeding ground for thunderstorms. Being east of the Blue Ridge also means thunderstorms that sometimes form along the Blue Ridge are an hour or two more mature when they reach Danville – which brings us to the next item on the list.  

·        Lee trough often near U.S. 29 corridor. A phenomenon that occurs as westerly winds blow over the mountains often sets up very near the U.S. 29 corridor running north and south through Lynchburg and Danville. The “lee trough” – resulting from processes of downslope warming of air descending the slope and spin developing as vertical columns of air elongate – is essentially a low-pressure system, usually faint, with at least somewhat added lift and shear that can focus thunderstorm development. Storms moving in from the west sometimes weaken or dissipate crossing the Alleghenies and the Blue Ridge, only to reform in the lee trough, or isolated storms can form there. The lee trough bolsters Danville’s propensity to be in or near more severe thunderstorms than much of our region.

·        Closer to ocean with little terrain interference.  Tropical systems from the Atlantic Ocean, only 200 miles of flat and slightly rolling terrain away, and the Gulf of Mexico, with sweeping easterly and southeasterly wind flow ahead of them tracking northeast, have more unobstructed access to Danville and Southside than to regions farther west. Danville is close enough that hurricanes that hit the Carolinas like Hazel in 1954 and Fran in 1996 can still have tight circulation centers passing nearby and strong tropical storm-force winds.

·        Dan River, Sandy River.  These are not so much contributors to local weather – perhaps subtly – as they are potential sources of mayhem from heavy rain, even that occurring some distance away in the rivers’ headwaters rather than right at Danville.

Danville’s climatic records going back to 1917 have many years with numerous missing days of records, so it’s difficult to discern graphs of local climatic trends. Most of the 1920s, ’30s and ’80s have 10 or more missing days of data.

The potential for heavier downpours and resulting floods as moisture is advected off warmer oceans is perhaps the greatest threat to Danville specifically from an overall warming global climate, something which the 2018 flood sparked greater preparedness for in the future.

The City of Danville’s emergency preparedness page, linked here, covers safety for flooding and tornadoes.

Tuesday’s severe storms

Enough ingredients for severe storms were present on Tuesday that NOAA’s Storm Prediction Center issued an “Enhanced” risk of severe storms, or level 3 of 5, for most of Southwest Virginia west of I-77, with the entire rest of Southwest and Southside Virginia in a “slight risk” or level 2 of 5. (Click here for a previous column explaining storm risk levels.)

Moisture, shear (changing wind speed with height in this case) and lift were all strong with the approach of a low-pressure system from the west, which pulled a warm front northward into our region while also swinging a cold front eastward. Instability varied across the region, but enough percolated with peekaboo sunshine to give the storms the afternoon boost they needed.

Largely missing was a strong veering profile, which would mean winds from the east or southeast at the surface, gradually shifting with each layer aloft to a westerly direction a few miles up.  That kept Tuesday from becoming a tornado outbreak. Though a few tornado warnings were issued, there were no immediate reports of actual tornadoes occurring. It is possible weather service surveys in days ahead may reveal a tornado path or a few. (UPDATE: Indeed that has proved to be the case, with EF1 tornadoes confirmed in Scott and Lee counties in Wednesday surveys. Still being surveyed is damage at Pennington Gap in Lee County, where a strip mall was badly damaged.)

Several small clusters of storms and individual supercells – discrete storms with rotating updrafts – developed ahead of a larger squall line that moved through in the early evening. There were numerous reports of wind damage, mostly trees blown over, in Southwest Virginia along and west of I-77 and also the southern tier of counties farther east. A single report of hail 1-inch in diameter was turned in from near Mountain Valley in Henry County.

Instability appeared to be weaker near the U.S. 460 corridor – Blacksburg, Roanoke, Bedford, Lynchburg – and storms never intensified much there, though there was some heavy evening rain, welcome to many gardeners. There was one curious tornado warning farther north for Alleghany County, after radar indicated a tight low-level circulation embedded deep in the rain shield southeast of Ronceverte, West Virginia, but were no immediate reports of a tornado having actually occurred.

Sunny, dry weather returns for the rest of the work week, with relatively cool temperatures, highs in the 60s to lower 70s and lows in the 40s to near 50.

Not our heat wave

You may see some discussion of a massive heat wave over the Pacific Northwest into western Canada. We will not feel direct impacts from that extreme heat. In fact, the high pressure over western North America will continue a northwest flow that will push cold fronts through our region on occasion and likely keep our temperatures in check, normal or slightly below normal, over the next 7-10 days.

Beyond that, the warm high center may relocation somewhat eastward into the north-central U.S., and that may start to make things a little more summerlike in our region, though a runaway May heat wave – like what we experienced last year, with several days of 90s, isn’t expected at this time.

We’ll talk more about summer heat, including a reader contest related to it, in the near future.

Journalist Kevin Myatt has been writing about weather for 19 years. His weekly column is sponsored by Oakey’s, a family-run, locally-owned funeral home with locations throughout the Roanoke Valley.