Category Archives: Weather Corner

Summer Rainfall… Feast or Famine

by Tom Ross, Meteorologist

Sometimes the weather gets a bum rap; however the following weather quote is quite telling, “Don’t knock the weather. If it didn’t change once in a while, nine out of ten people couldn’t start a conversation.” That is quite true here in the mountains of Western North Carolina, at least the part about the weather changing.

Taking a look at the last couple of months, we couldn’t buy any rain in June, with landscapes turning dusty and brown and increased sightings of crunchy brown lawns. The pattern started to change in July with rainfall in most places about average, and we saw an increase in thunderstorms and rainfall across the region. In August, those brown lawns and wilting landscapes gave way to lush lawns and vegetation and even some mold and mildew if one wasn’t careful. In fact, in the first half of August, I had more rain than I got during the entire two months of June and July.

One of the reasons for the sharp increase in rainfall in late July and August has been a switch in our prevailing winds both at the surface and aloft. In the early part of summer we had more of a west to northwest flow of winds both aloft and at the surface. This pattern tends to favor warm to hot dry days with little thunderstorm activity.

However, in late July and August that pattern started to switch and favor a more southeast to southerly pattern, which draws in more moisture from the Atlantic and Gulf. This usually leads to an increasing amount of showers and thunderstorms. If you combine this with the fact that the air coming in from the south and east is traveling uphill or rising as it hits the eastern spine of the Appalachians this provides extra uplift, or as we say ”upward motion,” which increases and enhances the rainfall as that uplifting air hits the escarpment and mountains east of the French Broad River Valley. This extra uplift, light upper-level winds, and heating cause heavy rains, thunderstorms and flash floods.

One such event affected part of the region during the beginning of August, dumping the heaviest rain east of Highway 74 toward Old Fort, Lake Lure and Marion. Looking at the chart below, which is the radar estimate of storm total rainfall, the rainfall amounts increase from west to east across the region as seen by the color scale on the right of the image. Less than one half of an inch of rain generally fell east of Highway 74 to more than 3 inches of rain south of Black Mountain. The time duration is shown on the lower right of the image. This pattern stayed with us most of August and certainly eased or erased many of the drought concerns we had in June.

weather-september-2016

Looking ahead to September, we will see a change to fall. Get set for that change, because it will come with cooler temperatures. Normal high and low temperatures at the beginning of the month are 80F and 60F, dropping to 72F and 50F by month’s end, and it is not unusual to have a couple of nights in the 40s.

However, the big wild card for September, as seen by our trivia question, is hurricanes. This is the month historically with the most activity in the Atlantic Ocean basin. Usually the peak of the hurricane season occurs in September and corresponds with low wind shear and warmest sea surface temperatures, and has an average of four tropical storms a year of which two become hurricanes. The month also has the highest probability of a hurricane making landfall — every two out of three.

Meteorologist Tom Ross managed NOAA’s Climate Database Modernization Program during his 25-year career at the National Climatic Data Center (NCDC) in Asheville. He was a senior weather forecaster at Accu Weather in Pennsylvania. Tom teaches classes on weather and climate at various local venues.