Greenville, South Carolina – As moisture from Isaias streams into the Southeast this weekend, the mountains of western North Carolina and the Upstate of South Carolina are about to show just how much terrain can affect rainfall. Forecast maps reveal a dramatic difference in rain totals between the upslope and downslope sides of the mountains. Communities on the southern and eastern-facing slopes could see 4 to 8 inches of rain, while areas tucked behind the mountains, such as Asheville, may get less than an inch. The Upstate is expected to see generally 2 to 4 inches, and some flooding issues are possible, especially in the mountains.
How Mountains Change Rainfall Totals
Moisture flowing in from both the Atlantic and the Gulf is being pushed northwest toward the mountains. When that moist air reaches the southern and eastern-facing slopes of the Blue Ridge, it is forced to rise. As air rises, it cools, and the moisture condenses into clouds and rain. This process, known as upslope enhancement, can significantly increase rainfall on the side of the mountains facing the incoming moisture.
On the opposite side of the mountains, the air sinks as it flows downhill. Sinking air warms and dries out, which reduces rainfall. This creates what is known as a rain shadow, where totals can be much lower even though the same storm is passing through.
Rain Enhanced Along the Blue Ridge Escarpment
The heaviest rain is expected along the Blue Ridge Escarpment, where the mountains rise sharply from the Upstate. Hendersonville is forecast to receive around 4.80 inches of rain, one of the highest totals in the region.
Farther southwest, Toccoa, Georgia could pick up about 3.45 inches, while Franklin, North Carolina is forecast to see around 3.15 inches. Some locations along the steepest south and east-facing slopes could see 4 to 8 inches, especially in areas where moisture is repeatedly forced upward.
Asheville in the Rain Shadow
In contrast, Asheville sits on the downslope side of the mountains and is forecast to receive only about 0.84 inches of rain. Waynesville is also expected to see a lower total, near 1.23 inches…