Good morning, everyone in the foothills. Currently, moderate to heavy rain is falling across the western foothills, and this rain will soon move into the rest of the foothill counties through the late morning. Today and tonight, a cold front will approach the area, making thunderstorms more likely as we move through the nighttime hours into Sunday. As the front pulls away, rain will turn to snow across the mountains while the foothills will dry out.
A large amount of moisture is causing widespread rainfall in the region as warm and moist air moves in. However, the best lift will remain to the west today, which may slow down the eastward progression of the rain. At 500mb levels high up in the atmosphere, a cutoff low-pressure system continues to strengthen over the Midwest. This system and its associated long wave trough will be the main driver for our weather over the next day. This afternoon, the widespread rainfall should temporarily taper back to scattered showers as orographic lift becomes the primary lifting mechanism. Temperatures are expected to reach the low 60s with the help of southerly winds. Light showers will persist in Polk and Western Rutherford Counties and south-facing escarpment areas in the western foothills throughout the afternoon.
As the trough and upper low move eastward, deeper moisture will surge from the Southwest tonight. According to the models, the early morning hours of Sunday will witness heavy rain and thunderstorms due to a small jet streak that will surge out ahead of the system. This jetstreak will create divergence in the atmosphere, which increases lift. As a result, we can expect heavier precipitation as the warm, moist air rises into cooler air and condensates readily. A few days ago, in our blog, we mentioned the possibility of thunderstorms along the Gulf of Mexico that could have reduced the amount of moisture transported into our area. However, it now appears that this will not happen. The weather models appear accurate and predict that there will be heavy rainfall and thunderstorms during the early morning hours of Sunday.
There has been some moderate rainfall this morning, and we expect scattered rainy conditions throughout the day along the southern-facing slopes. Due to this, there is a possibility of isolated flash flooding tonight and Sunday, with the highest risk in Polk and Rutherford Counties. Heavy rain bands could produce rainfall rates over 2 inches per hour. Additionally, due to the 850 MB (about 7500ft) flow out of the Southwest increasing to 50knots or so and the associated shear that produces, the surface-based CAPE (thunderstorm fuel) is likely to increase near daybreak across the western and southwestern foothills. There could be localized flash flooding in areas where thunderstorms repeatedly occur. There is a marginal risk (1 of 5) of severe thunderstorms due to the shear and developing CAPE on Sunday morning. There is a marginal risk (1 of 5) of severe thunderstorms due to the shear and developing CAPE on Sunday morning. Thanks to those southwesterly winds and the moisture pumping into the region, temperatures will be mild overnight tonight.
Although the heavier rain and thunderstorms have arrived earlier than expected, their speed will likely change throughout Sunday. A full-latitude trough will cut over the Eastern United States tonight, turning from positive (tilted right) to negative (tilted left). You can refer to the attached graphic to understand positive and negative tilts.
Above: Left: Positive Trough Tilt Right: Negative Trough Tilt
When a full-latitude trough turns positive to negative, it creates a pivot point that momentarily slows everything down. This physical process will shift over our area. So, although we expect thunderstorms in the early hours of Sunday, these storms will weaken and stall during the morning hours, all while continuing to produce a significant amount of rainfall. Flash flooding is possible across the entire area during the morning hours of Sunday. Still, the highest risk will be localized wherever the thunderstorms train as the trough pivots from positive to negative.
Above: Left: Forecast Rainfall Right: Model Projected Rainfall
We need to consider the possibility of severe weather on Sunday afternoon. Although the models still differ on the timing, a new line of thunderstorms (known as a QLCS) will likely form as the trough tilts to negative complete, possibly near Highway 16/Highway 321 corridors. As this QLCS moves east, it will interact with surface-based CAPE values that could boost up to around 500 j/kg, combined with significant wind shear. This suggests that the eastern and southern foothills could experience damaging straight-line winds and a few tornadoes. We need to monitor the QLCS to see where breaks in the line develop, as the southern storm along those breaks is where the best chance of tornadoes will occur. Look for temperatures to reach back up into the 60s again on Sunday.
Behind that line of thunderstorms and the associated frontal passage, the mountains’ dynamics continue to look very interesting. As the front passes, expect a rapid change from rain to snow across the mountains on Sunday Afternoon. The snow could sometimes be quite heavy as low pressure moves up the east coast. As the best-organized lift exits to the east Sunday evening, return to Northwest flow.
Many of you are discussing the NAM’s forecast for turning rain into snow along the I-40 corridor on Sunday afternoon. However, the precipitation shield will re-speed and move towards the northeast after the trough tilts from positive to negative. Despite this, there is still a 20% chance of seeing a few wet snowflakes from Hickory to Taylorsville and Wilkesboro, but no accumulation is expected. That said, temperatures will drop as we clear out Sunday Night into Monday. That means there could be some patchy black ice across the area, but the high winds Sunday Night should aid in drying the roadways out.
On Sunday night, there will be a strong Northwest flow across the mountains, which may lead to an inch or two of snow in Little Switzerland, Jonas Ridge, and Blowing Rock. Higher elevations along the Tennessee border may receive several inches of snow.
Area-wide temperatures will drop into the 20s on Sunday Night. Dry air moves back in Monday, ending precipitation across the mountains by late morning. Temperatures will struggle under ample sunshine. Expect highs in the mid-40s with gusty northwesterly winds.
The rest of the week looks dry, with temperatures moderating. We’ll watch next weekend’s weather, as rain may be in the forecast for the third weekend.