Grandfather Mountain / Linville

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Linville, NC

Forecast Discussion

Summary

SYNOPSIS

... Hot and humid high pressure dominates the region through the workweek, with daily afternoon and evening shower and thunderstorm chances through Friday. High pressure breaks down over the region and retreats west as a cold front approaches from the north this weekend, making for a showery but cooler weekend and start to next week.

NEAR TERM /THROUGH TONIGHT/... As of 300 am EDT: Isolated to scattered showers will continue through the early morning hours in and near the mountains given continued southerly upslope flow in and lingering 1000+ J/kg SBCAPE values. Also expect low clouds to form in the weak upslope in locations where mid and high clouds thin out through daybreak. These clouds and lower visibilities in fog should burn off quickly through mid-morning and not have an appreciable impact on maximum temperatures today, which will be shaded above guidance once again given recent performance.

Anticipate two general rounds of convection forming across the region this afternoon and evening. The earlier round should fire up along the mountains this afternoon in typical diurnal fashion with heating and then move off the terrain into the adjacent foothills, with lesser coverage in the Piedmont. A second round is then possible later in the evening, with the convection-allowing models showing more organized convection emerging from the Ohio River Valley and the central Appalachians surging southeast across the NC foothills and Piedmont through late evening. This later round would bring a better chance of severe wind gusts with a decent cold pool getting potentially established. Otherwise, 950 to 850 mb flow veering around to southwesterly should relieve some of the upslope forcing and localized flooding potential along the eastern escarpment as the day progresses. Still, instability should be a robust 3000+ J/kg of sbCAPE once again over the eastern mountains and immediate foothills this afternoon and evening, so isolated severe and hydro issues cannot be ruled out with any storms. The SPC Marginal severe and WPC Marginal excessive rainfall outlooks look well-placed.

Lingering convection should diminish quickly after midnight. Mainly mountain river valley nocturnal fog and low stratus are then likely tonight through daybreak Wednesday, with light SW to W mixing east of the mountains reducing fog chances there a bit. Muggy minimum temperatures will run 5 to 10 degrees above normal.

SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... As of 215 AM EDT Tuesday: Upper-level ridging remains dominant on Wednesday and Thursday, while a series of modest shortwaves shoot southeast across the forecast area. WNW flow at the surface should raise afternoon highs a solid two categories above normal, but increasingly it appears that any suppression of convection will be offset by waves of midlevel vorticity associated with the shortwaves. Curtailed thunder chances only slightly on Wednesday, with chance to likely PoTs now advertised on Thursday. While both evenings look convectively active, boasting sbCAPE values of 1800-2100 J/kg and slow enough storm motion to warrant WPC's marginal risk for heavy rainfall, Wednesday appears to be the more concerning setup in terms of severe potential, with the deterministic guidance all depicting a wave of loosely-organized thunderstorms trigging along a stationary front well to our north. This activity then travels south into NC during the late evening and overnight hours. Shear will be less-than-spectacular with ensembles in generally good agreement on no more than 10-15kts across the region; however, it looks significantly higher farther north, where 500mb flow will be constricted along the base of a lifting midlevel trough. This should keep the system alive long enough that it may still pack a punch upon its arrival in the Carolinas. Still some decent spread between where the system will track (ECWMF runs generally keep the more robust convection east of the I-77 corridor, where virtually every GEFS ensemble depicts a round of decent convection moving southeast across NC Piedmont overnight), so details won't become much clearer until we have a look at the CAMs.

Otherwise, summer heat remains through the short term, with afternoon highs well above normal both afternoons. Wednesday afternoon a smattering of heat index values of 100-105 degrees is likely east of the mountains, with much lower confidence on some isolated instances exceeding the 105 degree threshold. Downsloping is sure to keep temperatures up, but there is somewhat more question on how severely it will lower dewpoints and offset temperature increases with drier conditions.

LONG TERM /FRIDAY THROUGH MONDAY/... As of 300 AM EDT Tuesday: Midlevel shortwaves continue to chip away at the 500mb ridge over the Deep South on Friday, allowing it to gradually break down through the first part of the weekend. By Saturday afternoon, the arrival of a more impressive shortwave bumps the quasi-stationary front over the Mid-Atlantic south and east, pushing it through the Carolinas with relatively little fanfare during the wee hours of Saturday morning. Friday and Sasturday afternoons should feature seasonable convection, with guidance in semi-good agreement on 1500+ J/kg of sbCAPE. Despite the front to the north, there is no indication yet that any self-sustaining boundary-triggered convection will make it into the CWA by nightfall Saturday, and by the time the front pushes through overnight, instability will be significantly diminished.

Following the passage of the cold front, ridging will continue to break down, and the entire region will see widespread relief from high temperatures Sunday and into the first part of next week. Temperatures look to remain much more seasonable through Monday, with lower thicknesses across the eastern CONUS.

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