Harvey to Bring Heavy Rains and Severe Storms Upstate SC
Harvey is expected to weaken and move the Ohio River Valley this week, potentially bringing heavy rain and severer weather the Upstate of South Carolina. As the storm lifts north of Louisiana tonight, abundant moisture will spread across the Southeast, triggering heavy showers and thunderstorms across our area Thursday and Friday. Weak to moderate destabilization and strong shear profiles may be supportive strong to severe storms with a damaging wind and tornado threat in addition to heavy rainfall. Models feature several rounds of showers and storms during this time with rainfall amounts on the order of 1.5–2.5" with locally heavier amounts possible. Orographic lift will support 2"-4" rainfall totals with locally higher amounts possible across the mountains, however.
By this evening, a broad deformation band will over spread much of the Southeast as clouds increase from deep convection occurring along the AL/MS/FL coastline. As Harvey lifts into the northern Louisiana tonight, a warm front will develop as deep moisture is pulled north across the Gulf States. This front will likely approach the area by Thursday afternoon increasing our chances for shower and storms for the period.
By late Thursday, a northerly flow develops over the western Carolinas as a cold front, associated with an eastern Canadian high pressure settling in over the Great Lakes, advances south to the North Carolina/Tennesse. There has been some mention of a cold wedge developing over western Carolina’s by late Thursday evening. With the latest model run featuring considerably less shower and storm activity Wednesday-Thursday afternoon, I don’t see how there would be enough convection to sustain an effective cold air damming process. However, given the fact that there is a substantial 30–40kts 850 mb jet moving into the area by late Thursday evening, should CAD develop, enhanced lift along the surface boundary will increase the potential for heavy rain. 1200–1500 SBCape values along with strong 40–45 kt vertical shear may support a few severe thunderstorms. Strong low-level winds associated with Harvey’s circulation may contribute to strong to possible damaging wind gusts associated with stronger storms. As mentioned, models have toned down a lot of the convention for Thursday night, mitigating the potential for excessive rainfall. Hi-res models feature a round of storms pushing through Upstate, SC late Thursday. With 2.00" PWAT values down powers will be extremely heavy with 2–3" per hour rainfall rates
A greater potential for flash-flooding will exist in the mountains where enhanced lift along the south facing slopes of the NE Georgia, adjacent South Carolina, and western North Carolina mountains will more support more consistent [heavy] rainfall. WPC has included Upstate SC in a Day 2 Slight Risk of Excessive Rainfall. QPF for the Upstate has been reduced to 1.5'-2.5" and 2"-4" for the mountains. Of course, locally higher amounts possible with thunderstorms.
By Friday afternoon, the warm front should be well north of the region. Instability and deep moisture building into the area by Friday afternoon will likely support deep convection across the area as storms redevelop off to the west. Although there is uncertainty regarding the threat for severe weather, it does bear mentioning. The GFS/GEFS ensemble guidance builds 1800–2000 j/kg SBCAPE values and 0–3 km Helicity values of 200 m2/s2 over the area by mid-afternoon. NAM guidance is in agreement and features 1200/1500 j/kg MLCAPE values. If moderate destabilization occurs, there is a good chance for a few embedded discrete cells in linear segments/clustered storms. 30–40 kt SFC-500 mb Bulk Shear values and a 50–55 kt mid-level jet may sustain a few damaging wind gusts and isolated, brief tornadoes. There is an isolated chance of flash flooding with 2.0" PWAT values. SPC has featured Upstate SC in Day 3 marginal risk area. We will see if there are any upgrades tomorrow.
Labor Day Weekend
The threat for flash flooding will diminish Friday night as showers and storms move east and high pressure builds into the region. Our weekend clears out on by Saturday. Sunny skies, light winds, and good weather through Labor Day.
There is still some degree of uncertainty with this forecast, as dynamics often change rapidly with tropical systems. Keep updated on the severe weather threat by following Upstate Weather SC or @TheEllingwood on Twitter. Visit www.weather.gov/gsp for official information from the NWS Forecast Office in Greenville-Spartanburg. You can follow them on Twitter @NWSGSP.