textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Confidence increasing that heavy rainfall axis associated with the remnants of Arthur will occur anywhere from the southern Blue Ridge Escarpment to northeast Georgia to the South Carolina Upstate. Guidance is split on whether locally higher amounts will fall just along and north of I-85 or just along and south of I-85.

NBM QPF was running too low compared to the latest high-res guidance so blended in some of the NBM 90th for this evening and tonight. Thus, rainfall totals have trended higher compared to the previous forecast.

The severe risk was upgraded to a Slight risk across the North Carolina Piedmont and along and south of I-85 in South Carolina and northeast Georgia. A tornado risk was also added, mainly along and south of I-85. A Marginal risk remains in place elsewhere.

KEY MESSAGES

1. Rain chances ramp up later this afternoon into tonight as the remnants of Arthur track across the region. The main hazards with this system are heavy rainfall capable of producing isolated flash flooding and isolated severe storms capable of producing damaging wind gusts. However, there will also be an isolated tornado threat, mainly along and south of I-85. 2. Increased Fire Danger through 6 PM this evening across the North Carolina Piedmont. 3. Following a stint of dry and pleasant weather on Saturday and the first half of Sunday, we'll settle back into a typical summertime pattern, with warm, muggy conditions. Showers and thunderstorms are expected each afternoon from Monday onward.

DISCUSSION

Key message 1: Rain chances ramp up later this afternoon into tonight as the remnants of Arthur track across the region. The main hazards with this system are heavy rainfall capable of producing isolated flash flooding and isolated severe storms capable of producing damaging wind gusts. However, there will also be an isolated tornado threat, mainly along and south of I-85.

The remnants of Arthur will steadily lift northeast across the Deep South this afternoon as a cold front approaches out of the northwest. The remnants of Arthur will track across the western Carolinas and northeast Georgia late this afternoon into early Friday morning before the cold front pushes across the area behind the remnants on Friday. Mainly scattered convection can be expected through mid to late afternoon before convection gradually blossoms this evening into the overnight hours, becoming widespread. This system will be capable of producing isolated severe storms as well as isolated flash flooding.

The main hazard with any severe storms that develop is damaging wind gusts. However, there will also be a low-end and conditional isolated tornado threat. Timing for severe still appears to be anywhere from 5 PM this evening through 2 AM Friday morning. The severe risk was upgraded to a Slight risk across the North Carolina Piedmont and along and south of I-85 in South Carolina and northeast Georgia. A tornado risk was also added, mainly along and south of I- 85. A Marginal risk remains in place elsewhere. The damaging wind threat looks most likely late this afternoon into early this evening when instability is greater, especially across northeast Georgia and the South Carolina Upstate where SBCAPE is already ranging from 2,500-3,000 J/kg. The isolated tornado threat looks to be highest along and south of I-85 per the 12Z HRRR and NAMNest updraft helicity swaths (which matches up nicely with SPC's addition of the tornado risk). However, the tornado threat will be highly conditional and dependent on the exact timing and location of the 850 mb LLJ as well as on whether we remain destabilized ahead of a remnant MCS that is currently tracking across northeast Georgia. Depending on how well this remnant MCS holds together will determine whether the severe threat will continue through the late evening/early overnight hours. The damaging wind threat will gradually wane through the evening hours while the isolated tornado threat will linger through the early overnight hours.

As for the isolated flash flood threat with the remnants of Arthur, 1.5-2"+ PWATs are in place thanks to low-level southwesterly flow pulling in rich moisture from the Gulf. This will allow moderate to heavy downpours to develop at times this evening into early Friday morning. NBM trended up with QPF through Friday morning but it still appears too low compared to the 12Z high-res guidance. Thus, blended in 20% of the NBM 90th percentile QPF for this evening and tonight to better match up with the latest high-res trends. Ended up with 1.5-2.5" across the southern Blue Ridge Escarpment, northeast Georgia, and the South Carolina Upstate with lower amounts still expected along and north of I-40. 12Z high-res guidance is still split on where exactly the axis of heavy rainfall will occur. For now, it appears that anywhere from the southern Blue Ridge Escarpment down to northeast Georgia and the South Carolina Upstate will have the best chance to see locally higher rainfall totals of 3- 4"+. The 12Z CAMs are still split on whether the axis of heaviest rain will fall along and just north of I-85 or along and just south of I-85. So, confidence is low regarding the exact location of the axis of heavy rainfall, thus a Flood Watch will not be issued at this time. Although most of the ensemble guidance show low probabilities of >3" of rainfall through early Friday morning, we cannot rule out the potential for scattered flooding issues, mainly along/near I-85 but confidence remains low and will depend heavily on where the axis of heavy rain sets up.

Due to the low confidence on where exactly the flash flooding threat will occur, as well as whether the tornado threat will materialize, stay weather aware and make sure you have multiple ways to receive warnings, especially with the flash flood and tornado threat linger through late tonight.

Key message 2: Increased Fire Danger through 6 PM this evening across the North Carolina Piedmont.

In coordination with WFO RAH and the North Carolina Forest Service, a Special Weather Statement for Increased Fire Danger remains in effect across the North Carolina Piedmont through 6 PM this evening despite relative humidity values remaining above critical thresholds (>30%). The main concern with fire weather this afternoon and early evening will be the combination of ongoing drought, dry fuels, and gusty southwest winds (25-35 mph) working together to lead to an increased risk of fire ignition and adverse fire behavior.

Key message 3: Following a stint of dry and pleasant weather on Saturday and the first half of Sunday, we'll settle back into a typical summertime pattern, with warm, muggy conditions. Showers and thunderstorms are expected each afternoon from Monday onward.

By Saturday, a weakening frontal zone will be located just to our south. A few of 12Z CAMs depict some showers, perhaps even a rumble of thunder, south of I-85 Saturday afternoon, but generally think the influence of building high pressure across the Mid-Atlantic will keep convective threat limited...and severe threat virtually nil. Highs on Saturday will top out around or perhaps a degree or two below normal. By all accounts, a pleasant day!

On Sunday and Monday, a series of de-amplifying z500 shortwaves will dig out of the central Great Plains. Synoptic guidance is in good agreement that these waves will generally track across the central to upper Ohio Valley, remaining embedded within the stronger upper flow to our north through the first half of the week. As a result, there's no real impetus for any severe risk Sunday, Monday, and Tuesday, with only typical summertime diurnal convection anticipated.

By Wednesday and beyond, the upper pattern will become more amplified, as troughing deepens over the Midwest and heights fall across the Carolinas in response. There's not much ensemble consistency in depicting any specific impulses on Wednesday thru Friday...but the pattern will be such that a shortwave or two may eject out of the Ozarks and cross the area late in the week...possibly resulting in an uptick in convection and severe chances. Confidence remains low, however, on anything more than typical summertime afternoon showers and thunderstorms.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: Scattered showers are tracking northeastward across the western Carolinas this afternoon. Activity will gradually increase later this afternoon into this evening, becoming widespread, as the remnants of Arthur track over the terminals. Gusty southwest winds (20-30 kts) will linger through early this evening before tapering off late this evening. Wind direction will remain SW through late tonight before turning more WNW/NW behind/along a cold front mid-morning Friday. Winds will likely go VRB overnight as the remnants of Arthur track over the terminals. Widespread SHRA is expected from late this evening through the early morning hours Friday with isolated embedded TSRA possible at times. Confidence on TSRA overnight remains low so only have PROB30s to account for the thunder potential. Associated restrictions will develop as a result. Vsby restrictions should range mostly from MVFR to IFR with cig restrictions ranging mostly from MVFR to IFR. Could not rule out lower restrictions with heavier SHRA. A few strong to severe storms are possible, mainly from late afternoon through the early overnight hours (21Z Thur to 06Z Fri). The main hazard with any severe storms that develop are damaging wind gusts, but an isolated tornado cannot be ruled out. The severe threat will be higher for KCLT and the SC Upstate terminals. Confidence is low on whether a severe storm will directly impact a terminal. Drier conditions will return around daybreak Friday. Restrictions should lift within a few hours after the rain pushes east. Brief LLWS may develop early Thursday morning at KAVL along the front so have this accounted for in the 18Z TAF.

Outlook: A cold front may trigger shower and thunderstorms Friday afternoon and evening, mainly along and south of I-85. Dry and relatively inactive weather returns Friday night into Saturday night. Afternoon and evening showers and thunderstorms should return each day Sunday into early next week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...INCREASED FIRE DANGER until 6 PM EDT this evening for NCZ036- 037-056-057-068>072-082. SC...None.