textproduct: Columbia

WHAT HAS CHANGED

Removed today verbiage from Key Message 1. Aviation discussion updated for 00z TAFs.

KEY MESSAGES

- 1. Scattered afternoon showers and thunderstorms possible on Sunday and Monday. A few strong thunderstorms are possible on Sunday.

- 2. Dangerous heat possible again on Sunday, with a signal for a more impactful, intense heat event by the end of next week.

DISCUSSION

Key Message 1: Scattered afternoon showers and thunderstorms possible on Sunday and Monday. A few strong thunderstorms are possible on Sunday.

As upper-level ridging begins to build into the region, another weak shortwave is expected to move through the FA on Sunday. Dew points into the lower to mid 70s, PWATs around 2.0", and strong surface heating will promote robust destabilization in the afternoon, with similar conditions possible again on Monday. In this moisture rich environment with MLCAPE values between 1500-2000 J/kg, only subtle forcing will be needed to initiate showers and thunderstorms.

A few thunderstorms may become strong on Sunday as steep low- level lapse rates and modest low-level shear support storm clustering capable of producing damaging wind gusts. Deeper bounday layer mixing is expected tomorrow afternoon, resulting in inverted-V soundings and DCAPE values between 700-1000 J/kg, which would favor strong downdraft and damaging winds. The extent of Sundays convective threat will depend in part on how todays convection evolves, with the highest risk of severe weather expected north and east of I-26. The latest CAMs suggest a weakening MCS may move across neighboring FAs Sunday midday. While the system is expect to weaken as it approaches, it could still provide sufficient forcing for a few stronger thunderstorms, particularly if it arrives during peak heating. This potential is why the SPC has highlighted our northern and eastern CWA in a Marginal (1/5) severe weather risk on Sunday.

Lingering moisture is expected to bring another afternoon/evening with scattered showers and thunderstorms on Monday. However, there is less confidence for any isolated strong to severe thunderstorms.

Key Message 2: Dangerous heat possible again on Sunday, with a signal for a more impactful, intense heat event by the end of next week.

The primary story over the next 7-10 days will be a prolonged stretch of hot and humid conditions. A strengthening upper-level ridge will become established over the lower MS Valley this weekend before gradually expanding eastward. By the middle of next week, most deterministic and ensemble guidance places the center of the closed ridge over the eastern CONUS, supporting afternoon highs in the mid to upper 90s with dew points in the low to mid 70s. Heat indices are are expected to range between 100-105F today and tomorrow, although scattered afternoon convection could provide localized relief.

Monday and Tuesday will serve as transition days as a deep western US trough amplifies the downstream ridge to near the 99.5th percentile of the NAEFS climatology. Temperatures will remain hot, generally in the mid to upper 90s, but strengthening surface high pressure over the northeast should advect slightly drier air midweek. Lower dew points combined with deeper afternoon mixing and low-level subsidence beneath the ridge should allow heat indices to back off a bit and be closer to the upper 90s to around 100 degrees while also suppressing any convective development through much of the work week.

By late week, and into the holiday weekend, guidance continues to maintain a strong upper ridge, with 850-mb temperatures approaching or exceeding 20C. This pattern favors temperatures increasing again, with highs climbing into the upper 90s and potentially 100 degrees, especially on Friday and Saturday. Humidity is also likely to increase resulting in heat indices that could exceed 105 degrees and potentially reach our Heat Advisory Criteria. However, afternoon convection could locally temper max values, but confidence remains too low in coverage to significantly alter the heat forecast. Be sure to take heat safety precautions and monitor children, the elderly, and pets.

One last thing to note, as of this morning, NHC put out a 20% chance for cyclone formation over the next 7 days just off the SC coast. Confidence remains low, as ensemble guidance continues to show considerable spread on development and track. However, any system could alter the expected heat pattern heading into the Independence Day Holiday, making this feature worth monitoring over the coming days.

AVIATION /00Z SUNDAY THROUGH THURSDAY/

VFR conditions likely outside of afternoon convection Sunday.

Showers continue to diminish this evening with low chances of any impacts to the terminals after 00Z. A 20 kt LLJ tonight will keep the low levels relatively mixed with variable winds up to 5 kts. Convective debris cloud should also help to limit the threat of fog. Similar to the previous night we expect conditions to remain generally VFR with a chance of brief restrictions at OGB or AGS.

Sunday morning into mid-day, outflow from overnight convection in KY/TN is expected to push into the Carolinas. Models show any remaining showers or thunderstorms decaying through the morning and are unlikely to cause any impact to the terminals prior to 18Z. However there is a small chance of showers between 12Z and 18Z. After 18Z, new thunderstorms should develop along the old outflow boundary and will bring a threat of gusty winds and heavy rain. Therefore we have added a PROB30 group for Sunday afternoon to include thunderstorms in the forecast. West winds are expected Sunday after 15Z with gusts to 20 kts.

EXTENDED AVIATION OUTLOOK...Afternoon convection will develop again Monday but is less likely for the remainder of the week as ridging builds overhead. Brief early morning restrictions also possible early next week.

CAE WATCHES/WARNINGS/ADVISORIES

SC...None. GA...None.