textproduct: Columbia

WHAT HAS CHANGED

A low end severe threat is possible both today and Sunday with scattered convection. Aviation discussion updated for 18z TAFs.

KEY MESSAGES

- 1. Scattered afternoon showers and storms each day through Monday. Isolated severe storms possible today and Sunday.

- 2. Prolonged hot pattern expected with dangerous heat possible this weekend and a signal for a more impactful, intense heat event by the end of next week.

DISCUSSION

Key Message 1: Scattered afternoon showers and storms each day through Monday. Isolated severe storms possible today and Sunday.

As upper-level ridging begins to build into the region next week, a couple of weak shortwaves are expected to pass nearby today and Sunday. Dew points into the low to mid 70s, PWATs around 2.0", and strong surface heating will promote robust destabilization each afternoon this weekend, with similar conditions possible again on Monday. In this moisture rich environment with MLCAPE values between 1500-2000 J/kg, only subtle forcing is needed to initiate showers and thunderstorms. A few storms may become strong to severe, as steep low-level lapse rates and modest low-level shear support storm clustering capable of producing damaging wind gusts.

Another round of convection is possible on Sunday as an upper- level ridge strengthens over the lower MS Valley. This pattern will place the area beneath northwesterly flow, allowing convection to propagate downstream from the ridge axis. 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. 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 storms, particularly if it arrives during peak heating. SPC has just put much of the north and eastern portions of the CWA in a Marginal (Level 1) risk for severe weather tomorrow.

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

Key Message 2: Prolonged hot pattern expected with dangerous heat possible this weekend and 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 /18Z SATURDAY THROUGH THURSDAY/

VFR conditions likely into this evening with scattered storms expected to periodically impact sites.

Unsettled weather is forecast to continue today with another round of showers and thunderstorms developing currently. These storms are generally expected to remain relatively tame in the next hour or two but as convection becomes a bit more widespread, expect a strong storm or two to potentially give a TAF site a wind gust exceeding 30 knots. Hard to pinpoint when and where the storms are most likely at this point so have a generic Prob30 group in through this evening. This may need to be amended later to account for any widespread storms near a site. As we get through this evening, things should generally clear up after 00z-02z as the sun sets. Debris cloudiness is likely to hang around for several hours before clearing up overnight. Look for clear skies into tomorrow morning before cumulus develops again by late morning. Winds through the period should oscillate between light and variable and southwesterly between 6 and 12 knots.

EXTENDED AVIATION OUTLOOK...Diurnal convection and associated restrictions remain possible Sunday and into the early week before slightly drier air limits rain chances later in the week. Brief morning vsby/cig restrictions also possible into early next week.

CAE WATCHES/WARNINGS/ADVISORIES

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